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Multiple Andreev reflections in a quantum dot coupled to
superconductors: Effects of spin-orbit coupling: We study the out-of-equilibrium current through a multilevel quantum dot
contacted to two superconducting leads and in the presence of Rashba and
Dresselhaus spin-orbit couplings, in the regime of strong dot-lead coupling.
The multiple Andreev reflection (MAR) subgap peaks in the current voltage
characteristics are found to be modified (but not suppressed) by the spin-orbit
interaction, in a way that strongly depends on the shape of the dot confining
potential. In a perfectly isotropic dot the MAR peaks are enhanced when the
strength $\alpha_R$ and $\alpha_D$ of Rashba and Dresselhaus terms are equal.
When the anisotropy of the dot confining potential increases the dependence of
the subgap structure on the spin-orbit angle
$\theta=\arctan(\alpha_D/\alpha_R)$ decreases. Furthermore, when an in-plane
magnetic field is applied to a strongly anisotropic dot, the peaks of the
non-linear conductance oscillate as a function of the magnetic field angle, and
the location of the maxima and minima allows for a straightforward read-out of
the spin-orbit angle $\theta$. | cond-mat_mes-hall |
Emergence of a negative charging energy in a metallic dot capacitively
coupled to a superconducting island: We consider the hybrid setup formed by a metallic dot, capacitively coupled
to a superconducting island S connected to a bulk superconductor by a Josephson
junction. Charge fluctuations in S act as a dynamical gate and overscreen the
electronic repulsion in the metallic dot, producing an attractive interaction
between two additional electrons. As the offset charge of the metallic dot is
increased, the dot charging curve shows positive steps ($+2e$) followed by
negative ones ($-e$) signaling the occurrence of a negative differential
capacitance. A proposal for experimental detection is given, and potential
applications in nanoelectronics are mentioned. | cond-mat_mes-hall |
Spectral signatures of high-symmetry quantum dots and effects of
symmetry breaking: A sequence of photoluminescence spectroscopy based methods are used to
rigorously identify and study all the main spectral features (more than thirty
emission lines) of site controlled InGaAs/AlGaAs quantum dots (QDs) grown along
[111]B in inverted tetrahedral pyramids. The studied QDs reveal signatures of
one confined electron level, one heavy-hole-like level and one light-hole-like
level. The various heavy-light-hole hybrid exciton complexes formed in these
QDs are studied by polarization resolved spectroscopy, excitation power
dependence, crystal temperature dependence and temporal single photon
correlation measurements. The presented approach, which only requires a minimal
theoretical input, enables strict spectral identification of the fine structure
patterns including weak and spectrally overlapping emission lines. Furthermore,
it allows the involved electron-hole and hole-hole exchange interaction
energies to be deduced from measurements. Intricate fine structure patterns are
qualitatively understood by group theory and shown to be very sensitive to the
exact symmetry of the QD. Emission patterns influenced by hole-hole exchange
interactions are found to be particularly useful for identifying QDs with high
$C_{3v}$ symmetry and for probing symmetry breaking. | cond-mat_mes-hall |
Floquet metal to insulator phase transitions in semiconductor nanowires: We study steady-states of semiconductor nanowires subjected to strong
resonant time-periodic drives. The steady-states arise from the balance between
electron-phonon scattering, electron-hole recombination via photo-emission, and
Auger scattering processes. We show that tuning the strength of the driving
field drives a transition between an electron-hole metal (EHM) phase and a
Floquet insulator (FI) phase. We study the critical point controlling this
transition. The EHM-to-FI transition can be observed by monitoring the presence
of peaks in the density-density response function which are associated with the
Fermi momentum of the EHM phase, and are absent in the FI phase. Our results
may help guide future studies towards inducing novel non-equilibrium phases of
matter by periodic driving. | cond-mat_mes-hall |
Interaction of a graphene sheet with a ferromagnetic metal plate: Nanoscale surface forces such as Casimir and the van der Waals forces can
have a significant influence on fabrication, handling and assembly processes as
well as the performance of micro and nano devices. In this paper, the
investigation and the calculation of the Casimir force between a graphene sheet
and a ferromagnetic metal substrate in a vacuum are presented. The reflection
coefficients of graphene are graphene-conductivity dependent, and the
conductivity of graphene is described by the Kubo formalism. There is an effect
of magnetic properties of the metal on the Casimir interaction. The magnetic
effect plays a significant role at low temperatures or high value of chemical
potential. The numerical results also demonstrate that the thickness of a metal
slab has a minor influence on the Casimir force. The investigation and findings
about the Casimir force in this study would lead to useful information and
effective solutions for design and manufacturing of micro and nano devices,
especially in the areas of micro and nano machining, fabrication, manipulation,
assembly and metrology. | cond-mat_mes-hall |
Bright-exciton fine structures splittings in single perovskite
nanocrystals: Although both epitaxial quantum dots (QDs) and colloidal nanocrystals (NCs)
are quantum-confined semiconductor nanostructures, so far they have
demonstrated dramatically-different exciton fine structure splittings (FSSs) at
the cryogenic temperature. The single-QD photoluminescence (PL) is dominated by
the bright-exciton FSS, while it is the energy separation between bright and
dark excitons that is often referred to as the FSS in a single NC. Here we show
that, in single perovskite CsPbI3 NCs synthesized from a colloidal approach, a
bright-exciton FSS as large as hundreds of {\mu}eV can be resolved with two
orthogonally- and linearly-polarized PL peaks. This PL doublet could switch to
a single peak when a single CsPbI3 NC is photo-charged to eliminate the
electron-hole exchange interaction. The above findings have prepared an
efficient platform suitable for probing exciton and spin dynamics of
semiconductor nanostructures at the visible-wavelength range, from which a
variety of practical applications such as in entangled photon-pair source and
quantum information processing can be envisioned. | cond-mat_mes-hall |
Determination of the thickness and orientation of few-layer tungsten
ditelluride using polarized Raman spectroscopy: Orthorhombic tungsten ditelluride (WTe2), with a distorted 1T structure,
exhibits a large magnetoresistance that depends on the orientation, and its
electrical characteristics changes rom semimetallic to insulating as the
thickness decreases. Through polarized Raman spectroscopy in combination with
transmission electron diffraction, we establish a reliable method to determine
the thickness and crystallographic orientation of few-layer WTe2. The Raman
spectrum shows a pronounced dependence on the polarization of the excitation
laser. We found that the separation between two Raman peaks at ~90 cm-1 and at
80-86 cm-1, depending on thickness, is a reliable fingerprint for determination
of the thickness. For determination of the crystallographic orientation, the
polarization dependence of the A1 modes, measured with the 632.8-nm excitation,
turns out to be the most reliable. We also discovered that the polarization
behaviors of some of the Raman peaks depend on the excitation wavelength as
well as thickness, indicating a close interplay between the band structure and
anisotropic Raman scattering cross section. | cond-mat_mes-hall |
Controlled strong coupling and absence of dark polaritons in
microcavities with double quantum wells: We demonstrate an efficient switching between strong and weak exciton-photon
coupling regimes in microcavity-embedded asymmetric double quantum wells,
controlled by an applied electric field. We show that a fine tuning of the
electric field leads to drastic changes in the polariton properties, with the
polariton ground state being red-shifted by a few meV and having acquired
prominent features of a spatially indirect dipolar exciton. We study the
properties of dipolar exciton polaritons, called dipolaritons, on a microscopic
level and show that, unlike recent findings, they are not dark polaritons but,
owing to the finite size of the excition, are mixed states with comparable
contribution of the cavity photon, bright direct, and long-living indirect
exciton modes. | cond-mat_mes-hall |
Quantum Transport in Spin-1 Chiral Fermion: Self-Consistent Born
Approximation: Quantum transport for a spin-1 chiral fermion is studied within the
self-consistent Born approximation. We find characteristic properties around
zero energy, i.e., the peak structure of the density of states and significant
suppression of electrical conductivity. These structures originate from the
flat band and its interband effect. | cond-mat_mes-hall |
Velocity shift of surface acoustic waves due to interaction with
composite fermions in a modulated structure: We study the effect of a periodic density modulation on surface acoustic wave
(SAW) propagation along a 2D electron gas near Landau level filling $\nu=1/2$.
Within the composite fermion theory, the problem is described in terms of
fermions subject to a spatially modulated magnetic field and scattered by a
random magnetic field. We find that a few percent modulation induces a large
peak in the SAW velocity shift, as has been observed recently by Willett et al.
As further support of this theory we find the dc resistivity to be in good
agreement with recent data of Smet et al. | cond-mat_mes-hall |
Tunnel Barrier to Spin Filter: Electronic Transport Characteristics of
Transition Metal Atom Encapsulated in Smallest Cadmium Telluride Cage: We report first principles theory-based comparative electronic transport
studies performed for an atomic chain of Au, bare Cd9Te9 cage-like cluster and
single transition metal (TM) (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd)
atom encapsulated within the Cd9Te9 using Au(111) as electrodes. The bare
cluster is semiconducting and acts as a tunnel barrier up to a particular
applied bias and beyond that, the device has a linear current-voltage
relationship. Several TM (Ti, V, Cr, Mn, Fe) encapsulated in the cage show
half-metallic behavior and spin filtering effect in the I-V characteristics of
the device. A detailed qualitative and quantitative analysis of I-V
characteristics for metallic, semiconducting, and half-metallic nanostructures
has been carried out. | cond-mat_mes-hall |
Antiunitary symmetry protected higher order topological phases: Higher-order topological (HOT) phases feature boundary (such as corner and
hinge) modes of codimension $d_c>1$. We here identify an \emph{antiunitary}
operator that ensures the spectral symmetry of a two-dimensional HOT insulator
and the existence of cornered localized states ($d_c=2$) at precise zero
energy. Such an antiunitary symmetry allows us to construct a generalized HOT
insulator that continues to host corner modes even in the presence of a
\emph{weak} anomalous Hall insulator and a spin-orbital density wave orderings,
and is characterized by a quantized quadrupolar moment $Q_{xy}=0.5$. Similar
conclusions can be drawn for the time-reversal symmetry breaking HOT $p+id$
superconductor and the corner localized Majorana zero modes survive even in the
presence of weak Zeeman coupling and $s$-wave pairing. Such HOT insulators also
serve as the building blocks of three-dimensional second-order Weyl semimetals,
supporting one-dimensional hinge modes. | cond-mat_mes-hall |
Electron spin tomography through counting statistics: a quantum
trajectory approach: We investigate the dynamics of electron spin qubits in quantum dots.
Measurement of the qubit state is realized by a charge current through the dot.
The dynamics is described in the framework of the quantum trajectory approach,
widely used in quantum optics, and we show that it can be applied successfully
to problems in condensed matter physics. The relevant master equation dynamics
is unravelled to simulate stochastic tunneling events of the current through
the dot.Quantum trajectories are then used to extract the counting statistics
of the current. We show how, in combination with an electron spin resonance
(ESR) field, counting statistics can be employed for quantum state tomography
of the qubit state. Further, it is shown how decoherence and relaxation time
scales can be estimated with the help of counting statistics, in the time
domain. Finally, we discuss a setup for single shot measurement of the qubit
state without the need for spin-polarized leads. | cond-mat_mes-hall |
High Frequency Response of Volatile Memristors: In this theoretical study, we focus on the high-frequency response of the
electrothermal NbO2-Mott threshold switch, a real-world electronic device,
which has been proved to be relevant in several applications and is classified
as a volatile memristor. Memristors of this kind, have been shown to exhibit
distinctive non-linear behaviors crucial for cutting-edge neuromorphic
circuits. In accordance with well-established models for these devices, their
resistances depend on their body temperatures, which evolve over time following
Newton's Law of Cooling. Here, we demonstrate that HP's NbO2-Mott memristor can
manifest up to three distinct steady-state oscillatory behaviors under a
suitable high-frequency periodic voltage input, showcasing increased
versatility despite its volatile nature. Additionally, when subjected to a
high-frequency periodic voltage signal, the device body temperature oscillates
with a negligible peak-to-peak amplitude. Since, the temperature remains almost
constant over an input cycle, the devices under study behave as linear
resistors during each input cycle. Based on these insights, this paper presents
analytical equations characterizing the response of the NbO2-Mott memristor to
high-frequency voltage inputs, demarcating regions in the state space where
distinct initial conditions lead to various asymptotic oscillatory behaviors.
Importantly, the mathematical methods introduced in this manuscript are
applicable to any volatile electrothermal resistive switch. Additionally, this
paper presents analytical equations that accurately reproduce the temperature
time-waveform of the studied device during both its transient and steady-state
phases when subjected to a zero-mean sinusoidal voltage input oscillating in
the high-frequency limit. | cond-mat_mes-hall |
Double quantum dot as a probe of nonequilibrium charge fluctuations at
the quantum point contact: Absorption of energy quanta generated by quantum point contact results in the
inelastic current through the double quantum dot placed nearby. In contrast to
a single quantum dot, the inelastic current through the double quantum dot is
sensitive to the energy dependence of the quantum point contact transmission,
which can explain the experimentally observed features. We calculate the
inelastic current as a function of microscopic parameters of the circuit. | cond-mat_mes-hall |
Phonon-dressed Mollow triplet in the regime of cavity-QED: We study the resonance fluorescence spectra of a driven quantum dot placed
inside a high $Q$ semiconductor cavity and interacting with an acoustic phonon
bath. The dynamics is calculated using a time-convolutionless master equation
obtained in the polaron frame. We demonstrate pronounced spectral broadening of
the Mollow sidebands through cavity-emission which, for small cavity-coupling
rates, increases quadratically with the Rabi frequency. However, for larger
cavity coupling rates, this broadening dependence is found to be more complex.
This field-dependent Mollow triplet broadening is primarily a consequence of
the triplet peaks sampling different parts of the asymmetric phonon bath, and
agrees directly with recent experiments with semiconductor micropillars. The
influence from the detuned cavity photon bath and multi-photon effects is shown
to play a qualitatively important role on the fluorescence spectra. | cond-mat_mes-hall |
Effect of van-Hove singularities in single-walled carbon nanotube leads
on transport through double quantum dot system: The double quantum dot system with single-walled metallic armchair carbon
nanotube leads has been studied using Non-equilibrium Green function in the
Keldysh formalism. The effect of relative spacing between the energy levels of
the dots, interdot tunneling matrix-element, interdot Coulomb interaction and
van-Hove singularities in density of states characteristics of
quasi-one-dimensional carbon nanotube leads on the conductance of the double
quantum dot system has been studied. The conductance and dot occupancies are
calculated at finite temperature. It is observed that the density of states of
the carbon nanotube leads play a significant role in determining the
conductance profile. In particular, whenever the chemical potential of the
isolated double quantum dot system is aligned with the position of a van-Hove
singularity in the density of states of armchair carbon nanotube leads, the
height of the corresponding conductance peak falls considerably. It is further
observed that the suppression in the heights of the alternate peaks depends on
the relative positions of the energy levels of the dots and their magnitude of
separation. | cond-mat_mes-hall |
Models of Electrodes and Contacts in Molecular Electronics: Bridging the difference in atomic structure between experiments and
theoretical calculations and exploring quantum confinement effects in thin
electrodes (leads) are both important issues in molecular electronics. To
address these issues, we report here, by using Au-benzenedithiol-Au as a model
system, systematic investigations of different models for the leads and the
lead-molecule contacts: leads with different cross-sections, leads consisting
of infinite surfaces, and surface leads with a local nanowire or atomic chain
of different lengths. The method adopted is a non-equilibrium Green function
approach combined with density functional theory calculations for the
electronic structure and transport, in which the leads and molecule are treated
on the same footing. It is shown that leads with a small cross-section will
lead to large oscillations in the transmission function, T(E), which depend
significantly on the lead structure (orientation) because of quantum waveguide
effects. This oscillation slowly decays as the lead width increases, with the
average approaching the limit given by infinite surface leads. Local nanowire
structures around the contacts induce moderate fluctuations in T(E), while a Au
atomic chain (including a single Au apex atom) at each contact leads to a
significant conductance resonance. | cond-mat_mes-hall |
Efficient electrical spin readout of NV- centers in diamond: Using pulsed photoionization the coherent spin manipulation and echo
formation of ensembles of NV- centers in diamond are detected electrically
realizing contrasts of up to 17 %. The underlying spin-dependent ionization
dynamics are investigated experimentally and compared to Monte-Carlo
simulations. This allows the identification of the conditions optimizing
contrast and sensitivity which compare favorably with respect to optical
detection. | cond-mat_mes-hall |
Large Landau level splitting with tunable one-dimensional graphene
superlattice probed by magneto capacitance measurements: The unique zero energy Landau Level of graphene has a particle-hole symmetry
in the bulk, which is lifted at the boundary leading to a splitting into two
chiral edge modes. It has long been theoretically predicted that the splitting
of the zero-energy Landau level inside the {\it bulk} can lead to many
interesting physics, such as quantum spin Hall effect, Dirac like singular
points of the chiral edge modes, and others. However, so far the obtained
splitting with high-magnetic field even on a hBN substrate are not amenable to
experimental detection, and functionality. Guided by theoretical calculations,
here we produce a large gap zero-energy Landau level splitting ($\sim$ 150 meV)
with the usage of a one-dimensional (1D) superlattice potential. We have
created tunable 1D superlattice in a hBN encapsulated graphene device using an
array of metal gates with a period of $\sim$ 100 nm. The Landau level spectrum
is visualized by measuring magneto capacitance spectroscopy. We monitor the
splitting of the zeroth Landau level as a function of superlattice potential.
The observed splitting energy is an order higher in magnitude compared to the
previous studies of splitting due to the symmetry breaking in pristine
graphene. The origin of such large Landau level spitting in 1D potential is
explained with a degenerate perturbation theory. We find that owing to the
periodic potential, the Landau level becomes dispersive, and acquires sharp
peaks at the tunable band edges. Our study will pave the way to create the
tunable 1D periodic structure for multi-functionalization and device
application like graphene electronic circuits from appropriately engineered
periodic patterns in near future. | cond-mat_mes-hall |
Comment on "Do Intradot Electron-Electron Interactions Induce
Dephasing?": In a recent Letter, Jiang, Sun, Xie and Wang [Phys. Rev. Lett. 93, 076802
(2004), cond-mat/0408261] study transport through an interacting quantum dot
embedded in one arm of an Aharonov-Bohm interferometer. Based on a theoretical
analysis of the Aharonov-Bohm oscillation amplitude, Jiang {\it et al.} claim,
contrary to earlier work by two of us, that at finite temperature the intradot
interaction will {\em not} lead to any dephasing. Likewise, they claim that the
theoretically predicted and experimentally verified asymmetry of the
Aharonov-Bohm oscillation amplitude is {\em not} associated with dephasing. In
this Comment, we point out severe inconsistencies in the analysis of the Letter
by Jian {\it et al.}, and show that their conclusions are ill-founded. | cond-mat_mes-hall |
Spin noise of localized electrons interacting with optically cooled
nuclei: A microscopic theory of spin fluctuations of localized electrons interacting
with optically cooled nuclear spin bath has been developed. Since nuclear spin
temperature may stay low enough for macroscopically long time, the nuclear spin
system becomes very sensitive to an external magnetic field. This strongly
affects electron spin noise spectrum. It has been shown that in the case of
weak fields/relatively high nuclear spin temperature, a small degree of nuclear
spin polarization affect the electron spin fluctuations in the same way as an
additional external magnetic field. By contrast, the high degree of nuclear
polarization realized in relatively strong magnetic field and low nuclear spin
temperature leads to a suppression of hyperfine field fluctuations and to a
dramatic narrowing of precession-induced peak in the spin noise spectrum. The
experimental possibilities of nuclear spin system investigation by means of
spin noise spectroscopy are discussed. | cond-mat_mes-hall |
Strain engineering of the electronic states of silicon-based quantum
emitters: Light-emitting complex defects in silicon have been considered a potential
platform for quantum technologies based on spin and photon degrees of freedom
working at telecom wavelengths. Their integration in complex devices is still
in its infancy, and it was mostly focused on light extraction and guiding. Here
we address the control of the electronic states of carbon-related impurities
(G-centers) via strain engineering. By embedding them in patches of silicon on
insulator and topping them with SiN, symmetry breaking along [001] and [110]
directions is demonstrated, resulting in a controlled splitting of the zero
phonon line (ZPL), as accounted for by the piezospectroscopic theoretical
framework. The splitting can be as large as 18 meV and it is finely tuned by
selecting patch size or by moving in different positions on the patch. Some of
the split, strained ZPLs are almost fully polarized and their overall intensity
is enhanced up to 7 times with respect to the flat areas, whereas their
recombination dynamics is slightly affected. Our technique can be extended to
other impurities and Si-based devices such as suspended bridges, photonic
crystal microcavities, Mie resonators, and integrated photonic circuits. | cond-mat_mes-hall |
Optical Phonons in Twisted Bilayer Graphene with Gate-Induced Asymmetric
Doping: Twisted bilayer graphene (tBLG) devices with ion gel gate dielectrics are
studied using Raman spectroscopy in the twist angle regime where a resonantly
enhanced G band can be observed. We observe prominent splitting and intensity
quenching on the G Raman band when the carrier density is tuned away from
charge neutrality. This G peak splitting is attributed to asymmetric charge
doping in the two graphene layers, which reveals individual phonon self-energy
renormalization of the two weakly-coupled layers of graphene. We estimate the
effective interlayer capacitance at low doping density of tBLG using an
interlayer screening model. The anomalous intensity quenching of both G peaks
is ascribed to the suppression of resonant interband transitions between the
two saddle points (van Hove singularities), that are displaced in the momentum
space by gate-tuning. In addition, we observe a softening (hardening) of the R
Raman band, a superlattice-induced phonon mode in tBLG, in electron (hole)
doping. Our results demonstrate that gate modulation can be used to control the
optoelectronic and vibrational properties in tBLG devices. | cond-mat_mes-hall |
Distribution of waiting times between electron cotunnelings: In the resonant tunneling regime sequential processes dominate single
electron transport through quantum dots or molecules that are weakly coupled to
macroscopic electrodes. In the Coulomb blockade regime, however, cotunneling
processes dominate. Cotunneling is an inherently quantum phenomenon and thus
gives rise to interesting observations, such as an increase in the current shot
noise. Since cotunneling processes are inherently fast compared to the
sequential processes, it is of interest to examine the short time behaviour of
systems where cotunneling plays a role, and whether these systems display
nonrenewal statistics. We consider three questions in this paper. Given that an
electron has tunneled from the source to the drain via a cotunneling or
sequential process, what is the waiting time until another electron cotunnels
from the source to the drain? What are the statistical properties of these
waiting time intervals? How does cotunneling affect the statistical properties
of a system with strong inelastic electron-electron interactions? In answering
these questions, we extend the existing formalism for waiting time
distributions in single electron transport to include cotunneling processes via
an $n$-resolved Markovian master equation. We demonstrate that for a single
resonant level the analytic waiting time distribution including cotunneling
processes yields information on individual tunneling amplitudes. For both a SRL
and an Anderson impurity deep in the Coulomb blockade there is a nonzero
probability for two electrons to cotunnel to the drain with zero waiting time
inbetween. Furthermore, we show that at high voltages cotunneling processes
slightly modify the nonrenewal behaviour of an Anderson impurity with a strong
inelastic electron-electron interaction. | cond-mat_mes-hall |
Bound states in the continuum in a two-channel Fano-Anderson model: In this article, we study the formation of the bound states in the continuum
(BICs) in a two-channel Fano-Anderson model. We employ the Green's function
formalism, together with the equation of motion method, to analyze the relevant
observables, such as the transmission coefficient and the density of states.
Most importantly, our results show that the system hosts true BICs for the case
of a symmetric configuration with the degenerate impurity levels, and a
complete transmission channel is then suppressed. Finally, we argue that the
proposed mechanism could be relevant for the realization of BICs in the
electronic and photonic systems. | cond-mat_mes-hall |
Magnetization reversal in Py/Gd heterostructures: Using a combination of magnetometry and magnetotransport techniques, we
studied temperature and magnetic field behavior of magnetization in Py/Gd
heterostructures. It was shown quantitatively that proximity with Py enhances
magnetic order of Gd. Micromagnetic simulations demonstrate that a spin-flop
transition observed in a Py/Gd bilayer is due to exchange-spring rotation of
magnetization in the Gd layer. Transport measurements show that the
magnetoresistance of a [Py(2 nm)/Gd(2 nm)]25 multilayer changes sign at the
compensation temperature and below 20 K. The positive magnetoresistance above
the compensation temperature can be attributed to an in-plane domain-wall,
which appears because of the structural inhomogeneity of the film over its
thickness. By measuring the angular dependence of resistance we are able to
determine the angle between magnetizations in the multilayer and the magnetic
field at different temperatures. The measurement reveals that due to a change
of the chemical thickness profile, a non-collinear magnetization configuration
is only stable in magnetic fields above 10 kOe. | cond-mat_mes-hall |
Waveguide-integrated single-crystalline GaP resonators on diamond: Large-scale entanglement of nitrogen-vacancy (NV) centers in diamond will
require integration of NV centers with optical networks. Toward this goal, we
present the fabrication of single-crystalline gallium phosphide (GaP)
resonator-waveguide coupled structures on diamond. We demonstrate coupling
between 1 {\mu}m diameter GaP disk resonators and waveguides with a loaded Q
factor of 3,800, and evaluate their potential for efficient photon collection
if integrated with single photon emitters. This work opens a path toward
scalable NV entanglement in the hybrid GaP/diamond platform, with the potential
to integrate on-chip photon collection, switching, and detection for
applications in quantum information processing. | cond-mat_mes-hall |
Spectroscopy and level detuning of few-electron spin states in parallel
InAs quantum dots: We use tunneling spectroscopy to study the evolution of few-electron spin
states in parallel InAs nanowire double quantum dots (QDs) as a function of
level detuning and applied magnetic field. Compared to the much more studied
serial configuration, parallel coupling of the QDs to source and drain greatly
expands the probing range of excited state transport. Owing to a strong
confinement, we can here isolate transport involving only the very first
interacting single QD orbital pair. For the (2,0)-(1,1) charge transition, with
relevance for spin-based qubits, we investigate the excited (1,1) triplet, and
hybridization of the (2,0) and (1,1) singlets. An applied magnetic field splits
the (1,1) triplet, and due to spin-orbit induced mixing with the (2,0) singlet,
we clearly resolve transport through all triplet states near the avoided
singlet-triplet crossings. Transport calculations, based on a simple model with
one orbital on each QD, fully replicate the experimental data. Finally, we
observe an expected mirrored symmetry between the 1-2 and 2-3 electron
transitions resulting from the two-fold spin degeneracy of the orbitals. | cond-mat_mes-hall |
Quantum superposition of a single microwave photon in two different
"colour" states: The ability to coherently couple arbitrary harmonic oscillators in a
fully-controlled way is an important tool to process quantum information.
Coupling between quantum harmonic oscillators has previously been demonstrated
in several physical systems by use of a two-level system as a mediating
element. Direct interaction at the quantum level has only recently been
realized by use of resonant coupling between trapped ions. Here we implement a
tunable direct coupling between the microwave harmonics of a superconducting
resonator by use of parametric frequency conversion. We accomplish this by
coupling the mode currents of two harmonics through a superconducting quantum
interference device (SQUID) and modulating its flux at the difference (~ 7 GHz)
of the harmonic frequencies. We deterministically prepare a single-photon Fock
state and coherently manipulate it between multiple modes, effectively
controlling it in a superposition of two different "colours". This parametric
interaction can be described as a beam-splitter-like operation that couples
different frequency modes. As such, it could be used to implement linear
optical quantum computing protocols on-chip. | cond-mat_mes-hall |
Coherent Radiation by Quantum Dots and Magnetic Nanoclusters: The assemblies of either quantum dots or magnetic nanoclusters are studied.
It is shown that such assemblies can produce coherent radiation. A method is
developed for solving the systems of nonlinear equations describing the
dynamics of such assemblies. The method is shown to be general and applicable
to systems of different physical nature. Despite mathematical similarities of
dynamical equations, the physics of the processes for quantum dots and magnetic
nanoclusters is rather different. In a quantum dot assembly, coherence develops
due to the Dicke effect of dot interactions through the common radiation field.
For a system of magnetic clusters, coherence in the spin motion appears due to
the Purcell effect caused by the feedback action of a resonator. Self-organized
coherent spin radiation cannot arise without a resonator. This principal
difference is connected with the different physical nature of dipole forces
between the objects. Effective dipole interactions between the radiating
quantum dots, appearing due to photon exchange, collectivize the dot radiation.
While the dipolar spin interactions exist from the beginning, yet before
radiation, and on the contrary, they dephase spin motion, thus destroying the
coherence of moving spins. In addition, quantum dot radiation exhibits
turbulent photon filamentation that is absent for radiating spins. | cond-mat_mes-hall |
Negative differential magneto-resistance in ferromagnetic wires with
domain walls: A domain wall in a ferromagnetic one-dimensional nanowire experiences current
induced motion due to its coupling with the conduction electrons. When the
current is not sufficient to drive the domain wall through the wire, or it is
confined to a perpendicular layer, it nonetheless experiences oscillatory
motion. In turn, this oscillatory motion of the domain wall can couple
resonantly with the electrons in the system affecting the transport properties
further. We investigate the effect of the coupling between these domain wall
modes and the current electrons on the transport properties of the system and
show that such a system demonstrates negative differential magnetoresistance
due to the resonant coupling with the low-lying modes of the domain wall
motion. | cond-mat_mes-hall |
Mu-Metal Enhancement of Effects in Electromagnetic Fields Over Single
Emitters Near Topological Insulators: We focus on the transmission and reflection coefficients of light in systems
involving of topological insulators (TI). Due to the electro-magnetic coupling
in TIs, new mixing coefficients emerge leading to new components of the
electromagnetic fields of propagating waves. We have discovered a simple
heterostructure that provides a 100-fold enhancement of the mixing coefficients
for TI materials. Such effect increases with the TI's wave impedance. We also
predict a transverse deviation of the Poynting vector due to these mixed
coefficients contributing to the radiative electromagnetic field of an electric
dipole. Given an optimal configuration of the dipole-TI system, this deviation
could amount to $0.18\%$ of the Poynting vector due to emission near not
topological materials, making this effect detectable. | cond-mat_mes-hall |
Ultrafast switchable spin-orbit coupling for silicon spin qubits via
spin valves: Recent experimental breakthroughs, particularly for single-qubit and
two-qubit gates exceeding the error correction threshold, highlight silicon
spin qubits as leading candidates for fault-tolerant quantum computation. In
the existing architecture, intrinsic or synthetic spin-orbit coupling (SOC) is
critical in various aspects, including electrical control, addressability,
scalability, etc. However, the high-fidelity SWAP operation and quantum state
transfer (QST) between spin qubits, crucial for qubit-qubit connectivity,
require the switchable nature of SOC which is rarely considered. Here, we
propose a flexible architecture based on spin valves by electrically changing
its magnetization orientation within sub-nanoseconds to generate ultrafast
switchable SOC. Based on the switchable SOC architecture, both SWAP operation
of neighbor spin qubits and resonant QST between distant spins can be realized
with fidelity exceeding 99% while considering the realistic experimental
parameters. Benefiting from the compatible processes with the modern
semiconductor industry and experimental advances in spin valves and spin
qubits, our results pave the way for future construction of silicon-based
quantum chips. | cond-mat_mes-hall |
A semiclassical treatment of spinor topological effects in driven,
inhomogeneous insulators under external electromagnetic fields: Introducing internal degrees of freedom in the description of topological
insulators has led to a myriad of theoretical and experimental advances. Of
particular interest are the effects of periodic perturbations, either in time
or space, as they considerably enrich the variety of electronic responses, with
examples such as Thouless's charge pump and its higher dimensional cousins, or,
higher-order topological insulators. Here, we develop a semiclassical approach
to transport and accumulation of general spinor degrees of freedom, such as
physical spin, valley, or atomic orbits, in adiabatically driven, weakly
inhomogeneous insulators of dimensions one, two and three under external
electromagnetic fields. Specifically, we focus on physical spins and derive the
spin current and density up to third order in the spatio-temporal modulations
of the system. We, then, relate these contributions to geometrical and
topological objects -- the spin-Chern fluxes and numbers -- defined over the
higher-dimensional phase-space of the system, i.e., its combined
momentum-position-time coordinates. Furthermore, we provide a connection
between our semiclassical analysis and the modern theory of multipole moments
by introducing spin analogues of the electric dipole, quadrupole and octapole
moments. The results are showcased in concrete tight-binding models where the
induced responses are calculated analytically. | cond-mat_mes-hall |
Quantum oscillations in two coupled charge qubits: Despite an apparent progress in implementing individual solid-state qubits,
there have been no experimental reports so far on multi-bit gates required for
building a real quantum computer. Here we report a new circuit comprising two
coupled charge qubits. Using a pulse technique, we coherently mix quantum
states and observe quantum oscillations whose spectrum reflects interaction
between the qubits. Our results demonstrate the feasibility of coupling of
multiple solid-state qubits and indicate the existence of entangled two-qubit
states. | cond-mat_mes-hall |
Quantum Shuttle in Phase Space: We present a quantum theory of the shuttle instability in electronic
transport through a nanostructure with a mechanical degree of freedom. A phase
space formulation in terms of the Wigner function allows us to identify a
cross-over from the tunnelling to the shuttling regime, thus extending the
previously found classical results to the quantum domain. Further, a new
dynamical regime is discovered, where the shuttling is driven exclusively by
the quantum noise. | cond-mat_mes-hall |
Engineering single donor detectors in doped silicon: We demonstrate the possibility of engineering a single donor transistor
directly from a phosphorous doped quantum dot by making use of the intrinsic
glassy behaviour of the structure as well as the complex electron dynamics
during cooldown. Characterisation of the device at low temperatures and in
magnetic field shows single donors can be electrostatically isolated near one
of the tunnel barrier with either a single or a doubly occupancy. Such a model
is well supported by capacitance-based simulations. Ability of using the D0 of
such isolated donor as a charge detector is demonstrated by observing the
charge stability diagram of a nearby and capacitively coupled semi-connected
double quantum dot. | cond-mat_mes-hall |
Dielectric screening in two-dimensional insulators: Implications for
excitonic and impurity states in graphane: For atomic thin layer insulating materials we provide an exact analytic form
of the two-dimensional screened potential. In contrast to three-dimensional
systems where the macroscopic screening can be described by a static dielectric
constant in 2D systems the macroscopic screening is non local (q-dependent)
showing a logarithmic divergence for small distances and reaching the
unscreened Coulomb potential for large distances. The cross-over of these two
regimes is dictated by 2D layer polarizability that can be easily computed by
standard first-principles techniques. The present results have strong
implications for describing gap-impurity levels and also exciton binding
energies. The simple model derived here captures the main physical effects and
reproduces well, for the case of graphane, the full many-body GW plus
Bethe-Salpeter calculations. As an additional outcome we show that the impurity
hole-doping in graphane leads to strongly localized states, what hampers
applications in electronic devices. In spite of the inefficient and nonlocal
two-dimensional macroscopic screening we demonstrate that a simple
$\mathbf{k}\cdot\mathbf{p}$ approach is capable to describe the electronic and
transport properties of confined 2D systems. | cond-mat_mes-hall |
A topological look at the quantum spin Hall state: We propose a topological understanding of the quantum spin Hall state without
considering any symmetries, and it follows from the gauge invariance that
either the energy gap or the spin spectrum gap needs to close on the system
edges, the former scenario generally resulting in counterpropagating gapless
edge states. Based upon the Kane-Mele model with a uniform exchange field and a
sublattice staggered confining potential near the sample boundaries, we
demonstrate the existence of such gapless edge states and their robust
properties in the presence of impurities. These gapless edge states are
protected by the band topology alone, rather than any symmetries. | cond-mat_mes-hall |
Coulomb scattering cross-section in a 2D electron gas and production of
entangled electrons: We calculate the Coulomb scattering amplitude for two electrons injected with
opposite momenta in an interacting 2DEG. We include the effect of the Fermi
liquid background by solving the 2D Bethe-Salpeter equation for the
two-particle Green function vertex, in the ladder and random phase
approximations. This result is used to discuss the feasibility of producing
spin EPR pairs in a 2DEG by collecting electrons emerging from collisions at a
pi/2 scattering angle, where only the entangled spin-singlets avoid the
destructive interference resulting from quantum indistinguishability.
Furthermore, we study the effective 2D electron-electron interaction due to the
exchange of virtual acoustic and optical phonons, and compare it to the Coulomb
interaction. Finally, we show that the 2D Kohn-Luttinger pairing instability
for the scattering electrons is negligible in a GaAs 2DEG. | cond-mat_mes-hall |
Strong Band Hybridization between Silicene and Ag(111)Substrate: By using first-principles calculations, we systematically investigated
several observed phases of silicene on Ag(111) substrates and their electronic
structures. We find that the original Dirac cone of silicene is about 1.5-1.7
eV deeply below the Fermi level and severely destroyed by the band
hybridization between silicene and Ag in all the examined phases. Thus,
silicene synthesized on Ag(111) substrates could not preserve its excellent
electronic property and new method is needed to develop in synthesizing
silicene with its Dirac cone surviving. | cond-mat_mes-hall |
Fano-shaped impurity spectral density, electric-field-induced in-gap
state and local magnetic moment of an adatom on trilayer graphene: Recently, the existence of local magnetic moment in a hydrogen adatom on
graphene has been confirmed experimentally [Gonz\'{a}lez-Herrero et al.,
Science, 2016, 352, 437]. Inspired by this breakthrough, we theoretically
investigate the top-site adatom on trilayer graphene (TLG) by solving the
Anderson impurity model via self-consistent mean field method. The influence of
the stacking order, the adsorption site and external electric field are
carefully considered. We find that, due to its unique electronic structure, the
situation of the TLG is drastically different from that of the monolayer
graphene. Firstly, the adatom on rhombohedral stacked TLG (r-TLG) can have a
Fano-shaped impurity spectral density, instead of the normal Lorentzian-like
one, when the impurity level is around the Fermi level. Secondly, the impurity
level of the adatom on r-TLG can be tuned into an in-gap state by an external
electric field, which strongly depends on the direction of the applied electric
field and can significantly affect the local magnetic moment formation.
Finally, we systematically calculate the impurity magnetic phase diagrams,
considering various stacking orders, adsorption sites, doping and electric
field. We show that, because of the in-gap state, the impurity magnetic phase
of r-TLG will obviously depend on the direction of the applied electric field
as well. All our theoretical results can be readily tested in experiment, and
may give a comprehensive understanding about the local magnetic moment of
adatom on TLG. | cond-mat_mes-hall |
Transfer matrix solution of the Wako-Saitô-Muñoz-Eaton model
augmented by arbitrary short range interactions: The Wako-Sait{\^o}-Mu\~noz-Eaton (WSME) model, initially introduced in the
theory of protein folding, has also been used in modeling the RNA folding and
some epitaxial phenomena. The advantage of this model is that it admits exact
solution in the general inhomogeneous case (Bruscolini and Pelizzola, 2002)
which facilitates the study of realistic systems. However, a shortcoming of the
model is that it accounts only for interactions within continuous stretches of
native bonds or atomic chains while neglecting interstretch (interchain)
interactions. But due to the biopolymer (atomic chain) flexibility, the
monomers (atoms) separated by several non-native bonds along the sequence can
become closely spaced. This produces their strong interaction. The inclusion of
non-WSME interactions into the model makes the model more realistic and
improves its performance. In this study we add arbitrary interactions of finite
range and solve the new model by means of the transfer matrix technique. We can
therefore exactly account for the interactions which in proteomics are
classified as medium- and moderately long-range ones. | cond-mat_mes-hall |
A spin dephasing mechanism mediated by the interplay between the
spin-orbit coupling and the asymmetrical confining potential in semiconductor
quantum dot: Understanding the spin dephasing mechanism is of fundamental importance in
all potential applications of the spin qubit. Here we demonstrate a spin
dephasing mechanism in semiconductor quantum dot due to the $1/f$ charge noise.
The spin-charge interaction is mediated by the interplay between the spin-orbit
coupling and the asymmetrical quantum dot confining potential. The dephasing
rate is proportional to both the strength of the spin-orbit coupling and the
degree of the asymmetry of the confining potential. For parameters typical of
the InSb, InAs, and GaAs quantum dots with a moderate well-height $V_{0}=10$
meV, we find the spin dephasing times are ${\rm T}^{*}_{2}=7$ $\mu$s, $275$
$\mu$s, and $55$ ms, respectively. In particular, the spin dephasing can be
enhanced by lowering the well-height. When the well-height is as small as
$V_{0}=5$ meV, the spin depahsing times in the InSb, InAs, and GaAs quantum
dots are decreased to ${\rm T}^{*}_{2}=0.38$ $\mu$s, $18$ $\mu$s, and $9$ ms,
respectively. | cond-mat_mes-hall |
Origin of thermoelectric response fluctuations in single-molecule
junctions: The thermoelectric response of molecular junctions exhibits large
fluctuations, as observed in recent experiments [e.g. Malen J. A. {\sl et al.},
Nano Lett. {\bf 10}, 3406 (2009)]. These were attributed to fluctuations in the
energy alignment between the highest occupied molecular orbital (HOMO) and the
Fermi level at the electrodes. By analyzing these fluctuations assuming
resonant transport through the HOMO level, we demonstrate that fluctuations in
the HOMO level alone cannot account for the observed fluctuations in the
thermopower, and that the thermo-voltage distributions obtained using the most
common method, the Non-equilibrium Green's function method, are qualitatively
different than those observed experimentally. We argue that this inconsistency
between the theory and experiment is due to the level broadening, which is
inherently built-in to the method, and smears out any variations of the
transmission on energy scales smaller than the level broadening. We show that
although this smearing only weakly affects the transmission, it has a large
effect on the calculated thermopower. Using the theory of open quantum systems
we account for both the magnitude of the variations and the qualitative form of
the distributions, and show that they arise not only from variations in the
HOMO-Fermi level offset, but also from variations of the local density of
states at the contact point between the molecule and the electrode. | cond-mat_mes-hall |
Anomalous Dynamical Behavior of Freestanding Graphene Membranes: We report subnanometer, high-bandwidth measurements of the out-of-plane
(vertical) motion of atoms in freestanding graphene using scanning tunneling
microscopy. By tracking the vertical position over a long time period, a
1000-fold increase in the ability to measure space-time dynamics of atomically
thin membranes is achieved over the current state-of-the-art imaging
technologies. We observe that the vertical motion of a graphene membrane
exhibits rare long-scale excursions characterized by both anomalous
mean-squared displacements and Cauchy-Lorentz power law jump distributions. | cond-mat_mes-hall |
Hamiltonian Description of Composite Fermions: Magnetoexciton
Dispersions: A microscopic Hamiltonian theory of the FQHE, developed by Shankar and myself
based on the fermionic Chern-Simons approach, has recently been quite
successful in calculating gaps in Fractional Quantum Hall states, and in
predicting approximate scaling relations between the gaps of different
fractions. I now apply this formalism towards computing magnetoexciton
dispersions (including spin-flip dispersions) in the $\nu=1/3$, 2/5, and 3/7
gapped fractions, and find approximate agreement with numerical results. I also
analyse the evolution of these dispersions with increasing sample thickness,
modelled by a potential soft at high momenta. New results are obtained for
instabilities as a function of thickness for 2/5 and 3/7, and it is shown that
the spin-polarized 2/5 state, in contrast to the spin-polarized 1/3 state,
cannot be described as a simple quantum ferromagnet. | cond-mat_mes-hall |
Hinge solitons in three-dimensional second-order topological insulators: A second-order topological insulator in three dimensions refers to a
topological insulator with gapless states localized on the hinges, which is a
generalization of a traditional topological insulator with gapless states
localized on the surfaces. Here we theoretically demonstrate the existence of
stable solitons localized on the hinges of a second-order topological insulator
in three dimensions when nonlinearity is involved. By means of systematic
numerical study, we find that the soliton has strong localization in real space
and propagates along the hinge unidirectionally without changing its shape. We
further construct an electric network to simulate the second-order topological
insulator. When a nonlinear inductor is appropriately involved, we find that
the system can support a bright soliton for the voltage distribution
demonstrated by stable time evolution of a voltage pulse. | cond-mat_mes-hall |
Conduction Mechanism in a Molecular Hydrogen Contact: We present first principles calculations for the conductance of a hydrogen
molecule bridging a pair of Pt electrodes. The transmission function has a wide
plateau with T~1 which extends across the Fermi level and indicates the
existence of a single, robust conductance channel with nearly perfect
transmission. Through a detailed Wannier function analysis we show that the H2
bonding state is not involved in the transport and that the plateau forms due
to strong hybridization between the H2 anti-bonding state and states on the
adjacent Pt atoms. The Wannier functions furthermore allow us to derive a
resonant-level model for the system with all parameters determined from the
fully self-consistent Kohn-Sham Hamiltonian. | cond-mat_mes-hall |
Electronic structure of graphene-nanoribbons on hexagonal boron nitride: Hexagonal boron nitride is an ideal dielectric to form two-dimensional
heterostructures due to the fact that it can be exfoliated to be just few atoms
thick and its a very low density of defects. By placing graphene nanoribbons on
high quality hexagonal boron nitride it is possible to create ideal quasi one
dimensional (1D) systems with very high mobility. The availability of high
quality one-dimensional electronic systems is of great interest also given that
when in proximity to a superconductor they can be effectively engineered to
realize Majorana bound states. In this work we study how a boron nitride
substrate affects the electronic properties of graphene nanoribbons. We
consider both armchair and zigzag nanoribbons. Our results show that for some
stacking configurations the boron nitride can significantly affect the
electronic structure of the ribbons. In particular, for zigzag nanoribbons, due
to the lock between spin and sublattice degree of freedom at the edges, the
hexagonal boron nitride can induce a very strong spin-splitting of the spin
polarized, edge sates. We find that such spin-splitting can be as high as
40~meV. | cond-mat_mes-hall |
Multiwalled Carbon Nanotubes as Building Blocks in Nanoelectronics: Molecular level components, like carbon multiwalled nanotubes (MWNT), show
great potential for future nanoelectronics. At low frequencies, only the
outermost carbon layer determines the transport properties of the MWNT. Due to
the multiwalled structure and large capacitive interlayer coupling, also the
inner layers contribute to the conduction at high frequencies. Consequently,
the conduction properties of MWNTs are not very far from those of regular
conductors with well-defined electrical characteristics. In our work we have
experimentally utilized this fact in constructing various nanoelectronic
components out of MWNTs, such as single electron transistors (SET), lumped
resistors, and transmission lines. We present results on several nano- tube
samples, grown both using chemical vapor deposition as well as arc-discharge
vaporization. Our results show that SET-electrometers with a noise level as low
as 6x10^{-6}e/\sqrt{Hz} (at 45 Hz) can be built using arc-discharge-grown
carbon nanotubes. Moreover, short nanotubes with small contact areas are found
to work at 4.2 K with good gate modulation. Reactive ion etching on CVD tubes
is employed to produce nearly Ohmic components with a resistance of 200 kOhm
over a 2 micron section. At high frequencies, MWNTs work over micron distances
as special LC-transmission lines with high impedance, on the order of 5 kOhm. | cond-mat_mes-hall |
Non-Hermitian Chiral Magnetic Effect in Equilibrium: We analyze the Chiral Magnetic Effect for non-Hermitian fermionic systems
using the biorthogonal formulation of quantum mechanics. In contrast to the
Hermitian chiral counterparts, we show that the Chiral Magnetic Effect may take
place in thermal equilibrium of an open non-Hermitian system with, generally,
massive fermions. The key observation is that for non-Hermitian charged
systems, there is no strict charge conservation as understood in the Hermitian
case, so the Bloch theorem preventing currents in the thermodynamic limit in
equilibrium does not apply. | cond-mat_mes-hall |
Moiré Flat Bands of Twisted Few-layer Graphite: We report that the twisted few layer graphite (tFL-graphite) is a new family
of moir\'{e} heterostructures (MHSs), which has richer and highly tunable
moir\'{e} flat band structures entirely distinct from all the known MHSs. A
tFL-graphite is composed of two few-layer graphite (Bernal stacked multilayer
graphene), which are stacked on each other with a small twisted angle. The
moir\'{e} band structure of the tFL-graphite strongly depends on the layer
number of its composed two van der Waals layers. Near the magic angle, a
tFL-graphite always has two nearly flat bands coexisting with a few pairs of
narrowed dispersive (parabolic or linear) bands at the Fermi level, thus,
enhances the DOS at $E_F$. This coexistence property may also enhance the
possible superconductivity as been demonstrated in other multiband
superconductivity systems. Therefore, we expect strong multiband correlation
effects in tFL-graphite. Meanwhile, a proper perpendicular electric field can
induce several isolated nearly flat bands with nonzero valley Chern number in
some simple tFL-graphites, indicating that tFL-graphite is also a novel
topological flat band system. | cond-mat_mes-hall |
Robust Ferromagnetism in Silicene Nanoflakes through Patterned
Hydrogenation: Considerably different properties emerge in nanomaterials as a result of
quantum confinement and edge effects. In this study, the electronic and
magnetic properties of quasi zero dimensional silicene nanoflakes (SiNFs) are
investigated using first principles calculations. Whilst the zigzag edged
hexagonal SiNFs exhibit nonmagnetic semiconducting character, the zigzag edged
triangular SiNFs are magnetic semiconductors. One effective method of
harnessing the properties of silicene is hydrogenation owing to its
reversibility and controllability. From bare SiNFs to half hydrogenated and
then to fully hydrogenated, a triangular SiNF experiences a change from
ferrimagnetic to very strong ferromagnetic, and then to non-magnetic.
Nonetheless, a hexagonal SiNF undergoes a transfer from nonmagnetic to very
strong ferromagnetic, then to nonmagnetic. The half hydrogenated SiNFs produce
a large spin moment that is directly proportional to the square of the flakes
size. It has been revealed that the strong induced spin magnetizations align
parallel and demonstrates a collective character by large range ferromagnetic
exchange coupling, giving rise to its potential use in spintronic circuit
devices. Spin switch models are offered as an example of one of the potential
applications of SiNFs in tuning the transport properties by controlling the
hydrogen coverage. | cond-mat_mes-hall |
Impact of electrode density of states on transport through
pyridine-linked single molecule junctions: We study the impact of electrode band structure on transport through
single-molecule junctions by measuring the conductance of pyridine-based
molecules using Ag and Au electrodes. Our experiments are carried out using the
scanning tunneling microscope based break-junction technique and are supported
by density functional theory based calculations. We find from both experiments
and calculations that the coupling of the dominant transport orbital to the
metal is stronger for Au-based junctions when compared with Ag-based junctions.
We attribute this difference to relativistic effects, which results in an
enhanced density of d-states at the Fermi energy for Au compared with Ag. We
further show that the alignment of the conducting orbital relative to the Fermi
level does not follow the work function difference between two metals and is
different for conjugated and saturated systems. We thus demonstrate that the
details of the molecular level alignment and electronic coupling in
metal-organic interfaces do not follow simple rules, but are rather the
consequence of subtle local interactions. | cond-mat_mes-hall |
SNS junctions in nanowires with spin-orbit coupling: role of confinement
and helicity on the sub-gap spectrum: We study normal transport and the sub-gap spectrum of
superconductor-normal-superconductor (SNS) junctions made of semiconducting
nanowires with strong Rashba spin-orbit coupling. We focus, in particular, on
the role of confinement effects in long ballistic junctions. In the normal
regime, scattering at the two contacts gives rise to two distinct features in
conductance, Fabry-Perot resonances and Fano dips. The latter arise in the
presence of a strong Zeeman field $B$ that removes a spin sector in the leads
(\emph{helical} leads), but not in the central region. Conversely, a helical
central region between non-helical leads exhibits helical gaps of half-quantum
conductance, with superimposed helical Fabry-Perot oscillations. These normal
features translate into distinct subgap states when the leads become
superconducting. In particular, Fabry-Perot resonances within the helical gap
become parity-protected zero-energy states (parity crossings), well below the
critical field $B_c$ at which the superconducting leads become topological. As
a function of Zeeman field or Fermi energy, these zero-modes oscillate around
zero energy, forming characteristic loops, which evolve continuously into
Majorana bound states as $B$ exceeds $B_c$. The relation with the physics of
parity crossings of Yu-Shiba-Rusinov bound states is discussed. | cond-mat_mes-hall |
The importance of chemical potential in the determination of water slip
in nanochannels: We investigate the slip properties of water confined in graphite-like
nano-channels by non-equilibrium molecular dynamics simulations, with the aim
of identifying and analyze separately the influence of different physical
quantities on the slip length. In a system under confinement but connected to a
reservoir of fluid, the chemical potential is the natural control parameter: we
show that two nanochannels characterized by the same macroscopic contact angle
-- but a different microscopic surface potential -- do not exhibit the same
slip length unless the chemical potential of water in the two channels is
matched. Some methodological issues related to the preparation of samples for
the comparative analysis in confined geometries are also discussed. | cond-mat_mes-hall |
Emergent Electromagnetic Induction and Adiabatic Charge Pumping in Weyl
Semimetals: The photovoltaic effect in a Weyl semimetal due to the adiabatic quantum
phase is studied. We particularly focus on the case in which an external ac
electric field is applied to the semimetal. In this setup, we show that a
photocurrent is induced by the ac electric field. By considering a generalized
Weyl Hamiltonian with nonlinear terms, it is shown that the photocurrent is
induced by circularly, rather than linearly, polarized light. This photovoltaic
current can be understood as an emergent electromagnetic induction in the
momentum space; the Weyl node is a magnetic monopole in the momentum space, of
which the electric field is induced by the circular motion. This result is
distinct from conventional photovoltaic effects, and potentially useful for
experimentally identifying Weyl semimetals in chiral crystals. | cond-mat_mes-hall |
Decoherence of Cooper pairs and subgap magnetoconductance of
superconducting hybrids: We demonstrate that electron-electron interactions fundamentally restrict the
penetration length of superconducting correlations into a diffusive normal
metal (N) attached to a superconductor (S). We evaluate the subgap
magnetoconductance $G$ of SN hybrids in the presence of electron-electron
interactions and demonstrate that the effect of the magnetic field on $G$ is
twofold: It includes ($i$) additional temperature independent dephasing of
Cooper pairs and ($ii$) Zeeman splitting between the states with opposite
spins. The dephasing length of Cooper pairs can be directly extracted from
measurements of the subgap magnetoconductance in SN systems at low
temperatures. | cond-mat_mes-hall |
Thermal transport controlled by intra- and inter-dot Coulomb
interactions in sequential and cotunneling serially-coupled double quantum
dots: We study thermoelectric transport through a serial double quantum dot (DQD)
coupled to two metallic leads with different thermal energies. We take into
account the electron sequential and cotunneling effects via different master
equation approaches. In the absence of intra- and inter-dot Coulomb
interactions, a small peak in thermoelectric and heat currents is found for
$E_{\rm L} \text{=} E_{\rm R}$ indicating the Coulomb blockade DQD regime,
where $E_{\rm L}(E_{\rm R})$ is the energy of the state of the left(right)
quantum dot. In the presence of intra- and inter-dot Coulomb interactions with
strengths U$_{\rm intra}$, and U$_{\rm inter}$, respectively, avoided crossings
or resonance energies between the intra- and the inter-dot two-electron states,
2ES, are found. These resonances induce extra transport channels through the
DQD leading to strong side peaks in the thermoelectric and heat currents at $
E_{\rm L} \text{-} E_{\rm R} = \pm (U_{\rm intra} \text{-} U_{\rm inter})$ in
addition to the main peak generated at $E_{\rm L} \text{=} E_{\rm R}$. The
current side peaks are enhanced by increased strength of the Coulomb
interactions. Interestingly, the current side peaks are enhanced when
cotunneling terms are considered in which the resonances of the 2ESs assist the
electron cotunneling process through the system. Furthermore, the issue of
coherences is carefully checked in the DQD-leads system via different
approaches to the master equation, which are the Pauli, the Redfield, a first
order Lindblad, and the first- and second order von-Neumann methods. We realize
that the Pauli method gives a wrong results for the thermoelectric transport
when the role of the coherences is relevant. | cond-mat_mes-hall |
Cooling of a Micro-mechanical Resonator by the Back-action of Lorentz
Force: Using a semi-classical approach, we describe an on-chip cooling protocol for
a micro-mechanical resonator by employing a superconducting flux qubit. A
Lorentz force, generated by the passive back-action of the resonator's
displacement, can cool down the thermal motion of the mechanical resonator by
applying an appropriate microwave drive to the qubit. We show that this onchip
cooling protocol, with well-controlled cooling power and a tunable response
time of passive back-action, can be highly efficient. With feasible
experimental parameters, the effective mode temperature of a resonator could be
cooled down by several orders of magnitude. | cond-mat_mes-hall |
Charge-spin response and collective excitations in Weyl semimetals: Weyl semimetals are characterized by unconventional electromagnetic response.
We present analytical expressions for all components of the frequency- and
wave-vector-dependent charge-spin linear-response tensor of Weyl fermions. The
spin-momentum locking of the Weyl Hamiltonian leads to a coupling between
charge and longitudinal spin fluctuations, while transverse spin fluctuations
remain decoupled from the charge. A real Weyl semimetal with multiple Weyl
nodes can show this charge-spin coupling in equilibrium if its crystal symmetry
is sufficiently low. All Weyl semimetals are expected to show this coupling if
they are driven into a non-equilibrium stationary state with different
occupations of Weyl nodes, for example by exploiting the chiral anomaly. Based
on the response tensor, we investigate the low-energy collective excitations of
interacting Weyl fermions. For a local Hubbard interaction, the charge-spin
coupling leads to a dramatic change of the zero-sound dispersion: its velocity
becomes independent of the interaction strength and the chemical potential and
is given solely by the Fermi velocity. In the presence of long-range Coulomb
interactions, the coupling transforms the plasmon modes into spin plasmons. For
real Weyl semimetals with multiple Weyl nodes, the collective modes are
strongly affected by the presence of parallel static electric and magnetic
fields, due to the chiral anomaly. In particular, the zero-sound frequency at
fixed momentum and the spin content of the spin plasmons go through cusp
singularities as the chemical potential of one of the Weyl cones is tuned
through the Weyl node. We discuss possible experiments that could provide
smoking-gun evidence for Weyl physics. | cond-mat_mes-hall |
Heat dissipation and fluctuations in a driven quantum dot: While thermodynamics is a useful tool to describe the driving of large
systems close to equilibrium, fluctuations dominate the distribution of heat
and work in small systems and far from equilibrium. We study the heat generated
by driving a small system and change the drive parameters to analyse the
transition from a drive leaving the system close to equilibrium to driving it
far from equilibrium. Our system is a quantum dot in a GaAs/AlGaAs
heterostructure hosting a two-dimensional electron gas. The dot is
tunnel-coupled to one part of the two-dimensional electron gas acting as a heat
and particle reservoir. We use standard rate equations to model the driven
dot-reservoir system and find excellent agreement with the experiment.
Additionally, we quantify the fluctuations by experimentally test the
theoretical concept of the arrow of time, predicting our ability to distinguish
whether a process goes in the forward or backward drive direction. | cond-mat_mes-hall |
Dynamic vibronic coupling in InGaAs quantum dots: The electron-phonon coupling in self-assembled InGaAs quantum dots is
relatively weak at low light intensities, which means that the zero-phonon line
in emission is strong compared to the phonon sideband. However, the coupling to
acoustic phonons can be dynamically enhanced in the presence of an intense
optical pulse tuned within the phonon sideband. Recent experiments have shown
that this dynamic vibronic coupling can enable population inversion to be
achieved when pumping with a blue-shifted laser and for rapid de-excitation of
an inverted state with red detuning. In this paper we confirm the incoherent
nature of the phonon-assisted pumping process and explore the temperature
dependence of the mechanism. We also show that a combination of blue- and
red-shifted pulses can create and destroy an exciton within a timescale ~20 ps
determined by the pulse duration and ultimately limited by the phonon
thermalisation time. | cond-mat_mes-hall |
Landau Quantized Dynamics and Spectrum of the Diced Lattice: In this work the role of magnetic Landau quantization in the dynamics and
spectrum of Diced Lattice charge carriers is studied in terms of the associated
pseudospin 1 Green's function. The equations of motion for the 9 matrix
elements of this Green's function are formulated in position/frequency
representation and are solved explicitly in terms of a closed form integral
representation involving only elementary functions. The latter is subsequently
expanded in a Laguerre eigenfunction series whose frequency poles identify the
discretized energy spectrum for the Landau-quantized Diced Lattice as
$\epsilon_n = \pm\sqrt{2(2n+1)\alpha^2 eB}$ ($\alpha\sqrt{2}$ is the
characteristic speed for the Diced Lattice) which differs significantly from
the nonrelativistic linear dependence of $\epsilon_n$ on $B$, and is similar to
the corresponding $\sqrt{B}-$dependence of other Dirac materials (Graphene,
Group VI Dichalcogenides). | cond-mat_mes-hall |
Carrier Injection and Scattering in Atomically Thin Chalcogenides: Atomically thin two-dimensional chalcogenides such as MoS2 monolayers are
structurally ideal channel materials for the ultimate atomic electronics.
However, a heavy thickness dependence of electrical performance is shown in
these ultrathin materials, and the device performance normally degrades while
exhibiting a low carrier mobility as compared with corresponding bulks,
constituting a main hurdle for application in electronics. In this brief
review, we summarize our recent work on electrode/channel contacts and carrier
scattering mechanisms to address the origins of this adverse thickness
dependence. Extrinsically, the Schottky barrier height increases at the
electrode/channel contact area in thin channels owing to bandgap expansion
caused by quantum confinement, which hinders carrier injection and degrades
device performance. Intrinsically, thin channels tend to suffer from
intensified Coulomb impurity scattering, resulting from the reduced interaction
distance between interfacial impurities and channel carriers. Both factors are
responsible for the adverse dependence of carrier mobility on channel thickness
in two-dimensional semiconductors. | cond-mat_mes-hall |
Moiré Imaging in Twisted Bilayer Graphene Aligned on Hexagonal Boron
Nitride: Moir\'e superlattices (MSL) formed in angle-aligned bilayers of van der Waals
materials have become a promising platform to realize novel two-dimensional
electronic states. Angle-aligned trilayer structures can form two sets of MSLs
which could potentially interfere with each other. In this work, we directly
image the moir\'e patterns in both monolayer graphene aligned on hBN and
twisted bilayer graphene aligned on hBN, using combined scanning microwave
impedance microscopy and conductive atomic force microscopy. Correlation of the
two techniques reveals the contrast mechanism for the achieved ultrahigh
spatial resolution (<2 nm). We observe two sets of MSLs with different
periodicities in the trilayer stack. The smaller MSL breaks the 6-fold
rotational symmetry and exhibits abrupt discontinuities at the boundaries of
the larger MSL. Using a rigid atomic-stacking model, we demonstrate that the
hBN layer considerably modifies the MSL of twisted bilayer graphene. We further
analyze its effect on the reciprocal space spectrum of the dual-moir\'e system. | cond-mat_mes-hall |
Size dependent optical response in coupled systems of plasmons and
electron-hole pairs in metallic nanostructures: In bulk materials, the collective modes and individual modes are orthogonal
each other, and no connection occurs if there is no damping processes. In the
presence of damping, the collective modes, i.e., plasmons decay into the hot
carriers. In finite systems, the collective and individual modes are coupled by
the Coulomb interaction. Such couplings by longitudinal (L) field have been
intensively investigated, whereas a coupling via transverse (T) field has been
poorly studied although the plasmon is excited by an irradiated light on
surface and in finite nanostructures. Then, the T field would play a
significant role in the coupling between the collective and individual
excitations. In this study, we investigate how the T field mediates the
coherent coupling. This study is based on the recently developed microscopic
nonlocal theory of electronic systems in metals and the results of eigenmode
analyses by this theory. To tune the coupling strength in a single nanorod, we
examine three parameters: Rod length $L_z$, background refractive index $n_{\rm
b}$, and Fermi energy $\varepsilon_{\rm F}$. We discuss the modulation ratio of
the spectrum of optical response coefficients to evaluate the coupling by the T
field. The T field shifts the collective excitation energy, which causes a
finite modulation at both collective excitation and individual excitations. The
three parameters can change the energy distance between the collective and
individual excitations. Thus, the coherent coupling by the T field is enhanced
for a proper tuning of the parameters. The results of the investigation of
system parameter dependence would give insight into the guiding principle of
designing the materials for highly efficient hot carrier generation. | cond-mat_mes-hall |
Correlative nanoscale imaging of strained hBN spin defects: Spin defects like the negatively charged boron vacancy color center ($V_B^-$)
in hexagonal boron nitride (hBN) may enable new forms of quantum sensing with
near-surface defects in layered van der Waals heterostructures. Here, we reveal
the effect of strain associated with creases in hBN flakes on $V_B^-$ and $V_B$
color centers in hBN with correlative cathodoluminescence and photoluminescence
microscopies. We observe strong localized enhancement and redshifting of the
$V_B^-$ luminescence at creases, consistent with density functional theory
calculations showing $V_B^-$ migration toward regions with moderate uniaxial
compressive strain. The ability to manipulate these spin defects with highly
localized strain offers intriguing possibilities for future 2D quantum sensors. | cond-mat_mes-hall |
Optical Charge Injection and Full Coherent Control of Spin-Qubit in the
Telecom C-band Emitting Quantum Dot: Solid-state quantum emitters with manipulable spin-qubits are promising
platforms for quantum communication applications. Although such light-matter
interfaces could be realized in many systems only a few allow for light
emission in the telecom bands necessary for long-distance quantum networks.
Here, we propose and implement a new optically active solid-state spin-qubit
based on a hole confined in a single InAs/GaAs quantum dot grown on an InGaAs
metamorphic buffer layer emitting photons in the C-band. We lift the hole
spin-degeneracy using an external magnetic field and demonstrate hole
injection, initialization, read-out and complete coherent control using
picosecond optical pulses. These results showcase a new solid-state spin-qubit
platform compatible with preexisting optical fibre networks. | cond-mat_mes-hall |
Comparing two different descriptions of the I-V characteristic of
graphene: theory and experiment: The formalism of the nonperturbative description of transport phenomena in
graphene on the framework of the quantum kinetic equation for the
Schwinger-like process is compared with the description on the basis of
Zener-Klein tunneling. The regime of ballistic conductivity in a constant
electric field is considered.
In the latter case the interaction of carriers with electric field is
described in terms of the spatial dependence of their potential energy
(x-representation). The presented kinetic formalism uses an alternative method
of describing the interaction with a field through the introduction of a
quasimomentum $P=p-(e/c)A(t)$ where $A(t)$ is the vector potential
(t-representation). Both approaches should lead to the same physical
characteristics of the described process.
The measurement of the current in experiments is realized in static
conditions determined by the potential difference between the electrodes and
the distance between them. These parameters are native for the
x-representation. On the contrary, in the approach based on the
t-representation it is necessary to consider the situation in dynamics and
introduce the effective lifetime of the generated carriers. In the ballistic
regime this time depends on the distance between the electrodes.
We give a detailed comparison of these two descriptions of the current and
demonstrate good coincidence with the experimental data of the alternative
approach based on the t-representation. It provides a reliable foundation for
the application of nonperturbative methods adopted from strong field QED, that
allows to include in the consideration more general models of the field
(arbitrary polarization and time dependence) and to extend the scope of the
theory. | cond-mat_mes-hall |
Topological magnetic crystalline insulators and co-representation theory: Gapless surface states of time reversal invariant topological insulators are
protected by the anti-unitary nature of the time reversal operation. Very
recently, this idea was generalized to magnetic structures, in which time
reversal symmetry is explicitly broken, but there is still an anti-unitary
symmetry operation combining time reversal symmetry and crystalline symmetry.
These topological phases in magnetic structures are dubbed "topological
magnetic crystalline insulators". In this work, we present a general theory of
topological magnetic crystalline insulators in different types of magnetic
crystals based on the co-representation theory of magnetic crystalline symmetry
groups. We construct two concrete tight-binding models of topological magnetic
crystalline insulators, the $\hat{C}_4\Theta$ model and the $\hat{\bf
\tau}\Theta$ model, in which topological surface states and topological
invariants are calculated explicitly. Moreover, we check different types of
anti-unitary operators in magnetic systems and find that the systems with
$\hat{C}_4\Theta$, $\hat{C}_6\Theta$ and $\hat{\bf \tau}\Theta$ symmetry are
able to protect gapless surface states. Our work will pave the way to search
for topological magnetic crystalline insulators in realistic magnetic
materials. | cond-mat_mes-hall |
Fano resonance in discrete lattice models: controlling lineshapes with
impurities: The possibility of controlling Fano lineshapes in the electronic transmission
is addressed in terms of a simple discrete model within a tight binding
framework, in which a finite sized ordered chain is coupled from one side to an
infinite linear chain (the `backbone') at one lattice point. It is found that,
the profile of Fano resonance is strongly influenced by the presence of
impurity atoms in the backbone. We specifically discuss the case with just two
substitutional impurities sitting in the otherwise ordered backbone. Precise
analytical formulae relating the locations of these impurities to the size of
the side coupled chain have been presented. The nature of the transmission
spectrum and the reversal of the pole-zero structures in the Fano resonance are
discussed with the help of these formulae. | cond-mat_mes-hall |
Band Symmetries and Singularities in Twisted Multilayer Graphene: The electronic spectra of rotationally faulted graphene bilayers are
calculated using a continuum formulation for small fault angles that identifies
two distinct electronic states of the coupled system. The low energy spectra of
one state features a Fermi velocity reduction which ultimately leads to
pairwise annihilation and regeneration of its low energy Dirac nodes. The
physics in the complementary state is controlled by pseudospin selection rules
that prevent a Fermi velocity renormalization and produce second generation
symmetry-protected Dirac singularities in the spectrum. These results are
compared with previous theoretical analyses and with experimental data. | cond-mat_mes-hall |
Mechanical properties of carbynes investigated by ab initio total-energy
calculations: As sp carbon chains (carbynes) are relatively rigid molecular objects, can we
exploit them as construction elements in nanomechanics? To answer this
question, we investigate their remarkable mechanical properties by ab-initio
total-energy simulations. In particular, we evaluate their linear response to
small longitudinal and bending deformations and their failure limits for
longitudinal compression and elongation. | cond-mat_mes-hall |
Spin-valley filtering in strained graphene structures with artificially
induced carrier mass and spin-orbit coupling: The interplay of massive electrons with spin-orbit coupling in bulk graphene
results in a spin-valley dependent gap. Thus, a barrier with such properties
can act as a filter, transmitting only opposite spins from opposite valleys. In
this Letter we show that strain induced pseudomagnetic field in such a barrier
will enforce opposite cyclotron trajectories for the filtered valleys, leading
to their spatial separation. Since spin is coupled to the valley in the
filtered states, this also leads to spin separation, demonstrating a
spin-valley filtering effect. The filtering behavior is found to be
controllable by electrical gating as well as by strain. | cond-mat_mes-hall |
Mechanisms of radiation-induced degradation of hybryd perovskites based
solar cells and ways to increase their radiation tolerance: The basic processes of perovskite radiation resistance are discussed for
photo- and high-energy electron irradiation. It is shown that ionization of
iodine ions and a staged mechanism of elastic scattering (upon intermediate
scattering on light ions of an organic molecule) lead to the formation of a
recombination center Ii. The features of ionization degradation of interfaces
with both planar and fractal structures are considered. A special type of
fractality is identified, and its minimum possible level of photodegradation is
predicted. By using the methodology of classical radiation physics, the Hoke
effect was also studied, as well as the synergetics of cooperative phenomena in
tandem systems. The principal channels for counteracting the radiation
degradation of solar cells based on hybrid perovskites have been revealed. | cond-mat_mes-hall |
Slowing down of spin relaxation in two dimensional systems by quantum
interference effects: The effect of weak localization on spin relaxation in a two-dimensional
system with a spin-split spectrum is considered. It is shown that the spin
relaxation slows down due to the interference of electron waves moving along
closed paths in opposite directions. As a result, the averaged electron spin
decays at large times as $1/t$. It is found that the spin dynamics can be
described by a Boltzmann-type equation, in which the weak localization effects
are taken into account as nonlocal-in-time corrections to the collision
integral. The corrections are expressed via a spin-dependent return
probability. The physical nature of the phenomenon is discussed and it is shown
that the "nonbackscattering" contribution to the weak localization plays an
essential role. It is also demonstrated that the magnetic field, both
transversal and longitudinal, suppresses the power tail in the spin
polarization. | cond-mat_mes-hall |
Non-equilibrium spin-crossover in copper phthalocyanine: We demonstrate the tip induced control of the spin state of copper
phthalocyanine (CuPc) on an insulator coated substrate. Accounting for
electronic correlations, we find that, under the condition of energetic
proximity of neutral excited states to the anionic groundstate, the system can
undergo a population inversion towards these excited states. The resulting
state of the system is accompanied by a change in the total spin quantum
number. Experimental signatures of the crossover are the appearance of
additional nodal planes in the topographical STM images as well as a strong
suppression of the current near the center of the molecule. The robustness of
the effect against moderate charge conserving relaxation processes has also
been tested. | cond-mat_mes-hall |
Low field magnetotransport in strained Si/SiGe cavities: Low field magnetotransport revealing signatures of ballistic transport
effects in strained Si/SiGe cavities is investigated. We fabricated strained
Si/SiGe cavities by confining a high mobility Si/SiGe 2DEG in a bended nanowire
geometry defined by electron-beam lithography and reactive ion etching. The
main features observed in the low temperature magnetoresistance curves are the
presence of a zero-field magnetoresistance peak and of an oscillatory structure
at low fields. By adopting a simple geometrical model we explain the
oscillatory structure in terms of electron magnetic focusing. A detailed
examination of the zero-field peak lineshape clearly shows deviations from the
predictions of ballistic weak localization theory. | cond-mat_mes-hall |
Large insulating nitride islands on Cu3Au as a template for atomic spin
structures: We present controlled growth of c(2$\times$2)N islands on the (100) surface
of Cu$_3$Au, which can be used as an insulating surface template for
manipulation of magnetic adatoms. Compared to the commonly used
Cu(100)/c(2$\times$2)N surface, where island sizes do not exceed several
nanometers due to strain limitation, the current system provides better lattice
matching between metal and adsorption layer, allowing larger unstrained islands
to be formed. We show that we can achieve island sizes ranging from tens to
hundreds of nanometers, increasing the potential building area by a factor
10$^3$. Initial manipulation attempts show no observable difference in adatom
behaviour, either in manipulation or spectroscopy. | cond-mat_mes-hall |
Anomalously large g-factor of single atoms adsorbed on a metal substrate: We have performed inelastic scanning tunneling spectroscopy (ISTS) on
individual Fe atoms adsorbed on a Ag(111) surface. ISTS reveals a magnetization
excitation with a lifetime of about 400 fsec which decreases linearly upon
application of a magnetic field. Astoundingly, we find that the g-factor, which
characterizes the shift in energy of the excitation in a magnetic field, is g =
3.1 instead of the regular value of 2. This enhancement can be understood when
considering the complete electronic structure of both the Ag(111) surface state
and the Fe atom, as shown by ab initio calculations of the magnetic
susceptibility. | cond-mat_mes-hall |
Electricity Harvested from Ambient Heat across Silicon Surface: We report that electricity can be generated from limitless thermal motion of
ions by two dimensional (2D) surface of silicon wafer at room temperature. A
typical silicon device, on which asymmetric electrodes with Au and Ag thin
films were fabricated, can generate a typical open-circuit voltage up to 0.40 V
in 5 M CuCl2 solution and an output current over 11 {\mu}A when a 25 k{\Omega}
resistor was loaded into the circuit. Positive correlation between the output
current and the temperature, as well as the concentration, was observed. The
maximum output current and power density are 17 {\mu}A and 8.6 {\mu}W/cm2,
respectively. The possibility of chemical reaction was excluded by four groups
of control experiments. A possible dynamic drag mechanism was proposed to
explain the experimental results. This finding further demonstrates that
ambient heat in the environment can be harvested by 2D semiconductor surfaces
or low dimensional materials and would contribute significantly to the research
of renewable energy. However, this finding does not agree with the second law
of thermal dynamatics. A lot of future work will be needed to study the
mechanism behind this phenomenon. | cond-mat_mes-hall |
Ge/Si nanowire mesoscopic Josephson junctions: The controlled growth of nanowires (NWs) with dimensions comparable to the
Fermi wavelengths of the charge carriers allows fundamental investigations of
quantum confinement phenomena. Here, we present studies of proximity-induced
superconductivity in undoped Ge/Si core/shell NW heterostructures contacted by
superconducting leads. By using a top gate electrode to modulate the carrier
density in the NW, the critical supercurrent can be tuned from zero to greater
than 100 nA. Furthermore, discrete sub-bands form in the NW due to confinement
in the radial direction, which results in stepwise increases in the critical
current as a function of gate voltage. Transport measurements on these
superconductor-NW-superconductor devices reveal high-order (n = 25) resonant
multiple Andreev reflections, indicating that the NW channel is smooth and the
charge transport is highly coherent. The ability to create and control coherent
superconducting ordered states in semiconductor-superconductor hybrid
nanostructures allows for new opportunities in the study of fundamental
low-dimensional superconductivity. | cond-mat_mes-hall |
Scalar-Interchange Potential and Magnetic/Thermodynamic Properties of
Graphene-like Materials: By means of numerical simulations, we explore possible effects of a special
interparticle interaction potential which is a function of external and
internal conditions of graphene-like systems. In addition to the
electromagnetic interaction, we introduce a new potential due to the exchange
of a massive scalar, associated to the so-called Kekul\'e deformations; this
interaction displays a spin-dependent profile. It turns out that the magnitude
of Kekul\'e deformation may significantly affect physical properties of
graphene. A Monte Carlo analysis enables one to analyze the behavior of the
system under variation of the applied external field, temperature, and the
particular type of the exchanged excitation that induces the potential. We
pursue an investigation of the spin configurations, we analyze differences in
thermal equilibrium magnetization and we carry out calculations of the magnetic
susceptibility and the specific heat in the presence of the Kekul\'e-induced
new potential. | cond-mat_mes-hall |
Induced magneto-conductivity in a two-node Weyl semimetal under Gaussian
random disorder: Measuring the magnetoconductivity induced from impurities may help determine
the impurity distribution and reveal the structure of a Weyl semimetal sample.
To verify this, we utilized the Gaussian random disorder to simulate charged
impurities in a two-node Weyl semimetal model and investigate the impact of
charged impurities on magnetoconductivity in Weyl semimetals. We first compute
the longitudinal magnetic conductivity and find that it is positive and
increases proportionally with the parameter governing the Gaussian distribution
of charged impurities, suggesting the presence of negative longitudinal
magnetoresistivity (NLMR). Then we consider both the intravalley and
inter-valley scattering processes to calculate the induced transverse
magnetoconductivity in the model. Our findings indicate that both inter-valley
and intra-valley scattering processes play important roles in calculating the
transverse magnetoconductivity. The locations of Weyl nodes can also be
determined by magnetoconductivity measurements. This is possible if the
magnetic field strength and the density of charged impurities are known.
Alternatively, the measurement of magnetic conductivity may reveal the
distribution of charged impurites in a given sample once the locations of the
Weyl nodes have been determined. These findings can aid in detecting the
structure of a Weyl semimetal sample, enhancing comprehension of
magnetotransport in Weyl semimetals, and promoting the development of valley
electronics. | cond-mat_mes-hall |
Spin Hall magnetoresistance in antiferromagnet/heavy-metal
heterostructures: We investigate the spin Hall magnetoresistance in thin film bilayer
heterostructures of the heavy metal Pt and the antiferromagnetic insulator NiO.
While rotating an external magnetic field in the easy plane of NiO, we record
the longitudinal and the transverse resistivity of the Pt layer and observe an
amplitude modulation consistent with the spin Hall magnetoresistance. In
comparison to Pt on collinear ferrimagnets, the modulation is phase shifted by
90{\deg} and its amplitude strongly increases with the magnitude of the
magnetic field. We explain the observed magnetic field-dependence of the spin
Hall magnetoresistance in a comprehensive model taking into account magnetic
field induced modifications of the domain structure in antiferromagnets. With
this generic model we are further able to estimate the strength of the
magnetoelastic coupling in antiferromagnets. Our detailed study shows that the
spin Hall magnetoresistance is a versatile tool to investigate the magnetic
spin structure as well as magnetoelastic effects, even in antiferromagnetic
multidomain materials. | cond-mat_mes-hall |
Inverse design of reconfigurable piezoelectric topological phononic
plates: We present a methodology to perform inverse analysis on reconfigurable
topological insulators for flexural waves in plate-like structures. First the
unit cell topology of a phononic plate is designed, which offers two-fold
degeneracy in the band structure by topology optimization. In the second step,
piezoelectric patches bonded over the substrate plate are connected to an
external circuit and used appropriately to break space inversion symmetry. The
space inversion symmetry breaking opens a topological band gap by mimicking
quantum valley Hall effect. Numerical simulations demonstrate that the
topologically protected edge state exhibits wave propagation without
backscattering and is immune to disorders. Predominantly, the proposed idea
enables real-time reconfigurability of the topological interfaces in waveguide
applications. | cond-mat_mes-hall |
Temperature dependent electrical resistivity of a single strand of
ferromagnetic single crystalline nanowire: We have measured the electrical resistivity of a single strand of a
ferromagnetic Ni nanowire of diameter 55 nm using a 4-probe method in the
temperature range 3 K-300 K. The wire used is chemically pure and is a high
quality oriented single crystalline sample in which the temperature independent
residual resistivity is determined predominantly by surface scattering. Precise
evaluation of the temperature dependent resistivity ($\rho$) allowed us to
identify quantitatively the electron-phonon contribution (characterized by a
Debye temperature $\theta_R$) as well as the spin-wave contribution which is
significantly suppressed upon size reduction. | cond-mat_mes-hall |
Second- and third-order optical susceptibilities in bidimensional
semiconductors near excitons states: Semiconducting Transition Metal Dichalcogenides (TMDs) have significant
nonlinear optical effects. In this work we have used second-harmonic generation
(SHG) and the four-wave mixing (FWM) spectroscopy in resonance with the
excitons in MoS2, MoSe2, and WS2 monolayers to characterize the nonlinear
optical properties of these materials. We show that trions and excitons are
responsible for enhancing the nonlinear optical response, and determine the
exciton and trion energies by comparing with the photoluminescence spectra.
Moreover, we extract the second and third order optical sheet susceptibility
near exciton energies and compare with values found in the literature. We also
demonstrate the ability to generate different nonlinear effects in a wide
spectral range in the visible region for monolayer MoS2, opening the
possibility of using two-dimensional materials for nonlinear optoelectronic and
photonic applications. | cond-mat_mes-hall |
Unified theory of quantum phase transitions in quantum dots with gapped
host bands: We present a unified theory of quantum phase transitions for half-filled
quantum dots (QDs) coupled to gapped host bands. We augment the bands by
additional weakly coupled metallic lead which allows us to analyze the system
by using standard numerical renormalization group techniques. The ground state
properties of the systems without the additional metallic lead are then
extrapolated in a controlled way from the broadened subgap spectral functions.
We show that a broad class of narrow-gap-semiconductor tunneling densities of
states (TDOSs) support the existence of two distinct phases known from their
superconducting counterpart. Namely, $0$ phase which is marked by the singlet
ground state and the $\pi$ phase regime with the doublet ground state. To keep
a close analogy with the superconducting case, we focus on the influence of
particle-hole asymmetry of the TDOS of the subgap spectral features.
Nevertheless, we also discus the possibility of inducing singlet-doublet
quantum phase transitions in experimental setups by varying the filling of the
QD. In addition, for gapped TDOS functions with smoothed gap edges, we
demonstrate that all subgap peaks may leak out of the gap into the continuous
part of the spectrum, an effect which has no counterpart in the superconducting
Anderson model. | cond-mat_mes-hall |
Gate errors in solid state quantum computer architectures: We theoretically consider possible errors in solid state quantum computation
due to the interplay of the complex solid state environment and gate
imperfections. In particular, we study two examples of gate operations in the
opposite ends of the gate speed spectrum, an adiabatic gate operation in
electron-spin-based quantum dot quantum computation and a sudden gate operation
in Cooper pair box superconducting quantum computation. We evaluate
quantitatively the non-adiabatic operation of a two-qubit gate in a
two-electron double quantum dot. We also analyze the non-sudden pulse gate in a
Cooper-pair-box-based quantum computer model. In both cases our numerical
results show strong influences of the higher excited states of the system on
the gate operation, clearly demonstrating the importance of a detailed
understanding of the relevant Hilbert space structure on the quantum computer
operations. | cond-mat_mes-hall |
Describing non-Hermitian dynamics using a Generalized Three-Time NEGF
for a Partition-free Molecular Junction with Electron-Phonon Coupling: In this paper we develop the Non-Equilibrium Green's Function (NEGF)
formalism for a dissipative molecular junction that consists of a central
molecular system with one-dimensional electronic transport coupled to a phonon
environment and attached to multiple electronic leads. Our approach is
partitionless - initial preparation of the system places the whole system in
the correct canonical equilibrium state - and is valid for an external bias
with arbitrary time dependence. Using path integrals as an intermediary tool,
we apply a two-time Hubbard-Stratonovich transformation to the phonon influence
functional with mixed real and imaginary times to obtain an exact expression
for the electronic density matrix at the expense of introducing coloured
Gaussian noises whose properties are rigorously derived from the environment
action. This results in a unique stochastic Hamiltonian on each branch of the
Konstantinov-Perel' contour (upper, lower, vertical) such that the time
evolution operators in the Liouville equation no longer form a Hermitian
conjugate pair, thus corresponding to non-Hermitian dynamics. To account for
this we develop a generalized three-time NEGF which is sensitive to all
branches of the contour, and relate it to the standard NEGF in the absence of
phonons via a perturbative expansion of the noises. This approach is exact and
fully general, describing the non-equilibrium driven dynamics from an initial
thermal state while subject to inelastic scattering, and can be applied to
non-Hermitian dynamics in general. | cond-mat_mes-hall |
Few-electron eigenstates of concentric double quantum rings: Few-electron eigenstates confined in coupled concentric double quantum rings
are studied by the exact diagonalization technique. We show that the magnetic
field suppresses the tunnel coupling between the rings localizing the
single-electron states in the internal ring, and the few-electron states in the
external ring. The magnetic fields inducing the ground-state angular momentum
transitions are determined by the distribution of the electron charge between
the rings. The charge redistribution is translated into modifications of the
fractional Aharonov-Bohm period. We demonstrate that the electron distribution
can be deduced from the cusp pattern of the chemical potentials governing the
single-electron charging properties of the system. The evolution of the
electron-electron correlations to the high field limit of a classical Wigner
molecule is discussed. | cond-mat_mes-hall |
One-Dimensional Quantum Confinement Effect Modulated Thermoelectric
Properties in InAs Nanowires: We report electrical conductance and thermopower measurements on InAs
nanowires synthesized by chemical vapor deposition. Gate modulation of the
thermopower of individual InAs nanowires with diameter around 20nm is obtained
over T=40 to 300K. At low temperatures (T< ~100K), oscillations in the
thermopower and power factor concomitant with the stepwise conductance
increases are observed as the gate voltage shifts the chemical potential of
electrons in InAs nanowire through quasi-one-dimensional (1D) sub-bands. This
work experimentally shows the possibility to modulate semiconductor nanowire's
thermoelectric properties through the peaked 1D electronic density of states in
the diffusive transport regime, a long-sought goal in nanostructured
thermoelectrics research. Moreover, we point out the importance of scattering
(or disorder) induced energy level broadening in smearing out the 1D
confinement enhanced thermoelectric power factor at practical temperatures
(e.g. 300K). | cond-mat_mes-hall |
Combining micro- and macroscopic probes to untangle single-ion and
spatial exchange anisotropies in a $S = 1$ quantum antiferromagnet: The magnetic ground state of the quasi-one-dimensional spin-1
antiferromagnetic chain is sensitive to the relative sizes of the single-ion
anisotropy ($D$) and the intrachain ($J$) and interchain ($J'$) exchange
interactions. The ratios $D/J$ and $J'/J$ dictate the material's placement in
one or other of three competing phases: a Haldane gapped phase, a quantum
paramagnet and an XY-ordered state, with a quantum critical point at their
junction. We have identified [Ni(HF)$_2$(pyz)$_2]$SbF$_6$, where pyz =
pyrazine, as a candidate in which this behavior can be explored in detail.
Combining neutron scattering (elastic and inelastic) in applied magnetic fields
of up to 10~tesla and magnetization measurements in fields of up to 60~tesla
with numerical modeling of experimental observables, we are able to obtain
accurate values of all of the parameters of the Hamiltonian [$D = 13.3(1)$~K,
$J = 10.4(3)$~K and $J' = 1.4(2)$~K], despite the polycrystalline nature of the
sample. Density-functional theory calculations result in similar couplings ($J
= 9.2$~K, $J' = 1.8$~K) and predict that the majority of the total spin
population of resides on the Ni(II) ion, while the remaining spin density is
delocalized over both ligand types. The general procedures outlined in this
paper permit phase boundaries and quantum-critical points to be explored in
anisotropic systems for which single crystals are as yet unavailable. | cond-mat_mes-hall |
Determination of universal critical exponents using Lee-Yang theory: Lee-Yang zeros are points in the complex plane of an external control
parameter at which the partition function vanishes for a many-body system of
finite size. In the thermodynamic limit, the Lee-Yang zeros approach the
critical value on the real-axis, where a phase transition occurs. Partition
function zeros have for many years been considered a purely theoretical
concept, however, the situation is changing now as Lee-Yang zeros have been
determined in several recent experiments. Motivated by these developments, we
here devise a direct pathway from measurements of partition function zeros to
the determination of critical points and universal critical exponents of
continuous phase transitions. To illustrate the feasibility of our approach, we
extract the critical exponents of the Ising model in two and three dimensions
from the fluctuations of the total energy and the magnetization in lattices of
finite size. Importantly, the critical exponents can be determined even if the
system is away from the phase transition. Moreover, in contrast to standard
methods based on Binder cumulants, it is not necessary to drive the system
across the phase transition. As such, our method provides an intriguing
perspective for investigations of phase transitions that may be hard to reach
experimentally, for instance at very low temperatures or at very high
pressures. | cond-mat_mes-hall |
Coherent transport in linear arrays of quantum dots: the effects of
period doubling and of quasi-periodicity: We evaluate the phase-coherent transport of electrons along linear structures
of varying length, which are made from two types of potential wells set in
either a periodic or a Fibonacci quasi-periodic sequence. The array is
described by a tight-binding Hamiltonian and is reduced to an effective dimer
by means of a decimation-renormalization method, extended to allow for
connection to external metallic leads, and the transmission coefficient is
evaluated in a T-matrix scattering approach. Parallel behaviors are found for
the energy dependence of the density of electron states and of the
transmittivity of the array. In particular, we explicitly show that on
increasing its length the periodic array undergoes a metal-insulator transition
near single occupancy per dot, whereas prominent pseudo-gaps emerge away from
the band center in the Fibonacci-ordered array. | cond-mat_mes-hall |
Orbital gyrotropic magneto-electric effect and its strain engineering in
monolayer Nb$X_2$: Electrical control of the orbital degrees of freedom is an important area of
research in the emerging field of "orbitronics." Orbital {\it gyrotropic}
magneto-electric effect (OGME) is the generation of an orbital magnetization in
a nonmagnetic metal by an applied electric field. Here, we show that strain
induces a large GME in the monolayer Nb$X_2$ ($X =$ S, Se) normal to the plane,
primarily driven by the orbital moments of the Bloch bands as opposed to the
conventional spin magnetization, without any need for spin-orbit coupling. The
key physics is captured within an effective two-band valley-orbital model and
it is shown to be driven by three key ingredients: the intrinsic valley orbital
moment, broken $C_{3z}$ symmetry, and strain-induced Fermi surface changes. The
effect can be furthermore switched by changing the strain condition, with
potential for future device applications. | cond-mat_mes-hall |
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