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Influence of structural deformations on the reentrant conductance
feature in semiconducting nanowires: Helical states can be measured through the observation of the reentrant
behaviour, which is a dip in the conductance probed in semiconducting nanowires
(NWs) with strong spin-orbit coupling (SOC) under the presence of an external
perpendicular magnetic field. We investigate the effects of deformations in the
electronic transport in NWs considering the coupling between different
transverse modes. Within this approach, we show that the dip in the conductance
of a NW is affected by the presence of a local constriction. Moreover, we find
that the reentrant feature in the conductance can appear in NWs with a local
expansion of its radius, even in the absence of SOC and magnetic field.
Furthermore, we develop a numerical approach to calculate transport properties,
which is able to include the deformation and the coupling among several
scattering channels. | cond-mat_mes-hall |
Electron drift velocity control in GaAs-in-Al2O3 quantum wire transistor
structure due to the electron scattering rate alteration: Electron transport in the transistor structure based on thin undoped
GaAs-in-Al2O3 quantum wire is simulated by ensemble Monte-Carlo method taking
into account electron scattering by the phonons and surface roughness. The
influence of surface roughness height on electron drift velocity at 77 and 300
K is investigated for the values of longitudinal electric field strength of 0.1
and 1.0 kV/cm. A possibility of electron drift velocity control due to
variation of the bias applied to the gates, which results in the electron
scattering rate alteration, is ascertained. | cond-mat_mes-hall |
Optical control and decoherence of spin qubits in quantum dots: We discuss various methods of all-optical spin control in semiconductor
quantum dots. We present different ways of rotating a single confined electron
spin by optical coupling to a trion state. We also discuss a method for
controlling the polarization of a confined exciton via a two-photon transition.
Finally, we analyze the effect of phonon-induced decoherence on the fidelity of
these optical spin control protocols. | cond-mat_mes-hall |
Electronic Conduction in Short DNA Wires: A strict method is used to calculate the current-voltage characteristics of a
double-stranded DNA. A more reliable model considering the electrostatic
potential drop along an individual DNA molecular wire between the contacts is
considered and the corresponding Green's Function is obtained analytically
using Generating Function method, which avoids difficult numerical evaluations.
The obtained results indicate that the electrostatic drop along the wire always
increases the conductor beyond the threshold than without considering it, which
is in agreement with recent experiments. The present method can also be used to
calculate the current-voltage characteristics for other molecular wires of
arbitrary length. | cond-mat_mes-hall |
Exceptional rings protected by emergent symmetry for mechanical systems: We propose mechanical systems, described by Newton's equation of motion, as
suited platforms for symmetry protection of non-Hermitian topological
degeneracies. We point out that systems possess emergent symmetry, which is a
unique properties of mechanical systems. Because of the emergent symmetry, in
contrast to other systems, fine-tuning of parameters (e.g., gain and loss) is
not required to preserve the symmetry protecting exceptional rings in two
dimensions. The presence of symmetry-protected exceptional rings (SPERs) in two
dimensions is numerically demonstrated for a mechanical graphene with friction.
Furthermore, classification of symmetry-protected non-Hermitian degeneracies is
addressed by taking into account the above special characteristics of
mechanical systems. | cond-mat_mes-hall |
Non-equilibrium dynamics in coupled quantum dots: The aim of this work is to study the non-equilibrium dynamics of electrons in
a coupled quantum well pair. To achieve this aim, we consider a non-symmetric
distribution of electrons in a double quantum well. We derive the nonlinear
dynamical evolution of the carrier wave functions considering electron-phonon
interactions and a time-dependent Hartree potential in multielectron quantum
dots. We show the possibility of having an electrostatic trap for part of the
electrons which are injected into one of the quantum wells. | cond-mat_mes-hall |
All epitaxial self-assembly of vertically-confined silicon color centers
using ultra-low temperature epitaxy: Silicon-based color centers (SiCCs) have recently emerged as a source of
quantum light that could be well combined with existing telecom-based Si
Photonics platforms. However, considering the current SiCC fabrication
processes, deterministic control over the vertical emitter position is
impossible due to the stochastic nature of the ion implantation deployed for
color center formation. To bypass this bottleneck towards high-yield
integration, we demonstrate an entirely different creation method for various
SiCCs, that relies solely on the epitaxial growth of Si and C-doped Si at
untypically low temperatures in a pristine growth environment. Careful
adjustment to the SiCC's thermal budget allows the emitters to be confined
within a layer thickness of less than 1 nm embedded at an arbitrary vertical
position within a highly crystalline Si matrix. Depending on the SiCC layer
growth conditions and doping, different types of color centers, such as W
centers, T centers, or G'-centers can be created, some of which are
particularly promising as Si-based single photon sources and for spin-photon
interfaces. We show that the zero-phonon emission from G'-center ensembles can
be conveniently tuned by changing the C-doping concentration, characterized by
a systematic wavelength shift and significant linewidth narrowing towards low
emitter densities. | cond-mat_mes-hall |
Magnetic nanoparticle sensing: decoupling the magnetization from the
excitation field: Remote sensing of magnetic nanoparticles has exciting applications for
magnetic nanoparticle hyperthermia and molecular detection. We introduce,
simulate, and experimentally demonstrate an innovation---a sensing coil that is
geometrically decoupled from the excitation field---for magnetic nanoparticle
spectroscopy that increases the flexibility and capabilities of remote
detection. The decoupling enhances the sensitivity absolutely; to small amounts
of nanoparticles, and relatively; to small changes in the nanoparticle
dynamics. We adapt a previous spectroscopic method that measures the relaxation
time of nanoparticles and demonstrate a new measurement of nanoparticle
temperature that could potentially be used concurrently during hyperthermia. | cond-mat_mes-hall |
Ground state correlations in a trapped quasi one-dimensional Bose Gas: We review the basic concepts of a non-equilibrium kinetic theory of a trapped
bosonic gas. By extending the successful mean-field concept of the
Gross-Pitaevskii equation with the effects of non-local, two particle quantum
correlations, one obtains a renormalized binary interaction and allows for the
dynamic establishment of non-classical many-particle quantum correlations.
These concepts are illustrated by self-consistent numerical calculations of the
first and second order ground state quantum correlations of a harmonically
trapped, quasi one-dimensional bosonic gas. We do find a strong suppression of
the density fluctuations or, in other words, an enhanced number squeezing with
decreasing particle density. | cond-mat_mes-hall |
Unconventional Bloch-Grüneisen scattering in hybrid Bose-Fermi
systems: We report on the novel mechanism of electron scattering in hybrid Bose-Fermi
systems consisting of a two-dimensional electron gas in the vicinity of an
exciton condensate: We show that a pair-of-bogolons--mediated scattering proves
to be dominating over the conventional acoustic phonon channel and over the
single-bogolon scattering, even if the screening is taken into account. We
develop a microscopic theory of this effect, focusing on GaAs and MoS$_2$
materials, and find the principal temperature dependence of resistivity,
distinct from the conventional phonon--mediated processes. Further, we
scrutinize parameters and suggest a way to design composite samples with
predefined electron mobilities and propose a mechanism of electron pairing for
superconductivity. | cond-mat_mes-hall |
Ar+-sputtered Ge (001) surface nanostructuring at high implant
temperature: Ion sputtering induced nanoscale pattern formation on Ge (001) surface by 500
eV Ar+ bombardment has been investigated for a wide range of ion incidence
angles at temperature of 300 deg.C. A fourfold symmetric topography forms in
the angular regime 0 - 65 deg. Above 65 deg, they show a remarkable transition
into highly regular one-dimensional (1D) asymmetric pattern, known as
perpendicular mode ripples. In order to understand growth dynamics of both kind
of patterns, we have investigated their temporal evolution as a function of ion
fluence in a wide range from 1*10^{17} to 1*10^{20} ions cm-2. In addition, we
study the effect of substrate rotation on Ge surface morphology in whole
angular range. The four-fold symmetric patterns effect does not found to alter
their symmetry, while the ripples degenerate into hole structure with a weak
fourfold symmetric pattern. The origin of square topographies and their
symmetry independency on ion incident angle in the range 0 to 65 deg can be
attributed to the growth process due to biased diffusion of vacancies arising
from Ehrlich-Schwoebel barrier. Whereas, the ripple formation at grazing
incidence angles indicates the dominance of curvature dependent surface
instability induced by the incident ion direction. | cond-mat_mes-hall |
DFT modelling of the effect of strong magnetic field on Aniline molecule: Aniline is an organic compound with the stoichiometric expression
$C_{6}H_{5}NH_{2}$; consisting of a phenyl structure attached to an amino
group. It is colorless, but it slowly oxidizes and resinifies in air, giving a
red-brown tint to aged samples. Until now, there are only few researches on
Aniline considering low magnetic fields. In this work, we study Aniline
molecule under different high magnetic fields using density functional theory
methods including independent particle and interacting particle approaches. We
obtain charge density distrubitions, energy dispersions, dipol moments and
forces as functions of position and magnetic field. Our numerical results show
that magnetic field affects electron density of the considered molecule. As a
result, it is observed that there are strong fluctuations in energy dispersion. | cond-mat_mes-hall |
Energetics of metal slabs and clusters: the rectangle-box model: An expansion of energy characteristics of wide thin slab of thickness L in
power of 1/L is constructed using the free-electron approximation and the model
of a potential well of finite depth. Accuracy of results in each order of the
expansion is analyzed. Size dependences of the work function and electronic
elastic force for Au and Na slabs are calculated. It is concluded that the work
function of low-dimensional metal structure is always smaller that of
semi-infinite metal sample.
A mechanism for the Coulomb instability of charged metal clusters, different
from Rayleigh's one, is discussed. The two-component model of a metallic
cluster yields the different critical sizes depending on a kind of charging
particles (electrons or ions). For the cuboid clusters, the electronic spectrum
quantization is taken into account. The calculated critical sizes of
Ag_{N}^{2-} and Au_{N}^{3-} clusters are in a good agreement with experimental
data. A qualitative explanation is suggested for the Coulomb explosion of
positively charged Na_{\N}^{n+} clusters at 3<n<5. | cond-mat_mes-hall |
Voltage tunable quantum dot array by patterned Ge-nanowire based
metal-oxide-semiconductor (MOS) devices: Semiconductor quantum dots (QDs) are being regarded as the primary unit for a
wide range of advanced and emerging technologies including electronics,
optoelectronics, photovoltaics and biosensing applications as well as the
domain of q-bits based quantum information processing. Such QDs are suitable
for several novel device applications for their unique property of confining
carriers 3-dimensionally creating discrete quantum states. However, the
realization of such QDs in practice exhibits serious challenge regarding their
fabrication in array with desired scalability and repeatability as well as
control over the quantum states at room temperature. In this context, the
current work reports the fabrication of an array of highly scaled Ge-nanowire
(radius ~25 nm) based vertical metal-oxide-semiconductor devices that can
operate as voltage tunable quantum dots at room temperature. The electrons in
such nanowire experience a geometrical confinement in the radial direction,
whereas, they can be confined axially by tuning the applied bias in order to
manipulate the quantum states. Such quantum confinement of electrons has been
confirmed from the step-like responses in the room temperature
capacitance-voltage (C-V) characteristics at relatively low frequency (200
kHz). Each of such steps has observed to encompass convolution of the quantized
states occupying ~6 electronic charges. The details of such carrier confinement
are analyzed in the current work by theoretically modeling the device transport
properties based on non-equilibrium Green's function (NEGF) formalism. | cond-mat_mes-hall |
Dirac Spin-Orbit Torques at the Surface of Topological Insulators: We address the nature of spin-orbit torques at the magnetic surfaces of
topological insulators, using both Kubo formula and Keldysh formalism. Through
the analysis of the derived spin-charge equations, we find that the so-called
Dirac torques possess a different symmetry compared to their Rashba
counterpart, as well as a high anisotropy as a function of the magnetization
direction. In particular, the damping torque vanishes when the magnetization
lies in the plane of the topological insulator surface. This peculiarity has
important consequences in terms of magnetization dynamics and switching, as
demonstrated numerically via a macrospin model. | cond-mat_mes-hall |
Zero-field Time Correlation Functions of Four Classical Heisenberg Spins
on a Ring: A model relevant for the study of certain molecular magnets is the ring of
N=4 classical spins with equal near-neighbor isotropic Heisenberg exchange
interactions. Assuming classical Heisenberg spin dynamics, we solve explicitly
for the time evolution of each of the spins. Exact triple integral
representations are derived for the auto, near-neighbor, and
next-nearest-neighbor time correlation functions for any temperature. At
infinite temperature, the correlation functions are reduced to quadrature. We
then evaluate the Fourier transforms of these functions in closed form, which
are double integrals. At low temperatures, the Fourier transform functions
explicitly demonstrate the presence of magnons. Our exact results for the
infinite temperature correlation functions in the long-time asymptotic limit
differ qualitatively from those obtained assuming diffusive spin dynamics.
Whether such explicitly non-hydrodynamic behavior would be maintained for
large-N rings is discussed. | cond-mat_mes-hall |
Interplay of spin-orbit coupling and Coulomb interaction in ZnO-based
electron system: Spin-orbit coupling (SOC) is pivotal for various fundamental spin-dependent
phenomena in solids and their technological applications. In semiconductors,
these phenomena have been so far studied in relatively weak electron-electron
interaction regimes, where the single electron picture holds. However, SOC can
profoundly compete against Coulomb interaction, which could lead to the
emergence of unconventional electronic phases. Since SOC depends on the
electric field in the crystal including contributions of itinerant electrons,
electron-electron interactions can modify this coupling. Here we demonstrate
the emergence of SOC effect in a high-mobility two-dimensional electron system
in a simple band structure MgZnO/ZnO semiconductor. This electron system
features also strong electron-electron interaction effects. By changing the
carrier density with Mg-content, we tune the SOC strength and achieve its
interplay with electron-electron interaction. These systems pave a way to
emergent spintronic phenomena in strong electron correlation regime and to the
formation of novel quasiparticles with the electron spin strongly coupled to
the density. | cond-mat_mes-hall |
Thermoelectric response of fractional quantized Hall and re-entrant
insulating states in the N=1 Landau level: Detailed measurements of the longitudinal thermopower of two-dimensional
electrons in the first excited Landau level are reported. Clear signatures of
numerous fractional quantized Hall states, including those at $\nu = 5/2$ and
7/3, are observed in the magnetic field and temperature dependence of the
thermopower. An abrupt collapse of the thermopower is observed below about $T=
40$ mK at those filling factors where re-entrant insulating electronic states
have been observed in conventional resistive transport studies. The thermopower
observed at $\nu = 5/2$ is discussed in the context of recent theories which
incorporate non-abelian quasiparticle exchange statistics. | cond-mat_mes-hall |
Conductance saturation in a series of highly transmitting molecular
junctions: Understanding the properties of electronic transport across metal-molecule
interfaces is of central importance for controlling a large variety of
molecular-based devices such as organic light emitting diodes, nanoscale
organic spin-valves and single-molecule switches. One of the primary
experimental methods to reveal the mechanisms behind electronic transport
through metal-molecule interfaces is the study of conductance as a function of
molecule length in molecular junctions. Previous studies focused on transport
governed either by tunneling or hopping, both at low conductance. However, the
upper limit of conductance across molecular junctions has not been explored,
despite the great potential for efficient information transfer, charge
injection and recombination processes. Here, we study the conductance
properties of highly transmitting metal-molecule-metal interfaces, using a
series of single-molecule junctions based on oligoacenes with increasing
length. We find that the conductance saturates at an upper limit where it is
independent of molecule length. Furthermore, we show that this upper limit can
be controlled by the character of the orbital hybridization at the
metal-molecule interface. Using two prototype systems, in which the molecules
are contacted by either Ag or Pt electrodes, we reveal two different origins
for the saturation of conductance. In the case of Ag-based molecular junctions,
the conductance saturation is ascribed to a competition between energy level
alignment and level broadening, while in the case of Pt-based junctions, the
saturation is attributed to a band-like transport. The results are explained by
an intuitive model, backed by ab-initio transport calculations. Our findings
shed light on the mechanisms that constrain the conductance at the high
transmission limit, providing guiding principles for the design of highly
conductive metal-molecule interfaces. | cond-mat_mes-hall |
Rashba effect and magnetic field in quantum wires: We investigate the influence of a perpendicular magnetic field on the
spectral and spin properties of a ballistic quasi-one-dimensional electron
system with Rashba effect. The magnetic field strongly alters the spin-orbit
induced modification to the subband structure when the magnetic length becomes
comparable to the lateral confinement. A new subband-dependent energy splitting
at k=0 is found which can be much larger than the Zeeman splitting. This is due
to the breaking of a combined spin orbital-parity symmetry. | cond-mat_mes-hall |
Transport phenomena in helical edge states interferometers. A Green's
function approach: We analyze the current and the shot-noise of an electron interferometer made
of the helical edge states of a two-dimensional topological insulator within
the framework of non-equilibrium Green's functions formalism. We study in
detail setups with a single and with two quantum point contacts inducing
scattering between the different edge states. We consider processes preserving
the spin as well as the effect of spin-flip scattering. In the case of a single
quantum point contact, a simple test based on the shot-noise measurement is
proposed to quantify the strength of the spin-flip scattering. In the case of
two single point contacts with the additional ingredient of gate voltages
applied within a finite-size region at the top and bottom edges of the sample,
we identify two type of interference processes in the behavior of the currents
and the noise. One of such processes is analogous to that taking place in a
Fabry-P\'erot interferometer, while the second one corresponds to a
configuration similar to a Mach-Zehnder interferometer. In the helical
interferometer these two processes compete. | cond-mat_mes-hall |
Electrical transport between epitaxial manganites and carbon nanotubes: The possibility of performing spintronics at the molecular level may be
realized in devices that combine fully spin polarized oxides such as manganites
with carbon nanotubes. However, it is not clear whether electrical transport
between such different material systems is viable. Here we show that the room
temperature conductance of manganite-nanotube-manganite devices is only half
the value recorded in similar palladium-nanotube-palladium devices.
Interestingly, the former shows a pseudogap in the conductivity below the
relatively high temperature of 200 K. Our results suggest the possibility of
new spintronics heterostructures that exploit fully spin polarized sources and
drains. | cond-mat_mes-hall |
On the radiative lifetime of free-moving two-dimensional excitons: A simple microscopic mechanism explaining the linear dependence of the
radiative lifetime of free-moving two-dimensional excitons on their effective
temperature is suggested. It is shown that there exists a characteristic
effective temperature (of about few Kelvin) defined by the exciton-acoustic
phonon interaction at which the radiative lifetime is minimal. Below this
temperature the lifetime starts to increase with decreasing temperature. The
correspondence with previous theoretical and experimental results is discussed. | cond-mat_mes-hall |
Resonant dynamics of skyrmion lattices in thin film multilayers:
Localised modes and spin wave emission: The spectral signatures of magnetic skyrmions under microwave field
excitation are of fundamental interest and can be an asset for high frequency
applications. These topological solitons can be tailored in multilayered thin
films, but the experimental observation of their spin wave dynamics remains
elusive, in particular due to large damping. Here, we study Pt/FeCoB/AlO$_x$
multilayers hosting dense and robust skyrmion lattices at room temperature with
Gilbert damping of $\sim 0.02$. We use magnetic force microscopy to
characterise their static magnetic phases and broadband ferromagnetic resonance
to probe their high frequency response. Micromagnetic simulations reproduce the
experiments with accuracy and allow us to identify distinct resonant modes
detected in the skyrmion lattice phase. Low ($<$ 2 GHz) and intermediate
frequency ($2-8$ GHz) modes involve excitations localised to skyrmion edges in
conjunction with precession of the uniform background magnetisation, while a
high frequency ($>$ 12 GHz) mode corresponds to in-phase skyrmion core
precession emitting spin waves into uniform background with wavelengths in the
50--80 nm range commensurate with the lattice structure. These findings could
be instrumental in the investigation of room temperature wave scattering and
the implementation of novel microwave processing schemes in reconfigurable
arrays of solitons. | cond-mat_mes-hall |
Ideal Chern bands are Landau levels in curved space: We prove that all the criteria proposed in the literature to identify a Chern
band hosting exact fractional Chern insulating ground states, in fact, describe
an equivalence with a lowest Landau level defined in curved space under a
non-uniform magnetic field. In addition, we design an operational test for the
most general instance of such lowest Landau level mapping, which only relies on
the computationally inexpensive evaluation of Bloch wavefunctions' derivatives.
Our work clarifies the common origin of various Chern-idealness criteria,
proves that these criteria exhaust all possible lowest Landau levels, and hints
at classes of Chern bands that may posses interesting phases beyond Landau
level physics. | cond-mat_mes-hall |
Current-driven ferromagnetic resonance, mechanical torques and rotary
motion in magnetic nanostructures: We study theoretically the detection and possible utilization of electric
current-induced mechanical torques in ferromagnetic-normal metal
heterostructures that are generated by spin-flip scattering or the absorption
of transverse spin currents by a ferromagnet. To this end, we analyze the DC
voltage signals over a spin valve that is driven by an AC current. In agreement
with recent studies, this "rectification", measured as a function of AC
frequency and applied magnetic field, contains important information on the
magnetostatics and --dynamics. Subsequently, we show that the vibrations
excited by spin-transfer to the lattice can be detected as a splitting of the
DC voltage resonance. Finally, we propose a concept for a spin-transfer-driven
electric nanomotor based on integrating metallic nanowires with carbon
nanotubes, in which the current-induced torques generate a rotary motion. | cond-mat_mes-hall |
Robust polaritons in magnetic monolayers of CrI3: We show that the regime of strong-light matter coupling with remarkable
magnetic properties can be realized in systems based on monolayers of chromium
triiodide (CrI3). This two-dimensional material combines the presence of
ferromagnetic ordering with the possibility of forming strongly-bound excitonic
complexes even at room temperature. Using microscopic first-principle
calculations we reveal a rich spectrum of optical transitions, corresponding to
both Wannier- and Frenkel-type excitons, including those containing electrons
with a negative effective mass. We show that excitons of different
polarizations efficiently hybridize with a photonic mode of a planar
microcavity, and due to the peculiar selection rules polariton modes become
well resolved in circular polarizations. The combination of very strong optical
oscillator strength of excitons and cavity confinement leads to large values of
the Rabi splitting, reaching 35 meV for a single monolayer, and giant Zeeman
splitting between polariton modes of up to 20 meV. This makes CrI3 an excellent
platform for magnetopolaritonic applications. | cond-mat_mes-hall |
Two-Dimensional Wide-Band-Gap II-V Semiconductors with a Dilated
Graphene-like Structure: Since the advent of graphene, two-dimensional (2D) materials become very
attractive and there is growing interest to explore new 2D beyond graphene.
Here, through density functional theory (DFT) calculations, we predict 2D
wide-band-gap II-V semiconductor materials of M$_3$X$_2$ (M=Zn, Cd and X=N, P,
As) with a dilated graphene-like honeycomb structure. The structure features
that the group-V X atoms form two X-atomic planes symmetrically astride the
centering group-IIB M atomic plane. The 2D Zn$_3$N$_2$, Zn$_3$P$_2$, and
Zn$_3$As$_2$ are shown to have direct band gaps of 2.87, 3.81, and 3.55 eV,
respectively, and the 2D Cd$_3$N$_2$, Cd$_3$P$_2$, and Cd$_3$As$_2$ exhibit
indirect band gaps of 2.74, 3.51, and 3.29 eV, respectively. Each of the six 2D
materials is shown to have effective carrier (either hole or electron) masses
down to $0.03\sim 0.05$ $m_0$. The structural stability and feasibility of
experimental realization of these 2D materials has been shown in terms of DFT
phonon spectra and total energy comparison with related existing bulk
materials. On the experimental side, there already are many similar
two-coordinate structures of Zn and other transition metals in various organic
materials, which can be considered to support our DFT prediction. Therefore,
these 2D semiconductors can enrich the family of 2D electronic materials and
may have promising potential for achieving novel transistors and optoelectronic
devices. | cond-mat_mes-hall |
Dipole representation of half-filled Landau level: We introduce a variant of dipole representation for composite fermions in a
half-filled Landau level, taking into account the symmetry under exchange of
particles and holes. This is implemented by a special constraint on composite
fermion and composite hole degree of freedom (of an enlarged space), that makes
the resulting composite particle, dipole, a symmetric object. We study an
effective Hamiltonian, that commutes with the constraint on the physical space,
and fulfills the requirement for boost invariance on the Fermi level. The
calculated Fermi liquid parameter F2 is in a good agreement with numerical
investigations in [Phys. Rev. Lett. 121, 147601 (2018)]. | cond-mat_mes-hall |
Emergent topological fields and relativistic phonons within the
thermoelectricity in topological insulators: Topological edge states are predicted to be responsible for the high
efficient thermoelectric response of topological insulators, currently the best
thermoelectric materials. However, to explain their figure of merit the
coexistence of topological electrons, entropy and phonons can not be considered
independently. In a background that puts together electrodynamics and topology,
through an expression for the topological intrinsic field, we treat
relativistic phonons within the topological surface showing their ability to
modulate the Berry curvature of the bands and then playing a fundamental role
in the thermoelectric effect. Finally, we show how the topological insulators
under such relativistic thermal excitations keep time reversal symmetry
allowing the observation of high figures of merit at high temperatures. The
emergence of this new intrinsic topological field and other constraints are
suitable to have experimental consequences opening new possibilities of
improving the efficiency of this topological effect for their based technology. | cond-mat_mes-hall |
OLEDs as models for bird magnetoception: detecting electron spin
resonance in geomagnetic fields: Certain species of living creatures are known to orientate themselves in the
geomagnetic field. Given the small magnitude of approximately 48 {\mu}T, the
underlying quantum mechanical phenomena are expected to exhibit coherence times
approaching the millisecond regime. In this contribution, we show sensitivity
of organic light-emitting diodes (OLEDs) to magnetic fields far below Earth's
magnetic field, suggesting that coherence times of the spins of charge-carrier
pairs in these devices can be similarly long. By electron paramagnetic
resonance (EPR) experiments, a lower bound for the coherence time can be
assessed directly. Moreover, this technique offers the possibility to determine
the distribution of hyperfine fields within the organic semiconductor layer. We
extend this technique to a material system exhibiting both fluorescence and
phosphorescence, demonstrating stable anticorrelation between optically
detected magnetic resonance (ODMR) spectra in the singlet (fluorescence) and
triplet (phosphorescence) channel. The experiments demonstrate the extreme
sensitivity of OLEDs to both static as well as dynamic magnetic fields and
suggest that coherent spin precession processes of Coulombically bound electron
spin pairs may play a crucial role in the magnetoreceptive ability of living
creatures. | cond-mat_mes-hall |
Deflection of (anti)ferromagnetic skyrmions at heterochiral interfaces: Devising magnetic nanostructures with spatially heterogeneous
Dzyaloshinskii-Moriya interaction (DMI) is a promising pathway towards advanced
confinement and control of magnetic skyrmions in potential devices. Here we
discuss theoretically how a skyrmion interacts with a heterochiral interface
using micromagnetic simulations and analytic arguments. We show that a
heterochiral interface deflects the trajectory of ferromagnetic (FM) skyrmions,
and that the extent of such deflection is tuned by the applied spin-polarized
current and the difference in DMI across the interface. Further, we show that
this deflection is characteristic for the FM skyrmion, and is completely absent
in the antiferromagnetic (AFM) case. In turn, we reveal that the AFM skyrmion
achieves much higher velocities than its FM counterpart, yet experiences far
stronger confinement in nanoengineered heterochiral tracks, which reinforces
AFM skyrmions as a favorable choice for skyrmion-based devices. | cond-mat_mes-hall |
Excess electron screening of remote donors and mobility in modern
GaAs/AlGaAs herostructures: In modern GaAs/Al$_x$Ga$_{1-x}$As heterostructures with record high
mobilities, a two-dimensional electron gas (2DEG) in a quantum well is provided
by two remote donor $\delta$-layers placed on both sides of the well. Each
$\delta$-layer is located within a narrow GaAs layer, flanked by narrow AlAs
layers which capture excess electrons from donors but leave each of them
localized in a compact dipole atom with a donor. Still excess electrons can hop
between host donors to minimize their Coulomb energy. As a result they screen
the random potential of donors dramatically. We numerically model the
pseudoground state of excess electrons at a fraction $f$ of filled donors and
find both the mobility and the quantum mobility limited by scattering on remote
donors as universal functions of $f$. We repeat our simulations for devices
with additional disorder such as interface roughness of the doping layers, and
find the quantum mobility is consistent with measured values. Thus, in order to
increase the quantum mobility this additional disorder should be minimized. | cond-mat_mes-hall |
Large low-frequency resistance noise in chemical vapor deposited
graphene: We report a detailed investigation of resistance noise in single layer
graphene films on Si/SiO$_2$ substrates obtained by chemical vapor deposition
(CVD) on copper foils. We find that noise in these systems to be rather large,
and when expressed in the form of phenomenological Hooge equation, it
corresponds to Hooge parameter as large as $0.1 - 0.5$. We also find the
variation in the noise magnitude with the gate voltage (or carrier density) and
temperature to be surprisingly weak, which is also unlike the behavior of noise
in other forms of graphene, in particular those from exfoliation. | cond-mat_mes-hall |
Robustness of Helical Edge States Under Edge Reconstruction: The helical edge states of time-reversal invariant two-dimensional
topological insulators are protected against backscattering in idealized
models. In more realistic scenarios with a shallow confining potential at the
sample boundary, additional strongly interacting edge states may arise, that
could interfere with the topological protection of edge conduction. We find
that interaction effects within the reconstructed edges are well described by
the Luttinger liquid model. While interactions between this Luttinger liquid
and the helical edge states can in principle give rise to dynamical spin
polarization and the breaking of time-reversal symmetry, we demonstrate that
random spin-orbit coupling strongly suppresses such dynamical spin
polarization, resulting in the persistence of near quantized edge conduction. | cond-mat_mes-hall |
Anisotropy-assisted magnon condensation in ferromagnetic thin films: We theoretically demonstrate that adding an easy-axis magnetic anisotropy
facilitates magnon condensation in thin yttrium iron garnet (YIG) films.
Dipolar interactions in a quasi-equilibrium state stabilize room-temperature
magnon condensation in YIG. Even though the out-of-plane easy-axis anisotropy
generally competes with the dipolar interactions, we show that adding such
magnetic anisotropy may even assist the generation of the magnon condensate
electrically via the spin transfer torque mechanism. We use analytical
calculations and micromagnetic simulations to illustrate this effect. Our
results may explain the recent experiment on Bi-doped YIG and open a pathway
toward applying current-driven magnon condensation in quantum spintronics. | cond-mat_mes-hall |
Quantum criticality in a double quantum-dot system: We discuss the realization of the quantum-critical non-Fermi liquid state,
originally discovered within the two-impurity Kondo model, in double
quantum-dot systems. Contrary to the common belief, the corresponding fixed
point is robust against particle-hole and various other asymmetries, and is
only unstable to charge transfer between the two dots. We propose an
experimental set-up where such charge transfer processes are suppressed,
allowing a controlled approach to the quantum critical state. We also discuss
transport and scaling properties in the vicinity of the critical point. | cond-mat_mes-hall |
Radio-frequency methods for Majorana-based quantum devices: fast charge
sensing and phase diagram mapping: Radio-frequency (RF) reflectometry is implemented in hybrid
semiconductor-superconductor nanowire systems designed to probe Majorana zero
modes. Two approaches are presented. In the first, hybrid nanowire-based
devices are part of a resonant circuit, allowing conductance to be measured as
a function of several gate voltages ~40 times faster than using conventional
low-frequency lock-in methods. In the second, nanowire devices are capacitively
coupled to a nearby RF single-electron transistor made from a separate
nanowire, allowing RF detection of charge, including charge-only measurement of
the crossover from 2e inter-island charge transitions at zero magnetic field to
1e transitions at axial magnetic fields above 0.6 T, where a topological state
is expected. Single-electron sensing yields signal-to-noise exceeding 3 and
visibility 99.8% for a measurement time of 1 {\mu}s. | cond-mat_mes-hall |
Sign reversal of magnetoresistivity in massive nodal-line semimetals due
to Lifshitz transition of Fermi surface: Topological nodal-line semimetals offer an interesting research platform to
explore novel phenomena associated with its torus-shaped Fermi surface. Here,
we study magnetotransport in the massive nodal-line semimetal with spin-orbit
coupling and finite Berry curvature distribution which exists in many
candidates. The magnetic field leads to a deformation of the Fermi torus
through its coupling to the orbital magnetic moment, which turns out to be the
main scenario of the magnetoresistivity (MR) induced by the Berry curvature
effect. We show that a small deformation of the Fermi surface yields a positive
MR $\propto B^2$, different from the negative MR by pure Berry curvature effect
in other topological systems. As the magnetic field increases to a critical
value, a topological Lifshitz transition of the Fermi surface can be induced,
and the MR inverts its sign at the same time. The temperature dependence of the
MR is investigated, which shows a totally different behavior before and after
the Lifshitz transition. Our work uncovers a novel scenario of the MR induced
solely by the deformation of the Fermi surface and establishes a relation
between the Fermi surface topology and the sign of the MR. | cond-mat_mes-hall |
Non-Boltzmann classical correction to the velocity auto-correlation
function for isotropic scattering in two dimensions: The classical correction to the velocity auto-correlation function of
non-interacting particles due to memory effects, which are beyond the Boltzmann
equation, is calculated both analytically and numerically for the case of
isotropic scattering in two dimensions. | cond-mat_mes-hall |
Magnetoelectric effects and valley controlled spin quantum gates in
transition metal dichalcogenide bilayers: In monolayer group-VI transition metal dichalcogenides (TMDC), charge
carriers have spin and valley degrees of freedom, both associated with magnetic
moments. On the other hand, the layer degree of freedom in multilayers is
associated with electrical polarization. Here, we show that TMDC bilayers offer
an unprecedented platform to realize a strong coupling between the spin, layer
pseudospin, and valley degrees of freedom of holes. Such coupling not only
gives rise to the spin Hall effect and spin circular dichroism in inversion
symmetric bilayer, but also leads to a variety of magnetoelectric effects
permitting quantum manipulation of these electronic degrees of freedom.
Oscillating electric and magnetic fields can both drive the hole spin resonance
where the two fields have valley-dependent interference, making possible a
prototype interplay between the spin and valley as information carriers for
potential valley-spintronic applications. We show how to realize quantum gates
on the spin qubit controlled by the valley bit. | cond-mat_mes-hall |
Interplay of valley, layer and band topology towards interacting quantum
phases in bilayer graphene moire superlattice: A Bilayer of semiconducting 2D electronic systems has long been a versatile
platform to study electronic correlation with tunable interlayer tunneling,
Coulomb interactions and layer imbalance. In the natural graphite bilayer,
Bernal-stacked bilayer graphene (BBG), the Landau level gives rise to an
intimate connection between the valley and layer. Adding a moire superlattice
potential enriches the BBG physics with the formation of topological minibands,
potentially leading to tunable exotic quantum transports. Here, we present
magnetotransport measurements of a high-quality bilayer graphene-hexagonal
boron nitride (hBN) heterostructure. The zero-degree alignment generates a
strong moire superlattice potential for the electrons in BBG and the resulting
Landau fan diagram of longitudinal and Hall resistance displays a Hofstadter
butterfly pattern with an unprecedented level of detail. We demonstrate that
the intricate relationship between valley and layer degrees of freedom controls
the topology of moire-induced bands, significantly influencing the energetics
of interacting quantum phases in the BBG superlattice. We further observe
signatures of field-induced correlated insulators and clear fractional
quantizations of interaction driven topological quantum phases, such as
fractional Chern insulators. Our results highlight the BBG/hBN heterostructure
as an ideal platform for studying the delicate interplay between topology and
electron correlation. | cond-mat_mes-hall |
Mechanical Mixing in Nonlinear Nanomechanical Resonators: Nanomechanical resonators, machined out of Silicon-on-Insulator wafers, are
operated in the nonlinear regime to investigate higher-order mechanical mixing
at radio frequencies, relevant to signal processing and nonlinear dynamics on
nanometer scales. Driven by two neighboring frequencies the resonators generate
rich power spectra exhibiting a multitude of satellite peaks. This nonlinear
response is studied and compared to $n^{th}$-order perturbation theory and
nonperturbative numerical calculations. | cond-mat_mes-hall |
Surface Recombination Limited Lifetimes of Photoexcited Carriers in
Few-Layer Transition Metal Dichalcogenide MoS2: We present results on photoexcited carrier lifetimes in few-layer transition
metal dichalcogenide MoS2 using nondegenerate ultrafast optical pump-probe
technique. Our results show a sharp increase of the carrier lifetimes with the
number of layers in the sample. Carrier lifetimes increase from few tens of
picoseconds in monolayer samples to more than a nanosecond in 10-layer samples.
The inverse carrier lifetime was found to scale according to the probability of
the carriers being present at the surface layers, as given by the carrier
wavefunction in few layer samples, which can be treated as quantum wells. The
carrier lifetimes were found to be largely independent of the temperature and
the inverse carrier lifetimes scaled linearly with the photoexcited carrier
density. These observations are consistent with defect-assisted carrier
recombination, in which the capture of electrons and holes by defects occurs
via Auger scatterings. Our results suggest that carrier lifetimes in few-layer
samples are surface recombination limited due to the much larger defect
densities at the surface layers compared to the inner layers. | cond-mat_mes-hall |
Quasiparticle diffusion based heating in superconductor tunneling
micro-coolers: In a hybrid Superconductor - Insulator - Normal metal tunnel junction biased
just below the gap, the extraction of hot electrons out of the normal metal
results in electronic cooling effect. The quasiparticles injected in the
superconductor accumulate near the tunnel interface, thus increasing the
effective superconductor temperature. We propose a simple model for the
diffusion of excess quasiparticles in a superconducting strip with an
additional trap junction. This diffusion model has a complete analytic
solution, which depends on experimentally accessible parameters. We find that
the accumulated quasiparticles near the junction reduce the efficiency of the
device. This study is also relevant to more general situations making use of
superconducting tunnel junctions, as low temperature detectors. | cond-mat_mes-hall |
Fingerprints of Qubit Noise in Transient Cavity Transmission: Noise affects the coherence of qubits and thereby places a bound on the
performance of quantum computers. We theoretically study a generic two-level
system with fluctuating control parameters in a photonic cavity and find that
basic features of the noise spectral density are imprinted in the transient
transmission through the cavity. We obtain analytical expressions for generic
noise and proceed to study the cases of quasistatic, white and $1/f^{\alpha}$
noise in more detail. Additionally, we propose a way of extracting the spectral
density for arbitrary noise in a frequency band only bounded by the range of
the qubit-cavity detuning and with an exponentially decaying error due to
finite measurement times. Our results suggest that measurements of the
time-dependent transmission probability represent a novel way of extracting
noise characteristics. | cond-mat_mes-hall |
From chaos to disorder: Statistics of the eigenfunctions of microwave
cavities: We study the statistics of the experimental eigenfunctions of chaotic and
disordered microwave billiards in terms of the moments of their spatial
distributions, such as the Inverse Participation Ratio (IPR) and
density-density auto-correlation. A path from chaos to disorder is described in
terms of increasing IPR. In the chaotic, ballistic limit, the data correspond
well with universal results from random matrix theory. Deviations from
universal distributions are observed due to disorder induced localization, and
for the weakly disordered case the data are well-described by including finite
conductance and mean free path contributions in the framework of nonlinear
sigma models of supersymetry. | cond-mat_mes-hall |
The next nearest neighbor effect on the 2D materials properties: In this work, the effect of introducing next nearest neighbor (NNN) hopping
to the 2D materials was studied using the graphene 2D honeycomb two sublattice
as an example. It is found that introducing NNN to the 2D materials removes the
symmetry around the Fermi level and shifts it, at a small value of NNN hopping.
This effect increases with increasing NNN hopping. If the NNN hopping becomes
competitive with nearest neighbor (NN) hopping, the dispersion relations of the
2D materials changes completely from NN hopping dispersion relations. The
results show that the 2D material sensitivity for NNN hopping effect is much
larger in the 2D honeycomb lattice than 2D square lattice. This is due to the
fact that the number of NNN sites is equal to six, which is the double of NN
sites in the 2D honeycomb lattice. Meanwhile, the number of NNN sites is equal
to four which is equal to NN sites in 2D square lattice. We therefore conclude
that by changing the ratio between NNN and NN sites in the 2D lattice one can
tune the sensitivity for NNN hopping effects. | cond-mat_mes-hall |
Dephasing effect promotes the appearance of quantized Hall plateaus: The quantum Hall effect (QHE) is a topologically protected phenomenon which
has been observed in various systems. In experiments, the size of Hall bar
device to realize the QHE is generally much larger than the phase coherence
length, in which the quantum coherence of electrons is destroyed. Here, we
theoretically study the influence of dephasing effect on the quantized Hall
(QH) plateaus. We find that the QH plateau disappears in perfectly quantum
coherent systems if the coupling between leads and central region is imperfect.
The Hall resistance is very large and strongly oscillates instead of presenting
the QH plateau in this case. However, by introducing the dephasing, the Hall
resistance decreases and the QH plateau appears gradually. Similar results can
also be observed for the quantum anomalous Hall effect. Our results propose
that dephasing effect promotes the appearance of QH plateaus, which opens a new
topic of the dephasing effect on topological systems. | cond-mat_mes-hall |
Quantum magnetism and topological superconductivity in Yu-Shiba-Rusinov
chains: Chains of magnetic adatoms on superconductors have been discussed as
promising systems for realizing Majorana end states. Here, we show that dilute
Yu-Shiba-Rusinov (YSR) chains are also a versatile platform for quantum
magnetism and correlated electron dynamics, with widely adjustable spin values
and couplings. Focusing on subgap excitations, we derive an extended $t-J$
model for dilute quantum YSR chains and use it to study the phase diagram as
well as tunneling spectra. We explore the implications of quantum magnetism for
the formation of a topological superconducting phase, contrasting it to
existing models assuming classical spin textures. | cond-mat_mes-hall |
Giant Magnetoresistance Oscillations Induced by Microwave Radiation and
a Zero-Resistance State in a 2D Electron System with a Moderate Mobility: The effect of a microwave field in the frequency range from 54 to 140
$\mathrm{GHz}$ on the magnetotransport in a GaAs quantum well with AlAs/GaAs
superlattice barriers and with an electron mobility no higher than $10^6$
$\mathrm{cm^2/Vs}$ is investigated. In the given two-dimensional system under
the effect of microwave radiation, giant resistance oscillations are observed
with their positions in magnetic field being determined by the ratio of the
radiation frequency to the cyclotron frequency. Earlier, such oscillations had
only been observed in GaAs/AlGaAs heterostructures with much higher mobilities.
When the samples under study are irradiated with a 140-$\mathrm{GHz}$ microwave
field, the resistance corresponding to the main oscillation minimum, which
occurs near the cyclotron resonance, appears to be close to zero. The results
of the study suggest that a mobility value lower than $10^6$ $\mathrm{cm^2/Vs}$
does not prevent the formation of zero-resistance states in magnetic field in a
two-dimensional system under the effect of microwave radiation. | cond-mat_mes-hall |
Charge frustration in a triangular triple quantum dot: We experimentally investigate the charge (isospin) frustration induced by a
geometrical symmetry in a triangular triple quantum dot. We observe the
ground-state charge configurations of six-fold degeneracy, the manifestation of
the frustration. The frustration results in omnidirectional charge transport,
and it is accompanied by nearby nontrivial triple degenerate states in the
charge stability diagram. The findings agree with a capacitive interaction
model. We also observe unusual transport by the frustration, which might be
related to elastic cotunneling and the interference of trajectories through the
dot. This work demonstrates a unique way of studying geometrical frustration in
a controllable way. | cond-mat_mes-hall |
Anomalous Characteristics of the Generation - Recombination Noise in
Quasi-One-Dimensional Van der Waals Nanoribbons: We describe the low-frequency current fluctuations, i.e. electronic noise, in
quasi-one-dimensional ZrTe3 van der Waals nanoribbons, which have recently
attracted attention owing to their extraordinary high current carrying
capacity. Whereas the low-frequency noise spectral density reveals 1/f behavior
near room temperature, it is dominated by the Lorentzian bulges of the
generation - recombination noise at low temperatures (f is the frequency).
Unexpectedly, the corner frequency of the observed Lorentzian peaks shows
strong sensitivity to the applied source - drain bias. This dependence on
electric field can be explained by the Frenkel-Poole effect in the scenario
where the voltage drop happens predominantly on the defects, which block the
quasi-1D conduction channels. We also have found that the activation energy of
the characteristic frequencies of the G-R noise in quasi-1D ZrTe3 is defined
primarily by the temperature dependence of the capture cross-section of the
defects rather than by their energy position. These results are important for
the application of quasi-1D van der Waals materials in ultimately downscaled
electronics. | cond-mat_mes-hall |
Anisotropic contribution to the van der Waals and the Casimir-Polder
energies for CO$_2$ and CH$_4$ molecules near surfaces and thin films: In order to understand why carbon dioxide (CO$_2$) and methane (CH$_4$)
molecules interact differently with surfaces, we investigate the Casimir-Polder
energy of a linearly polarizable CO$_2$ molecule and an isotropically
polarizable CH$_4$ molecule in front of an atomically thin gold film and an
amorphous silica slab. We quantitatively analyze how the anisotropy in the
polarizability of the molecule influences the van der Waals contribution to the
binding energy of the molecule. | cond-mat_mes-hall |
Machine learning nonequilibrium electron forces for adiabatic spin
dynamics: We present a generalized potential theory of nonequilibrium torques for the
Landau-Lifshitz equation. The general formulation of exchange forces in terms
of two potential energies allows for the implementation of accurate machine
learning models for adiabatic spin dynamics of out-of-equilibrium itinerant
magnetic systems. To demonstrate our approach, we develop a deep-learning
neural network that successfully learns the forces in a driven s-d model
computed from the nonequilibrium Green's function method. We show that the
Landau-Lifshitz dynamics simulations with forces predicted from the neural-net
model accurately reproduce the voltage-driven domain-wall propagation. Our work
opens a new avenue for multi-scale modeling of nonequilibrium dynamical
phenomena in itinerant magnets and spintronics based on machine-learning
models. | cond-mat_mes-hall |
Super-fermion representation of the Lindblad master equation for the
electron transport problem: We discuss the use of super-fermion formalism to represent and solve quantum
kinetic equations for the electron transport problem. Starting with the
Lindblad master equation for the molecule connected to two metal electrodes, we
convert the problem of finding the nonequilibrium steady state to the many-body
problem with non-Hermitian Liouvillian in super-Fock space. We transform the
Liouvillian to the normal ordered form, introduce nonequilibrium quasiparticles
by a set of canonical nonunitary transformations and develop general many-body
theory for the electron transport through the interacting region. The approach
is applied to the electron transport through a single level. We consider a
minimal basis hydrogen atom attached to two metal leads in Coulomb blockade
regime (out of equilibrium Anderson model) within the nonequilibrium
Hartree-Fock approximation as an example of the system with electron
interaction. Our approach agrees with exact results given by the Landauer
theory for the considered models. | cond-mat_mes-hall |
Optical conductivity as a probe of the interaction-driven metal in
rhombohedral trilayer graphene: Study of the strongly correlated states in van der Waals heterostructures is
one of the central topics in modern condensed matter physics. Among these, the
rhombohedral trilayer graphene (RTG) occupies a prominent place since it hosts
a variety of interaction-driven phases, with the metallic ones yielding exotic
superconducting orders upon doping. Motivated by these experimental findings,
we show within the framework of the low-energy Dirac theory that the optical
conductivity can distinguish different candidates for a paramagnetic metallic
ground state in this system. In particular, this observable shows a single peak
in the fully gapped valence-bond state. On the other hand, the bond-current
state features two pronounced peaks in the optical conductivity as the probing
frequency increases. Finally, the rotational symmetry breaking charge-density
wave exhibits a minimal conductivity with the value independent of the
amplitude of the order parameter, which corresponds precisely to the splitting
of the two cubic nodal points at the two valleys into two triplets of the band
touching points featuring linearly dispersing quasiparticles. These features
represent the smoking gun signatures of different candidate order parameters
for the paramagnetic metallic ground state, which should motivate further
experimental studies of the RTG. | cond-mat_mes-hall |
Electron-hole spin flip-flop in semiconductor quantum dots: We use temporally resolved intensity cross-correlation measurements to
identify the biexciton-exciton radiative cascades in a negatively charged QD.
The polarization sensitive correlation measurements show unambiguously that the
excited two electron triplet states relax non-radiatively to their singlet
ground state via a spin non conserving flip-flop with the ground state heavy
hole. We explain this mechanism in terms of resonant coupling between the
confined electron states and an LO phonon. This resonant interaction together
with the electron-hole exchange interaction provides an efficient mechanism for
this, otherwise spin-blockaded, electronic relaxation. | cond-mat_mes-hall |
Breakdown of topological protection due to non-magnetic edge disorder in
two-dimensional materials in the Quantum Spin Hall phase: We study the suppression of the conductance quantization in quantum spin Hall
systems by a combined effect of electronic interactions and edge disorder, that
is ubiquitous in exfoliated and CVD grown 2D materials. We show that the
interplay between the electronic localized states due to edge defects and
electron-electron interactions gives rise to local magnetic moments, that break
time-reversal symmetry and the topological protection of the edge states in 2D
topological systems. Our results suggest that edge disorder leads to small
deviations of a perfect quantized conductance in short samples and to a strong
conductance suppression in long ones. Our analysis is based on on the Kane-Mele
model, an unrestricted Hubbard mean field Hamiltonian and on a self-consistent
recursive Green's functions technique to calculate the transport quantities. | cond-mat_mes-hall |
Topological Quantum Computation Based on Chiral Majorana Fermions: Chiral Majorana fermion is a massless self-conjugate fermion which can arise
as the edge state of certain two-dimensonal topological matters. It has been
theoretically predicted and experimentally observed in a hybrid device of
quantum anomalous Hall insulator and a conventional superconductor. Its closely
related cousin, Majorana zero mode in the bulk of the corresponding topological
matter, is known to be applicable in topological quantum computations. Here we
show that the propagation of chiral Majorana fermions lead to the same unitary
transformation as that in the braiding of Majorana zero modes, and propose a
new platform to perform quantum computation with chiral Majorana fermions. A
Corbino ring junction of the hybrid device can utilize quantum coherent chiral
Majorana fermions to implement the Hadamard gate and the phase gate, and the
junction conductance yields a natural readout for the qubit state. | cond-mat_mes-hall |
Bolometric arrays and infrared sensitivity of VO2 films with varying
stoichiometry: Here we propose a linear microbolometric array based on VOx thin films. The
linear microbolometric array is fabricated by using micromachining technology,
and its thermo-sensitive VOx thin film has excellent infrared response spectrum
and TCR characteristics. Nano-scale VOx thin films deposited on SiO2/Si
substrates were obtained by e-beam vapor deposition. The VOx films were then
annealed at temperatures between 300 to 500 C with various deposition duration
time. The crystal structures and microstructures were examined by XRD, SEM and
ESCA. These films showed a predominant phase of rhombohedral VO2 and the
crystallinity of the VO2 increased as the annealing temperature increased.
Integrated with CMOS circuit, an experimentally prototypical monolithic linear
microbolometric array is designed and fabricated. The testing results of the
experimental linear array show that the responsivity of linear array can
approach 18KV/W and is potential for infrared image systems. | cond-mat_mes-hall |
Generalized Bloch theorem and topological characterization: The Bloch theorem enables reduction of the eigenvalue problem of the
single-particle Hamiltonian that commutes with translational group. Based on a
group theory analysis we present generalization of the Bloch theorem that
incorporates all additional symmetries of a crystal. The generalized Bloch
theorem constrains the form of the Hamiltonian which becomes manifestly
invariant under additional symmetries. In the case of isotropic interactions
the generalized Bloch theorem gives a unique Hamiltonian. This Hamiltonian
coincides with the Hamiltonian in the periodic gauge. In the case of
anisotropic interactions the generalized Bloch theorem allows a family of
Hamiltonians. Due to the continuity argument we expect that even in this case
the Hamiltonian in the periodic gauge defines observables, such as Berry
curvature, in the inverse space. For both cases we present examples and
demonstrate that the average of the Berry curvatures of all possible
Hamiltonians in the Bloch gauge is the Berry curvature in the periodic gauge. | cond-mat_mes-hall |
Mechanisms of optical orientation of an individual Mn$^{2+}$ ion spin in
a II-VI quantum dot: We provide a theoretical description of the optical orientation of a single
Mn$^{2+}$ ion spin under quasi-resonant excitation demonstrated experimentally
by Goryca et al. [Phys. Rev. Lett. 103, 087401 (2009)]. We build and analyze a
hierarchy of models by starting with the simplest assumptions (transfer of
perfectly spin-polarized excitons from Mn-free dot to the other dot containing
a single Mn$^{2+}$ spin, followed by radiative recombination) and subsequently
adding more features, such as spin relaxation of electrons and holes.
Particular attention is paid to the role of the influx of the dark excitons and
the process of biexciton formation, which are shown to contribute significantly
to the orientation process in the quasi-resonant excitation case. Analyzed
scenarios show how multiple features of the excitonic complexes in
magnetically-doped quantum dots, such as the values of exchange integrals, spin
relaxation times, etc., lead to a plethora of optical orientation processes,
characterized by distinct dependencies on light polarization and laser
intensity, and occurring on distinct timescales. Comparison with experimental
data shows that the correct description of the optical orientation mechanism
requires taking into account Mn$^{2+}$ spin-flip processes occurring not only
when the exciton is already in the orbital ground state of the light-emitting
dot, but also those that happen during the exciton transfer from high-energy
states to the ground state. Inspired by the experimental results on energy
relaxation of electrons and holes in nonmagnetic dots, we focus on the process
of biexciton creation allowed by mutual spin-flip of an electron and the
Mn$^{2+}$ spin, and we show that by including it in the model, we obtain good
qualitative and quantitative agreement with the experimental data on
quasi-resonantly driven Mn$^{2+}$ spin orientation. | cond-mat_mes-hall |
Element-specific soft X-ray spectroscopy, scattering and imaging studies
of skyrmion-hosting compound Co$_8$Zn$_8$Mn$_4$: A room-temperature skyrmion-hosting compound Co$_8$Zn$_8$Mn$_4$ has been
examined by means of soft X-ray absorption spectroscopy, resonant small-angle
scattering and extended reference holography. An element-selective study was
performed by exciting the $2p$-to-$3d$ transitions near Co and Mn $L_{2,3}$
absorption edges. By utilizing the coherence of soft X-ray beams the
element-specific real-space distribution of local magnetization at different
temperatures has been reconstructed using iterative phase retrieval and
holography with extended reference. It was shown that the magnetic moments of
Co and Mn are ferromagnetically coupled and exhibit similar magnetic patterns.
Both imaging methods provide a real-space resolution of 30 nm and allowed to
record a magnetic texture in the temperature range between $T\,=\,20$ K and
$T\,=120\,$ K, demonstrating the elongation of the skyrmions along the
principal crystallographic axes at low temperatures. Micromagnetic simulations
have shown that such deformation is driven by decreasing ratio of symmetric
exchange interaction to antisymmetric Dzyaloshinskii-Moriya interaction in the
system and effect of the cubic anisotropy. | cond-mat_mes-hall |
Modified Confinement Model for Size Dependent Raman Shift and Linewidth
of Silicon Nanocrystals: A modified phonon confinement model considering the size distribution, an
improved phonon dispersion curve and a confinement function is developed for
the calculation of size dependent Raman spectra of the silicon (Si)
nanocrystals. The model is capable in simultaneous calculation of the Raman
shift, intensity and linewidth. The calculated size dependent redshift and
linewidth of Raman spectra are in good agreement with the available
experimental data in literature and better than previously reported theoretical
results. The rapid rise in the redshift and linewidth for relatively smaller Si
nanocrystals are well reproduced. The asymmetric behavior of Raman spectra is
also obtained from the present model. | cond-mat_mes-hall |
Antiferromagnetic Magnonic Crystals: We describe the features of magnonic crystals based upon antiferromagnetic
elements. Our main results are that with a periodic modulation of either
magnetic fields or system characteristics, such as the anisotropy, it is
possible to tailor the spin wave spectra of antiferromagnetic systems into a
band-like organization that displays a segregation of allowed and forbidden
bands. The main features of the band structure, such as bandwidths and
bandgaps, can be readily manipulated. Our results provide a natural link
between two steadily growing fields of spintronics: antiferromagnetic
spintronics and magnonics. | cond-mat_mes-hall |
Non-quantized square-root topological insulators: a realization in
photonic Aharonov-Bohm cages: Topological Insulators are a novel state of matter where spectral bands are
characterized by quantized topological invariants. This unique quantized
non-local property commonly manifests through exotic bulk phenomena and
corresponding robust boundary effects. In our work, we report a new type of
topological insulator exhibiting spectral bands with non-quantized topological
properties, but with a quantization that arises in a corresponding system where
the square of the Hamiltonian is taken. We provide a thorough theoretical
analysis as well as an experimental demonstration based on photonic
Aharonov-Bohm cages to highlight the bulk and boundary properties of this
neophyte state of matter. | cond-mat_mes-hall |
Energetics and stability of vacancies in carbon nanotubes: In this work we present ab initio calculations of the formation energies and
stability of different types of multi-vacancies in carbon nanotubes. We
demonstrate that, as in the case of graphene, the reconstruction of the defects
has drastic effects on the energetics of the tubes. In particular, the
formation of pentagons eliminates the dangling bonds thus lowering the
formation energy. This competition leads to vacancies having an even number of
carbon atoms removed to be more stable. Finally the appearance of magic numbers
indicating more stable defects can be represented by a model for the formation
energies that is based on the number of dangling bonds of the unreconstructed
system, the pentagons and the relaxation of the final form of the defect formed
after the relaxation. | cond-mat_mes-hall |
On magnetic-field-induced dissipationless electric current in helicoidal
graphene nanoribbons: We argue that twisted (helicoidal) graphene nanoribbons may support
dissipationless electric current in the presence of static uniform magnetic
field. The non-resistive charge transfer in this parity-odd system should be
enhanced by thermal fluctuations. | cond-mat_mes-hall |
Giant Zeeman splitting inducing near-unity valley polarization in van
der Waals heterostructures: Monolayers of semiconducting transition metal dichalcogenides exhibit
intriguing fundamental physics of strongly coupled spin and valley degrees of
freedom for charge carriers. While the possibility of exploiting these
properties for information processing stimulated concerted research activities
towards the concept of valleytronics , maintaining control over spin-valley
polarization proved challenging in individual monolayers. A promising
alternative route explores type II band alignment in artificial van der Waals
heterostructures. The resulting formation of interlayer excitons combines the
advantages of long carrier lifetimes and spin-valley locking . Here, we
demonstrate direct magnetic manipulation of valley polarization in a WSe2/MoSe2
heterostructure through giant valley Zeeman splitting of interlayer
transitions. Remarkably, even after non-selective injection, the observed $g$
factor as large as $-15$ induces near-unity polarization of long-lived excitons
with 100 ns lifetimes under magnetic fields. The demonstrated control of the
spin-valley physics highlights the exceptional aspects of novel, artificially
designed material systems and their promise for atomically-thin valleytronic
devices. | cond-mat_mes-hall |
Thermodynamic properties of tunneling quasiparticles in graphene-based
structures: Thermodynamic properties of quasiparticles in a graphene-based structures are
investigated. Two graphene superconducting layers (one superconducting
component is placed on the top layeredgraphene structure and the other
component in the bottom) separated by oxide dielectric layers and one normal
graphene layer in the middle. The quasiparticle flow emerged due to external
gate voltage, we considered it as a gas of electron-hole pairs whose components
belong to different layers. This is a striking result in view of the complexity
of these systems: we have established that specific heat exhibits universal
(-T3) behaviour at low T, independent from the gate voltage and the
superconducting gap. The experimental observation of this theoretical
prediction would be an important step towards our understanding of critical
massless matter. | cond-mat_mes-hall |
Resources of polarimetric sensitivity in spin noise spectroscopy: We attract attention to the fact that the ultimate (shot-noise-limited)
polarimetric sensitivity can be enhanced by orders of magnitude leaving the
photon flux incident onto the photodetector on the same low level. This
opportunity is of crucial importance for present-day spin noise spectroscopy,
where a direct increase of sensitivity by increasing the probe beam power is
strongly restricted by the admissible input power of the broadband
photodetectors. The gain in sensitivity is achieved by replacing the 45-deg
polarization geometry commonly used in conventional schemes with balanced
detectors by geometries with stronger polarization extinction. The efficiency
of these high-extinction polarization geometries with enhancement of the
detected signal by more than an order of magnitude is demonstrated by
measurements of the spin noise spectra of bulk n:GaAs in the spectral range
835-918 nm. It is shown that the inevitable growth of the probe beam power with
the sensitivity gain makes spin noise spectroscopy much more perturbative, but,
at the same time, opens up fresh opportunities for studying nonlinear
interactions of strong light fields with spin ensembles. | cond-mat_mes-hall |
Spin Currents in Metallic Nanostructures; Explicit Calculations: In ultrathin ferromagnets deposited on metallic substrates, excitation of
precessional motion of the spins produces a spin current in the substrate that
transports angular momentum out of the film. This phenomenon is referred to as
spin pumping, and is a source of damping of the spin motion. Spin pumping
enters importantly in the description of spin dynamics in other nanoscale and
subnanoscale systems as well. In this paper, we present an approach based on
the Kubo formalism that allows the explicit calculation of this spin current
and its spatial variation. We use the formalism to explore features of the spin
current generated by spin motions in a simple model system. | 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 |
Non-linear transport and heat dissipation in metallic carbon nanotubes: We show that the local temperature dependence of thermalized electron and
phonon populations along metallic carbon nanotubes is the main reason behind
this non-linear transport characteristics in the high bias regime. Our model
that considers optical and zone boundary phonon emission as well as absorption
by charge carriers is based on the solution of the Boltzmann transport equation
that assumes a local temperature along the nanotube, determined
self-consistently with the heat transport equation. By using realistic
transport parameters, our results not only reproduce experimental data for
electronic transport, but also provide a coherent interpretation of thermal
breakdown under electric stress. In particular, electron and phonon
thermalization prohibits ballistic transport in short nanotubes. | cond-mat_mes-hall |
Novel Exotic Magnetic Spin-order in Co5Ge3 Nano-size Materials: The Cobalt-germanium (Co-Ge) is a fascinating complex alloy system that has
unique structure and exhibit range of interesting magnetic properties which
would change when reduce to nanoscale dimension. At this experimental work, the
high-aspect-ratio Co5Ge3 nanoparticle with average size of 8nm was synthesized
by gas aggregation-type cluster-deposition technology. The nanostructure
morphology of the as-made binary Co5Ge3 nanoparticles demonstrate excellent
single-crystalline hexagonal structure with mostly preferable growth along
(110) and (102) directions. In contrast the bulk possess Pauli paramagnetic
spin-order at all range of temperature, here we discover size-driven new
magnetic ordering of as-synthesized Co5Ge3 nanoparticles exhibiting
ferromagnetism at room temperature with saturation magnetization of Ms = 32.2
emu/cm3. This is first report of observing such new magnetic spin ordering in
this kind of material at nano-size which the magnetization has lower
sensitivity to thermal energy fluctuation and exhibit high Curie temperature
close to 850 K. This ferromagnetic behavior along with higher Curie temperature
at Co5Ge3 nanoparticles are attributes to low-dimension and quantum-confinement
effect which imposes strong spin coupling and provides a new set of size-driven
spin structures in Co5Ge3 nanoparticle which no such magnetic behavior being
present in the bulk of same material. This fundamental scientific study
provides important insights into the formation, structural, and the magnetic
property of sub 10nm Co5Ge3 nanostructure which shall lead to promising
practical versatile applications for magneto- germanide based nano-devices. | cond-mat_mes-hall |
Electronic transport in normal-conductor/graphene/normal-conductor
junctions and conditions for insulating behavior at a finite charge-carrier
density: We investigate the conductance of normal-conductor/graphene/normal-conductor
(NGN) junctions for arbitrary on-site potentials in the normal and graphitic
parts of the system. We find that a ballistic NGN junction can display
insulating behavior even when the charge-carrier density in the graphene part
is finite. This effect originates in the different k intervals supporting
propagating modes in graphene and a normal conductor, and persists for moderate
levels of bulk, edge, or interface disorder. The ensuing conductance thresholds
could be used as an electronic tool to map out details of the graphene band
structure in absolute k space. | cond-mat_mes-hall |
A new method to epitaxially grow long-range ordered self-assembled InAs
quantum dots on (110) GaAs: We report on a new approach for positioning of self-assembled InAs quantum
dots on (110) GaAs with nanometer precision. By combining self-assembly of
quantum dots with molecular beam epitaxy on in-situ cleaved surfaces
(cleaved-edge overgrowth) we have successfully fabricated arrays of long-range
ordered InAs quantum dots. Both atomic force microscopy and
micro-photoluminescence measurements demonstrate the ability to control size,
position, and ordering of the quantum dots. Furthermore, single dot
photoluminescence investigations confirm the high optical quality of the
quantum dots fabricated. | cond-mat_mes-hall |
Probing the helical edge states of a topological insulator by
Cooper-pair injection: We consider the proximity effect between a singlet s-wave superconductor and
the edge of a Quantum Spin Hall (QSH) topological insulator. We establish that
Andreev reflection at a QSH edge state/superconductor interface is perfect
while nonlocal Andreev processes through the superconductor are totally
suppressed. We compute the corresponding conductance and noise. | cond-mat_mes-hall |
Spin Hall Insulator: Recent theories predict dissipationless spin current induced by an electric
field in doped semiconductors. Nevertheless, the charge current is still
dissipative in these systems. In this work, we theoretically predict the
dissipationless spin Hall effect, without any accompanying charge current, in
some classes of band insulators, including zero-gap semiconductors such as HgTe
and narrow-gap semiconductors such as PbTe. This effect is similar to the
quantum Hall effect in that all the states below the gap contribute and there
occurs no dissipation. However the spin Hall conductance is not quantized even
in two dimensions. This is the first example of a nontrivial topological
structure in a band insulator without any magnetic field. | cond-mat_mes-hall |
Absence of magnetic-proximity effect at the interface of Bi$_2$Se$_3$
and (Bi,Sb)$_2$Te$_3$ with EuS: We performed x-ray magnetic circular dichroism (XMCD) measurements on
heterostructures comprising topological insulators (TIs) of the
(Bi,Sb)$_2$(Se,Te)$_3$ family and the magnetic insulator EuS. XMCD measurements
allow us to investigate element-selective magnetic proximity effects at the
very TI/EuS interface. A systematic analysis reveals that there is neither
significant induced magnetism within the TI nor an enhancement of the Eu
magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te,
and Se sites are lower than the estimated detection limit of the XMCD
measurements of $\sim\!10^{-3}$ $\mu_\mathrm{B}$/at. | cond-mat_mes-hall |
Magnetoelectric control of topological phases in graphene: Topological antiferromagnetic (AFM) spintronics is an emerging field of
research, which involves the topological electronic states coupled to the AFM
order parameter known as the N$\acute{\rm e}$el vector. The control of these
states is envisioned through manipulation of the N$\acute{\rm e}$el vector by
spin-orbit torques driven by electric currents. Here we propose a different
approach favorable for low-power AFM spintronics, where the control of the
topological states in a two-dimensional material, such as graphene, is
performed via the proximity effect by the voltage induced switching of the
N$\acute{\rm e}$el vector in an adjacent magnetoelectric AFM insulator, such as
chromia. Mediated by the symmetry protected boundary magnetization and the
induced Rashba-type spin-orbit coupling at the interface between graphene and
chromia, the emergent topological phases in graphene can be controlled by the
N$\acute{\rm e}$el vector. Using density functional theory and tight-binding
Hamiltonian approaches, we model a graphene/Cr2O3 (0001) interface and
demonstrate non-trivial band gap openings in the graphene Dirac bands
asymmetric between the K and K' valleys. This gives rise to an unconventional
quantum anomalous Hall effect (QAHE) with a quantized value of $2e^2/h$ and an
additional step-like feature at a value close to $e^2/2h$, and the emergence of
the spin-polarized valley Hall effect (VHE). Furthermore, depending on the
N$\acute{\rm e}$el vector orientation, we predict the appearance and
transformation of different topological phases in graphene across the
$180^{\circ}$ AFM domain wall, involving the QAHE, the valley-polarized QAHE
and the quantum VHE (QVHE), and the emergence of the chiral edge state along
the domain wall. These topological properties are controlled by voltage through
magnetoelectric switching of the AFM insulator with no need for spin-orbit
torques. | cond-mat_mes-hall |
Unified Semi-Classical Description of Intrinsic Spin-Hall Effect in
Spintronic, Optical, and Graphene Systems: A semi-classical description of the intrinsic spin-Hall effect (SHE) is
presented which is relevant for a wide class of systems. A heuristic model for
the SHE is developed, starting with a fully quantum mechanical treatment, from
which we construct an intuitive expression for the spin-Hall current and
conductivity. Our method makes transparent the physical mechanism which drives
the effect, and unifies the SHE across several spintronic and optical systems.
Finally, we propose an analogous effect in bilayer graphene. | cond-mat_mes-hall |
Backscattering off a driven Rashba impurity at the helical edge: The spin degree of freedom is crucial for both understanding and exploiting
the particular properties of the edges of two-dimensional topological
insulators. In the absence of superconductivity and magnetism, Rashba coupling
is the most relevant single particle perturbation in this system. Since Rashba
coupling does not break time reversal symmetry, its influence on transport
properties is only visible if processes that do not conserve the single
particle energy are included. Paradigmatic examples of such processes are
electron-electron interactions and time dependent external drivings. We analyze
the effects of a periodically driven Rashba impurity at the helical edge, in
the presence of electron-electron interactions. Interactions are treated by
means of bosonization and the backscattering current is computed perturbatively
up to second order in the impurity strength. We show that the backscattering
current is non-monotonic in the driving frequency. This property is a
fingerprint of the Rashba impurity, being absent in the case of a magnetic
impurity in the helical liquid. Moreover, the non-monotonic behaviour allows us
to directly link the backscattering current to the Luttinger parameter $K$,
encoding the strength of electron-electron interactions. | cond-mat_mes-hall |
Electrons imitating light: Frustrated supercritical collapse in charged
arrays on graphene: The photon-like electronic dispersion of graphene bestows its charge carriers
with unusual confinement properties that depend strongly on the geometry and
strength of the surrounding potential. Here we report bottom-up synthesis of
atomically-precise one-dimensional (1D) arrays of point charges aimed at
exploring supercritical confinement of carriers in graphene for new geometries.
The arrays were synthesized by arranging F4TCNQ molecules into a 1D lattice on
back-gated graphene devices, allowing precise tuning of both the molecular
charge state and the array periodicity. Dilute arrays of ionized F4TCNQ
molecules are seen to behave like isolated subcritical charges but dense arrays
show emergent supercriticality. In contrast to compact supercritical clusters,
extended 1D charge arrays exhibit both supercritical and subcritical
characteristics and belong to a new physical regime termed frustrated
supercritical collapse. Here carriers in the far-field are attracted by a
supercritical charge distribution, but have their fall to the center frustrated
by subcritical potentials in the near-field, similar to the trapping of light
by a dense cluster of stars in general relativity. | cond-mat_mes-hall |
Exact microscopic wave function for a topological quantum membrane: The higher dimensional quantum Hall liquid constructed recently supports
stable topological membrane excitations. Here we introduce a microscopic
interacting Hamiltonian and present its exact ground state wave function. We
show that this microscopic ground state wave function describes a topological
quantum membrane. We also construct variational wave functions for excited
states using the non-commutative algebra on the four sphere. Our approach
introduces a non-perturbative method to quantize topological membranes. | cond-mat_mes-hall |
The Synthesis and Electrical Transport of Ligand-Protected Au13 Clusters: The ligand-protected Au13 clusters have been synthesized by using
meso-2,3-imercaptosuccinic acid as the reducing and stabilizing agent.
Transmission electron microscopic analysis shows a size distribution of 0.6nm.
Optical spectrum shows an absorbance peak at 390 nm. The electrical transport
measurement devices are fabricated using the electro-migration method. Coulomb
blockade is observed at the temperature of 1.6 K,revealing the formation of the
tunneling junction. The Coulomb oscillation on-off ratio is nearly 5. Three
peaks are extracted in the dI-dV data and attributed to the energy levels of
Au13 clusters, gapped by about 60 meV. First principle calculations are carried
out to interpret the energy diagram. | cond-mat_mes-hall |
Zero-Energy Modes from Coalescing Andreev States in a Two-Dimensional
Semiconductor-Superconductor Hybrid Platform: We investigate zero-bias conductance peaks that arise from coalescing subgap
Andreev states, consistent with emerging Majorana zero modes, in hybrid
semiconductor-superconductor wires defined in a two-dimensional InAs/Al
heterostructure using top-down lithography and gating. The measurements
indicate a hard superconducting gap, ballistic tunneling contact, and in-plane
critical fields up to $3$~T. Top-down lithography allows complex geometries,
branched structures, and straightforward scaling to multicomponent devices
compared to structures made from assembled nanowires. | cond-mat_mes-hall |
Programmable two-qubit gates in capacitively coupled flopping-mode spin
qubits: Recent achievements in the field of gate defined semiconductor quantum dots
reinforce the concept of a spin-based quantum computer consisting of nodes of
locally connected qubits which communicate with each other via superconducting
circuit resonator photons. In this work we theoretically demonstrate a
versatile set of quantum gates between adjacent spin qubits defined in
semiconductor quantum dots situated within the same node of such a spin-based
quantum computer. The electric dipole acquired by the spin of an electron that
moves across a double quantum dot potential in a magnetic field gradient has
enabled strong coupling to resonator photons and low-power spin control. Here
we show that this flopping-mode spin qubit also provides with the tunability to
program multiple two-qubit gates. Since the capacitive coupling between these
qubits brings about additional dephasing, we calculate the estimated infidelity
of different two-qubit gates in the most immediate possible experimental
realizations. | cond-mat_mes-hall |
Electronic Noise of a Single Skyrmion: To enable the practical use of skyrmion-based devices, it is essential to
achieve a balance between energy efficiency and thermal stability, while also
ensuring reliable electrical detection against noise. Understanding how a
skyrmion interacts with material disorder and external perturbations is thus
essential. Here we investigate the electronic noise of a single skyrmion under
the influence of thermal fluctuations and spin currents in a magnetic thin
film. We detect the thermally induced noise with a 1/f signature in the strong
pinning regime but a random telegraph noise in the intermediate pinning regime.
Both the thermally dominated and current-induced telegraph-like signals are
detected in the weak pinning regime. Our results provide a comprehensive
electronic noise picture of a single skyrmion, demonstrating the potential of
noise fluctuation as a valuable tool for characterizing the pinning condition
of a skyrmion. These insights could also aid in the development of low-noise
and reliable skyrmion-based devices. | cond-mat_mes-hall |
Charging effects in the inductively shunted Josephson junction: The choice of impedance used to shunt a Josephson junction determines if the
charge transferred through the circuit is quantized: a capacitive shunt renders
the charge discrete, whereas an inductive shunt leads to continuous charge.
This discrepancy leads to a paradox in the limit of large inductances L. We
show that while the energy spectra of the capacitively and inductively shunted
junction are vastly different, their high-frequency responses become identical
for large L. Inductive shunting thus opens the possibility to observe charging
effects unimpeded by charge noise. | cond-mat_mes-hall |
Enhanced thermoelectric properties in hybrid graphene-boron nitride
nanoribbons: The thermoelectric properties of hybrid graphene-boron nitride nanoribbons
(BCNNRs) are investigated using the non-equilibrium Green's function (NEGF)
approach. We find that the thermoelectric figure of merit (ZT) can be
remarkably enhanced by periodically embedding hexagonal BN (h-BN) into graphene
nanoribbons (GNRs). Compared to pristine GNRs, the ZT for armchair-edged BCNNRs
with width index 3p+2 is enhanced up to 10~20 times while the ZT of nanoribbons
with other widths is enhanced just by 1.5~3 times. As for zigzag-edge
nanoribbons, the ZT is enhanced up to 2~3 times. This improvement comes from
the combined increase in the Seebeck coefficient and the reduction in the
thermal conductivity outweighing the decrease in the electrical conductance. In
addition, the effect of component ratio of h-BN on the thermoelectric transport
properties is discussed. These results qualify BCNNRs as a promising candidate
for building outstanding thermoelectric devices. | cond-mat_mes-hall |
Light Induced Aggregation of Specific Single Walled Carbon Nanotubes: We report optically induced aggregation and consequent separation of specific
diameter of pristine single walled carbon nanotubes (SWNT) from stable
solution. Well dispersed solution of pristine SWNTs, without any surfactant or
functionalization, show rapid aggregation by uniform exposure to UV, visible
and NIR illumination. Optically induced aggregation linearly increases with
consequent increase in the intensity of light. Aggregated SWNTs were separated
from the dispersed supernatant and characterized using absorption and Raman
spectroscopy. Separated SWNTs distinctly show enrichment of specific SWNTs
under UV visible and NIR illumination. | cond-mat_mes-hall |
Dissipation-driven quantum phase transitions in a Tomonaga-Luttinger
liquid electrostatically coupled to a metallic gate: The dissipation induced by a metallic gate on the low-energy properties of
interacting 1D electron liquids is studied. As function of the distance to the
gate, or the electron density in the wire, the system undergoes a quantum phase
transition from the Tomonaga-Luttinger liquid state to two kinds of dissipative
phases, one of them with a finite spatial correlation length. We also define a
dual model, which describes an attractive one dimensional metal with a
Josephson coupling to a dirty metallic lead. | cond-mat_mes-hall |
Raman Photogalvanic Effect: photocurrent at inelastic light scattering: We show theoretically that electromagnetic waves propagating in the
transparency region of a non-centrosymmetric medium can induce a dc electric
current. The origin of the effect is the Raman scattering of light by free
carriers in the system. Due to the photon scattering, electrons undergo real
quantum transitions resulting in the formation of their anisotropic momentum
distribution and in shifts of electronic wavepackets giving rise to a steady
state photocurrent. We present microscopic theory of the Raman Photogalvanic
effect (RPGE) focusing on two specific situations: (i) generic case of a bulk
gyrotropic semiconductor and (ii) a quantum well structure where the light is
scattered by intersubband excitations. We uncover the relation of the predicted
RPGE and the traditional photogalvanic effect at the light absorption. | cond-mat_mes-hall |
Study of intrinsic spin and orbital Hall effects in Pt based on a (6s,
6p, 5d) tight-binding model: We study the origin of the intrinsic spin Hall conductivity (SHC) and the
d-orbital Hall conductivity (OHC) in Pt based on a multiorbital tight-binding
model with spin-orbit interaction. We find that the SHC reaches 1000
\hbar/e\Omega cm when the resistivity \rho is smaller than ~10 \mu\Omega cm,
whereas it decreases to 300 \hbar/e\Omega cm when \rho ~ 100 \mu\Omega cm. In
addition, the OHC is still larger than the SHC. The origin of huge SHE and OHE
in Pt is the large ``effective magnetic flux'' that is induced by the
interorbital transition between d_{xy}- and d_{x2-y2}-orbitals with the aid of
the strong spin-orbit interaction. | cond-mat_mes-hall |
Ultrafast mapping of optical polarization states onto spin coherence of
localized electrons in a semiconductor: We experimentally demonstrate an ultrafast method for preparing spin states
of donor-bound electrons in GaAs with single laser pulses. Each polarization
state of a preparation pulse has a direct mapping onto a spin state, with
bijective correspondence between the Poincar\'{e}-sphere (for photon
polarization) and Bloch-sphere (for spin) state representations. The
preparation is governed by a stimulated Raman process and occurs orders of
magnitude faster than the spontaneous emission and spin dephasing. Similar
dynamics governs our ultrafast optical Kerr detection of the spin coherence,
thus getting access to spin state tomography. Experiments with double
preparation pulses show an additive character for the preparation method.
Utilization of these phenomena is of value for quantum information schemes. | cond-mat_mes-hall |
Modeling ultrafast all-optical switching in synthetic ferrimagnets: Based on numerical simulations, we demonstrate thermally induced magnetic
switching in synthetic ferrimagnets composed of multilayers of rare-earth and
transition metals. Our findings show that deterministic magnetization reversal
occurs above a certain threshold temperature if the ratio of transition metal
atoms to rare-earth atoms is sufficiently large. Surprisingly, the total
thickness of the multilayer system has little effect on the occurence of
switching. We further provide a simple argument to explain the temperature
dependence of the reversal process. | cond-mat_mes-hall |
Size Dependence of the Multiple Exciton Generation Rate in CdSe Quantum
Dots: The multiplication rates of hot carriers in CdSe quantum dots are quantified
using an atomistic pseudopotential approach and first order perturbation
theory. Both excited holes and electrons are considered, and electron-hole
Coulomb interactions are accounted for. We find that holes have much higher
multiplication rates than electrons with the same excess energy due to the
larger density of final states (positive trions). When electron-hole pairs are
generated by photon absorption, however, the net carrier multiplication rate is
dominated by photogenerated electrons, because they have on average much higher
excess energy. We also find, contrary to earlier studies, that the effective
Coulomb coupling governing carrier multiplication is energy dependent. We show
that smaller dots result in a decrease in the carrier multiplication rate for a
given absolute photon energy. However, if the photon energy is scaled by the
volume dependent optical gap, then smaller dots exhibit an enhancement in
carrier multiplication for a given relative energy. | cond-mat_mes-hall |
Rectification and nonlinear transport in chaotic dots and rings: We investigate the nonlinear current-voltage characteristic of mesoscopic
conductors and the current generated through rectification of an alternating
external bias. To leading order in applied voltages both the nonlinear and the
rectified current are quadratic. This current response can be described in
terms of second order conductance coefficients and for a generic mesoscopic
conductor they fluctuate randomly from sample to sample. Due to Coulomb
interactions the symmetry of transport under magnetic field inversion is broken
in a two-terminal setup. Therefore, we consider both the symmetric and
antisymmetric nonlinear conductances separately. We treat interactions
self-consistently taking into account nearby gates.
The nonlinear current is determined by different combinations of second order
conductances depending on the way external voltages are varied away from an
equilibrium reference point (bias mode). We discuss the role of the bias mode
and circuit asymmetry in recent experiments. In a photovoltaic experiment the
alternating perturbations are rectified, and the fluctuations of the nonlinear
conductance are shown to decrease with frequency. Their asymptotical behavior
strongly depends on the bias mode and in general the antisymmetric conductance
is suppressed stronger then the symmetric conductance.
We next investigate nonlinear transport and rectification in chaotic rings.
To this extent we develop a model which combines a chaotic quantum dot and a
ballistic arm to enclose an Aharonov-Bohm flux. In the linear two-probe
conductance the phase of the Aharonov-Bohm oscillation is pinned while in
nonlinear transport phase rigidity is lost. We discuss the shape of the
mesoscopic distribution of the phase and determine the phase fluctuations. | cond-mat_mes-hall |
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