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A simple view on the quantum Hall system: The physics of the quantum Hall system becomes very simple when studied on a
thin torus. Remarkably, however, the very rich structure still exists in this
limit and there is a continuous route to the bulk system. Here we review recent
progress in understanding various features of the quantum Hall system in terms
of a simple one-dimensional model corresponding to the thin torus. | cond-mat_mes-hall |
Spin waves in zigzag graphene nanoribbons and the stability of edge
ferromagnetism: We study the low energy spin excitations of zigzag graphene nanoribbons of
varying width. We find their energy dispersion at small wave vector to be
dominated by antiferromagnetic correlations between the ribbon's edges, in
accrodance with previous calculations. We point out that spin wave lifetimes
are very long due to the semi-conducting nature of the electrically neutral
nanoribbons. However, application of very modest gate voltages cause a
discontinuous transition to a regime of finite spin wave lifetime. By further
increasing doping the ferromagnetic alignments along the edge become unstable
against transverse spin fluctuations. This makes the experimental detection of
ferromagnetism is this class of systems very delicate, and poses a difficult
challenge to the possible uses of these nanoribbons as basis for spintronic
devices. | cond-mat_mes-hall |
Saturation and bistability of defect-mode intersubband polaritons: In this article we report about linear and nonlinear optical properties of
intersubband cavity polariton samples, where the resonant photonic mode is a
defect state in a metallo-dielectric photonic crystal slab. By tuning a single
geometric parameter of the resonator, the cavity Q-factor can reach values as
large as 85, with a consequent large cooperativity for the light-matter
interaction. We show that a device featuring large cooperativity leads to sharp
saturation, or even bistability, of the polariton states. This nonlinear
dynamics occurs at the crossover between the weak and the strong coupling
regimes, where the weak critical coupling concept plays a fundamental role. | cond-mat_mes-hall |
Ligand effects on the electronic structure and magnetism of magnetite
surfaces: We address the effect of functionalization on the electronic and magnetic
properties of magnetite surface as an indicator of the same properties in
nanoparticles too big for a direct ab-initio approach. Using well-established
methods and references (namely LDA+U on magnetite surfaces) we could verify the
validity of our approach, and using two typical ligands, dopamine and citrate,
namely pi and sigma electron donors, we could predict that those ligands would
induce a different change in the electronic properties of the systems, but in
both cases an enhancement of magnetization. | cond-mat_mes-hall |
High temperature magnetism and microstructure of semiconducting
ferromagnetic alloy (GaSb)$_{1-x}$(MnSb)$_{x}$: We have studied the properties of relatively thick (about 120 nm) magnetic
composite films grown by pulsed laser deposition method using
(GaSb)$_{0.59}$(MnSb)$_{0.41}$ eutectic compound as a target for sputtering.
For the studied films we have observed ferromagnetism and anomalous Hall effect
above the room temperature, it manifests the presence of spin-polarized
carriers. Electron microscopy, atomic and magnetic force microscopy results
suggests that films under study have homogenous columnar structure in the bulk
while MnSb inclusions accumulate near it's surface. This is in good agreement
with high mobility values of charge carriers. Based on our data we conclude
that room temperature magnetic and magnetotransport properties of the films are
defined by MnSb inclusions. | cond-mat_mes-hall |
Quantum Coherent Multielectron Processes in an Atomic Scale Contact: The light emission from a scanning tunneling microscope operated on a Ag(111)
surface at 6 K is analyzed from low conductances to values approaching the
conductance quantum. Optical spectra recorded at a sample voltages V reveal
emission with photon energies hv> 2eV. A model of electrons interacting
coherently via a localized plasmon-polariton mode reproduces the experimental
data, in particular the kinks in the spectra at eV and 2eV as well as the
scaling of the intensity at low and intermediate conductances. | cond-mat_mes-hall |
Anomalous Spin Dephasing in (110) GaAs Quantum Wells: Anisotropy and
Intersubband Effects: A strong anisotropy of electron spin decoherence is observed in GaAs/(AlGa)As
quantum wells grown on (110) oriented substrate. The spin lifetime of spins
perpendicular to the growth direction is about one order of magnitude shorter
compared to spins along (110). The spin lifetimes of both spin orientations
decrease monotonically above a temperature of 80 and 120 K, respectively. The
decrease is very surprising for spins along (110) direction and cannot be
explained by the usual Dyakonov Perel dephasing mechanism. A novel spin
dephasing mechanism is put forward that is based on scattering of electrons
between different quantum well subbands. | cond-mat_mes-hall |
On the nature of the spin polarization limit in the warped Dirac cone of
the Bi2Te3: The magnitude of electron spin polarization in topologically protected
surface states is an important parameter with respect to spintronics
applications. In order to analyze the warped spin texture in Bi$_2$Te$_3$ thin
films, we combine angle- and spin-resolved photoemission experiments with
theoretical \textit{ab initio} calculations. We find an \textit{in-plane} spin
polarization of up to $\sim$~45\% in the topologically protected Dirac cone
states near the Fermi level. The Fermi surface of the Dirac cone state is
warped and shows an \textit{out-of-plane} spin polarization of $\sim$~15\%.
These findings are in quantitative agreement with dedicated simulations which
find electron density of the Dirac cone delocalized over the first quintuple
layer with spin reversal occurring in the surface atomic layer. | cond-mat_mes-hall |
Vorticity and quantum turbulence in the merging of superfluid Helium
nanodroplets: We have studied the merging of two $^4$He droplets at zero temperature,
caused by their Van der Waals mutual attraction. During the early stages of the
merging, density structures appear which closely match the experimental
observations by Vicente et al. [J. Low Temp. Phys. 121, 627 (2000)]. When the
droplets are merging, quantized vortex-antivortex ring pairs nucleate at the
surface and annihilate inside the merged droplet producing a roton burst. We
also observe the nucleation of quantized vortex-antivortex rings that wrap the
droplet surface and remain localized on the surface until they eventually decay
into short-wavelength surface waves. Analysis of the kinetic energy spectrum
discloses the existence of a regime where turbulence caused by vortex
interaction and annihilation is characterized by a Kolmogorov power law. This
is followed by another regime where roton radiation (produced by
vortex-antivortex annihilation) dominates, whose hallmark is a weak, turbulent
surface dynamics. We suggest that similar processes might appear in superfluid
helium droplets after they capture impurities or if they are produced by
hydrodynamic instability of a liquid jet. Experiments on collisions between
recently-discovered self-bound Bose-Einstein condensates should display a
similar phenomenology. | cond-mat_mes-hall |
Voltage switching and domain relocation in semiconductor superlattices: A numerical study of domain wall relocation during voltage switching with
different ramping times is presented for weakly coupled, doped semiconductor
superlattices exhibiting multistable domain formation in the first plateau of
their current-voltage characteristics. Stable self-oscillations of the current
at the end of stable stationary branches of the current-voltage characteristics
have been found. These oscillations are due to periodic motion of charge
dipoles near the cathode that disappear inside the SL, before they can reach
the receiving contact. Depending on the dc voltage step, the type of
multistability between static branches and the duration of voltage switching,
unusual relocation scenarios are found including changes of the current that
follow adiabatically the stable I--V branches, different faster episodes
involving charge tripoles and dipoles, and even small amplitude oscillations of
the current near the end of static I--V branches followed by dipole-tripole
scenarios. | cond-mat_mes-hall |
Metastability and dynamics in remanent states of square artificial spin
ice with long-range dipole interactions: After removal of an applied magnetic field, artificial square spin ice can be
left in a metastable remanent state, with nonzero residual magnetization and
excess energy above the ground state. Using a model of magnetic islands with
dipoles of fixed magnitude and local anisotropies, the remanent states are
precisely determined here, including all long-range dipole interactions. Small
deviations away from remanent states are analyzed and the frequencies of modes
of oscillation are determined. Some modes reach zero frequency at high symmetry
wave vectors, such that the stability limits are found, as determined by the
local anisotropy strength relative to the dipolar coupling strength. | cond-mat_mes-hall |
Periodic negative differential conductance in a single metallic
nano-cage: We report a bi-polar multiple periodic negative differential conductance
(NDC) effect on a single cage-shaped Ru nanoparticle measured using scanning
tunneling spectroscopy. This phenomenon is assigned to the unique
multiply-connected cage architecture providing two (or more) defined routes for
charge flow through the cage. This, in turn, promotes a self- gating effect,
where electron charging of one route affects charge transport along a
neighboring channel, yielding a series of periodic NDC peaks. This picture is
established and analyzed here by a theoretical model. | cond-mat_mes-hall |
Thermally-Activated Phase Slips in Superfluid Spin Transport in Magnetic
Wires: We theoretically study thermally-activated phase slips in superfluid spin
transport in easy-plane magnetic wires within the stochastic
Landau-Lifshitz-Gilbert phenomenology, which runs parallel to the
Langer-Ambegaokar-McCumber-Halperin theory for thermal resistances in
superconducting wires. To that end, we start by obtaining the exact solutions
for free-energy minima and saddle points. We provide an analytical expression
for the phase-slip rate in the zero spin-current limit, which involves detailed
analysis of spin fluctuations at extrema of the free energy. An experimental
setup of a magnetoeletric circuit is proposed, in which thermal phase slips can
be inferred by measuring nonlocal magnetoresistance. | cond-mat_mes-hall |
Critical current of spin transfer torque-driven magnetization dynamics
in magnetic multilayers: The critical current of the spin transfer torque-driven magnetization
dynamics was studied by taking into account both spin pumping and the finite
penetration depth of the transverse spin current. We successfully reproduced
the recent experimental results obtained by Chen et al. [Phys. Rev. B {\bf 74},
144408 (2006)] and found that the critical current remains finite even in the
zero-thickness limit of the free layer. We showed that the remaining value of
the critical current is determined mainly by spin pumping. | cond-mat_mes-hall |
Non-equilibrium theory for strongly coupled quantum dot with arbitrary
on-site correlation strength: An analytical expression for the current through a single level quantum dot
for arbitrary strength of the on-site electron-electron interaction is derived
beyond standard mean-field theory. By describing the localised states in terms
of many-body operators, the employed diagrammatic technique for strong coupling
enables inclusion of electron correlation effects into the description of the
local dynamics, which provides transport properties that are consistent with
recent experimental data. | cond-mat_mes-hall |
Spin-Torque and Spin-Hall Nano-Oscillators: This paper reviews the state of the art in spin-torque and spin Hall effect
driven nano-oscillators. After a brief introduction to the underlying physics,
the authors discuss different implementations of these oscillators, their
functional properties in terms of frequency range, output power, phase noise,
and modulation rates, and their inherent propensity for mutual synchronization.
Finally, the potential for these oscillators in a wide range of applications,
from microwave signal sources and detectors to neuromorphic computation
elements, is discussed together with the specific electronic circuitry that has
so far been designed to harness this potential. | cond-mat_mes-hall |
Persistent currents and magnetic flux trapping in fragments of carbon
deposits containing multiwalled nanotubes: It is found that the magnetization curves of samples of fragments of cathode
carbon deposits with a high content of multiwalled nanotubes exhibit a
pronounced irreversible character, attesting to the induction of persistent
currents in the samples and to magnetic flux trapping, as happens in a multiply
connected superconducting structure. A decrease of the trapped flux in time
could not be observed at low (helium) temperatures with a measurement time of
about 20 h. For intermediate (~30K) and room temperatures the trapped magnetic
flux decays slowly with characteristic relaxation times of the order of 150 and
15 h, respectively. | cond-mat_mes-hall |
Geometrical Effect Explains Graphene Membrane Stiffening at Finite
Vacancy Concentrations: The presence of defects such as vacancies in solids has prominent effects on
their mechanical properties. It not only modifies the stiffness and strength of
materials, but also changes their morphologies. The latter effect is extremely
significant for low- dimensional materials such as graphene. We show in this
work that graphene swells while point defects such as vacancies are created at
finite concentrations. The distorted geometry resulted from this areal
expansion, in combination with the in-plane softening effect, predicts an
unusual defect concentration dependence of stiffness measured for supported
graphene membrane in nanoindentation tests, which explains the defect- induced
stiffening phenomenon reported recently. The mechanism is elucidated through an
analytical membrane model as well as numerical simulations at atomistic and
continuum levels. In addition to elucidate the counter-intuitive observations
in experiments and computer simulations, our findings also highlight the role
of defect- modulated morphology engineering that can be quite effective in
designing nanoscale material and structural applications. | cond-mat_mes-hall |
The Atomic and Electronic structure of 0° and 60° grain
boundaries in MoS2: We have investigated atomic and electronic structure of grain boundaries in
monolayer MoS2, where relative angles between two different grains are 0 and 60
degree. The grain boundaries with specific relative angle have been formed with
chemical vapor deposition growth on graphite and hexagonal boron nitride
flakes; van der Waals interlayer interaction between MoS2 and the flakes
restricts the relative angle. Through scanning tunneling microscopy and
spectroscopy measurements, we have found that the perfectly stitched structure
between two different grains of MoS2 was realized in the case of the 0 degree
grain boundary. We also found that even with the perfectly stitched structure,
valence band maximum and conduction band minimum shows significant blue shift,
which probably arise from lattice strain at the boundary. | cond-mat_mes-hall |
On the Relevance of Disorder for Dirac Fermions with Imaginary Vector
Potential: We consider the effects of disorder in a Dirac-like Hamiltonian. In order to
use conformal perturbation theory, we argue that one should consider disorder
in an imaginary vector potential. This affects significantly the signs of the
lowest order $\beta$eta functions. We present evidence for the existence of two
distinct universality classes, depending on the relative strengths of the gauge
field verses impurity disorder strengths. In one class all disorder is driven
irrelevant by the gauge field disorder. | cond-mat_mes-hall |
Kinetic Monte Carlo Approach to Non-equilibrium Bosonic Systems: We consider the use of a Kinetic Monte Carlo approach for the description of
non-equilibrium bosonic systems, taking non-resonantly excited
exciton-polariton condensates and bosonic cascade lasers as examples. In the
former case, the considered approach allows the study of the cross-over between
incoherent and coherent regimes, which represents the formation of a
quasi-condensate that forms purely from the action of energy relaxation
processes rather than interactions between the condensing particles themselves.
In the latter case, we show that a bosonic cascade can theoretically develop an
output coherent state. | cond-mat_mes-hall |
Non-adiabatic current generation in a finite width semiconductor ring: We consider a model of a semiconductor quantum ring of finite width in a
constant perpendicular magnetic field. We show how a current of the same order
as the persistent current can be generated non-adiabatically by a short
intensive pulse in the Tera-Hertz regime. | cond-mat_mes-hall |
Quantum gates by periodic driving: Topological quantum computation has been extensively studied due to its
robustness against decoherence. A conventional way to realize it is by
adiabatic operations---it requires relatively long time to accomplish so that
the speed of quantum computation slows down. In this work, we present a method
to realize topological quantum computation by periodic driving. Compared to the
adiabatic evolution, the total operation time can be regulated arbitrarily by
the amplitude and frequency of the periodic driving. For the sinusoidal
driving, we give an expression for the total operation time in the
high-frequency limit. For the square wave driving, we derive an exact
analytical expression for the evolution operator without any approximations,
and show that the amplitude and frequency of driving field depend on its period
and total operation time. This could provide a new direction in regulations of
the operation time in topological quantum computation. | cond-mat_mes-hall |
Microscopic theory of quantum-transport phenomena in mesoscopic systems:
A Monte Carlo approach: A theoretical investigation of quantum-transport phenomena in mesoscopic
systems is presented. In particular, a generalization to ``open systems'' of
the well-known semiconductor Bloch equations is proposed. The presence of
spatial boundary conditions manifest itself through self-energy corrections and
additional source terms in the kinetic equations, whose form is suitable for a
solution via a generalized Monte Carlo simulation. The proposed approach is
applied to the study of quantum-transport phenomena in double-barrier
structures as well as in superlattices, showing a strong interplay between
phase coherence and relaxation. | cond-mat_mes-hall |
Voltage-Controlled Low-Energy Switching of Nanomagnets through
Ruderman-Kittel-Kasuya-Yosida Interactions for Magnetoelectric Device
Applications: In this letter, we consider through simulation Ruderman-Kittel-Kasuya-Yosida
(RKKY) interactions between nanomagnets sitting on a conductive surface, and
voltage-controlled gating thereof for low-energy switching of nanomagnets for
possible memory and nonvolatile logic applications. For specificity, we
consider nanomagnets with perpendicular anisotropy on a three-dimensional
topological insulator. We model the possibility and dynamics of RKKY-based
switching of one nanomagnet by coupling to one or more nanomagnets of set
orientation. Applications for both memory and nonvolatile logic are considered,
with follower, inverter and majority gate functionality shown. Sub-attojoule
switching energies, far below conventional spin transfer torque (STT)-based
memories and even below CMOS logic appear possible. Switching times on the
order of a few nanoseconds, comparable to times for STT switching, are
estimated for ferromagnetic nanomagnets. | cond-mat_mes-hall |
Photon Absorption of Two-dimensional Nonsymmorphic Dirac Semimetals: Two-dimensional Dirac semimetals have attracted much attention because of
their linear energy dispersion and non-trivial Berry phase. Graphene-like 2D
Dirac materials are gapless only within certain approximations, e.g., if
spin-orbit coupling (SOC) is neglected. It has recently been reported that
materials with nonsymmorphic crystal lattice possess symmetry-enforced
Dirac-like band dispersion around certain high-symmetry momenta even in the
presence of SOC. Here we calculate the optical absorption coefficient of
nonsymmorphic semimetals, such as $\alpha$-bismuthene, which hosts two
anisotropic Dirac cones with different Fermi velocities along $x$ and $y$
directions.We find that the optical absorption coefficient depends strongly on
the anisotropy factor and the photon polarization. When a magnetic field is
applied perpendicular to the plane of the material, the absorption coefficient
also depends on an internal parameter we termed the mixing angle of the band
structure. We further find that an in-plane magnetic field, while leaving the
system gapless, can induce a Van-Hove singularity in the joint density of
states: this causes a significant enhancement of the optical absorption at the
frequency of the singularity for one direction of polarization but not for the
orthogonal one, making the optical properties even more strongly dependent on
polarization. Due to the anisotropy present in our model, the Dirac cones at
two high-symmetry momenta in the Brillouin zone contribute very differently to
the optical absorbance. Consequently, it might be possible to preferentially
populate one valley or the other by varying photon polarization and frequency.
These results suggest that nonsymmorphic 2D Dirac semimetals are excellent
candidate materials for tunable magneto-optic devices. | cond-mat_mes-hall |
Surface segregation in nanoparticles from first principles: FePt nanoparticles are known to exhibit reduced L1$_0$ order with decreasing
particle size. The reduction in order reduces the magnetic anisotropy and the
thermal stability of the direction of magnetization of the particle. The
phenomenon is addressed by investigating the thermodynamic driving forces for
surface segregation using a local (inhomogeneous) cluster expansion fitted to
ab initio data which accurately represents interatomic interactions in both the
bulk and surface regions. Subsequent Monte Carlo simulations reveal that first
surface layer Pt segregation is compensated by Pt depletion in the second
subsurface layer. This indicates that the core's ordered state is not affected
by surface thermodynamics as much as previously thought. Thus, the weak
ordering experimentally observed is likely not due to fundamental thermodynamic
limitations but rather to kinetic effects. | cond-mat_mes-hall |
Deterministic formation of highly coherent nitrogen-vacancy centers
using a focused electron irradiation technique: We demonstrate fully three-dimensional and patterned localization of
nitrogen-vacancy (NV) centers in diamond with coherence times in excess of 1
ms. Nitrogen {\delta}-doping during CVD diamond growth vertically confines
nitrogen to 4 nm while electron irradiation with a transmission electron
microscope (TEM) laterally confines vacancies to less than 1 {\mu}m. We
characterize the effects of electron energy and dose on NV formation.
Importantly, our technique enables the formation of reliably high-quality NV
centers inside diamond nanostructures, with applications in quantum information
and sensing. | cond-mat_mes-hall |
Magneto-Electric Effect for Multiferroic Thin Film by Monte Carlo
Simulation: Magneto-electric effect in a multiferroic heterostructure film, i.e. a
coupled ferromagnetic-ferroelectric thin film, has been investigated through
the use of the Metropolis algorithm in Monte Carlo simulations. A classical
Heisenberg model describes the energy stored in the ferromagnetic film, and we
use a pseudo-spin model with a transverse Ising Hamiltonian to characterise the
energy of electric dipoles in the ferroelectric film. The purpose of this
article is to demonstrate the dynamic response of polarisation is driven by an
external magnetic field, when there is a linear magneto-electric coupling at
the interface between the ferromagnetic and ferroelectric components. | cond-mat_mes-hall |
Effect of laser on thermopower of chiral carbon nanotube: An investigation of laser stimulated thermopower in chiral CNT is presented.
The thermopower of a chiral CNT is calculated using a tractable analytical
approach. This is done by solving the Boltzmann kinetic equation with energy
dispersion relation obtained in the tight binding approximation to determine
the electrical and thermal properties of chiral carbon nanotubes. The
differential thermoelectric power {\alpha} along the circumferential and axial
axes are obtained. The results obtained are numerically analyzed and {\alpha}
is found to oscillate in the presence of laser radiations. We have also noted
that Laser source above 4.6 x 107V/m lowered the thermopower otherwise there is
no change. Varying delta s and delta z the thermopower changes from positive to
negative. | cond-mat_mes-hall |
Excited State Quantum Couplings and Optical Switching of an Artificial
Molecule: We optically probe the spectrum of ground and excited state transitions of an
individual, electrically tunable self-assembled quantum dot molecule.
Photocurrent absorption measurements show that the spatially direct neutral
exciton transitions in the upper and lower dots are energetically separated by
only ~2 meV. Excited state transitions ~8-16 meV to higher energy exhibit
pronounced anticrossings as the electric field is tuned due to the formation of
hybridized electron states. We show that the observed excited state transitions
occur between these hybridized electronic states and different hole states in
the upper dot. By simultaneously pumping two different excited states with two
laser fields we demonstrate a strong (88% on-off contrast) laser induced
switching of the optical response. The results represent an electrically
tunable, discrete coupled quantum system with a conditional optical response. | cond-mat_mes-hall |
Fano resonance in Raman scattering of graphene: Fano resonances and their strong doping dependence are observed in Raman
scattering of single-layer graphene (SLG). As the Fermi level is varied by a
back-gate bias, the Raman G band of SLG exhibits an asymmetric line shape near
the charge neutrality point as a manifestation of a Fano resonance, whereas the
line shape is symmetric when the graphene sample is electron or hole doped.
However, the G band of bilayer graphene (BLG) does not exhibit any Fano
resonance regardless of doping. The observed Fano resonance can be interpreted
as interferences between the phonon and excitonic many-body spectra in SLG. The
absence of a Fano resonance in the Raman G band of BLG can be explained in the
same framework since excitonic interactions are not expected in BLG. | cond-mat_mes-hall |
Electron transport in semiconducting carbon nanotubes with
hetero-metallic contacts: We present an atomistic self-consistent study of the electronic and transport
properties of semiconducting carbon nanotube in contact with metal electrodes
of different work functions, which shows simultaneous electron and hole doping
inside the nanotube junction through contact-induced charge transfer. We find
that the band lineup in the nanotube bulk region is determined by the effective
work function difference between the nanotube channel and source/drain
electrodes, while electron transmission through the SWNT junction is affected
by the local band structure modulation at the two metal-nanotube interfaces,
leading to an effective decoupling of interface and bulk effects in electron
transport through nanotube junction devices. | cond-mat_mes-hall |
Using nonlocal surface transport to identify the axion insulator: The axion is a hypothetical but experimentally undetected particle. Recently,
the antiferromagnetic topological insulator MnBi$_2$Te$_4$ has been predicted
to host the axion insulator, but the experimental evidence remains elusive.
Specifically, the axion insulator is believed to carry "half-quantized" chiral
currents running antiparallel on its top and bottom surfaces. However, it is
challenging to measure precisely the half-quantization. Here, we propose a
nonlocal surface transport device, in which the axion insulator can be
distinguished from normal insulators without a precise measurement of the
half-quantization. More importantly, we show that the nonlocal surface
transport, as a qualitative measurement, is robust in realistic situations when
the gapless side surfaces and disorder come to play. Moreover, thick electrodes
can be used in the device of MnBi$_2$Te$_4$ thick films, enhancing the
feasibility of the surface measurements. This proposal will be insightful for
the search of the axion insulator and axion in topological matter. | cond-mat_mes-hall |
Quantum Confinement in Si and Ge Nanostructures: We apply perturbative effective mass theory as a broadly applicable
theoretical model for quantum confinement (QC) in all Si and Ge nanostructures
including quantum wells (QWs), wires (Q-wires) and dots (QDs). Within the
limits of strong, medium, and weak QC, valence and conduction band edge energy
levels (VBM and CBM) were calculated as a function of QD diameters, QW
thicknesses and Q-wire diameters. Crystalline and amorphous quantum systems
were considered separately. Calculated band edge levels with strong, medium and
weak QC models were compared with experimental VBM and CBM reported from X-ray
photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS) or
photoluminescence (PL). Experimentally, the dimensions of the nanostructures
were determined directly, by transmission electron microscopy (TEM), or
indirectly, by x-ray diffraction (XRD) or by XPS. We found that crystalline
materials are best described by a medium confinement model, while amorphous
materials exhibit strong confinement regardless of the dimensionality of the
system. Our results indicate that spatial delocalization of the hole in
amorphous versus crystalline nanostructures is the important parameter
determining the magnitude of the band gap expansion, or the strength of the
quantum confinement. In addition, the effective masses of the electron and hole
are discussed as a function of crystallinity and spatial confinement. | cond-mat_mes-hall |
Dark Exciton Giant Rabi Oscillations with no External Magnetic Field: Multi-phonon physics is an emerging field that serves as a test bed for
fundamental quantum physics and several applications in metrology, on-chip
communication, among others. Quantum acoustic cavities or resonators are
devices that are being used to study multi-phonon phenomena both theoretically
and experimentally. In particular, we study a system consisting of a
semiconductor quantum dot pumped by a driving laser, and coupled to an acoustic
cavity. This kind of systems has proven to yield interesting multi-phonon
phenomena, but the description of the quantum dot has been limited to a
two-level system. This limitation restrains the complexity that a true
semiconductor quantum dot can offer. Instead, in this work we consider a model
where the quantum dot can have both bright and dark excitons, the latter being
particularly useful due to their lower decoherence rates, because they do not
present spontaneous photon emission. In this setup, we demonstrate that by
fine-tuning the driving laser frequency, one is able to realise giant Rabi
oscillations between the vacuum state and a dark exciton state with $N$-phonon
bundles. From this, we highlight two outstanding features: first, we are able
to create dark states excitations in the quantum dot without the usual external
magnetic field needed to do so; and second, in a dissipative scenario where the
acoustic cavity and the quantum dot suffer from losses, the system acts as a
phonon gun able to emit $N$-phonon bundles. | cond-mat_mes-hall |
Linear-response magnetoresistance effects in chiral systems: The chirality-induced spin selectivity (CISS) effect enables the detection of
chirality as electrical charge signals. It is often studied using a
two-terminal circuit geometry where a ferromagnet is connected to a chiral
component, and a change of electrical resistance is reported upon magnetization
reversal. This is however not expected in the linear response regime because of
compensating reciprocal processes, limiting the interpretation of experimental
results. Here we show that magnetoresistance effects can indeed appear even in
the linear response regime, either by changing the magnitude or the direction
of the magnetization or an applied magnetic field. We illustrate this in a
spin-valve device and in a chiral thin film as the CISS-induced Hanle
magnetoresistance (CHMR) effect. This effect helps to distinguish
spin-transport-related effects from other effects, and can thereby provide
further insight into the origin of CISS. | cond-mat_mes-hall |
Band geometry from position-momentum duality at topological band
crossings: We show that the position-momentum duality offers a transparent
interpretation of the band geometry at the topological band crossings. Under
this duality, the band geometry with Berry connection is dual to the
free-electron motion under gauge field. This identifies the trace of quantum
metric as the dual energy in momentum space. The band crossings with Berry
defects thus induce the dual energy quantization in the trace of quantum
metric. For the $\mathbb Z$ nodal-point and nodal-surface semimetals in three
dimensions, the dual Landau level quantization occurs owing to the Berry
charges. Meanwhile, the two-dimensional (2D) Dirac points exhibit the Berry
vortices, leading to the quantized dual axial rotational energies. Such a
quantization naturally generalizes to the three-dimensional (3D) nodal-loop
semimetals, where the nodal loops host the Berry vortex lines. The $\mathbb
Z_2$ monopoles bring about additional dual axial rotational energies, which
originate from the links with additional nodal lines. Nontrivial band geometry
generically induces finite spread in the Wannier functions. While the spread
manifest quantized lower bounds from the Berry charges, logarithmic divergences
occur from the Berry vortices. The band geometry at the band crossings may be
probed experimentally by a periodic-drive measurement. | cond-mat_mes-hall |
Coherent tunnelling across a quantum point contact in the quantum Hall
regime: The unique properties of quantum Hall devices arise from the ideal
one-dimensional edge states that form in a two-dimensional electron system at
high magnetic field. Tunnelling between edge states across a quantum point
contact (QPC) has already revealed rich physics, like fractionally charged
excitations, or chiral Luttinger liquid. Thanks to scanning gate microscopy, we
show that a single QPC can turn into an interferometer for specific potential
landscapes. Spectroscopy, magnetic field and temperature dependences of
electron transport reveal a quantitatively consistent interferometric behavior
of the studied QPC. To explain this unexpected behavior, we put forward a new
model which relies on the presence of a quantum Hall island at the centre of
the constriction as well as on different tunnelling paths surrounding the
island, thereby creating a new type of interferometer. This work sets the
ground for new device concepts based on coherent tunnelling. | cond-mat_mes-hall |
Electrical Transport Across an Individual Magnetic Domain Wall in
(Ga,Mn)As Microdevices: Recent studies demonstrate that an individual magnetic domain wall (DW) can
be trapped and reproducibly positioned within multiterminal (Ga,Mn)As
microdevices. The electrical resistance obtained from such measurements is
found to be measurably altered by the presence of this single entity. To
elucidate these observations we develop a simple model for the electrical
potential distribution along a multiterminal device in the presence of a single
DW. This is employed to calculate the effect of a single DW upon the
longitudinal and transverse resistance. The model provides very good agreement
with experimental observations, and serves to highlight important deviations
from simple theory. We show that measurements of transverse resistance along
the channel permits establishing the position and the shape of the DW contained
within it. An experimental scheme is developed that enables unambiguous
extraction of the intrinsic DW resistivity. This permits the intrinsic
contribution to be differentiated from resistivities originating from the bulk
and from magnetic anisotropy - effects that are generally manifested as large
backgrounds in the experiments. | cond-mat_mes-hall |
2D MoS2-Graphene-based multilayer van der Waals heterostructures:
Enhanced charge transfer and optical absorption, and electric-field tunable
Dirac point and band gap: Multilayer van der Waals (vdWs) heterostructures assembled by diverse
atomically thin layers have demonstrated a wide range of fascinating phenomena
and novel applications. Understanding the interlayer coupling and its
correlation effect is paramount for designing novel vdWs heterostructures with
desirable physical properties. Using a detailed theoretical study of 2D
MoS2-graphene (GR)-based heterostructures based on state-of-the-art hybrid
density functional theory, we reveal that for 2D few-layer heterostructures,
vdWs forces between neighboring layers depend on the number of layers. Compared
to that in bilayer, the interlayer coupling in trilayer vdW heterostructures
can significantly be enhanced by stacking the third layer, directly supported
by short interlayer separations and more interfacial charge transfer. The
trilayer shows strong light absorption over a wide range (<700 nm), making it
very potential for solar energy harvesting and conversion. Moreover, the Dirac
point of GR and band gaps of each layer and trilayer can be readily tuned by
external electric field, verifying multilayer vdWs heterostructures with unqiue
optoelectronic properties found by experiments. These results suggest that
tuning the vdWs interaction, as a new design parameter, would be an effective
strategy for devising particular 2D multilayer vdWs heterostructures to meet
the demands in various applications. | cond-mat_mes-hall |
Positive longitudinal spin magnetoconductivity in $\mathbb{Z}_{2}$
topological Dirac semimetals: Recently, a class of Dirac semimetals, such as \textrm{Na}$_{\mathrm{3}}%
$\textrm{Bi} and \textrm{Cd}$_{\mathrm{2}}$\textrm{As}$_{\mathrm{3}}$, are
discovered to carry $\mathbb{Z}_{2}$ monopole charges. We present an
experimental mechanism to realize the $\mathbb{Z}_{2}$ anomaly in regard to the
$\mathbb{Z}_{2}$ topological charges, and propose to probe it by
magnetotransport measurement. In analogy to the chiral anomaly in a Weyl
semimetal, the acceleration of electrons by a spin bias along the magnetic
field can create a $\mathbb{Z}_{2}$ charge imbalance between the Dirac points,
the relaxation of which contributes a measurable positive longitudinal spin
magnetoconductivity (LSMC) to the system. The $\mathbb{Z}_{2}$ anomaly induced
LSMC is a spin version of the longitudinal magnetoconductivity (LMC) due to the
chiral anomaly, which possesses all characters of the chiral anomaly induced
LMC. While the chiral anomaly in the topological Dirac semimetal is very
sensitive to local magnetic impurities, the $\mathbb{Z}_{2}$ anomaly is found
to be immune to local magnetic disorder. It is further demonstrated that the
quadratic or linear field dependence of the positive LMC is not unique to the
chiral anomaly. Base on this, we argue that the periodic-in-$1/B$ quantum
oscillations superposed on the positive LSMC can serve as a fingerprint of the
$\mathbb{Z}_{2}$ anomaly in topological Dirac semimetals. | cond-mat_mes-hall |
Andreev-Coulomb Drag in Coupled Quantum Dots: The Coulomb drag effect has been observed as a tiny current induced by both
electron-hole asymmetry and interactions in normal coupled quantum dot devices.
In the present work we show that the effect can be boosted by replacing one of
the normal electrodes by a superconducting one. Moreover, we show that at low
temperatures and for sufficiently strong coupling to the superconducting lead,
the Coulomb drag is dominated by Andreev processes, is robust against details
of the system parameters and can be controlled with a single gate voltage. This
mechanism can be distinguished from single-particle contributions by a sign
inversion of the drag current. | cond-mat_mes-hall |
Effects of mechanical rotation on spin currents: We study the Pauli--Schr\"odinger equation in a uniformly rotating frame of
reference to describe a coupling of spins and mechanical rotations. The
explicit form of the spin-orbit interaction (SOI) with the inertial effects due
to the mechanical rotation is presented. We derive equations of motion for a
wavepacket of electrons in two-dimensional planes subject to the SOI. The
solution is a superposition of two cyclotron motions with different frequencies
and a circular spin current is created by the mechanical rotation. | cond-mat_mes-hall |
Tunable terahertz radiation from graphene induced by moving electrons: Based on a structure consisting of a single graphene layer situated on a
periodic dielectric grating, we show theoretically that intense terahertz (THz)
radiations can be generated by an electron bunch moving atop the graphene
layer. The underlying physics lies in the fact that a moving electron bunch
with rather low electron energy ($\sim$1 keV) can efficiently excite graphene
plasmons (GPs) of THz frequencies with a strong confinement of near-fields. GPs
can be further scattered into free space by the grating for those satisfying
the phase matching condition. The radiation patterns can be controlled by
varying the velocity of the moving electrons. Importantly, the radiation
frequencies can be tuned by varying the Fermi level of the graphene layer,
offering tunable THz radiations that can cover a wide frequency range. Our
results could pave the way toward developing tunable and miniature THz
radiation sources based on graphene. | cond-mat_mes-hall |
Metamorphosis of Andreev bound states into Majorana bound states in
pristine nanowires: We show theoretically that in the generic finite chemical potential
situation, the clean superconducting spin-orbit-coupled nanowire has two
distinct nontopological regimes as a function of Zeeman splitting (below the
topological quantum phase transition): one is characterized by finite-energy
in-gap Andreev bound states, while the other has only extended bulk states. The
Andreev bound state regime is characterized by strong features in the tunneling
spectra creating a "gap closure" signature, but no "gap reopening" signature
should be apparent above the topological quantum phase transition, in agreement
with most recent experimental observations. The gap closure feature is actually
the coming together of the Andreev bound states at high chemical potential
rather than a simple trivial gap of extended bulk states closing at the
transition. Our theoretical finding establishes the generic intrinsic Andreev
bound states on the trivial side of the topological quantum phase transition as
the main contributors to the tunneling conductance spectra, providing a generic
interpretation of existing experiments in clean Majorana nanowires. Our work
also explains why experimental tunnel conductance spectra generically have gap
closing features below the topological quantum phase transition, but no gap
opening features above it. | cond-mat_mes-hall |
Cyclotron resonance of single valley Dirac fermions in gapless HgTe
quantum well: We report on Landau level spectroscopy studies of two HgTe quantum wells
(QWs) near or at the critical well thickness, where the band gap vanishes. In
magnetic fields up to $B$=16T, oriented perpendicular to the QW plane, we
observe a $\sqrt{B}$ dependence for the energy of the dominant cyclotron
resonance (CR) transition characteristic of two-dimensional Dirac fermions. The
dominant CR line exhibits either a single or double absorption lineshape for
the gapless or gapped QW. Using an effective Dirac model, we deduce the band
velocity of single valley Dirac fermions in gapless HgTe quantum wells,
$v_F=6.4 \times10^5$ m/s, and interpret the double absorption of the gapped QW
as resulting from the addition of a small relativistic mass. | cond-mat_mes-hall |
Quantum anomalies in nodal line semimetals: Topological semimetals is a new class of condensed matter systems with
nontrivial electronic structure topology. Their unusual observable properties
may often be understood in terms of quantum anomalies. In particular, Weyl and
Dirac semimetals, which have point band touching nodes, are characterized by
the chiral anomaly, which leads to the Fermi arc surface states, anomalous Hall
effect, negative longitudinal magnetoresistance and planar Hall effect. In this
paper we explore analogous phenomena in nodal line semimetals. We demonstrate
that such semimetals realize a three dimensional analog of the parity anomaly,
which is a known property of two dimensional Dirac semimetals arising, for
example, on the surface of a three dimensional topological insulator. We relate
one of the characteristic properties of nodal line semimetals, namely the
drumhead surface states, to this anomaly, and derive the field theory, which
encodes the corresponding anomalous response. | cond-mat_mes-hall |
Magnetotransport properties of granular oxide-segregated CoPtCr films
for applications in future magnetic memory technology: Magnetotransport properties of granular oxide-segregated CoPtCr films were
studied on both macroscopic and microscopic length scales by performing bulk
and point-contact magnetoresistance measurements, respectively. Such a
perpendicular magnetic medium is used in state-of-the-art hard disc drives and
if combined with magnetoresistive phenomena (for read/write operations) may
lead to a novel concept for magnetic recording with high areal density. While
the bulk measurements on the films showed only small variations in dc
resistance as a function of applied magnetic field (magnetoresistance of less
than 0.02 %), the point-contact measurements revealed
giant-magnetoresistance-like changes in resistance with up to 50,000 % ratios.
The observed magnetorestive effect could be attributed to a tunnel
magnetoresistance between CoPtCr grains with different coercivity. The
tunneling picture of electronic transport in our granular medium was confirmed
by the observation of tunneling-like current-voltage characteristics and bias
dependence of magnetoresistance; both the point-contact resistance and
magnetoresistance were found to decrease with the applied dc bias. | cond-mat_mes-hall |
Generating surface states in a Weyl semi-metal by applying
electromagnetic radiation: We show that the application of circularly polarized electromagnetic
radiation on the surface of a Weyl semi-metal can generate states at that
surface. These states can be characterized by their surface momentum. The
Floquet eigenvalues of these states come in complex conjugate pairs rather than
being equal to $\pm 1$. If the amplitude of the radiation is small, we find
some unusual bulk-boundary relations: the Floquet eigenvalues of the surface
states lie at the extrema of the Floquet eigenvalues of the bulk system, and
the peaks of the Fourier transforms of the surface state wave functions lie at
the momenta where the bulk Floquet eigenvalues have extrema. For the case of
zero surface momentum, we can analytically derive scaling relations between the
decay length of the surface states and the amplitude and penetration length of
the radiation. For topological insulators, we again find that circularly
polarized radiation can generate states on the surfaces; these states have much
larger decay lengths than the surface states which are present even in the
absence of radiation. Finally, we show that radiation can generate surface
states for trivial insulators also. | cond-mat_mes-hall |
Extending the spin excitation lifetime of a magnetic molecule on a
proximitized superconductor: Magnetic molecules deposited on surfaces are a promising platform to
individually address and manipulate spins. Long spin excitation lifetimes are
necessary to utilize them in quantum information processing and data storage.
Normally, coupling of the molecular spin with the conduction electrons of
metallic surfaces causes fast relaxation of spin excitations into the ground
state. However, the presence of superconducting paring effects in the substrate
can protect the excited spin from decaying. In this work, we show that a
proximity-induced superconducting gold film can sustain spin excitations of a
FeTPP-Cl molecule for more than 80ns. This long value was determined by
studying inelastic spin excitations of the S=5/2 multiplet of FeTPP-Cl on Au
films over V(100) using scanning tunneling spectroscopy. The spin lifetime
decreases with increasing film thickness, in apparent connection with the
gradual gap-closing of a pair of de Gennes-Saint James resonances found inside
the superconducting gap. Our results elucidate the use of proximitized gold
electrodes for addressing quantum spins on surfaces, envisioning new routes for
tuning the value of their spin lifetime. | cond-mat_mes-hall |
Valley-selective exciton bistability in a suspended monolayer
semiconductor: We demonstrate robust power- and wavelength-dependent optical bistability in
fully suspended monolayers of WSe2 near the exciton resonance. Bistability has
been achieved under continuous-wave optical excitation at an intensity level of
10^3 W/cm^2. The observed bistability is originated from a photo-thermal
mechanism, which provides both optical nonlinearity and passive feedback, two
essential elements for optical bistability. Under a finite magnetic field, the
exciton bistability becomes helicity dependent, which enables repeatable
switching of light purely by its polarization. | cond-mat_mes-hall |
Overlapping Andreev states in semiconducting nanowires: competition of
1D and 3D propagation: The recent proposals of devices with overlapping Andreev bound states (ABS)
open up the opportunities to control and fine-tune their spectrum, that can be
used in various applications. In this Article, we study the ABS in a device
consisting of a semiconducting nanowire covered with three superconducting
leads. The ABS are formed at two junctions where the wire is not covered. They
overlap in the wire where the electron propagation is 1D, and in one of the
leads where the propagation is 3D. We identify a number of regimes where these
two overlaps either dominate or compete, depending on the junction separation
$L$ as compared to the correlation lengths $\xi_{\rm w}$, $\xi_{\rm s}$ in the
wire and in the lead, respectively. We utilize a simple model of 1D electron
spectrum in the nanowire and take into account the quality of the contact
between the nanowire and the superconducting lead. We present the spectra for
different $L$, detailing the transition from a single-ABS in the regime of
strong 1D hybridization to two almost independent ABS hybridized at the
degeneracy points, in the regime of weak 1D hybridization. We present the
details of merging the upper ABS with the continuous spectrum upon decreasing
$L$. We study in detail the effect of quantum interference due to the phase
accumulated during the electron passage between the junctions. We develop a
perturbation theory for analytical treatment of hybridization. We address an
interesting separate case of fully transparent junctions. We derive and
exemplify a perturbation theory suitable for the competition regime
demonstrating the interference of 1D and two 3D transmission amplitudes. | cond-mat_mes-hall |
Anomalous Thermal Transport in Quantum Wires: We study thermal transport in a one-dimensional quantum wire, connected to
reservoirs. Despite of the absence of electron backscattering, interactions in
the wire strongly influence thermal transport. Electrons propagate with unitary
transmission through the wire and electric conductance is not affected. Energy,
however, is carried by bosonic excitations (plasmons) which suffer from
scattering even on scales much larger than the Fermi wavelength. If the
electron density varies randomly, plasmons are localized and {\em charge-energy
separation} occurs. We also discuss the effect of plasmon-plasmon interaction
using Levinson's theory of nonlocal heat transport. | cond-mat_mes-hall |
Transport through quantum spin Hall insulator/metal junctions in
graphene ribbons: Quantum spin Hall insulator/metal interfaces are formed in graphene ribbons
with intrinsic spin-orbit coupling by selectively doping two regions creating a
potential step. For a clean graphene ribbon, the transmission of the
topological edge states through a n-n or p-p junction is perfect irrespective
of the ribbon termination, width, and potential step parameters due to the
orthogonality of incoming and outgoing edge channels. This is shown numerically
for an arbitrary crystallographic orientation of the ribbon and proven
analytically for zigzag and metallic armchair boundary conditions. In
disordered ribbons, the orthogonality between left- and right-movers is in
general destroyed and backscattering sets in. However, transmission approaches
one by increasing the ribbon's width, even in the presence of strong edge
roughness. | cond-mat_mes-hall |
Switching 2D Magnetic States via Pressure Tuning of Layer Stacking: The physical properties of two-dimensional van der Waals (2D vdW) crystals
depend sensitively on the interlayer coupling, which is intimately connected to
the stacking arrangement and the interlayer spacing. For example, simply
changing the twist angle between graphene layers can induce a variety of
correlated electronic phases, which can be controlled further in a continuous
manner by applying hydrostatic pressure to decrease the interlayer spacing. In
the recently discovered 2D magnets, theory suggests that the interlayer
exchange coupling strongly depends on layer separation, while the stacking
arrangement can even change the sign of the magnetic exchange, thus drastically
modifying the ground state. Here, we demonstrate pressure tuning of magnetic
order in the 2D magnet CrI3. We probe the magnetic states using tunneling and
scanning magnetic circular dichroism microscopy measurements. We find that the
interlayer magnetic coupling can be more than doubled by hydrostatic pressure.
In bilayer CrI3, pressure induces a transition from layered antiferromagnetic
to ferromagnetic phases. In trilayer CrI3, pressure can create coexisting
domains of three phases, one ferromagnetic and two distinct antiferromagnetic.
The observed changes in magnetic order can be explained by changes in the
stacking arrangement. Such coupling between stacking order and magnetism
provides ample opportunities for designer magnetic phases and functionalities. | cond-mat_mes-hall |
Tunable strong coupling of mechanical resonance between spatially
separated FePS$_3$ nanodrums: Coupled nanomechanical resonators made of two-dimensional materials are
promising for processing information with mechanical modes. However, the
challenge for these types of systems is to control the coupling. Here, we
demonstrate strong coupling of motion between two suspended membranes of the
magnetic 2D material FePS$_3$. We describe a tunable electromechanical
mechanism for control over both the resonance frequency and the coupling
strength using a gate voltage electrode under each membrane. We show that the
coupling can be utilized for transferring data from one drum to the other by
amplitude modulation. Finally, we also study the temperature dependence of the
coupling, and in particular how it is affected by the antiferromagnetic phase
transition characteristic of this material. The presented electrical coupling
of resonant magnetic 2D membranes holds promise of transferring mechanical
energy over a distance at low electrical power, thus enabling novel data
readout and information processing technologies. | cond-mat_mes-hall |
Polarization bistability and resultant spin rings in semiconductor
microcavities: The transmission of a pump laser resonant with the lower polariton branch of
a semiconductor microcavity is shown to be highly dependent on the degree of
circular polarization of the pump. Spin dependent anisotropy of
polariton-polariton interactions allows the internal polarization to be
controlled by varying the pump power. The formation of spatial patterns, spin
rings with high degree of circular polarization, arising as a result of
polarization bistability, is observed. A phenomenological model based on spin
dependent Gross-Pitaevskii equations provides a good description of the
experimental results. Inclusion of interactions with the incoherent exciton
reservoir, which provides spin-independent blueshifts of the polariton modes,
is found to be essential. | cond-mat_mes-hall |
Moderate bandgap and high carrier mobility simultaneously realized in
bilayer silicene by oxidation: Semiconductors simultaneously possessing high carrier mobility, moderate
bandgap, and ambient environment stability are so important for the modern
industry, and Si-based semiconducting materials can match well with the
previous silicon based electronic components. Thus, searching for such Si-based
semiconductors has been one hot project due to the lack of them nowadays. Here,
with the help of density functional theory, we found that the oxidized bilayer
silicene exhibits high carrier mobility with a moderate direct bandgap of 1.02
eV. The high carrier mobility is caused by the remaining of big pi bond, and
the moderate bandgap is opened by the saturation of dangling Si 3p bonds.
Originated from the formation of strong Si-O and Si-Si bonds, the sample
exhibits strong thermodynamic and dynamical stabilities. Our work indicates
that the oxidized bilayer silicene has many potential applications in modern
electronic fields. | cond-mat_mes-hall |
Full Counting Statistics of a Non-adiabatic Electron Pump: Non-adiabatic charge pumping through a single-level quantum dot with
periodically modulated parameters is studied theoretically. By means of a
quantum-master-equation approach the full counting statistics of the system is
obtained. We find a trinomial-probability distribution of the charge transfer,
which adequately describes the reversal of the pumping current by sweeping the
driving frequency. Further, we derive equations of motion for current and
noise, and solve those numerically for two different driving schemes. Both show
interesting features which can be fully analyzed due to the simple and generic
model studied. | cond-mat_mes-hall |
Step-like features on caloric effects of graphenes: We considered a graphene nano-ribbon with a longitudinal electric field
(along $x$ direction) and a transversal magnetic field (along $z$ direction),
and then observe (i) the electrocaloric effect ruled by an applied magnetic
field and (ii) the magnetocaloric effect ruled by an applied electric field. We
focused our attention to the limit of low temperatures, and then observed
interesting step-like features. For each filled Landau level $n$, created by
the applied magnetic field, both caloric effects increase proportionally to
$n+1/2$; and this step measures either important graphene properties (like
Fermi velocity) or quantum fundamental quantities (like Planck constant and
magnetic flux quantum). | cond-mat_mes-hall |
Size-dependent spatial magnetization profile of manganese-zinc ferrite
Mn0.2Zn0.2Fe2.6O4 nanoparticles: We report the results of an unpolarized small-angle neutron scattering (SANS)
study on Mn-Zn ferrite (MZFO) magnetic nanoparticles with the aim to elucidate
the interplay between their particle size and the magnetization configuration.
We study different samples of single-crystalline MZFO nanoparticles with
average diameters ranging between 8 to 80 nm, and demonstrate that the smallest
particles are homogeneously magnetized. However, with increasing nanoparticle
size, we observe the transition from a uniform to a nonuniform magnetization
state. Field-dependent results for the correlation function confirm that the
internal spin disorder is suppressed with increasing field strength. The
experimental SANS data are supported by the results of micromagnetic
simulations, which confirm an increasing inhomogeneity of the magnetization
profile of the nanoparticle with increasing size. The results presented
demonstrate the unique ability of SANS to detect even very small deviations of
the magnetization state from the homogeneous one. | cond-mat_mes-hall |
Near-field infrared nano-spectroscopy of surface phonon-polariton
resonances: We present combined experimental and numerical work on light-matter
interactions at nanometer length scales. We report novel numerical simulations
of near-field infrared nanospectroscopy that consider, for the first time,
detailed tip geometry and have no free parameters. Our results match published
spectral shapes of amplitude and phase measurements even for strongly resonant
surface phonon-polariton (SPhP) modes. They also verify published absolute
scattering amplitudes for the first time. A novel, ultrabroadband light source
enables near-field amplitude and phase acquisition into the far-infrared
spectral range. This allowed us to discover a strong SPhP resonance in the
polar dielectric SrTiO3 (STO) at approximately 24 micrometer wavelength of
incident light. | cond-mat_mes-hall |
Instanton Approach to Josephson Tunneling between Trapped Condensates: An instanton method is proposed to investigate the quantum tunneling between
two weakly-linked Bose-Einstein condensates confined in double-well potential
traps. We point out some intrinsic pathologies in the earlier treatments of
other authors and make an effort to go beyond these very simple zero order
models. The tunneling amplitude may be calculated in the Thomas-Fermi
approximation and beyond it; we find it depends on the number of the trapped
atoms, through the chemical potential. Some suggestions are given for the
observation of the Josephson oscillation and the MQST. | cond-mat_mes-hall |
Universal dephasing in a chiral 1D interacting fermion system: We consider dephasing by interactions in a one-dimensional chiral fermion
system (e.g. a Quantum Hall edge state). For finite-range interactions, we
calculate the spatial decay of the Green's function at fixed energy, which sets
the contrast in a Mach-Zehnder interferometer. Using a physically transparent
semiclassical ansatz, we find a power-law decay of the coherence at high
energies and zero temperature (T=0), with a universal asymptotic exponent of 1,
independent of the interaction strength. We obtain the dephasing rate at T>0
and the fluctuation spectrum acting on an electron. | cond-mat_mes-hall |
Acoustic plasmons in graphene sandwiched between two metallic slabs: We study the effect of two metallic slabs on the collective dynamics of
electrons in graphene positioned between the two slabs. We show that if the
slabs are perfect conductors the plasmons of graphene display a linear
dispersion relation. The velocity of these acoustic plasmons crucially depends
on the distance between the two metal gates and the graphene sheet. In the case
of generic slabs, the dispersion relation of graphene plasmons is much more
complicated but we find that acoustic plasmons can still be obtained under
specific conditions. | cond-mat_mes-hall |
Flat bands and chiral optical response of moiré insulators: We present a low-energy model describing the reconstruction of the electronic
spectrum in twisted bilayers of honeycomb crystals with broken sublattice
symmetry. The resulting moir\'e patterns are classified into two families with
different symmetry. In both cases, flat bands appear at relatively large
angles, without any magic angle condition. Transitions between them give rise
to sharp resonances in the optical absorption spectrum at frequencies well
below the gap of the monolayer. Owing to their chiral symmetry, twisted
bilayers display circular dichroism, i.e., different absorption of left and
right circularly-polarized light. This optical activity is a nonlocal property
determined by the stacking. In hexagonal boron nitride, sensitivity to the
stacking leads to strikingly different circular dichroism in the two types of
moir\'es. Our calculations exemplify how subtle properties of the electronic
wavefunctions encoded in current correlations between the layers control
physical observables of moir\'e materials. | cond-mat_mes-hall |
Field-effect tunneling transistor based on vertical graphene
heterostructures: We report a bipolar field effect tunneling transistor that exploits to
advantage the low density of states in graphene and its one atomic layer
thickness. Our proof-of-concept devices are graphene heterostructures with
atomically thin boron nitride acting as a tunnel barrier. They exhibit room
temperature switching ratios ~50, a value that can be enhanced further by
optimizing the device structure. These devices have potential for high
frequency operation and large scale integration. | cond-mat_mes-hall |
Interference of diffusing photons and level crossing spectroscopy: We show that a new interference effect appears in the intensity fluctuations
of photons multiply scattered by an atomic gas of large optical depth b. This
interference occurs only for scattering atoms that are Zeeman degenerate and it
leads to a deviation from the Rayleigh law. The fluctuations measured by their
variance, display a resonance peak as a function of an applied magnetic field.
The resonance width is proportional to the small factor 1/b. We derive closed
analytic expressions for all these physical quantities which are directly
accessible experimentally. | cond-mat_mes-hall |
Disorder-mediated Kondo effect in graphene: We study the emergence of strongly correlated states and Kondo physics in
disordered graphene. Diluted short range disorder gives rise to localized
midgap states at the vicinity of the system charge neutrality point. We show
that long-range disorder, ubiquitous in graphene, allows for the coupling of
these localized states to an effective (disorder averaged) metallic band. The
system is described by an Anderson-like model. We use the numerical
renormalization group (NRG) method to study the distributions of Kondo
temperatures $P(T_K)$. The results show that disorder can lead to long
logarithmic tails in $P(T_K)$, consistent with a quantum Griffiths phase. | cond-mat_mes-hall |
Giant excitonic magneto-optical Faraday rotation in single semimagnetic
CdTe/Cd_{1-x}Mn_{x}Te quantum ring: Magnetic tuning of the bound exciton states and corresponding giant Zeeman
splitting (GZS) between {\sigma}^{+} and {\sigma}^{-} excitonic transitions in
CdTe/Cd_{1-x}Mn_{x}Te quantum ring has been investigated in the Faraday
configuration for various concentrations of Mn^{2+} ions, using the variational
technique in the effective mass approximation. The sp-d exchange interaction
between the localized magnetic impurity ions and the delocalized charge
carriers has been accounted via mean-field theory with the inclusion of a
modified Brillouin function. The enhancement of the GZS, and in turn, the
effective g-factor with the application of an external magnetic field, is
strikingly manifested in type-I - type-II transition in the band structure,
which has been well explained by computing the overlap integral between the
electron and hole, and the in-plane exciton radius. This highlights the
extraordinary magneto-optical properties, including the giant Faraday rotation
and associated Verdet constant, which have been calculated using single
oscillator model. The oscillator strength and exciton lifetime have been
estimated, and are found to be larger than in the bulk diluted magnetic
semiconductors (DMS) and quantum wells, reflecting stronger confinement inside
the quantum ring. The results show that the DMS-based quantum ring exhibits
more extensive Zeeman splitting, which gives rise to ultra-high Verdet constant
of 2.6 \times 10^{9}rad/Tesla/m, which are a few orders of magnitude larger
than in the existing quantum systems and magneto-optical materials. | cond-mat_mes-hall |
Topological qubits in graphenelike systems: The fermion-doubling problem can be an obstacle to getting half-a-qubit in
two-dimensional fermionic tight-binding models in the form of Majorana zero
modes bound to the core of superconducting vortices. We argue that the number
of such Majorana zero modes is determined by a Z_2 x Z_2 topological charge for
a family of two-dimensional fermionic tight-binding models ranging from
noncentrosymmetric materials to graphene. This charge depends on the dimension
of the representation (i.e., the number of species of Dirac fermions -- where
the doubling problem enters) and the parity of the Chern number induced by
breaking time-reversal symmetry. We show that in graphene there are as many as
ten order parameters that can be used in groups of four to change the
topological number from even to odd. | cond-mat_mes-hall |
Novel mechanisms to enhance the capacitance beyond the classical limits
in capacitors with free-electron-like electrodes: The so-called negative electron compressibility refers to the lowering of the
chemical potential of a metallic system when the carrier density increases.
This effect has often been invoked in the past to explain the enhancement of
the capacitance beyond the classical limits in capacitors with two-dimensional
electron gases as electrodes. Based on experiments on strongly confined
semiconductor quantum wells (QWs), it has been traditionally ascribed to the
electron exchange energy as the main driving force. Recent research, however,
has revealed that analogous effects can occur in other classes of materials
systems, such as polar oxide interfaces, whose characteristics drastically
depart from those of the previously considered cases. To rationalize these new
results, it is necessary to revisit the established theory of confined electron
gases, and test whether its conclusions are valid beyond the specifics of
semiconductor-based QWs. Here we find, based on first-principles calculations
of jellium slabs, that one must indeed be very careful when extrapolating
existing results to other realistic physical systems. In particular, we
identify a number of additional, previously overlooked mechanisms (e.g.,
related to the displacement of the electronic cloud and to the multiband
structure of the delocalized gas), that enter into play and become new sources
of negative capacitance in the weak-confinement regime. Our detailed analysis
of these emerging contributions, supported by analytic models and multiple test
cases, will provide a useful guidance in the ongoing quest for nanometric
capacitors with enhanced performance. | cond-mat_mes-hall |
Valley degeneracy in biaxially strained aluminum arsenide quantum wells: This paper details a complete formalism for calculating electron subband
energy and degeneracy in strained multi-valley quantum wells grown along any
orientation with explicit results for the AlAs quantum well case. A
standardized rotation matrix is defined to transform from the conventional-
cubic-cell basis to the quantum-well-transport basis whereby effective mass
tensors, valley vectors, strain matrices, anisotropic strain ratios, and
scattering vectors are all defined in their respective bases. The specific
cases of (001)-, (110)-, and (111)-oriented aluminum arsenide (AlAs) quantum
wells are examined, as is the unconventional (411) facet, which is of
particular importance in AlAs literature. Calculations of electron confinement
and strain in the (001), (110), and (411) facets determine the critical well
width for crossover from double- to single-valley degeneracy in each system.
The notation is generalized to include miscut angles, and can be adapted to
other multi-valley systems. To help classify anisotropic inter-valley
scattering events, a new primitive unit cell is defined in momentum space which
allows one to distinguish purely in-plane inter-valley scattering events from
those that requires an out-of-plane momentum scattering component. | cond-mat_mes-hall |
Kinetics of Exciton Emission Patterns and Carrier Transport: We report on the measurements of the kinetics of expanding and collapsing
rings in the exciton emission pattern. The rings are found to preserve their
integrity during expansion and collapse, indicating that the observed kinetics
is controlled by charge carrier transport rather than by a much faster process
of exciton production and decay. The relation between ring kinetics and carrier
transport, revealed by our experiment and confirmed by comparison with a
theoretical model, is used to determine electron and hole transport
characteristics in a contactless fashion. | cond-mat_mes-hall |
Heat transport in harmonic lattices: We work out the non-equilibrium steady state properties of a harmonic lattice
which is connected to heat reservoirs at different temperatures. The heat
reservoirs are themselves modeled as harmonic systems. Our approach is to write
quantum Langevin equations for the system and solve these to obtain steady
state properties such as currents and other second moments involving the
position and momentum operators. The resulting expressions will be seen to be
similar in form to results obtained for electronic transport using the
non-equilibrium Green's function formalism. As an application of the formalism
we discuss heat conduction in a harmonic chain connected to self-consistent
reservoirs. We obtain a temperature dependent thermal conductivity which, in
the high-temperature classical limit, reproduces the exact result on this model
obtained recently by Bonetto, Lebowitz and Lukkarinen. | cond-mat_mes-hall |
Scaling for rectification of bipolar nanopores as a function of a
modified Dukhin number: the case of 1:1 electrolytes: The scaling behavior for the rectification of bipolar nanopores is studied
using the Nernst-Planck equation coupled to the Local Equilibrium Monte Carlo
method. The bipolar nanopore's wall carries $\sigma$ and $-\sigma$ surface
charge densities in its two half regions axially. Scaling means that the device
function (rectification) depends on the system parameters (pore length, $H$,
pore radius, $R$, concentration, $c$, voltage, $U$, and surface charge density,
$\sigma$) via a single scaling parameter that is a smooth analytical function
of the system parameters. Here, we suggest using a modified Dukhin number,
$\mathrm{mDu}=|\sigma|l_{\mathrm{B}}^{*}\lambda_{\mathrm{D}}HU/(RU_{0})$, where
$l_{\mathrm{B}}^{*}=8\pi l_{\mathrm{B}}$, $l_{\mathrm{B}}$ is the Bjerrum
length, $\lambda_{\mathrm{D}}$ is the Debye length, and $U_{0}$ is a reference
voltage. We show how scaling depends on $H$, $U$, and $\sigma$ and through what
mechanisms these parameters influence the pore's behavior. | cond-mat_mes-hall |
Scaling properties of induced density of chiral and non-chiral Dirac
fermions in magnetic fields: We find that a repulsive potential of graphene in the presence of a magnetic
field has bound states that are peaked inside the barrier with tails extending
over \ell(N + 1), where \ell and N are the magnetic length and Landau level(LL)
index. We have investigated how these bound states affect scaling properties of
the induced density of filled Landau levels of massless Dirac fermions. For
chiral fermions we find, in strong coupling regime, that the density inside the
repulsive potential can be greater than the value in the absence of the
potential while in the weak coupling regime we find negative induced density.
Similar results hold also for non-chiral fermions. As one moves from weak to
strong coupling regimes the effective coupling constant between the potential
and electrons becomes more repulsive, and then it changes sign and becomes
attractive. Different power-laws of induced density are found for chiral and
non-chiral fermions. | cond-mat_mes-hall |
Minimal model for charge transfer excitons at the dielectric interface: Theoretical description of the charge transfer (CT) exciton across the
donor-acceptor interface without the use of a completely localized hole (or
electron) is a challenge in the field of organic solar cells. We calculate the
total wavefunction of the CT exciton by solving an effective two-particle
Schrodinger equation for the inhomogeneous dielectric interface. We formulate
the magnitude of the CT and construct a minimal model of the CT exciton under
the breakdown of inversion symmetry. We demonstrate that both a light hole mass
and a hole localization along the normal to the dielectric interface are
crucial to yield the CT exciton. | cond-mat_mes-hall |
Robustness of the quantum Hall effect, sample size versus sample
topology, and quality control management of III-V molecular beam epitaxy: We measure the IQHE on macroscopic (1.5cm x 1.5cm) "quick 'n' dirty" prepared
III-V heterostructure samples with van der Pauw and modified Corbino geometries
at 1.3 K. We compare our results with (i) data taken on smaller specimens,
among them samples with a standard Hall bar geometry, (ii) results of our
numerical analysis taking inhomogenities of the 2DEG into account. Our main
finding is a confirmation of the expected robustness of the IQHE which favours
the development of wide plateaux for small filling factors and very large
sample sizes (here with areas 10,000 times larger than in standard
arrangements). | cond-mat_mes-hall |
Realization of a Laughlin quasiparticle interferometer: Observation of
fractional statistics: In two dimensions, the laws of physics permit existence of anyons, particles
with fractional statistics which is neither Fermi nor Bose. That is, upon
exchange of two such particles, the quantum state of a system acquires a phase
which is neither 0 nor \pi, but can be any value. The elementary excitations
(Laughlin quasiparticles) of a fractional quantum Hall fluid have fractional
electric charge and are expected to obey fractional statistics. Here we report
experimental realization of a novel Laughlin quasiparticle interferometer,
where quasiparticles of the 1/3 fluid execute a closed path around an island of
the 2/5 fluid and thus acquire statistical phase. Interference fringes are
observed as conductance oscillations as a function of magnetic flux, similar to
the Aharonov-Bohm effect. We observe the interference shift by one fringe upon
introduction of five magnetic flux quanta (5h/e) into the island. The
corresponding 2e charge period is confirmed directly in calibrated gate
experiments. These results constitute direct observation of fractional
statistics of Laughlin quasiparticles. | cond-mat_mes-hall |
Double refraction and spin splitter in a normal-hexagonal semiconductor
junction: In analogy with light refraction at optical boundary, ballistic electrons
also undergo refraction when propagate across a semiconductor junction.
Establishing a negative refractive index in conventional optical materials is
difficult, but the realization of negative refraction in electronic system is
conceptually straightforward, which has been verified in graphene p-n junctions
in recent experiments. Here, we propose a model to realize double refraction
and double focusing of electric current by a normal-hexagonal semiconductor
junction. The double refraction can be either positive or negative, depending
on the junction being n-n type or p-n type. Based on the valley-dependent
negative refraction, a spin splitter (valley splitter) is designed at the p-n
junction system, where the spin-up and spin-down electrons are focused at
different regions. These findings may be useful for the engineering of double
lenses in electronic system and have underlying application of spin splitter in
spintronics. | cond-mat_mes-hall |
Electron quantum dynamics in closed and open potentials at high magnetic
fields: Quantization and lifetime effects unified by semicoherent states: We have developed a Green's function formalism based on the use of an
overcomplete semicoherent basis of vortex states, specially devoted to the
study of the Hamiltonian quantum dynamics of electrons at high magnetic fields
and in an arbitrary potential landscape smooth on the scale of the magnetic
length. This formalism is used here to derive the exact Green's function for an
arbitrary quadratic potential in the special limit where Landau level mixing
becomes negligible. This solution remarkably embraces under a unified form the
cases of confining and unconfining quadratic potentials. This property results
from the fact that the overcomplete vortex representation provides a more
general type of spectral decomposition of the Hamiltonian operator than usually
considered. Whereas confining potentials are naturally characterized by
quantization effects, lifetime effects emerge instead in the case of
saddle-point potentials. Our derivation proves that the appearance of lifetimes
has for origin the instability of the dynamics due to quantum tunneling at
saddle points of the potential landscape. In fact, the overcompleteness of the
vortex representation reveals an intrinsic microscopic irreversibility of the
states synonymous with a spontaneous breaking of the time symmetry exhibited by
the Hamiltonian dynamics. | cond-mat_mes-hall |
Generating quantizing pseudomagnetic fields by bending graphene ribbons: We analyze the mechanical deformations that are required to create uniform
pseudomagnetic fields in graphene. It is shown that, if a ribbon is bent
in-plane into a circular arc, this can lead to fields exceeding 10T, which is
sufficient for the observation of pseudo-Landau quantization. The arc geometry
is simpler than those suggested previously and, in our opinion, has much better
chances to be realized experimentally soon. The effects of a scalar potential
induced by dilatation in this geometry is shown to be negligible. | cond-mat_mes-hall |
Dynamic frequency dependence of bias activated negative capacitance in
semiconductor diodes under high forward bias: We observed qualitatively dissimilar frequency dependence of negative
capacitive response under high charge injection in two sets of junction diodes
which are functionally different from each other i.e. electroluminescent diodes
and non-luminescent Si-based diodes. Using the technique of bias-activated
differential capacitance response, we investigated the mutual dynamics of
different rate processes in different diodes. We explain these observations as
the mutual competition of fast and slow electronic transition rates albeit
differently. This study provides a better understanding of the physics of
junction diodes operating under high charge carrier injection and may lead to
superior device functionalities. | cond-mat_mes-hall |
Weak antilocalization beyond the fully diffusive regime in Pb1-xSnxSe
topological quantum wells: We report the measurements and analysis of weak antilocalization (WAL) in
Pb1-xSnxSe topological quantum wells in a new regime where the elastic
scattering length is larger than the magnetic length. We achieve this regime
through the development of high-quality epitaxy and doping of topological
crystalline insulator (TCI) quantum wells. We obtain elastic scattering lengths
that exceeds 100nm and become comparable to the magnetic length. In this
transport regime, the Hikami-Larkin-Nagaoka model is no longer valid. We employ
the model of Wittmann and Schmid to extract the coherence time from the
magnetoresistance. We find that despite our improved transport characteristics,
the coherence time may be limited by scattering channels that are not strongly
carrier dependent, such as electron-phonon or defect scattering. | cond-mat_mes-hall |
Using single quantum states as spin filters to study spin polarization
in ferromagnets: By measuring electron tunneling between a ferromagnet and individual energy
levels in an aluminum quantum dot, we show how spin-resolved quantum states can
be used as filters to determine spin-dependent tunneling rates. We also observe
magnetic-field-dependent shifts in the magnet's electrochemical potential
relative to the dot's energy levels. The shifts vary between samples and are
generally smaller than expected from the magnet's spin-polarized density of
states. We suggest that they are affected by field-dependent charge
redistribution at the magnetic interface. | cond-mat_mes-hall |
Quantum properties of a strongly driven Josephson junction: A Josephson junction embedded in a dissipative circuit can be externally
driven to induce nonlinear dynamics of its phase. Classically, under
sufficiently strong driving and weak damping, dynamic multi-stability emerges
associated with dynamical bifurcations so that the often used modeling as a
Duffing oscillator, which can exhibit bi-stability at the most, is
insufficient. The present work analyzes in this regime corresponding quantum
properties by mapping the problem onto a highly-nonlinear quasi-energy operator
in a rotating frame. This allows us to identify in detail parameter regions
where simplifications such as the Duffing approximation are valid, to explore
classical-quantum correspondences, and to study how quantum fluctuations impact
the effective junction parameters as well as the dynamics around higher
amplitude classical fixed points. | cond-mat_mes-hall |
Spatiotemporal spin fluctuations caused by spin-orbit-coupled Brownian
motion: We develop a theory of thermal fluctuations of spin density emerging in a
two-dimensional electron gas. The spin fluctuations probed at spatially
separated spots of the sample are correlated due to Brownian motion of
electrons and spin-obit coupling. We calculate the spatiotemporal correlation
functions of the spin density for both ballistic and diffusive transport of
electrons and analyze them for different types of spin-orbit interaction
including the isotropic Rashba model and persistent spin helix regime. The
measurement of spatial spin fluctuations provides direct access to the
parameters of spin-orbit coupling and spin transport in conditions close to the
thermal equilibrium. | cond-mat_mes-hall |
Ab initio simulation of the structure and transport properties of
zirconium and ferromagnetic cobalt contacts on the two-dimensional
semiconductor WS_2: Using density-functional theory calculations, the atomic and electronic
structure of single-layer WS_2 attached to Zr and Co contacts are determined.
Both metals form stable interfaces that are promising as contacts for injection
of n-type carriers into the conduction band of WS_2 with Schottky barriers of
0.45eV and 0.62eV for Zr and Co, respectively. With the help of quantum
transport calculations, we address the conductive properties of a free-standing
WS_2 sheet suspended between two Zr contacts. It is found that such a device
behaves like a diode with steep I-V characteristics. Spin-polarized transport
is calculated for such a device with a floating-gate Co electrode added.
Depending on the geometrical shape of the Co gate and the energy of the
carriers in WS_2, the transmission of spin majority and minority electrons may
differ by up to an order of magnitude. Thus the steep I-V characteristics of
the nanoscale device makes it possible to realize a spin filter. | cond-mat_mes-hall |
Large-Scale Schrödinger-Cat States and Majorana Bound States in
Coupled Circuit-QED Systems: We have studied the low-lying excitations of a chain of coupled circuit-QED
systems, and report several intriguing properties of its two nearly degenerate
ground states. The ground states are Schr\"odinger cat states at a truly large
scale, involving maximal entanglement between the resonator and the qubit, and
are mathematically equivalent to Majorana bound states. With a suitable design
of physical qubits, they are protected against local fluctuations and
constitute a non-local qubit. Further, they can be probed and manipulated
coherently by attaching an empty resonator to one end of the circuit-QED chain. | cond-mat_mes-hall |
High Mobility Free-Standing InSb Nanoflags Grown On InP Nanowire Stems
For Quantum Devices: High quality heteroepitaxial two-dimensional (2D) InSb layers are very
difficult to realize owing to the large lattice mismatch with other widespread
semiconductor substrates. A way around this problem is to grow free-standing 2D
InSb nanostructures on nanowire (NW) stems, thanks to the capability of NWs to
efficiently relax elastic strain along the sidewalls when lattice-mismatched
semiconductor systems are integrated. In this work, we optimize the morphology
of free-standing 2D InSb nanoflags (NFs). In particular, robust NW stems,
optimized growth parameters, and the use of reflection high-energy electron
diffraction (RHEED), to precisely orient the substrate for preferential growth,
are implemented to increase the lateral size of the 2D InSb NFs. Transmission
electron microscopy (TEM) analysis of these NFs reveals defect-free zinc blend
crystal structure, stoichiometric composition, and relaxed lattice parameters.
The resulting NFs are large enough to fabricate Hall-bar contacts with suitable
length-to-width ratio enabling precise electrical characterization. An electron
mobility of ~29,500 cm2/Vs is measured, which is the highest value reported for
free-standing 2D InSb nanostrutures in literature. We envision the use of 2D
InSb NFs for fabrication of advanced quantum devices. | cond-mat_mes-hall |
Thermally-Activated Phase Slips in Superfluid Spin Transport in Magnetic
Wires: We theoretically study thermally-activated phase slips in superfluid spin
transport in easy-plane magnetic wires within the stochastic
Landau-Lifshitz-Gilbert phenomenology, which runs parallel to the
Langer-Ambegaokar-McCumber-Halperin theory for thermal resistances in
superconducting wires. To that end, we start by obtaining the exact solutions
for free-energy minima and saddle points. We provide an analytical expression
for the phase-slip rate in the zero spin-current limit, which involves detailed
analysis of spin fluctuations at extrema of the free energy. An experimental
setup of a magnetoeletric circuit is proposed, in which thermal phase slips can
be inferred by measuring nonlocal magnetoresistance. | cond-mat_mes-hall |
Multiphoton excitation and high harmonic generation in rectangular
graphene quantum dot: The multiphoton excitation and high harmonic generation (HHG) processes are
considered using the microscopic quantum theory of nonlinear interaction of
strong coherent electromagnetic (EM) radiation with rectangular graphene
quantum dot (RGQD). The dynamic Hartree-Fock approximation is developed for the
consideration of the quantum dot-laser field nonlinear interaction at the
nonadiabatic multiphoton excitation regime. The many-body Coulomb interaction
is described in the extended Hubbard approximation. By numerical results, we
show the significance of the RGQD lateral size, shape, and EM wavefield
orientation in RGQD of the zigzag edge compear to the armchair edge in the HHG
process allowing for increasing the cutoff photon energy and the quantum yield
of higher harmonics. | cond-mat_mes-hall |
Commensurate and incommensurate double moiré interference in twisted
trilayer graphene: Twisted graphene multi-layers have been recently demonstrated to share
several correlation-driven behaviours with twisted bilayer graphene. In
general, the van Hove singularities (VHSs) can be used as a proxy of the
tendency for correlated behaviours. In this paper, we adopt an atomistic method
by combining tight-binding method with the semi-classical molecular dynamics to
investigate the electronic structures of twisted trilayer graphene (TTG) with
two independent twist angles. The two independent twist angles can lead to the
interference of the moir\'e patterns forming a variety of
commensurate/incommensurate complex supermoir\'e patterns. In particular, the
lattice relaxation, twist angle and angle disorder effects on the VHS are
discussed. We find that the lattice relaxation significantly influence the
position and magnitude of the VHSs. In the supermoir\'e TTG, the moir\'e
interference provides constructive or destructive effects depending on the
relative twist angle. By modulating the two independent twist angles, novel
superstructures, for instance, the Kagome-like lattice, could constructed via
the moir\'e pattern. Moreover, we demonstrate that a slight change in twist
angles (angle disorder) provides a significant suppression of the peak of the
VHSs. Apart from the moir\'e length, the evolution of the VHSs and the LDOS
mapping in real space could be used to identify the twist angles in the
complicated TTG. In practice, our work could provide a guide for exploring the
flat band behaviours in the supermoir\'e TTG experimentally. | cond-mat_mes-hall |
Anisotropic 2D materials for tunable hyperbolic plasmonics: Motivated by the recent emergence of a new class of anisotropic 2D materials,
we examine their electromagnetic modes and demonstrate that a broad class of
the materials can host highly directional hyperbolic plasmons. Their
propagation direction can be manipulated on-the-spot by gate doping, enabling
hyperbolic beams reflection, refraction and bending. The realization of these
natural 2D hyperbolic media opens up a new avenue in dynamic control of
hyperbolic plasmons not possible in the 3D version. | cond-mat_mes-hall |
Resonant Coherent Phonon Generation in Single-Walled Carbon Nanotubes
through Near-Band-Edge Excitation: We have observed large-amplitude coherent phonon oscillations of radial
breathing modes (RBMs) in single-walled carbon nanotubes excited through the
lowest-energy (E11) interband transitions. In contrast to the
previously-studied coherent phonons excited through higher-energy (E22)
transitions, these RBMs show comparable intensities between (n-m) mod 3 = 1 and
-1 nanotubes. We also find novel non-resonantly excited RBMs over an excitation
range of ~300 meV above the E11 transition, which we attribute to multi-phonon
replicas arising from strong exciton-phonon coupling. | cond-mat_mes-hall |
Quantum Hall Effect in a Graphene p-n Junction: We report on the fabrication and transport studies of a single-layer graphene
p-n junction. Carrier type and density in two adjacent regions are individually
controlled by electrostatic gating using a local top gate and a global back
gate. A functionalized Al203 oxide that adheres to graphene and does not
significantly affect its electronic properties is described. Measurements in
the quantum Hall regime reveal new plateaus of two-terminal conductance across
the junction at 1 and 3/2 times the quantum of conductance, e2/h, consistent
with theory. | cond-mat_mes-hall |
The dynamical bulk boundary correspondence and dynamical quantum phase
transitions in the Benalcazar-Bernevig-Hughes model: In this article we demonstrate that dynamical quantum phase transitions occur
for an exemplary higher order topological insulator, the
Benalcazar-Bernevig-Hughes model, following quenches across a topological phase
boundary. A dynamical bulk boundary correspondence is also seen both in the
eigenvalues of the Loschmidt overlap matrix and the boundary return rate. The
latter is found from a finite size scaling analysis for which the relative
simplicity of the model is crucial. Contrary to the usual two dimensional case
the dynamical quantum phase transitions in this model show up as cusps in the
return rate, as for a one dimensional model, rather than as cusps in its
derivative as would be typical for a two dimensional model. We explain the
origin of this behaviour. | cond-mat_mes-hall |
Robust Type-II Weyl Semimetal Phase in Transition Metal Diphosphides
XP$_2$ (X = Mo, W): The recently discovered type-II Weyl points appear at the boundary between
electron and hole pockets. Type-II Weyl semimetals that host such points are
predicted to exhibit a new type of chiral anomaly and possess thermodynamic
properties very different from their type-I counterparts. In this Letter, we
describe the prediction of a type-II Weyl semimetal phase in the transition
metal diphosphides MoP$_2$ and WP$_2$. These materials are characterized by
relatively simple band structures with four pairs of type-II Weyl points.
Neighboring Weyl points have the same chirality, which makes the predicted
topological phase robust with respect to small perturbations of the crystalline
lattice. In addition, this peculiar arrangement of the Weyl points results in
long topological Fermi arcs, thus making them readily accessible in
angle-resolved photoemission spectroscopy. | cond-mat_mes-hall |
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