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Coulomb-enhanced dynamic localization and Bell state generation in
coupled quantum dots: We investigate the dynamics of two interacting electrons in coupled quantum
dots driven by an AC field. We find that the two electrons can be trapped in
one of the dots by the AC field, in spite of the strong Coulomb repulsion. In
particular, we find that the interaction may enhance the localization effect.
We also demonstrate the field excitation procedure to generate the maximally
entangled Bell states. The generation time is determined by both analytic and
numerical solutions of the time dependent Schrodinger equation. | cond-mat_mes-hall |
Purcell effect at metal-insulator transitions: We investigate the spontaneous emission rate of a two-level quantum emitter
next to a composite medium made of randomly distributed metallic inclusions
embedded in a dielectric host matrix. In the near-field, the Purcell factor can
be enhanced by two-orders of magnitude relative to the case of an homogeneous
metallic medium, and reaches its maximum precisely at the insulator-metal
transition. By unveiling the role of the decay pathways on the emitter's
lifetime, we demonstrate that, close to the percolation threshold, the
radiation emission process is dictated by electromagnetic absorption in the
heterogeneous medium. We show that our findings are robust against change in
material properties, shape of inclusions, and apply for different effective
medium theories as well as for a wide range of transition frequencies. | cond-mat_mes-hall |
Interlayer binding energy of graphite -- A direct experimental
determination: Despite interlayer binding energy is one of the most important material
properties for graphite, there is still lacking report on its direct
experimental determination. In this paper, we present a novel experimental
method to directly measure the interlayer binding energy of highly oriented
pyrolytic graphite (HOPG). The obtained values of the binding energy are
0.27($\pm $0.02)J/m$^{2}$, which can serve as a benchmark for other theoretical
and experimental works. | cond-mat_mes-hall |
Observation of collapse of pseudospin order in bilayer quantum Hall
ferromagnets: The Hartree-Fock paradigm of bilayer quantum Hall states with finite
tunneling at filling factor $\nu$=1 has full pseudospin ferromagnetic order
with all the electrons in the lowest symmetric Landau level. Inelastic light
scattering measurements of low energy spin excitations reveal major departures
from the paradigm at relatively large tunneling gaps. The results indicate the
emergence of a novel correlated quantum Hall state at $\nu$=1 characterized by
reduced pseudospin order. Marked anomalies occur in spin excitations when
pseudospin polarization collapses by application of in-plane magnetic fields. | cond-mat_mes-hall |
Resonant plasmonic effects in periodic graphene antidot arrays: We show that a graphene sheet perforated with micro- or nano-size antidots
have prominent absorption resonances in the microwave and terahertz regions.
These resonances correspond to surface plasmons of a continuous sheet
"perturbed" by a lattice. They are excited in different diffraction orders, in
contrast to cavity surface plasmon modes existing in disconnected graphene
structures. The resonant absorption by the antidot array can essentially exceed
the absorption by a continuous graphene sheet, even for high antidot
diameter-to-period aspect ratios. Surface plasmon-enhanced absorption and
suppressed transmission is more efficient for higher relaxation times of the
charge carriers. | cond-mat_mes-hall |
Floquet approach to bichromatically driven cavity optomechanical systems: We develop a Floquet approach to solve time-periodic quantum Langevin
equations in steady state. We show that two-time correlation functions of
system operators can be expanded in a Fourier series and that a generalized
Wiener-Khinchin theorem relates the Fourier transform of their zeroth Fourier
component to the measured spectrum. We apply our framework to bichromatically
driven cavity optomechanical systems, a setting in which mechanical oscillators
have recently been prepared in quantum-squeezed states. Our method provides an
intuitive way to calculate the power spectral densities for time-periodic
quantum Langevin equations in arbitrary rotating frames. | cond-mat_mes-hall |
A-geometrical approach to Topological Insulators with defects: The study of the propagation of electrons with a varying spinor orientability
is performed using the coordinate transformation method. Topological Insulators
are characterized by an odd number of changes of the orientability in the
Brillouin zone. For defects the change in orientability takes place for closed
orbits in real space. Both cases are characterized by nontrivial spin
connections. Using this method , we derive the form of the spin connections for
topological defects in three dimensional Topological Insulators. On the surface
of a Topological Insulator, the presence an edge dislocation gives rise to a
spin connection controlled by torsion. We find that electrons propagate along
two dimensional regions and confined circular contours. We compute for the edge
dislocations the tunneling density of states. The edge dislocations violates
parity symmetry resulting in a current measured by the in-plane component of
the spin on the surface. | cond-mat_mes-hall |
Dynamics of quantum cellular automata electron transition in triple
quantum dots: The quantum cellular automata (QCA) effect is a transition in which multiple
electron move coordinately by Coulomb interactions and observed in multiple
quantum dots. This effect will be useful for realizing and improving quantum
cellular automata and information transfer using multiple electron transfer. In
this paper, we investigate the real-time dynamics of the QCA charge transitions
in a triple quantum dot by using fast charge-state readout realized by rf
reflectometry. We observe real-time charge transitions and analyze the
tunneling rate comparing with the first-order tunneling processes. We also
measure the gate voltage dependence of the QCA transition and show that it can
be controlled by the voltage. | cond-mat_mes-hall |
Microwave Rectification at the Boundary between Two-Dimensional Electron
Systems: Rectification of microwave radiation (20-40 GHz) by a line boundary between
two two-dimensional metals on a silicon surface was observed and investigated
at different temperatures, in-plane magnetic fields and microwave powers. The
rectified voltage $V_{dc}$ is generated whenever the electron densities
$n_{1,2}$ of the two metals are different, changing polarity at $n_1 \approx
n_2$. Very strong nonlinear response is found when one of the two 2D metals is
close to the electron density corresponding to the reported magnetic
instability in this system. | cond-mat_mes-hall |
Quantum electrodynamic approach to the conductivity of gapped graphene: The electrical conductivity of graphene with a nonzero mass-gap parameter is
investigated starting from the first principles of quantum electrodynamics in
(2+1)-dimensional space-time at any temperature. The formalism of the
polarization tensor defined over the entire plane of complex frequency is used.
At zero temperature we reproduce the results for both real and imaginary parts
of the conductivity, obtained previously in the local approximation, and
generalize them taking into account the effects of nonlocality. At nonzero
temperature the exact analytic expressions for real and imaginary parts of the
longitudinal and transverse conductivities of gapped graphene are derived, as
well as their local limits and approximate expressions in several asymptotic
regimes. Specifically, a simple local result for the real part of conductivity
of gapped graphene valid at any temperature is obtained. According to our
results, the real part of the conductivity is not equal to zero for frequencies
exceeding the width of the gap and goes to the universal conductivity with
increasing frequency. The imaginary part of conductivity of gapped graphene
varies from infinity at zero frequency to minus infinity at the frequency
defined by the gap parameter and then goes to zero with further increase of
frequency. The analytic expressions are accompanied by the results of numerical
computations. Possible future generalization of the used formalism is
discussed. | cond-mat_mes-hall |
Non-invasive detection of charge-rearrangement in a quantum dot in high
magnetic fields: We demonstrate electron redistribution caused by magnetic field on a single
quantum dot measured by means of a quantum point contact as non-invasive
detector. Our device which is fabricated by local anodic oxidation allows to
control independently the quantum point contact and all tunnelling barriers of
the quantum dot. Thus we are able to measure both the change of the quantum dot
charge and also changes of the electron configuration at constant number of
electrons on the quantum dot. We use these features to exploit the quantum dot
in a high magnetic field where transport through the quantum dot displays the
effects of Landau shells and spin blockade. We confirm the internal
rearrangement of electrons as function of the magnetic field for a fixed number
of electrons on the quantum dot. | cond-mat_mes-hall |
Monolithically integrated single quantum dots coupled to bowtie
nanoantennas: Deterministically integrating semiconductor quantum emitters with plasmonic
nano-devices paves the way towards chip-scale integrable, true nanoscale
quantum photonics technologies. For this purpose, stable and bright
semiconductor emitters are needed, which moreover allow for CMOS-compatibility
and optical activity in the telecommunication band. Here, we demonstrate
strongly enhanced light-matter coupling of single near-surface ($<10\,nm$) InAs
quantum dots monolithically integrated into electromagnetic hot-spots of
sub-wavelength sized metal nanoantennas. The antenna strongly enhances the
emission intensity of single quantum dots by up to $\sim16\times$, an effect
accompanied by an up to $3.4\times$ Purcell-enhanced spontaneous emission rate.
Moreover, the emission is strongly polarised along the antenna axis with
degrees of linear polarisation up to $\sim85\,\%$. The results unambiguously
demonstrate the efficient coupling of individual quantum dots to
state-of-the-art nanoantennas. Our work provides new perspectives for the
realisation of quantum plasmonic sensors, step-changing photovoltaic devices,
bright and ultrafast quantum light sources and efficent nano-lasers. | cond-mat_mes-hall |
Emission and absorption asymmetry in the quantum noise of a Josephson
junction: We measure current fluctuations of mesoscopic devices in the quantum regime,
when the frequency is of the order of or higher than the applied voltage or
temperature. Detection is designed to probe separately the absorption and
emission contributions of current fluctuations, i.e. the positive and negative
frequencies of the Fourier transformed nonsymmetrized noise correlator. It
relies on measuring the quasiparticles photon assisted tunneling current across
a superconductor-insulator-superconductor junction (the detector junction)
caused by the excess current fluctuations generated by quasiparticles tunneling
across a Josephson junction (the source junction). We demonstrate unambiguously
that the negative and positive frequency parts of the nonsymmetrized noise
correlator are separately detected and that the excess current fluctuations of
a voltage biased Josephson junction present a strong asymmetry between emission
and absorption. | cond-mat_mes-hall |
Bilayer WSe$_2$ as a natural platform for interlayer exciton condensates
in the strong coupling limit: Exciton condensates (EC) are macroscopic coherent states arising from
condensation of electron-hole pairs. Bilayer heterostructures, consisting of
two-dimensional electron and hole layers separated by a tunnel barrier, provide
a versatile platform to realize and study EC. The tunnel barrier suppresses
recombination yielding long-lived excitons. However, this separation also
reduces interlayer Coulomb interactions, limiting the exciton binding strength.
Here, we report the observation of EC in naturally occurring 2H-stacked bilayer
WSe$_2$. In this system, the intrinsic spin-valley structure suppresses
interlayer tunneling even when the separation is reduced to the atomic limit,
providing access to a previously unattainable regime of strong interlayer
coupling. Using capacitance spectroscopy, we investigate magneto-EC, formed
when partially filled Landau levels (LL) couple between the layers. We find
that the strong-coupling EC show dramatically different behaviour compared with
previous reports, including an unanticipated variation of the EC robustness
with the orbital number, and find evidence for a transition between two types
of low-energy charged excitations. Our results provide a demonstration of
tuning EC properties by varying the constituent single-particle wavefunctions. | cond-mat_mes-hall |
Cavity-enhanced measurements of defect spins in silicon carbide: The identification of new solid-state defect qubit candidates in widely used
semiconductors has the potential to enable the use of nanofabricated devices
for enhanced qubit measurement and control operations. In particular, the
recent discovery of optically active spin states in silicon carbide thin films
offers a scalable route for incorporating defect qubits into on-chip photonic
devices. Here we demonstrate the use of 3C silicon carbide photonic crystal
cavities for enhanced excitation of color center defect spin ensembles in order
to increase measured photoluminescence signal count rates, optically detected
magnetic resonance signal intensities, and optical spin initialization rates.
We observe up to a factor of 30 increase in the photoluminescence and ODMR
signals from Ky5 color centers excited by cavity resonant excitation and
increase the rate of ground-state spin initialization by approximately a factor
of two. Furthermore, we show that the small excitation mode volume and enhanced
excitation and collection efficiencies provided by the structures can be used
to study inhomogeneous broadening in defect qubit ensembles. These results
highlight some of the benefits that nanofabricated devices offer for
engineering the local photonic environment of color center defect qubits to
enable applications in quantum information and sensing. | cond-mat_mes-hall |
Topological Hall signatures of magnetic hopfions: Magnetic hopfions are topologically protected three-dimensional solitons that
are constituted by a tube which exhibits a topologically nontrivial spin
texture in the cross-section profile and is closed to a torus. Here we show
that the hopfion's locally uncompensated emergent field leads to a topological
Hall signature, although the topological Hall effect vanishes on the global
level. The topological Hall signature is switchable by magnetic fields or
electric currents and occurs independently of the anomalous and conventional
Hall effects. It can therefore be exploited to electrically detect hopfions in
experiments and even to distinguish them from other textures like skyrmion
tubes. Furthermore, it can potentially be utilized in spintronic devices.
Exemplarily, we propose a hopfion-based racetrack data storage device and
simulate the electrical detection of the hopfions as carriers of information. | cond-mat_mes-hall |
Quasiparticle scattering off phase boundaries in epitaxial graphene: We investigate the electronic structure of terraces of single layer graphene
(SLG) by scanning tunneling microscopy (STM) on samples grown by thermal
decomposition of 6H-SiC(0001) crystals in ultra-high vacuum. We focus on the
perturbations of the local density of states (LDOS) in the vicinity of edges of
SLG terraces. Armchair edges are found to favour intervalley quasiparticle
scattering, leading to the (\surd3\times\surd3)R30{\deg} LDOS superstructure
already reported for graphite edges and more recently for SLG on SiC(0001).
Using Fourier transform of LDOS images, we demonstrate that the intrinsic
doping of SLG is responsible for a LDOS pattern at the Fermi energy which is
more complex than for neutral graphene or graphite, since it combines local
(\surd3\times\surd3)R30{\deg} superstructure and long range beating modulation.
Although these features were already reported by Yang et al. Nanoletters 10,
943 (2010), we propose here an alternative interpretation based on simple
arguments classically used to describe standing wave patterns in standard
two-dimensional systems. Finally, we discuss the absence of intervalley
scattering off other typical boundaries: zig-zag edges and SLG/bilayer graphene
junctions. | cond-mat_mes-hall |
High-fidelity quantum memory utilizing inhomogeneous nuclear
polarization in a quantum dot: We numerically investigate the encoding and retrieval processes for a quantum
memory realized in a semiconductor quantum dot, by focusing on the effect of
inhomogeneously polarized nuclear spins whose polarization depends on the local
hyperfine coupling strength. We find that the performance of the quantum memory
is significantly improved by the inhomogeneous nuclear polarization, as
compared to the homogeneous one. Moreover, the narrower the nuclear
polarization distribution is, the better the performance of the quantum memory
is. We ascribe the performance improvement to the full harnessing of the highly
polarized and strongly coupled nuclear spins, by carefully studying the entropy
change of individual nuclear spins during encoding process. Our results shed
new light on the implementation of a quantum memory in a quantum dot. | cond-mat_mes-hall |
Photon properties of single graphene nanoribbon microcavity laser: In this work, I propose a scheme about a single graphene nanoribbon (GNR)
emitter in a microcavity, and focus on a fully-quantum-mechanical treatment
model with the excitonic interaction included to investigate the photon
properties and lasing action. When the single armchair-edged GNRs (AGNRs)
microcavity system is pumped, the exciton-photon coupling provides more photons
and enhances the photon emission process, making it essentially a lasing
object. The theoretical results demonstrated that single AGNR in a
semiconductor microcavity system maybe serve as a nanolaser with ultralow
lasing threshold. | cond-mat_mes-hall |
Spatial Confinement, Magnetic Localization and Their Interactions on
Massless Dirac Fermions: It is of keen interest to researchers understanding different approaches to
confine massless Dirac fermions in graphene, which is also a central problem in
making electronic devices based on graphene. Here, we studied spatial
confinement, magnetic localization and their interactions on massless Dirac
fermions in an angled graphene wedge formed by two linear graphene p-n
boundaries with an angle 34. Using scanning tunneling microscopy, we visualized
quasibound states temporarily confined in the studied graphene wedge. Large
perpendicular magnetic fields condensed the massless Dirac fermions in the
graphene wedge into Landau levels (LLs). The spatial confinement of the wedge
affects the Landau quantization, which enables us to experimentally measure the
spatial extent of the wave functions of the LLs. The magnetic fields induce a
sudden and large increase in energy of the quasibound states because of a pi
Berry phase jump of the massless Dirac fermions in graphene. Such a behavior is
the hallmark of the Klein tunneling in graphene. Our experiment demonstrated
that the angled wedge is a unique system with the critical magnetic fields for
the pi Berry phase jump depending on distance from summit of the wedge. | cond-mat_mes-hall |
Elastic strain field due to an inclusion of a polyhedral shape with a
non-uniform lattice misfit: An analytical solution in a closed form is obtained for the three-dimensional
elastic strain distribution in an unlimited medium containing an inclusion with
a coordinate-dependent lattice mismatch (an eigenstrain). Quantum dots
consisting of a solid solution with a spatially varying composition are
examples of such inclusions. It is assumed that both the inclusion and the
surrounding medium (the matrix) are elastically isotropic and have the same
Young modulus and Poisson ratio. The inclusion shape is supposed to be an
arbitrary polyhedron, and the coordinate dependence of the lattice misfit, with
respect to the matrix, is assumed to be a polynomial of any degree. It is shown
that, both inside and outside the inclusion, the strain tensor is expressed as
a sum of contributions of all faces, edges and vertices of the inclusion. Each
of these contributions, as a function of the observation point's coordinates,
is a product of some polynomial and a simple analytical function, which is the
solid angle subtended by the face from the observation point (for a
contribution of a face), or the potential of the uniformly charged edge (for a
contribution of an edge), or the distance from the vertex to the observation
point (for a contribution of a vertex). The method of constructing the relevant
polynomial functions is suggested. We also found out that similar expressions
describe an electrostatic or gravitational potential, as well as its first and
second derivatives, of a polyhedral body with a charge/mass density that
depends on coordinates polynomially. | cond-mat_mes-hall |
Hot exciton relaxation in coupled ultra-thin CdTe/ZnTe quantum well
structures: The photoluminescence (PL) and PL excitation (PLE) spectra of CdTe/ZnTe
asymmetric double quantum well (QW) structures are studied on a series of
samples containing two CdTe layers with nominal thicknesses of 2 and 4
monolayers (ML) in the ZnTe matrix. The samples differ in the thickness of the
ZnTe spacer between CdTe QWs which is 45, 65 and 75 ML. It has been found that
at above-barrier excitation the PL from a shallow QW at sufficiently weak
excitation intensities is determined by recombination of hot excitons. It is
shown that under these conditions, when PL is excited by lasers with different
wavelengths, the ratio of the PL intensities from shallow and deep QWs
decreases exponentially with an increase of the initial kinetic energy of hot
excitons. It is found that energy relaxation of hot excitons with LO phonon
emission determine the shape of the PLE spectrum of shallow QW in the range of
exciton kinetic energies up to more than 20 LO phonons above ZnTe bandgap. We
have shown that the results obtained are well described by the model of charge
and energy transfer between QWs. | cond-mat_mes-hall |
Influence of Nuclear Quadrupole Moments on Electron Spin Coherence in
Semiconductor Quantum Dots: We theoretically investigate the influence of the fluctuating Overhauser
field on the spin of an electron confined to a quantum dot (QD). The
fluctuations arise from nuclear angular momentum being exchanged between
different nuclei via the nuclear magnetic dipole coupling. We focus on the role
of the nuclear electric quadrupole moments (QPMs), which generally cause a
reduction in internuclear spin transfer efficiency in the presence of electric
field gradients. The effects on the electron spin coherence time are studied by
modeling an electron spin echo experiment. We find that the QPMs cause an
increase in the electron spin coherence time and that an inhomogeneous
distribution of the quadrupolar shift, where different nuclei have different
shifts in energy, causes an even larger increase in the electron coherence time
than a homogeneous distribution. Furthermore, a partial polarization of the
nuclear spin ensemble amplifies the effect of the inhomogeneous quadrupolar
shifts, causing an additional increase in electron coherence time, and provides
an alternative to the experimentally challenging suggestion of full dynamic
nuclear spin polarization. | cond-mat_mes-hall |
Ultrafast Relaxation Dynamics of Photoexcited Dirac Fermion in The Three
Dimensional Dirac Semimetal Cadmium Arsenide: Three dimensional (3D) Dirac semimetals which can be seen as 3D analogues of
graphene have attracted enormous interests in research recently. In order to
apply these ultrahigh-mobility materials in future electronic/optoelectronic
devices, it is crucial to understand the relaxation dynamics of photoexcited
carriers and their coupling with lattice. In this work, we report ultrafast
transient reflection measurements of the photoexcited carrier dynamics in
cadmium arsenide (Cd3As2), which is one of the most stable Dirac semimetals
that have been confirmed experimentally. By using low energy probe photon of
0.3 eV, we probed the dynamics of the photoexcited carriers that are
Dirac-Fermi-like approaching the Dirac point. We systematically studied the
transient reflection on bulk and nanoplate samples that have different doping
intensities by tuning the probe wavelength, pump power and lattice temperature,
and find that the dynamical evolution of carrier distributions can be retrieved
qualitatively by using a two-temperature model. This result is very similar to
that of graphene, but the carrier cooling through the optical phonon couplings
is slower and lasts over larger electron temperature range because the optical
phonon energies in Cd3As2 are much lower than those in graphene. | cond-mat_mes-hall |
Carbon Nanotube Thermal Transport: Ballistic to Diffusive: We propose to use l_0/(l_0+L) for the energy transmission covering both
ballistic and diffusive regimes, where l_0 is mean free path and L is system
length. This formula is applied to heat conduction in carbon nanotubes (CNTs).
Calculations of thermal conduction show: (1) Thermal conductance at room
temperature is proportional to the diameter of CNTs for single-walled CNTs
(SWCNTs) and to the square of diameter for multi-walled CNTs (MWCNTs). (2)
Interfaces play an important role in thermal conduction in CNTs due to the
symmetry of CNTs vibrational modes. (3) When the phonon mean free path is
comparable with the length L of CNTs in ballistic-diffusive regime, thermal
conductivity \kappa goes as L^{\alpha} . The effective exponent \alpha is
numerically found to decrease with increasing temperature and is insensitive to
the diameter of SWCNTs for Umklapp scattering process. For short SWCNTs (<0.1
\mu m) we find \alpha \approx 0.8 at room temperature. These results are
consistent with recent experimental findings. | cond-mat_mes-hall |
Practical Guide to Quantum Phase Transitions in Quantum-Dot-Based
Tunable Josephson Junctions: Quantum dots attached to BCS superconducting leads exhibit a $0-\pi$ impurity
quantum phase transition, which can be experimentally controlled either by the
gate voltage or by the superconducting phase difference. For the pertinent
superconducting single-impurity Anderson model, we newly present two simple
analytical formulae describing the position of the phase boundary in parameter
space for the weakly correlated and Kondo regime, respectively. Furthermore, we
show that the two-level approximation provides an excellent description of the
low temperature physics of superconducting quantum dots near the phase
transition. We discuss reliability and mutual agreement of available finite
temperature numerical methods (Numerical Renormalization Group and Quantum
Monte Carlo) and suggest a novel approach for efficient determination of the
quantum phase boundary from measured finite temperature data. Our results
enable fast and efficient, yet reliable characterization and design of such
nanoscopic tunable Josephson junction devices. | cond-mat_mes-hall |
Giant Magneto-Oscillations of Electric-Field-Induced Spin Polarization
in 2DEG: We consider a disordered two-dimensional electron gas with spin-orbit
coupling placed in a perpendicular magnetic field and calculate the magnitude
and direction of the electric-field-induced spin polarization. We find that in
strong magnetic fields the polarization becomes an oscillatory function of the
magnetic field and that the amplitude of these oscillations is parametrically
larger than the polarization at zero magnetic field. We show that the enhanced
amplitude of the polarization is a consequence of strong electron-hole
asymmetry in a quantizing magnetic field. | cond-mat_mes-hall |
Magnetic Response in Mesoscopic Rings and Moebius Strips: A Theoretical
Study: We investigate magnetic response in mesoscopic rings and moebius strips
penetrated by magnetic flux $\phi$. Based on a simple tight-binding framework
all the calculations are performed numerically which describe persistent
current and low-field magnetic susceptibility as functions of magnetic flux
$\phi$, total number of electrons $N_e$, system size $N$ and disorder strength
$W$. Our exact analysis may provide some important signatures to study magnetic
response in nano-scale loop geometries. | cond-mat_mes-hall |
Shot noise in diffusive conductors: A quantitative analysis of
electron-phonon interaction effects: Using the 'drift-diffusion-Langevin' equation, we have quantitatively
analyzed the effects of electron energy relaxation via their interaction with
phonons, generally in presence of electron-electron interaction, on shot noise
in diffusive conductors. We have found that the noise power $ S_I(\omega )$
(both at low and high observation frequencies $\omega $) drops to half of its
'mesoscopic' value only at $\beta \gtrsim 100,$ where $\beta $ is the ratio of
the sample length $L$ to the energy relaxation length $l_{% {\rm ph}}$ (the
latter may be much larger then the dephasing length). It means in particular
that at low temperatures the shot noise may be substantial even when $L\sim
10^{-2}$ -- $10^{-1}$ cm, and the conductor is 'macroscopic' in any other
respect. | cond-mat_mes-hall |
Magnetic translations for a spatially periodic magnetic field: It is shown that in the case of free electron in a spatially periodic
magnetic field the concept of magnetic translations operators is still valid
and, moreover, these operators can be defined in the same way as for a Bloch
electron in a uniform magnetic field. The results can be a useful tool in
investigation of lately observed phenomena in 2DEG with spatially modulated
density. | cond-mat_mes-hall |
PT Symmetric Floquet Topological Phase: In this paper, we study the existence of Floquet topological insulators for
PT symmetric non-Hermitian Hamiltonians. We consider an array of waveguide in
1D with periodically changing non-Hermitian potential and predict the existence
of Floquet topological insulators in the system. We also extend the concept of
Floquet topological phase to a two dimensional non-Hermitian system. | cond-mat_mes-hall |
Enhanced longevity of the spin helix in low-symmetry quantum wells: In a semiconductor, collective excitations of spin textures usually decay
rather fast due to D'yakonov-Perel' spin relaxation. The latter arises from
spin-orbit coupling, which induces wave-vector-dependent spin rotations that,
in conjunction with random disorder scattering, generate spin decoherence.
However, symmetries occurring under certain conditions can prevent the
relaxation of particular homogeneous and inhomogeneous spin textures. The
inhomogeneous spin texture, termed as persistent spin helix, is especially
appealing as it enables us to manipulate the spin orientation while retaining a
long spin lifetime. Recently, it was predicted that such symmetries can be
realized in zinc-blende two-dimensional electron gases if at least two
growth-direction Miller indices agree in modulus and the coefficients of the
Rashba and linear Dresselhaus spin-orbit couplings are suitably matched [PRL
117, 236801 (2016)]. In the present paper, we systematically analyze the impact
of the symmetry-breaking cubic Dresselhaus spin-orbit coupling, which
generically coexists in these systems, on the stability of the emerging spin
helices with respect to the growth direction. We find that, as an interplay
between orientation and strength of the effective magnetic field induced by the
cubic Dresselhaus terms, the spin relaxation is weakest for a low-symmetry
growth direction that can be well approximated by a [225] lattice vector. These
quantum wells yield a 30\% spin-helix lifetime enhancement compared to
[001]-oriented electron gases and, remarkably, require a negligible Rashba
coefficient. The rotation axis of the corresponding spin helix is only slightly
tilted out of the quantum-well plane. This makes the experimental study of the
spin-helix dynamics readily accessible for conventional optical spin
orientation measurements where spins are excited and detected along the
quantum-well growth direction. | cond-mat_mes-hall |
Optimized Tersoff and Brenner empirical potential parameters for lattice
dynamics and phonon thermal transport in carbon nanotubes and graphene: We have examined the commonly used Tersoff and Brenner empirical interatomic
potentials in the context of the phonon dispersions in graphene. We have found
a parameter set for each empirical potential that provides improved fits to
some structural data and to the in-plane phonon dispersion data for graphite.
These optimized parameter sets yield values of the acoustic phonon velocities
that are in better agreement with measured data. They also provide lattice
thermal conductivity values in single-walled carbon nanotubes that are
considerably improved compared to those obtained from the original parameter
sets. | cond-mat_mes-hall |
Magnetic Splitting of the Zero Bias Peak in a Quantum Point Contact with
a Variable Aspect Ratio: We report a zero-bias peak in the differential conductance of a Quantum Point
Contact (QPC), which splits in an external magnetic field. The peak is observed
over a range of device conductance values starting significantly below
$2e^2/h$. The observed splitting closely matches the Zeeman energy and shows
very little dependence on gate voltage, suggesting that the mechanism
responsible for the formation of the peak involves electron spin. Precision
Zeeman energy data for the experiment are obtained from a separately patterned
single-electron transistor located a short distance away from the QPC. The QPC
device has four gates arranged in a way that permits tuning of the longitudinal
potential, and is fabricated in a GaAs/AlGaAs heterostructure containing
2-dimenional electron gas. We show that the agreement between the peak
splitting and the Zeeman energy is robust with respect to moderate distortions
of the QPC potential. We also show that the mechanism that leads to the
formation of the ZBP is different from the conventional Kondo effect found in
quantum dots. | cond-mat_mes-hall |
Transport Study of Charge Carrier Scattering in Monolayer WSe$_2$: Employing flux-grown single crystal WSe$_2$, we report charge carrier
scattering behaviors measured in $h$-BN encapsulated monolayer field effect
transistors. We perform quantum transport measurements across various hole
densities and temperatures and observe a non-monotonic change of transport
mobility $\mu$ as a function of hole density in the degenerately doped sample.
This unusual behavior can be explained by energy dependent scattering amplitude
of strong defects calculated using the T-matrix approximation. Utilizing long
mean-free path ($>$500 nm), we demonstrate the high quality of our electronic
devices by showing quantized conductance steps from an
electrostatically-defined quantum point contact. Our results show the potential
for creating ultra-high quality quantum optoelectronic devices based on
atomically thin semiconductors. | cond-mat_mes-hall |
Magnetically induced oscillations of the spin polarization in the
Datta-Das geometry: The control of intrinsic magnetic degrees of freedom is very important as it
offers a practical means to manipulate and probe electron spin transport.
Tunable spin-orbit effect in quantum wires can in principle serve as a means to
achieve this goal. Here, we investigate within the scattering matrix approach
the effect of an in-plane magnetic field on the conductance of the quantum wire
in the Datta-Das geometry and show that the interplay of the spin-orbit
interaction with the magnetic field provides enhanced control over the electron
spin polarization. In particular, we predict a novel effect of magnetically
induced oscillations of the electron spin in a certain range of magnetic field. | cond-mat_mes-hall |
Vacancy-induced localized modes and impurity band formation in the
Haldane model: a quantum dot analogy: In this study, the Haldane model's edge states are utilized to illustrate
that a zero-energy localized state forms around a single vacancy in the model.
In order to complete this task, the conventional unit cell associated to the
Haldane hexagonal structure is transferred onto a two-leg ladder in momentum
space, effectively forming an extended Su-Schrieffer-Heeger~(SSH) lattice
through a one-dimensional Fourier transform. Through the application of a
suitable unitary transformation, the two-leg SSH ladder in momentum space is
converted into an equivalent lattice with two distinct on-site states with
different momentum that are suitable for the calculations. Ultimately, the
desired zero-energy localized mode formed around the vacant-site is represented
by a combination of the armchair edge states. Furthermore, the scenario
involving two vacant sites is investigated and it is revealed that an effective
hopping interaction exists between the localized states formed around the
on-site vacancies created along a zigzag chain in the lattice. This structure
can be likened to the structure of a quantum dot with two none-degenerate
energy levels. Such a hopping interaction is absent for the same vacancies
created on the armchair chains. Finally, it is shown that introducing vacancies
periodically on the sites of a zigzag row along a finite-width ribbon with the
Haldane structure leads to the emergence of an impurity band within the energy
gap. | cond-mat_mes-hall |
Cryogenic spin Seebeck effect: We present a theory of the non-linearities of the spin Seebeck effect (SSE)
in a ferromagnetic nanowire at cryogenic temperatures. We adopt a microscopic
quantum noise model based on a collection of two-level systems. At certain
positions of Pt detectors to the wire, the transverse SSE changes sign as a
function of temperature and/or temperature gradient. On the other hand, the
longitudinal SSE does not show significant non-linearities even far outside the
regime of validity of linear response theory. | cond-mat_mes-hall |
Tunable ferroelectricity in hBN intercalated twisted double-layer
graphene: Van der Waals (vdW) assembly of two-dimensional materials has been long
recognized as a powerful tool to create unique systems with properties that
cannot be found in natural compounds. However, among the variety of vdW
heterostructures and their various properties, only a few have revealed
metallic and ferroelectric behaviour signatures. Here we show ferroelectric
semimetal made of double-gated double-layer graphene separated by an atomically
thin crystal of hexagonal boron nitride, which demonstrating high room
temperature mobility of the order of 10 m$^2$V$^{-1}$s$^{-1}$ and exhibits
robust ambipolar switching in response to the external electric field. The
observed hysteresis is tunable, reversible and persists above room temperature.
Our fabrication method expands the family of ferroelectric vdW compounds and
offers a route for developing novel phase-changing devices. | cond-mat_mes-hall |
On the Implications of Discrete Symmetries for the Beta Function of
Quantum Hall Systems: We argue that the large discrete symmetry group of quantum Hall systems is
insufficient in itself to determine the complete beta function for the scaling
of the conductivities, $\sigma_{xx}$ and $\sigma_{xy}$. We illustrate this
point by showing that a recent ansatz for this function is one of a
many-parameter family. A clean prediction for the delocalization exponents for
these systems therefore requires the specification of more information, such as
past proposals that the beta function is either holomorphic or
quasi-holomorphic in the variable $z = (\hbar/e^2)(\sigma_{xy} +
i\sigma_{xx})$. | cond-mat_mes-hall |
Large quantum nonlinear dynamic susceptibility of single-molecule
magnets: The nonlinear dynamical response of Mn$_{12}$ single-molecule magnets is
experimentally found to be very large, quite insensitive to the spin-lattice
coupling constant, and displaying peaks reversed with respect to classical
superparamagnets. It is shown that these features are caused by the strong
field dependence of the relaxation rate due to the detuning of energy levels
between which tunneling takes place. The nonlinear susceptibility technique,
previously overlooked, is thus proposed as a privileged probe to ascertain the
occurrence of quantum effects in mesoscopic magnetic systems. | cond-mat_mes-hall |
The electronic properties of doped single walled carbon nanotubes and
carbon nanotube sensors: We present ab initio calculations on the band structure and density of states
of single wall semiconducting carbon nanotubes with high degrees (up to 25%) of
B, Si and N substitution. The doping process consists of two phases: different
carbon nanotubes (CNTs) for a constant doping rate and different doping rates
for the zigzag (8, 0) carbon nanotube. We analyze the doping dependence of
nanotubes on the doping rate and the nanotube type. Using these results, we
select the zigzag (8, 0) carbon nanotube for toxic gas sensor calculation and
obtain the total and partial densities of states for CNT (8, 0). We have
demonstrated that the CNT (8, 0) can be used as toxic gas sensors for CO and NO
molecules, and it can partially detect Cl$_2$ toxic molecules but cannot detect
H$_2$S. To overcome these restrictions, we created the B and N doped CNT (8, 0)
and obtained the total and partial density of states for these structures. We
also showed that B and N doped CNT (8, 0) can be used as toxic gas sensors for
such molecules as CO, NO, Cl$_2$ and H$_2$S. | cond-mat_mes-hall |
Trigonal distortion of topologically confined channels in bilayer
Graphene: In this work we show that the trigonal warping of the electronic bands in
bilayer graphene dramatically modifies the behavior of the one-dimensional
modes topologically confined due to an inhomogeneous bias that changes sign
across a channel. The topologically protected states are present but their
behavior is disrupted from the predicted in the isotropic approximation. We
present detailed studies of the electronic properties of the 1D channel in
function of the orientation of the channel. | cond-mat_mes-hall |
Oligothiophene nano-rings as electron resonators for whispering gallery
modes: Structural and electronic properties of oligothiophene nano-wires and rings
synthesized on a Au(111) surface are investigated by scanning tunneling
microscopy. The spectroscopic data of the linear and cyclic oligomers show
remarkable differences which, to a first approximation, can be accounted by
considering electronic states confinement to one-dimensional (1D) boxes having
respectively fixed and periodic boundary conditions. A more detailed analysis
shows that polythiophene must be treated as a ribbon (i.e. having an effective
width) rather than a purely 1D structure. A fascinating consequence is that the
molecular nano-rings act as whispering gallery mode resonators for electrons,
opening the way for new applications in quantum-electronics. | cond-mat_mes-hall |
Dynamical separation of bulk and edge transport in HgTe-based 2D
topological insulators: Topological effects in edge states are clearly visible on short lengths only,
thus largely impeding their studies. On larger distances, one may be able to
dynamically enhance topological signatures by exploiting the high mobility of
edge states with respect to bulk carriers. Our work on microwave spectroscopy
highlights the responses of the edges which host very mobile carriers, while
bulk carriers are drastically slowed down in the gap. Though the edges are
denser than expected, we establish that charge relaxation occurs on short
timescales, and suggests that edge states can be addressed selectively on
timescales over which bulk carriers are frozen. | cond-mat_mes-hall |
Magneto-elastic universal logic gate: A non-volatile, error-resilient
Boolean logic gate with ultra-low energy-delay product: A long-standing goal of computer technology is to process and store digital
information with the same device in order to implement new architectures. One
way to accomplish this is to use nanomagnetic `non-volatile' logic gates that
can perform Boolean operations and then store the output data in the
magnetization states of nanomagnets, thereby doubling as both logic and memory.
Unfortunately, many proposed nanomagnetic gates do not possess the seven
essential characteristics of a Boolean logic gate: concatenability,
non-linearity, isolation between input and output, gain, universal logic
implementation, scalability and error resilience. More importantly, their
energy-delay products and error-rates vastly exceed that of conventional
transistor-based logic gates, which is a drawback. Here, we propose a
non-volatile voltage-controlled nanomagnetic logic gate that possesses all the
necessary characteristics of a logic gate and whose energy-delay product is ~2
orders of magnitude less than that of other nano-magnetic (non-volatile) logic
gates and ~1 order of magnitude less than that of (volatile) CMOS-based logic
gates. The error-resilience is also superior to that of other known
nanomagnetic gates. | cond-mat_mes-hall |
Superfluidity of Dipolar Excitons in a Black Phosphorene Double Layer: We study the formation of dipolar excitons and their superfluidity in a black
phosphorene double layer. The analytical expressions for the single dipolar
exciton energy spectrum and wave function are obtained. It is predicted that a
weakly interacting gas of dipolar excitons in a double layer of black
phosphorus exhibits superfluidity due to the dipole-dipole repulsion between
the dipolar excitons. In calculations are employed the Keldysh and Coulomb
potentials for the interaction between the charge carriers to analyze the
influence of the screening effects on the studied phenomena. It is shown that
the critical velocity of superfluidity, the spectrum of collective excitations,
concentrations of the superfluid and normal component, and mean field critical
temperature for superfluidity are anisotropic and demonstrate the dependence on
the direction of motion of dipolar excitons. The critical temperature for
superfluidity increases if the exciton concentration and the interlayer
separation increase. It is shown that the dipolar exciton binding energy and
mean field critical temperature for superfluidity are sensitive to the electron
and hole effective masses. The proposed experiment to observe a directional
superfluidity of excitons is addressed. | cond-mat_mes-hall |
Optical generation and detection of pure valley current in monolayer
transition metal dichalcogenides: We propose a practical scheme to generate a pure valley current in monolayer
transition metal dichalcogenides by one-photon absorption of linearly polarized
light. We show that the pure valley current can be detected by either
photoluminescence measurements or the ultrafast pump-probe technique. Our
method, together with the previously demonstrated generation of valley
polarization, opens up the exciting possibility of ultrafast optical-only
manipulation of the valley index. The tilted field effect on the valley current
in experiment is also discussed. | cond-mat_mes-hall |
Quantum oscillations observed in graphene at microwave frequencies: We have measured the microwave conductance of mechanically exfoliated
graphene at frequencies up to 8.5 GHz. The conductance at 4.2 K exhibits
quantum oscillations, and is independent of the frequency. | cond-mat_mes-hall |
Time Reversal Invariant Topologically Insulating Circuits: From studies of exotic quantum many-body phenomena to applications in
spintronics and quantum information processing, topological materials are
poised to revolutionize the condensed matter frontier and the landscape of
modern materials science. Accordingly, there is a broad effort to realize
topologically non-trivial electronic and photonic materials for fundamental
science as well as practical applications. In this work, we demonstrate the
first simultaneous site- and time- resolved measurements of a time reversal
invariant topological band-structure, which we realize in a radio frequency
(RF) photonic circuit. We control band-structure topology via local permutation
of a traveling wave capacitor-inductor network, increasing robustness by going
beyond the tight-binding limit. We observe a gapped density of states
consistent with a modified Hofstadter spectrum at a flux per plaquette of
$\phi=\pi/2$. In-situ probes of the band-gaps reveal spatially-localized
bulk-states and de-localized edge-states. Time-resolved measurements reveal
dynamical separation of localized edge-excitations into spin-polarized
currents. The RF circuit paradigm is naturally compatible with non-local
coupling schemes, allowing us to implement a M\"{o}bius strip topology
inaccessible in conventional systems. This room-temperature experiment
illuminates the origins of topology in band-structure, and when combined with
circuit quantum electrodynamics (QED) techniques, provides a direct path to
topologically-ordered quantum matter. | cond-mat_mes-hall |
Domain wall motion at low current density in a synthetic antiferromagnet
nanowire: The current-driven motion of magnetic domain walls (DWs) is the working
principle of magnetic racetrack memories. In this type of spintronic
technology, high current densities are used to propel DW motion in magnetic
nanowires, causing significant wire heating. Synthetic antiferromagnets are
known to show very fast DW motion at high current densities, but lower current
densities around onset of motion have received less attention. Here we use
scanning transmission x-ray microscopy to study the response of DWs in a SAF
multilayer to currents. We observe that the DWs depin at $\sim 3 \times
10^{11}$~A/m$^2$ and move more quickly in response to 5~ns duration current
pulses than in comparable conventional multilayers. The results suggest that
DWs in SAF structures are superior to conventional N\'{e}el DWs for low energy
consumption racetrack technologies. | cond-mat_mes-hall |
Persistent current of correlated electrons in mesoscopic ring with
impurity: The persistent current of correlated electrons in a continuous
one-dimensional ring with a single scatterer is calculated by solving the
many-body Schrodinger equation for several tens of electrons interacting via
the electron-electron (e-e) interaction of finite range. The problem is solved
by the configuration-interaction (CI) and diffusion Monte Carlo (DMC) methods.
The CI and DMC results are in good agreement. In both cases, the persistent
current $I$ as a function of the ring length $L$ exhibits the asymptotic
dependence $I \propto L^{-1-\alpha}$ typical of the Luttinger liquid, where the
power $\alpha$ depends only on the e-e interaction. The numerical values of
$\alpha$ agree with the known formula of the renormalisation-group theory. | cond-mat_mes-hall |
Current-induced forces in mesoscopic systems: a scattering matrix
approach: Nanoelectromechanical systems are characterized by an intimate connection
between electronic and mechanical degrees of freedom. Due to the nanoscopic
scale, current flowing through the system noticeably impacts the vibrational
dynamics of the device, complementing the effect of the vibrational modes on
the electronic dynamics. We employ the scattering matrix approach to quantum
transport to develop a unified theory of nanoelectromechanical systems out of
equilibrium. For a slow mechanical mode, the current can be obtained from the
Landauer-B\"uttiker formula in the strictly adiabatic limit. The leading
correction to the adiabatic limit reduces to Brouwer's formula for the current
of a quantum pump in the absence of the bias voltage. The principal result of
the present paper are scattering matrix expressions for the current-induced
forces acting on the mechanical degrees of freedom. These forces control the
Langevin dynamics of the mechanical modes. Specifically, we derive expressions
for the (typically nonconservative) mean force, for the (possibly negative)
damping force, an effective "Lorentz" force which exists even for time reversal
invariant systems, and the fluctuating Langevin force originating from Nyquist
and shot noise of the current flow. We apply our general formalism to several
simple models which illustrate the peculiar nature of the current-induced
forces. Specifically, we find that in out of equilibrium situations the current
induced forces can destabilize the mechanical vibrations and cause limit-cycle
dynamics. | cond-mat_mes-hall |
Microwave Experiments Simulating Quantum Search and Directed Transport
in Artificial Graphene: A series of quantum search algorithms have been proposed recently providing
an algebraic speedup compared to classical search algorithms from $N$ to
$\sqrt{N}$, where $N$ is the number of items in the search space. In
particular, devising searches on regular lattices has become popular in
extending Grover's original algorithm to spatial searching. Working in a
tight-binding setup, it could be demonstrated, theoretically, that a search is
possible in the physically relevant dimensions 2 and 3 if the lattice spectrum
possesses Dirac points. We present here a proof of principle experiment
implementing wave search algorithms and directed wave transport in a graphene
lattice arrangement. The idea is based on bringing localized search states into
resonance with an extended lattice state in an energy region of low spectral
density---namely, at or near the Dirac point. The experiment is implemented
using classical waves in a microwave setup containing weakly coupled dielectric
resonators placed in a honeycomb arrangement, i.e., artificial graphene.
Furthermore, we investigate the scaling behavior experimentally using linear
chains. | cond-mat_mes-hall |
In-plane gate single-electron transistor in Ga[Al]As fabricated by
scanning probe lithography: A single-electron transistor has been realized in a Ga[Al]As heterostructure
by oxidizing lines in the GaAs cap layer with an atomic force microscope. The
oxide lines define the boundaries of the quantum dot, the in-plane gate
electrodes, and the contacts of the dot to source and drain. Both the number of
electrons in the dot as well as its coupling to the leads can be tuned with an
additional, homogeneous top gate electrode. Pronounced Coulomb blockade
oscillations are observed as a function of voltages applied to different gates.
We find that, for positive top-gate voltages, the lithographic pattern is
transferred with high accuracy to the electron gas. Furthermore, the dot shape
does not change significantly when in-plane voltages are tuned. | cond-mat_mes-hall |
Plasmons in realistic graphene/hexagonal boron nitride moiré patterns: Van der Waals heterostructures employing graphene and hexagonal boron nitride
(hBN) crystals have emerged as a promising platform for plasmonics thanks to
the tunability of their collective modes with carrier density and record values
for plasmonics figures of merit. In this Article we investigate theoretically
the role of moir\'e-pattern superlattices in nearly aligned graphene on hBN by
using continuum-model Hamiltonians derived from ab initio calculations. We
calculate the system's energy loss function for a variety of chemical potential
values that are accessible in gated devices. Our calculations reveal that the
electron-hole asymmetry of the moir\'e bands leads to a remarkable asymmetry of
the plasmon dispersion between positive and negative chemical potentials,
showcasing the intricate band structure and rich absorption spectrum across the
secondary Dirac point gap for the hole bands. | cond-mat_mes-hall |
Non-equilibrium fractional quantum Hall states visualized by optically
detected MRI: Using photoluminescence microscopy enhanced by MRI, we visualize in real
space both electron and nuclear polarization occurring in non-equilibrium FQH
liquids. We observe stripe-like regions comprising FQH excited states which
discretely form when the FQH liquid is excited by a source-drain current. These
regions are topologically protected and deformable, and give rise to
bidirectionally polarized nuclear spins as spin-resolved electrons flow across
their boundaries. | cond-mat_mes-hall |
Directional emission of a deterministically fabricated quantum dot -
Bragg reflection multi-mode waveguide system: We report on the experimental study and numerical analysis of chiral
light-matter coupling in deterministically fabricated quantum dot (QD)
waveguide structures. We apply in-situ electron beam lithography to
deterministically integrate single InGaAs/GaAs QDs into GaAs-DBR waveguides to
systematically explore the dependence of chiral coupling on the position of the
QD inside the waveguide. By a series of micro-photoluminescence measurements,
we determine the directionality contrast of emission into left and right
traveling waveguide modes revealing a maximum of 0.93 for highly off-center QDs
and an oscillatory dependence of this contrast on the QD position. In numerical
simulations we obtain insight into chiral light-matter coupling by computing
the light field emitted by a circularly polarized source and its overlap with
multiple guided modes of the structure, which enables us to calculate
directional $\beta$-factors for the quantum emitters. The calculated dependence
of the directionality on the off-center QD position is in good agreement with
the experimental data. It confirms the control of chiral effects in
deterministically fabricated QD-waveguide systems with high potential for
future non-reciprocal on-chip systems required for quantum information
processing. | cond-mat_mes-hall |
Strong parametric coupling between two ultra-coherent membrane modes: We demonstrate parametric coupling between two modes of a silicon nitride
membrane. We achieve the coupling by applying an oscillating voltage to a sharp
metal tip that approaches the membrane surface to within a few 100 nm. When the
voltage oscillation frequency is equal to the mode frequency difference, the
modes exchange energy periodically and much faster than their free energy decay
rate. This flexible method can potentially be useful for rapid state control
and transfer between modes, and is an important step towards parametric spin
sensing experiments with membrane resonators. | cond-mat_mes-hall |
Room-temperature coherent optical manipulation of single-hole spins in
solution-grown perovskite quantum dots: Manipulation of solid-state spin coherence is an important paradigm for
quantum information processing. Current systems either operate at very low
temperatures or are difficult to scale-up. Developing low-cost, scalable
materials whose spins can be coherently manipulated at room temperature is thus
highly-attractive for a sustainable future of quantum information science. Here
we report ambient-condition all-optical initialization, manipulation and
readout of single-hole spins in an ensemble of solution-grown CsPbBr3
perovskite QDs. Single-hole spins are obtained by sub-picosecond electron
scavenging following a circularly-polarized femtosecond-pulse excitation. A
transversal magnetic field induces spin precession, and a second off-resonance
femtosecond-pulse coherently rotates hole spins via strong light-matter
interaction. These operations accomplish nearly complete quantum-state control
of single-hole spins at room temperature. | cond-mat_mes-hall |
Micromagnetic Simulation of Amorphous Ferrimagnetic TbFeCo Films with
Exchange Coupled Nanophases: Amorphous ferrimagnetic TbFeCo thin films are found to exhibit exchange bias
effect near the compensation temperature by magnetic hysteresis loop
measurement. The observed exchange anisotropy is believed to originate from the
exchange interaction between the two nanoscale amorphous phases distributed
within the films. Here, we present a computational model of phase-separated
TbFeCo using micromagnetic simulation. Two types of cells with different Tb
concentration are distributed within the simulated space to obtain a
heterogeneous structure consisting of two nanoscale amorphous phases. Each cell
contains separated Tb and FeCo components, forming two antiferromagnetically
coupled sublattices. Using this model, we are able to show the existence of
exchange bias effect, and the shift in hysteresis loops is in agreement with
experiment. The micromagnetic model developed herein for a heterogeneous
magnetic material may also account for some recent measurements of exchange
bias effect in crystalline films. | cond-mat_mes-hall |
Photogalvanic effect in Weyl semimetals: We theoretically study the impact of impurities on the photogalvanic effect
(PGE) in Weyl semimetals with weakly tilted Weyl cones. Our calculations are
based on a two-nodes model with an inversion symmetry breaking offset and we
employ a kinetic equation approach in which both optical transitions as well as
particle-hole excitations near the Fermi energy can be taken into account. We
focus on the parameter regime with a single photoactive node and control the
calculation in small impurity concentration. Internode scattering is treated
generically and therefore our results allow to continuously interpolate between
the cases of short range and long range impurities. We find that the time
evolution of the circular PGE may be nonmonotonic for intermediate internode
scattering. Furthermore, we show that the tilt vector introduces three
additional linearly independent components to the steady state photocurrent.
Amongst them, the photocurrent in direction of the tilt takes a particular role
inasmuch it requires elastic internode scattering or inelastic intranode
scattering to be relaxed. It may therefore be dominant. The tilt also generates
skew scattering which leads to a current component perpendicular to both the
incident light and the tilt. We extensively discuss our findings and comment on
the possible experimental implications. | cond-mat_mes-hall |
Topological Hofstadter Insulators in a Two-Dimensional Quasicrystal: We investigate the properties of a two-dimensional quasicrystal in the
presence of a uniform magnetic field. In this configuration, the density of
states (DOS) displays a Hofstadter butterfly-like structure when it is
represented as a function of the magnetic flux per tile. We show that the
low-DOS regions of the energy spectrum are associated with chiral edge states,
in direct analogy with the Chern insulators realized with periodic lattices. We
establish the topological nature of the edge states by computing the
topological Chern number associated with the bulk of the quasicrystal. This
topological characterization of the non-periodic lattice is achieved through a
local (real-space) topological marker. This work opens a route for the
exploration of topological insulating materials in a wide range of non-periodic
lattice systems, including photonic crystals and cold atoms in optical
lattices. | cond-mat_mes-hall |
True amplification of spin waves in magnonic nano-waveguides: Magnonic nano-devices exploit magnons -- quanta of spin waves -- to transmit
and process information within a single integrated platform that has the
potential to outperform traditional semiconductor-based electronics for low
power applications. The main missing cornerstone of this information
nanotechnology is an efficient scheme for the direct amplification of
propagating spin waves. The recent discovery of spin-orbit torque provided an
elegant mechanism for propagation losses compensation. While partial
compensation of the spin-wave damping has allowed for spin-wave signal
modulation, true amplification - the exponential increase in the spin-wave
intensity during propagation - has so far remained elusive. Here we evidence
the operating conditions to achieve unambiguous amplification using clocked
nanoseconds-long spin-orbit torque pulses in sub-micrometer wide magnonic
waveguides, where the effective magnetization has been engineered to be close
to zero to suppress the detrimental magnon-magnon scattering. As a result, we
achieve an exponential increase in the intensity of propagating spin waves up
to 500 % at a propagation distance of several micrometers. These results pave
the way towards the implementation of energy efficient, cascadable magnonic
architectures for wave-based information processing and complex on-chip
computation. | cond-mat_mes-hall |
Pair-wise decoherence in coupled spin qubit networks: Experiments involving phase coherent dynamics of networks of spins, such as
echo experiments, will only work if decoherence can be suppressed. We show
here, by analyzing the particular example of a crystalline network of Fe8
molecules, that most decoherence typically comes from pairwise interactions
(particularly dipolar interactions) between the spins, which cause `correlated
errors'. However at very low T these are strongly suppressed. These results
have important implications for the design of quantum information processing
systems using electronic spins. | cond-mat_mes-hall |
Hot electrons in a tunnel structure based on metal nanoclusters: We study the effect of temperature on the tunnel current in a structure based
on gold clusters taking into consideration their discrete electronic spectra.
We suggest that an overheating of electron subsystem leads to the disappearance
of a current gap and gradual smoothing of current--voltage curves that is
observed experimentally. | cond-mat_mes-hall |
Kondo Effect in a Many-Electron Quantum Ring: The Kondo effect is investigated in a many-electron quantum ring as a
function of magnetic field. For fields applied perpendicular to the plane of
the ring a modulation of the Kondo effect with the Aharonov-Bohm period is
observed. This effect is discussed in terms of the energy spectrum of the ring
and the parametrically changing tunnel coupling. In addition, we use gate
voltages to modify the ground-state spin of the ring. The observed splitting of
the Kondo-related zero-bias anomaly in this configuration is tuned with an
in-plane magnetic field. | cond-mat_mes-hall |
Spin-Hall Effect in A Symmetric Quantum Wells by A Random Rashba Field: Changes dopant ion concentrations in the sides of a symmetric quantum well
are known to create a random Rashba-type spin-orbit coupling. Here we
demonstrate that, as a consequence, a finite size spin-Hall effect is also
present. Our numerical algorithm estimates the result of the Kubo formula for
the spin-Hall conductivity, by using a tight-binding approximation of the
Hamiltonian in the framework of a time-dependent Green's function formalism,
well suited for very large systems. | cond-mat_mes-hall |
Coulomb drag in graphene near the Dirac point: We study Coulomb drag in double-layer graphene near the Dirac point. A
particular emphasis is put on the case of clean graphene, with transport
properties dominated by the electron-electron interaction. Using the quantum
kinetic equation framework, we show that the drag becomes $T$-independent in
the clean limit, $T\tau \to \infty$, where $T$ is temperature and $1/\tau$
impurity scattering rate. For stronger disorder (or lower temperature), $T\tau
\ll 1/\alpha^2$, where $\alpha$ is the interaction strength, the kinetic
equation agrees with the leading-order ($\alpha^2$) perturbative result. At
still lower temperatures, $T\tau \ll 1$ (diffusive regime) this contribution
gets suppressed, while the next-order ($\alpha^3$) contribution becomes
important; it yields a peak centered at the Dirac point with a magnitude that
grows with lowering $T\tau$. | cond-mat_mes-hall |
Hot electron cooling by acoustic phonons in graphene: We have investigated the energy loss of hot electrons in metallic graphene by
means of GHz noise thermometry at liquid helium temperature. We observe the
electronic temperature T / V at low bias in agreement with the heat diffusion
to the leads described by the Wiedemann-Franz law. We report on
$T\propto\sqrt{V}$ behavior at high bias, which corresponds to a T4 dependence
of the cooling power. This is the signature of a 2D acoustic phonon cooling
mechanism. From a heat equation analysis of the two regimes we extract accurate
values of the electron-acoustic phonon coupling constant $\Sigma$ in monolayer
graphene. Our measurements point to an important effect of lattice disorder in
the reduction of $\Sigma$, not yet considered by theory. Moreover, our study
provides a strong and firm support to the rising field of graphene bolometric
detectors. | cond-mat_mes-hall |
Micromagnetic view on ultrafast magnon generation by femtosecond spin
current pulses: In this Article we discuss a micromagnetic modelling approach to describe the
ultrafast spin-transfer torque excitation of coherent and incoherent magnons on
the nanoscale. Implementing the action of a femtosecond spin current pulse
entering an orthogonally magnetized thin ferromagnetic film, we reproduce
recent experimental results and reveal the factors responsible for the unequal
excitation efficiency of various spin waves. Our findings are in an excellent
agreement with the results of an analytical description of spin-wave excitation
based on classical kinetic equations. Furthermore, we suggest an experimental
design allowing for the excitation of laterally propagating spin waves beyond
the optical diffraction limit. Our findings demonstrate that the classical
micromagnetic picture retains its predictive and interpretative power on
femtosecond temporal and nanometer spatial scales. | cond-mat_mes-hall |
"Smoking gun" signatures of topological milestones in trivial materials
by measurement fine-tuning and data postselection: Exploring the topology of electronic bands is a way to realize new states of
matter with possible implications for information technology. Because bands
cannot always be observed directly, a central question is how to tell that a
topological regime has been achieved. Experiments are often guided by a
prediction of a unique signal or a pattern, called "the smoking gun". Examples
include peaks in conductivity, microwave resonances, and shifts in interference
fringes. However, many condensed matter experiments are performed on relatively
small, micron or nanometer-scale, specimens. These structures are in the
so-called mesoscopic regime, between atomic and macroscopic physics, where
phenomenology is particularly rich. In this paper, we demonstrate that the
trivial effects of quantum confinement, quantum interference and charge
dynamics in nanostructures can reproduce accepted smoking gun signatures of
triplet supercurrents, Majorana modes, topological Josephson junctions and
fractionalized particles. The examples we use correspond to milestones of
topological quantum computing: qubit spectroscopy, fusion and braiding. None of
the samples we use are in the topological regime. The smoking gun patterns are
achieved by fine-tuning during data acquisition and by subsequent data
selection to pick non-representative examples out of a fluid multitude of
similar patterns that do not generally fit the "smoking gun" designation.
Building on this insight, we discuss ways that experimentalists can rigorously
delineate between topological and non-topological effects, and the effects of
fine-tuning by deeper analysis of larger volumes of data. | cond-mat_mes-hall |
Liquid exfoliation of solvent-stabilised black phosphorus: applications
beyond electronics: Few layer black phosphorus is a new two-dimensional material which is of
great interest for applications, mainly in electronics. However, its lack of
stability severely limits our ability to synthesise and process this material.
Here we demonstrate that high-quality, few-layer black phosphorus nanosheets
can be produced in large quantities by liquid phase exfoliation in the solvent
N-cyclohexyl-2-pyrrolidone (CHP). We can control nanosheet dimensions and have
developed metrics to estimate both nanosheet size and thickness
spectroscopically. When exfoliated in CHP, the nanosheets are remarkably stable
unless water is intentionally introduced. Computational studies show the
degradation to occur by reaction with water molecules only at the nanosheet
edge, leading to the removal of phosphorus atoms and the formation of phosphine
and phosphorous acid. We demonstrate that liquid exfoliated black phosphorus
nanosheets are potentially useful in a range of applications from optical
switches to gas sensors to fillers for composite reinforcement. | cond-mat_mes-hall |
Diffusion of photo-excited holes in viscous electron fluid: The diffusion of photo-generated holes is studied in a high-mobility
mesoscopic GaAs\ channel where electrons exhibit hydrodynamic properties. It is
shown that the injection of holes into such an electron system leads to the
formation of a hydrodynamic three-component mixture consisted of electrons and
photo-generated heavy and light holes. The obtained results are analyzed within
the framework of ambipolar diffusion, which reveals characteristics of a
viscous flow. Both hole types exhibit similar hydrodynamic characteristics. In
such a way the diffusion lengths, ambipolar diffusion coefficient and the
effective viscosity of the electron-hole system are determined. | cond-mat_mes-hall |
Temperature effect in the conductance of hydrogen molecule: We present a many-body calculation for the conductance of a conducting bridge
of a simple hydrogen molecule between $Pt$ electrodes.The experimental results
showed that the conductance $G=dI/dV$ has the maximum value near the quantum
unit $G_{0}=2e^{2}/h$.
The $I-V$ dependence presents peak and dip and we consider that the
electron-phonon interaction is responsible for this behavior. At T=0 there is a
step in this dependence for the energy of phonons $\omega_{0}$ which satisfies
$eV=\omega_{0}$. We calculated the conductance at finite temperature and showed
that $dG(T)/dV\propto 1/4T\cosh^{2}\frac{eV-\omega_{0}}{2T}$. | cond-mat_mes-hall |
Current and Shot Noise in a Quantum Dot Coupled to Ferromagnetic Leads
in the Large U Limit: Using the Keldysh nonequilibrium Green function technique, we study the
current and shot noise spectroscopy of a single interacting quantum dot coupled
to two ferromagnetic leads with different polarizations. The polarizations of
leads can be both parallel and antiparallel alignments. General formulas of
current and shot noise are obtained, which can be applied in both the parallel
and antiparallel alignment cases. We show that for large polarization value,
the differential conductance and shot noise are completely diferent for spin up
and spin down configurations in the parallel alignment case. However, the
differential conductance and shot noise have the similar properties for
parallel alignment case in the small polarization value and for antiparallel
alignment case in any polarization value. | cond-mat_mes-hall |
Electron spin resonance on a 2-dimensional electron gas in a single AlAs
quantum well: Direct electron spin resonance (ESR) on a high mobility two dimensional
electron gas in a single AlAs quantum well reveals an electronic $g$-factor of
1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 Gauss. The
ESR amplitude and its temperature dependence suggest that the signal originates
from the effective magnetic field caused by the spin orbit-interaction and a
modulation of the electron wavevector caused by the microwave electric field.
This contrasts markedly to conventional ESR that detects through the microwave
magnetic field. | cond-mat_mes-hall |
Near-Field Radiative Heat Transfer between Drift-biased Graphene through
Nonreciprocal Surface Plasmons: In this Rapid Communication, we theoretically demonstrate that near-field
radiative heat transfer (NFRHT) can be modulated and enhanced by a new energy
transmission mode of evanescent wave, i.e. the nonreciprocal surface plasmons
polaritons (NSPPs). In addition to the well-known coupled surface plasmon
polaritons (SPPs), applying a drift current on a graphene sheet leads to an
extremely asymmetric photonic transmission model, which has never been noted in
the noncontact heat exchanges at nanoscale before. The coupling of plasmons in
the infrared bands dominates the NFRHT, associated with low loss (high loss and
ultrahigh confinement) traveling along (against) the current. The dependence of
NSPPs on the drift-current velocity as well as the vacuum gap is analyzed. It
is found that the coupling of NSPPs at smaller and larger gap sizes exhibits
different nonreciprocities. Finally, we also demonstrate that the prominent
influence of the drift current on the radiative heat flux is found at a low
chemical potential. These findings will open a new way to spectrally control
NFRHT, which holds great potential for improving the performance of energy
systems like near-field thermophotovoltaics and thermal modulator. | cond-mat_mes-hall |
What do noise measurements reveal about fractional charge in FQH
liquids?: We present a calculation of noise in the tunneling current through junctions
between two two-dimensional electron gases (2DEG) in inequivalent Laughlin
fractional quantum Hall (FQH) states, as a function of voltage and temperature.
We discuss the interpretation of measurements of suppressed shot noise levels
of tunneling currents through a quantum point contact (QPC) in terms of
tunneling of fractionally charged states. We show that although this
interpretation is always possible, for junctions between different FQH states
the fractionally charged states involved in the tunneling process are not the
Laughlin quasiparticles of the isolated FQH states that make up the junction,
and should be regarded instead as solitons of the coupled system. The charge of
the soliton is, in units of the electron charge, the harmonic average of the
filling fractions of the individual Laughlin states, which also coincides with
the saturation value of the differential conductance of the QPC. For the
especially interesting case of a QPC between states at filling fractions
$\nu=1$ and $\nu={{1/3}}$, we calculate the noise in the tunneling current
exactly for all voltages and temperatures and investigate the crossovers. These
results can be tested by noise experiments on $(1,{{1/3}})$ QPCs. We present a
generalization of these results for QPC's of arbitrary Laughlin fractions in
their weak and strong coupling regimes. We also introduce generalized Wilson
ratios for the noise in the shot and thermal limits. These ratios are universal
scaling functions of $V/T$ that can be measured experimentally in a general QPC
geometry. | cond-mat_mes-hall |
Sub-to-super-Poissonian photon statistics in cathodoluminescence of
color center ensembles in isolated diamond crystals: Impurity-vacancy centers in diamond offer a new class of robust photon
sources with versatile quantum properties. While individual color centers
commonly act as single-photon sources, their ensembles have been theoretically
predicted to have tunable photon-emission statistics. Importantly, the
particular type of excitation affects the emission properties of a color center
ensemble within a diamond crystal. While optical excitation favors
non-synchronized excitation of color centers within an ensemble, electron-beam
excitation can synchronize the emitters and thereby provides a control of the
second-order correlation function $g_2(0)$. In this letter, we demonstrate
experimentally that the photon stream from an ensemble of color centers can
exhibit $g_2(0)$ both above and below unity. Such a photon source based on an
ensemble of few color centers in a diamond crystal provides a highly tunable
platform for informational technologies operating at room temperature. | cond-mat_mes-hall |
Field-induced dissociation of two-dimensional excitons in
transition-metal dichalcogenides: Generation of photocurrents in semiconducting materials requires dissociation
of excitons into free charge carriers. While thermal agitation is sufficient to
induce dissociation in most bulk materials, an additional push is required to
induce efficient dissociation of the strongly bound excitons in monolayer
transition-metal dichalcogenides (TMDs). Recently, static in-plane electric
fields have proven to be a promising candidate. In the present paper, we
introduce a numerical procedure, based on exterior complex scaling, capable of
computing field-induced exciton dissociation rates for a wider range of field
strengths than previously reported in literature. We present both Stark shifts
and dissociation rates for excitons in various TMDs calculated within the
Mott-Wannier model. Here, we find that the field induced dissociation rate is
strongly dependent on the dielectric screening environment. Furthermore,
applying weak-field asymptotic theory (WFAT) to the Keldysh potential, we are
able to derive an analytical expression for exciton dissociation rates in the
weak-field region. | cond-mat_mes-hall |
Non-stationary effects in the coupled quantum dots influenced by the
electron-phonon interaction: We analyzed time evolution of the localized charge in the system of two
interacting single level quantum dots (QDs) coupled with the continuous
spectrum states in the presence of electron-phonon interaction.
We demonstrated that electron-phonon interaction leads to the increasing of
localized charge relaxation rate. We also found that several time scales with
different relaxation rates appear in the system in the case of non-resonant
tunneling between the dots. We revealed the formation of oscillations in the
filling numbers time evolution caused by the emission and adsorption processes
of phonons. | cond-mat_mes-hall |
Finite-size scaling effect on Néel temperature of antiferromagnetic
Cr$_2$O$_3$-(0001) films in an exchange-coupled heterostructure: The scaling of antiferromagnetic ordering temperature of corundum-type
chromia films have been investigated. N\'eel temperature $T_N$ was determined
from the effect of perpendicular exchange-bias on the magnetization of a
weakly-coupled adjacent ferromagnet. For a thick-film case, the validity of
detection is confirmed by a susceptibility measurement. Detection of $T_N$ was
possible down to 1-nm-thin chromia films. The scaling of ordering temperature
with thickness was studied using different buffering materials, and compared
with Monte-Carlo simulations. The spin-correlation length and the corresponding
critical exponent were estimated, and they were consistent between experimental
and simulation results. The spin-correlation length is an order of magnitude
less than cubic antiferromagnets. We propose that the difference is from the
change of number of exchange-coupling links in the two crystal systems. | cond-mat_mes-hall |
Spin-orbit fields in asymmetric (001) quantum wells: We measure simultaneously the in-plane electron g-factor and spin relaxation
rate in a series of undoped inversion-asymmetric (001)-oriented GaAs/AlGaAs
quantum wells by spin-quantum beat spectroscopy. In combination the two
quantities reveal the absolute values of both the Rashba and the Dresselhaus
coefficients and prove that the Rashba coefficient can be negligibly small
despite huge conduction band potential gradients which break the inversion
symmetry. The negligible Rashba coefficient is a consequence of the
'isomorphism' of conduction and valence band potentials in quantum systems
where the asymmetry is solely produced by alloy variations. | cond-mat_mes-hall |
Dirac-point engineering and topological phase transitions in honeycomb
optical lattices: We study the electronic structure and the phase diagram of non-interacting
fermions confined to hexagonal optical lattices. In the first part, we compare
the properties of Dirac points arising in the eigenspectrum of either honeycomb
or triangular lattices. Numerical results are complemented by analytical
equations for weak and strong confinements. In the second part we discuss the
phase diagram and the evolution of Dirac points in honeycomb lattices applying
a tight-binding description with arbitrary nearest-neighbor hoppings. With
increasing asymmetry between the hoppings the Dirac points approach each other.
At a critical asymmetry the Dirac points merge to open an energy gap, thus
changing the topology of the eigenspectrum. We analyze the trajectory of the
Dirac points and study the density of states in the different phases.
Manifestations of the phase transition in the temperature dependence of the
specific heat and in the structure factor are discussed. | cond-mat_mes-hall |
Shot noise of series quantum point contacts intercalating chaotic
cavities: Shot noise of series quantum point contacts forming a sequence of cavities in
a two dimensional electron gas are studied theoretically and experimentally.
Noise in such a structure originates from local scattering at the point
contacts as well as from chaotic motion of the electrons in the cavities. We
found that the measured shot noise is in reasonable agreement with our
theoretical prediction taking the cavity noise into account. | cond-mat_mes-hall |
Giga-Hertz quantized charge pumping in bottom gate defined InAs nanowire
quantum dots: Semiconducting nanowires (NWs) are a versatile, highly tunable material
platform at the heart of many new developments in nanoscale and quantum
physics. Here, we demonstrate charge pumping, i.e., the controlled transport of
individual electrons through an InAs NW quantum dot (QD) device at frequencies
up to $1.3\,$GHz. The QD is induced electrostatically in the NW by a series of
local bottom gates in a state of the art device geometry. A periodic modulation
of a single gate is enough to obtain a dc current proportional to the frequency
of the modulation. The dc bias, the modulation amplitude and the gate voltages
on the local gates can be used to control the number of charges conveyed per
cycle. Charge pumping in InAs NWs is relevant not only in metrology as a
current standard, but also opens up the opportunity to investigate a variety of
exotic states of matter, e.g. Majorana modes, by single electron spectroscopy
and correlation experiments. | cond-mat_mes-hall |
Analytical description of the 1s exciton linewidth
temperature-dependence in transition metal dichalcogenides: We obtain an analytical expression for the linewidth of the 1s-exciton as a
function of temperature in transition metal dichalcogenides. The total
linewidth, as function of temperature, is dominated by three contributions: (i)
the radiative decay (essentially temperature independent); (ii) the
phonon-induced intravalley scattering; (iii) the phonon-induced intervalley
scattering. Our approach uses a variational \emph{Ansatz} to solve the Wannier
equation allowing for an analytical treatment of the excitonic problem,
including rates of the decay dynamics. Our results are in good agreement with
experimental data already present in the literature and can be used to readily
predict the value of the total linewidth at any temperature in the broad class
of excitonic two-dimensional materials. | cond-mat_mes-hall |
Topological Number of Edge States: We show that the edge states of the four-dimensional class A system can have
topological charges, which are characterized by Abelian/non-Abelian monopoles.
The edge topological charges are a new feature of relations among theories with
different dimensions. From this novel viewpoint, we provide a non-Abelian
analogue of the TKNN number as an edge topological charge, which is defined by
an SU(2) 't Hooft-Polyakov BPS monopole through an equivalence to Nahm
construction. Furthermore, putting a constant magnetic field yields an edge
monopole in a non-commutative momentum space, where D-brane methods in string
theory facilitate study of edge fermions. | cond-mat_mes-hall |
Controllable Spin-Transfer Torque on an Antiferromagnet in a Dual
Spin-Valve: We consider current-induced spin-transfer torque on an antiferromagnet in a
dual spin-valve setup. It is demonstrated that a net magnetization may be
induced in the AFM by partially or completely aligning the sublattice
magnetizations via a current-induced spin-transfer torque. This effect occurs
for current densities ranging below 10$^6$ A/cm$^2$. The direction of the
induced magnetization in the AFM is shown to be efficiently controlled by means
of the magnetic configuration of the spin-valve setup, with the anti-parallell
configuration yielding the largest spin-transfer torque. Interestingly, the
magnetization switching time-scale $\tau_\text{switch}$ itself has a strong,
non-monotonic dependence on the spin-valve configuration. These results may
point toward new ways to incorporate AFMs in spintronic devices in order to
obtain novel types of functionality. | cond-mat_mes-hall |
Signatures of spin-orbit coupling in scanning gate conductance images of
electron flow from quantum point contacts: Electron flow through a quantum point contact in presence of spin-orbit
coupling is investigated theoretically in the context of the scanning gate
microscopy (SGM) conductance mapping. Although in the absence of the floating
gate the spin-orbit coupling does not significantly alter the conductance, we
find that the angular dependence of the SGM images of the electron flow at the
conductance plateaux is substantially altered as the spin-orbit interaction
mixes the orbital modes that enter the quantum point contact. The radial
interference fringes that are obtained in the SGM maps at conductance steps are
essentially preserved by the spin-orbit interaction as backscattering by the
tip preserves the electron spin although the effects of the mode mixing are
visible. | cond-mat_mes-hall |
Probing the potential landscape inside a two-dimensional electron-gas: We report direct observations of the scattering potentials in a
two-dimensional electron-gas using electron-beam diffaction-experiments. The
diffracting objects are local density-fluctuations caused by the spatial and
charge-state distribution of the donors in the GaAs-(Al,Ga)As heterostructures.
The scatterers can be manipulated externally by sample illumination, or by
cooling the sample down under depleted conditions. | cond-mat_mes-hall |
Mechanism for self-formation of periodic structures on a plastic polymer
surface using a nanosecond and femtosecond laser pulses: The high UV laser dose at 193 nm induces grooves on poly allyl diglycol
carbonate PADC (CR39) at normal irradiation. The spatial period exhibits to be
nearly invariant for azimuth and polar angles indicating a loose dependence on
the incident angles but the LIPSS (Laser-induced periodic surface structures)
are always parallel to the P polarization component of the incident beam. The
most common approach to explain LIPSS formation is related to the Sipe theory
which does not account for all the observed phenomena especially LIPSS with
periodicity larger than the laser wavelength. In fact the LIPSS is a multi
parameter mechanism based on surface rippling, acoustic modulation and laser
ablation and etc. In experiment with CR-39 polymer, laser irradiation produce a
very tiny melting layer of mixture of monomer due to depolymerization on the
surface and it seems capillary wave is responsible for grooves formation. | cond-mat_mes-hall |
Control of Plasmons in Topological Insulators via Local Perturbations: We use a fully quantum mechanical approach to demonstrate control of
plasmonic excitations in prototype models of topological insulators by
molecule-scale perturbations. Strongly localized surface plasmons are present
in the host systems, arising from the topologically non-trivial single-particle
edge states. A numerical evaluation of the RPA equations for the perturbed
systems reveals how the positions and the internal electronic structure of the
added molecules affect the degeneracy of the locally confined collective
excitations, i.e., shifting the plasmonic energies of the host system and
changing their spatial charge density profile. In particular, we identify
conditions under which significant charge transfer from the host system to the
added molecules occurs. Furthermore, the induced field energy density in the
perturbed topological systems due to external electric fields is determined. | cond-mat_mes-hall |
One-Dimensional Luttinger Liquids in a Two-Dimensional Moiré Lattice: The Luttinger liquid (LL) model of one-dimensional (1D) electronic systems
provides a powerful tool for understanding strongly correlated physics
including phenomena such as spin-charge separation. Substantial theoretical
efforts have attempted to extend the LL phenomenology to two dimensions (2D),
especially in models of closely packed arrays of 1D quantum wires, each being
described as a LL. Such coupled-wire models have been successfully used to
construct 2D anisotropic non-Fermi liquids, quantum Hall states, topological
phases, and quantum spin liquids. However, an experimental demonstration of
high-quality arrays of 1D LLs suitable for realizing these models remains
absent. Here we report the experimental realization of 2D arrays of 1D LLs with
crystalline quality in a moir\'e superlattice made of twisted bilayer tungsten
ditelluride (tWTe$_{2}$). Originating from the anisotropic lattice of the
monolayer, the moir\'e pattern of tWTe$_{2}$ hosts identical, parallel 1D
electronic channels, separated by a fixed nanoscale distance, which is tunable
by the interlayer twist angle. At a twist angle of ~ 5 degrees, we find that
hole-doped tWTe$_{2}$ exhibits exceptionally large transport anisotropy with a
resistance ratio of ~ 1000 between two orthogonal in-plane directions. The
across-wire conductance exhibits power-law scaling behaviors, consistent with
the formation of a 2D anisotropic phase that resembles an array of LLs. Our
results open the door for realizing a variety of correlated and topological
quantum phases based on coupled-wire models and LL physics. | cond-mat_mes-hall |
Anomalous Nernst effect and field-induced Lifshitz transition in Weyl
semimetals TaP and TaAs: The discovery of Weyl fermions in transition metal monoarsenides/phosphides
without inversion symmetry represents an exceptional breakthrough in modern
condensed matter physics. However, exploring the inherent nature of these
quasiparticles is experimentally elusive because most of the experimental
probes rely on analysing Fermi arc topology or controversial signatures such as
the appearance of the chiral anomaly and the giant magnetoresistance. Here we
show that the prototypical type-I Weyl semimetals TaP and TaAs possess a giant
anomalous Nernst signal with a characteristic saturation plateau beyond a
critical field which can be understood as a direct consequence of the finite
Berry curvature originating from the Weyl points. Our results thus promote the
Nernst coefficient as an ideal bulk probe for detecting and exploring the
fingerprints of emergent Weyl physics. | cond-mat_mes-hall |
Tunnel magnetoresistance and temperature related effects in magnetic
tunnel junctions with embedded nanoparticles: Temperature dependence of the tunnel magnetoresistance (TMR) was calculated
in range of the quantum-ballistic model in the magnetic tunnel junctions (MTJs)
with embedded nanoparticles (NPs). The electron tunnel transport through NP was
simulated in range of double barrier approach, which was integrated into the
model of the magnetic point-like contact. The resonant TMR conditions and
temperature impact were explored in detail. Moreover, the possible reasons of
the temperature induced resonant conditions were discussed in the range of the
lead-tunneling cell-lead model near Kondo temperature. We also found that
redistribution of the voltage drop becomes crucial in this model. Furthermore,
the direct tunneling plays the dominant role and cannot be omitted in the
quantum systems with the total tunneling thickness up to 5-6 nm. Hence, Coulomb
blockade model cannot explain Kondo-induced TMR anomalies in nanometer-sized
tunnel junctions. | cond-mat_mes-hall |
Compensation of the Kondo effect in quantum dots coupled to
ferromagnetic leads within equation of motion approach: We propose a new approximation scheme within equation of motion approach
(EOM) to spin polarized transport through a quantum dot coupled to
ferromagnetic leads. It has some advantages over a widely used in the
literature standard EOM technique, in particular when we are interested in spin
polarized quantities. Namely, it gives the values of the dot spin polarization
which are closer to the ones obtained within numerical renormalization group
(NRG), than the standard EOM approach. While restoring the Kondo effect, the
spin polarization vanishes and the transport becomes unpolarized, in agreement
with NRG and a real time diagrammatic calculations. The standard EOM procedure
gives nonzero values of the spin polarization, and the transport is still spin
polarized. Both approximations give the same correct splitting of the Kondo
peaks due to ferromagnetism in the electrodes. | cond-mat_mes-hall |
Semiconductor quantum well irradiated by a two-mode electromagnetic
field as a terahertz emitter: We study theoretically the nonlinear optical properties of a semiconductor
quantum well (QW) irradiated by a two-mode electromagnetic wave consisting of a
strong resonant dressing field and a weak off-resonant driving field. In the
considered strongly coupled electron-field system, the dressing field opens
dynamic Stark gaps in the electron energy spectrum of the QW, whereas the
driving field induces electron oscillations in the QW plane. Since the gapped
electron spectrum restricts the amplitude of the oscillations, the emission of
a frequency comb from the QW appears. Therefore, the doubly-driven QW operates
as a nonlinear optical element which can be used, particularly, for optically
controlled generation of terahertz radiation. | cond-mat_mes-hall |
Mapping Spin Interactions from Conductance Peak Splitting in Coulomb
Blockade: We investigate the transport properties of a quantum dot coupled to leads
interacting with a multi-spin system using the generalized master equation
within the Coulomb blockade regime. We find that if two states for each
scattering region electron manifold are included, several signatures of the
interacting spin system appear in steady-state transport properties. We provide
a theoretical mapping of differential conductance peak signatures and all spin
Hamiltonian parameters related to the inclusion of excited state transitions
between uncharged and charged electron manifolds. Our predictions describe a
scheme of only using a quantum dot and differential conductance to measure
magnetic anisotropy, inter-spin exchange coupling, exchange coupling between
the spin system and itinerant electron, and applied magnetic field response. | cond-mat_mes-hall |
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