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Quantum Hall States in Graphene from Strain-Induced Nonuniform Magnetic
Fields: We examine strain-induced quantized Landau levels in graphene. Specifically,
arc-bend strains are found to cause nonuniform pseudomagnetic fields. Using an
effective Dirac model which describes the low-energy physics around the nodal
points, we show that several of the key qualitative properties of graphene in a
strain-induced pseudomagnetic field are different compared to the case of an
externally applied physical magnetic field. We discuss how using different
strain strengths allows us to spatially separate the two components of the
pseudospinor on the different sublattices of graphene. These results are
checked against a tight-binding calculation on the graphene honeycomb lattice,
which is found to exhibit all the features described. Furthermore, we find that
introducing a Hubbard repulsion on the mean-field level induces a measurable
polarization difference between the A and the B sublattices, which provides an
independent experimental test of the theory presented here. | cond-mat_mes-hall |
Euler equation of the optimal trajectory for the fastest magnetization
reversal of nano-magnetic structures: Based on the modified Landau-Lifshitz-Gilbert equation for an arbitrary
Stoner particle under an external magnetic field and a spin-polarized electric
current, differential equations for the optimal reversal trajectory, along
which the magnetization reversal is the fastest one among all possible reversal
routes, are obtained. We show that this is a Euler-Lagrange problem with
constrains. The Euler equation of the optimal trajectory is useful in designing
a magnetic field pulse and/or a polarized electric current pulse in
magnetization reversal for two reasons. 1) It is straightforward to obtain the
solution of the Euler equation, at least numerically, for a given magnetic
nano-structure characterized by its magnetic anisotropy energy. 2) After
obtaining the optimal reversal trajectory for a given magnetic nano-structure,
finding a proper field/current pulse is an algebraic problem instead of the
original nonlinear differential equation. | cond-mat_mes-hall |
All Spin Logic device with inbuilt Non-Reciprocity: The need for low power alternatives to digital electronic circuits has led to
increasing interest in logic devices where information is stored in
nanomagnets. This includes both nanomagnetic logic (NML) where information is
communicated through magnetic fields of nanomagnets and all-spin logic (ASL)
where information is communicated through spin currents. A key feature needed
for logic implementation is non-reciprocity, whereby the output is switched
according to the input but not the other way around, thus providing directed
information transfer. The objective of this paper is to draw attention to
possible ASL-based schemes that utilize the physics of spin-torque to build in
non-reciprocity similar to transistors that could allow logic implementation
without the need for special clocking schemes. We use an experimentally
benchmarked coupled spin-transport/ magnetization-dynamics model to show that a
suitably engineered single ASL unit indeed switches in a non-reciprocal manner.
We then present heuristic arguments explaining the origin of this directed
information transfer. Finally we present simulations showing that individual
ASL devices with inbuilt directionality can be cascaded to construct circuits. | cond-mat_mes-hall |
Dynamic screening of quasiparticles in WS$_2$ monolayers: We unravel the influence of quasiparticle screening in the non-equilibrium
exciton dynamics of monolayer WS$_2$. We report pump photon energy-dependent
exciton blue and red-shifts from time-resolved-reflectance contrast
measurements. Based on a phenomenological model, we isolate the effective
impact of excitons and free carriers on the renormalization of the quasi-free
particle band gap, exciton binding energy and linewidth broadening. By this,
our work does not only provide a comprehensive picture of the competing
phenomena governing the exciton dynamics in WS$_2$ upon photoexcitation, but
also demonstrates that exciton and carrier contributions to dynamic screening
of the Coulomb interaction differ significantly. | cond-mat_mes-hall |
Spaser as Nanoscale Quantum Generator and Ultrafast Amplifier: Nanoplasmonics has recently experienced explosive development with many novel
ideas and dramatic achievements in both fundamentals and applications. The
spaser has been predicted and observed experimentally as an active element --
generator of coherent local fields. Even greater progress will be achieved if
the spaser could function as a ultrafast nanoamplifier -- an optical
counterpart of the MOSFET (metal-oxide-semiconductor field-effect transistor).
A formidable problem with this is that the spaser has the inherent feedback
causing quantum generation of nanolocalized surface plasmons and saturation and
consequent elimination of the net gain, making it unsuitable for amplification.
We have overcome this inherent problem and shown that the spaser can perform
functions of an ultrafast nanoamplifier in two modes: transient and bistable.
On the basis of quantum density matrix (optical Bloch) equations we have shown
that the spaser amplifies with gain greater than 50, the switching time less or
on the order of 100 fs (potentially, 10 fs). This prospective spaser technology
will further broaden both fundamental and applied horizons of nanoscience, in
particular, enabling ultrafast microprocessors working at 10 to 100 THz clock
speed. Other prospective applications are in ultrasensing, ultradense and
ultrafast information storage, and biomedicine. The spasers are based on metals
and, in contrast to semiconductors, are highly resistive to ionizing radiation,
high temperatures, microwave radiation, and other adverse environments. | cond-mat_mes-hall |
Circulating persistent current and induced magnetic field in a fractal
network: We present the overall conductance as well as the circulating currents in
individual loops of a Sierpinski gasket (SPG) as we apply bias voltage via the
side attached electrodes. SPG being a self-similar structure, its manifestation
on loop currents and magnetic fields are examined in various generations of
this fractal and it has been observed that for a given configuration of the
electrodes, the physical quantities exhibit certain regularity as we go from
one generation to another. Also a notable feature is the introduction of
anisotropy in hopping causes an increase in magnitude of overall transport
current. These features are a subject of interest in this article. | cond-mat_mes-hall |
Ellipsometry studies of Si/Ge superlattices with embedded Ge dots: In this paper, we present an analysis for treating the spectroscopic
ellipsometry response of Si/Ge superlattices (SL) with embedded Ge dots.
Spectroscopic ellipsometry (SE) measurement at room temperature was used to
investigate optical and electronic properties of Si/Ge SL which were grown on
silicon (Si) wafers having <111> crystallographic orientation. The results of
the SE analysis between 200 nm and 1000 nm indicate that the SL system can
effectively be described using interdiffusion/intermixing model by assuming a
multicrystalline Si and Si1-xGex intermixing layers. The electronic transitions
deduced from analysis reveal Si, Ge and alloying related critical energy
points. | cond-mat_mes-hall |
Conductance of a quantum point contact based on spin-density-functional
theory: We present full quantum mechanical conductance calculations of a quantum
point contact (QPC) performed in the framework of the density functional theory
(DFT) in the local spin-density approximation (LDA). We show that a
spin-degeneracy of the conductance channels is lifted and the total conductance
exhibits a broad plateau-like feature at 0.5*2e^{2}/h. The lifting of the
spin-degeneracy is a generic feature of all studied QPC structures (both very
short and very long ones; with the lengths in the range 40<l<500 nm). The
calculated conductance also shows a hysteresis for forward- and backward sweeps
of the gate voltage. These features in the conductance can be traced to the
formation of weakly coupled quasi-bound states (magnetic impurities) inside the
QPC (also predicted in previous DFT-based studies). A comparison of obtained
results with the experimental data shows however, that while the spin-DFT based
"first-principle" calculations exhibits the spin polarization in the QPC, the
calculated conductance clearly does not reproduce the 0.7 anomaly observed in
almost all QPCs of various geometries. We critically examine major features of
the standard DFT-based approach to the conductance calculations and argue that
its inability to reproduce the 0.7 anomaly might be related to the infamous
derivative discontinuity problem of the DFT leading to spurious
self-interaction errors not corrected in the standard LDA. Our results indicate
that the formation of the magnetic impurities in the QPC might be an artefact
of the LDA when localization of charge is expected to occur. We thus argue that
an accurate description of the QPC structure would require approaches that go
beyond the standard DFT+LDA schemes. | cond-mat_mes-hall |
Order from Disorder in Graphene Quantum Hall Ferromagnet: Valley-polarized quantum Hall states in graphene are described by a
Heisenberg O(3) ferromagnet model, with the ordering type controlled by the
strength and sign of valley anisotropy. A mechanism resulting from electron
coupling to strain-induced gauge field, giving leading contribution to the
anisotropy, is described in terms of an effective random magnetic field aligned
with the ferromagnet z axis. We argue that such random field stabilizes the XY
ferromagnet state, which is a coherent equal-weight mixture of the $K$ and $K'$
valley states. Other implications such as the Berezinskii-Kosterlitz-Thouless
ordering transition and topological defects with half-integer charge are
discussed. | cond-mat_mes-hall |
Intrinsic Spin Swapping: Here, we study diffusive spin transport in two dimensions and demonstrate
that an intrinsic analog to a previously predicted extrinsic spin swapping
effect, where the spin polarization and the direction of flow are interchanged
due to spin-orbit coupling at extrinsic impurities, can be induced by intrinsic
(Rashba) spin-orbit coupling. The resulting accumulation of intrinsically
spin-swapped polarizations is shown to be much larger than for the extrinsic
effect. Intrinsic spin swapping is particularly strong when the system
dimensions exceed the spin-orbit precession length and the generated transverse
spin currents are of the order of the injected primary spin currents. In
contrast, spin accumulations and spin currents caused by extrinsic spin
swapping are proportional to the spin-orbit coupling. We present numerical and
analytical results for the secondary spin currents and accumulations generated
by intrinsic spin swapping, and we derive analytic expressions for the induced
spin accumulation at the edges of a narrow strip, where a long-range
propagation of spin polarizations takes place. | cond-mat_mes-hall |
Local density of states in disordered graphene: We study two lattice models, the honeycomb lattice (HCL) and a special square
lattice (SQL), both reducing to the Dirac equation in the continuum limit. In
the presence of disorder (gaussian potential disorder and random vector
potential), we investigate the behaviour of the density of states (DOS)
numerically and analytically. While an upper bound can be derived for the DOS
on the SQL at the Dirac point, which is also confirmed by numerical
calculations, no such upper limit exists for the HCL in the presence of random
vector potential. A careful investigation of the lowest eigenvalues indeed
indicate, that the DOS can possibly be divergent at the Dirac point on the HCL.
In spite of sharing a common continuum limit, these lattice models exhibit
different behaviour. | cond-mat_mes-hall |
Quantum interference and contact effects in dangling bond loops on
H-Si(100) surfaces: We perform electronic structure and quantum transport studies of dangling
bond loops created on H-passivated Si(100) surfaces and connected to carbon
nanoribbon leads. We model loops with straight and zigzag topologies as well as
with varying lenght with an efficient density-functional based tight-binding
electronic structure approach (DFTB) . Varying the length of the loop or the
lead coupling position we induce the drastic change in the transmission due to
the electron interference. Depending if the constructive or destructive
interference within the loop takes place we can noticeably change transport
properties by few orders of magnitude. These results propose a way to engineer
the closed electronically driven nanocircuits with high transport properties
and exploit the interference effects in order to control them. | cond-mat_mes-hall |
Phase biasing of a Josephson junction using Rashba-Edelstein effect: Manifestation of orbital coupling of spin degree of freedom in condensed
matter systems has opened up a new dimension for the field of spintronics. The
most appealing aspect of the spin-orbit coupling is the apparent Magnus force
sensed by a spin system which locks the Fermi momentum with electron spin in a
fascinating manner. In the current carrying state, the resulting macroscopic
spin polarization becomes directly accessible in the form of spin current or
spin density. At a Rashba interface, for example, a charge current shifts the
spin-locked Fermi surface, leading to a non-equilibrium spin density at the
interface, commonly known as the Rashba-Edelstein effect. Since the
Rashba-Edelstein effect is an intrinsically interface property, direct
detection of the spin moment is harder to set-up. Here we demonstrate that a
simple planar Josephson Junction geometry, realized by placing two closely
spaced superconducting electrodes on such a Rashba interface, allows a direct
estimation of strength of the non-equilibrium spin moment. Measurements of
Fraunhofer patterns of Nb-(Pt/Cu)-Nb planar Josephson junctions in a
perpendicular magnetic field showed a shift of the center of the Fraunhofer
pattern to a non-zero field value. By performing extensive control
measurements, we argue that the screening currents in the junction effectively
lock the external field with the spin moment of the Rashba-Edelstein effect
induced spin-density, leading to the observed shift in the Fraunhofer patterns.
This simple experiment offers a fresh perspective on direct detection of spin
polarization induced by various spin-orbit effects. Very interestingly, this
device platform also offers the possibility of retaining a controllable phase
at zero field in the junction without using any magnetic material, and thereby
useful as phase batteries for superconducting quantum circuits. | cond-mat_mes-hall |
Bulk-edge correspondence of Stiefel-Whitney and Euler insulators through
the entanglement spectrum and cutting procedure: We propose an unconventional bulk-edge correspondence for two-dimensional
Stiefel-Whitney insulators and Euler insulators, which are topological
insulators protected by the $PT$ symmetry. We find that, although the energy
spectrum under the open boundary condition is generally gapped, the
entanglement spectrum is gapless when the Stiefel-Whitney or Euler class is
nonzero. The robustness of the gapless spectrum for Stiefel-Whitney insulator
can be understood through an emergent anti-unitary particle-hole symmetry. For
the Euler insulators, we propose a conjecture, which is supported by our
numerical calculation, that the Euler class is equal to the number of crossing
in the entanglement spectrum, taking into account the degree of the crossings.
We also discuss that these crossings of the entanglement spectrum are related
to the gap closing points in the cutting procedure, which is the energy
spectrum as the magnitude of the boundary hopping is varied. | cond-mat_mes-hall |
Origin of the hysteresis in bilayer 2D systems in the quantum Hall
regime: The hysteresis observed in the magnetoresistance of bilayer 2D systems in the
quantum Hall regime is generally attributed to the long time constant for
charge transfer between the 2D systems due to the very low conductivity of the
quantum Hall bulk states. We report electrometry measurements of a bilayer 2D
system that demonstrate that the hysteresis is instead due to non-equilibrium
induced current. This finding is consistent with magnetometry and electrometry
measurements of single 2D systems, and has important ramifications for
understanding hysteresis in bilayer 2D systems. | cond-mat_mes-hall |
Observation of Hysteretic Transport Due to Dynamic Nuclear Spin
Polarization in a GaAs Lateral Double Quantum Dot: We report a new transport feature in a GaAs lateral double quantum dot that
emerges only for magnetic field sweeps and shows hysteresis due to dynamic
nuclear spin polarization (DNP). This DNP signal appears in the Coulomb
blockade regime by virtue of the finite inter-dot tunnel coupling and
originates from the crossing between ground levels of the spin triplet and
singlet extensively used for nuclear spin manipulations in pulsed gate
experiments. The unexpectedly large signal intensity is suggestive of
unbalanced DNP between the two dots, which opens up the possibility of
controlling electron and nuclear spin states via DC transport. | cond-mat_mes-hall |
Entangling electrons by splitting Cooper pairs: Two-particle conductance
resonance and time coincidence measurements: Entanglement, being at the heart of the Einstein-Podolsky-Rosen (EPR)
paradox, is a necessary ingredient in processing quantum information. Cooper
pairs in superconductors - being composites of two fully entangled electrons -
can be split adiabatically, thus forming entangled electrons. We fabricated
such electron splitter by contacting an aluminum superconductor strip at the
center of a suspended InAs nanowire; terminated at both ends with two normal
metallic drains. Intercepting each half of the nanowire by gate - induced
Coulomb blockaded quantum dot strongly impeded the flow of Cooper pairs due to
large charging energy, while still permitting passage of single electrons.
Here, we provide conclusive evidence of extremely high efficiency Cooper pairs
splitting via observing positive average (conductance) and time (shot noise)
correlations of the split electrons in the two opposite drains of the nanowire.
Moreover, The actual charge of the injected quasiparticles was verified by shot
noise measurements. | cond-mat_mes-hall |
Anyonic order parameters for discrete gauge theories on the lattice: We present a new family of gauge invariant non-local order parameters for
(non-abelian) discrete gauge theories on a Euclidean lattice, which are in
one-to-one correspondence with the excitation spectrum that follows from the
representation theory of the quantum double D(H) of the finite group H.
These combine magnetic flux-sector labeled by a conjugacy class with an
electric representation of the centralizer subgroup that commutes with the
flux. In particular cases like the trivial class for magnetic flux, or the
trivial irrep for electric charge, these order parameters reduce to the
familiar Wilson and the 't Hooft operators respectively. It is pointed out that
these novel operators are crucial for probing the phase structure of a class of
discrete lattice models we define, using Monte Carlo simulations. | cond-mat_mes-hall |
Strain engineering the charged-impurity-limited carrier mobility in
phosphorene: We investigate, based on the tight-binding model and in the linear
deformation regime, the strain dependence of the electronic band structure of
phosphorene, exposed to a uniaxial strain in one of its principle directions,
the normal, the armchair and the zigzag directions. We show that the electronic
band structure of strained phosphorene, for experimentally accessible carrier
densities and uniaxial strains, is well described by a strain-dependent
decoupled electron-hole Hamiltonian. Then, employing the decoupled Hamiltonian,
we consider the strain dependence of the charged-impurity-limited carrier
mobility in phosphorene, for both types of carrier, arbitrary carrier density
and in both armchair and zigzag directions. We show that a uniaxial tensile
(compressive) strain in the normal direction enhances (weakens) the anisotropy
of the carrier mobility, while a uniaxial strain in the zigzag direction acts
inversely. Moreover applying a uniaxial strain in the armchair direction is
shown to be ineffective on the anisotropy of the carrier mobility. These will
be explained based on the effect of the strain on the carrier effective mass. | cond-mat_mes-hall |
Broadband terahertz probes of anisotropic magnetoresistance disentangle
extrinsic and intrinsic contributions: Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of
magnetic order in contemporary spintronics research. Its origins are usually
ascribed to extrinsic effects (i.e. spin-dependent electron scattering),
whereas intrinsic (i.e. scattering-independent) contributions are neglected.
Here, we measure AMR of polycrystalline thin films of the standard ferromagnets
Co, Ni, Ni81Fe19 and Ni50Fe50 over the frequency range from DC to 28 THz. The
large bandwidth covers the regimes of both diffusive and ballistic intraband
electron transport and, thus, allows us to separate extrinsic and intrinsic AMR
components. Analysis of the THz response based on Boltzmann transport theory
reveals that the AMR of the Ni, Ni81Fe19 and Ni50Fe50 samples is of
predominantly extrinsic nature. However, the Co thin film exhibits a sizeable
intrinsic AMR contribution, which is constant up to 28 THz and amounts to more
than 2/3 of the DC AMR contrast of 1%. These features are attributed to the
hexagonal structure of the Co crystallites. They are interesting for
applications in terahertz spintronics and terahertz photonics. Our results show
that broadband terahertz electromagnetic pulses provide new and contact-free
insights into magneto-transport phenomena of standard magnetic thin films on
ultrafast time scales. | cond-mat_mes-hall |
Validity of the Lowest Landau Level Approximation for Rotating Bose
Gases: The energy spectrum for an ultracold rotating Bose gas in a harmonic trap is
calculated exactly for small systems, allowing the atoms to occupy several
Landau levels. Two vortex-like states and two strongly correlated states (the
Pfaffian and Laughlin) are considered in detail. In particular, their critical
rotation frequencies and energy gaps are determined as a function of particle
number, interaction strength, and the number of Landau levels occupied (up to
three). For the vortex-like states, the Lowest Landau level (LLL) approximation
is justified only if the interaction strength decreases with the number of
particles; nevertheless, the constant of proportionality increases rapidly with
the angular momentum per particle. For the strongly correlated states, however,
the interaction strength can increase with particle number without violating
the LLL condition. The results suggest that in large systems, the Pfaffian and
Laughlin states might be stabilized at rotation frequencies below the
centrifugal limit for sufficiently large interaction strengths, with energy
gaps a significant fraction of the trap energy. | cond-mat_mes-hall |
Frequency-Dependent Current Noise through Quantum-Dot Spin Valves: We study frequency-dependent current noise through a single-level quantum dot
connected to ferromagnetic leads with non-collinear magnetization. We propose
to use the frequency-dependent Fano factor as a tool to detect single-spin
dynamics in the quantum dot. Spin precession due to an external magnetic and/or
a many-body exchange field affects the Fano factor of the system in two ways.
First, the tendency towards spin-selective bunching of the transmitted
electrons is suppressed, which gives rise to a reduction of the low-frequency
noise. Second, the noise spectrum displays a resonance at the Larmor frequency,
whose lineshape depends on the relative angle of the leads' magnetizations. | cond-mat_mes-hall |
The two dimensional local density of states of a Topological Insulator
with an edge dislocation: We investigate the effect of a crystal edge dislocation on the metallic
surface of a Topological Insulator. The edge dislocation gives rise to torsion
which the electrons experience as a spin connection. As a result the electrons
propagate along confined two dimensional regions and circular contours. Due to
the edge dislocations the parity symmetry is violated resulting in a current
measured by the in-plane component of the spin on the surface.
The tunneling density of states for Burger vectors in the $y$ direction is
maximal along the $x$ direction. The evidence of the enhanced tunneling density
of states can be verified with the help of the scanning tunneling technique. | cond-mat_mes-hall |
Temperature dependence of spin polarization in ferromagnetic metals
using lateral spin valves: A high reproducibility in the performance of cobalt/copper and
permalloy/copper lateral spin valves with transparent contacts is obtained by
optimizing the interface quality and the purity of copper. This allows us to
study comprehensively the spin injection properties of both ferromagnetic
materials, as well as the spin transport properties of copper, which are not
affected by the used ferromagnetic material, leading to long spin diffusion
lengths. Spin polarizations of permalloy and cobalt are obtained as a function
of temperature. Analysis of the temperature dependence of both the spin
polarization and conductivity of permalloy using the standard two-channel model
for ferromagnetic metals suggests that a correction factor of ~2 is needed for
the spin polarization values obtained by lateral spin valve experiments. | cond-mat_mes-hall |
Exact solution for the dynamical decoupling of a qubit with telegraph
noise: We study the dissipative dynamics of a qubit that is afflicted by classical
random telegraph noise and it is subject to dynamical decoupling. We derive
exact formulas for the qubit dynamics at arbitrary working points in the limit
of infinitely strong control pulses (bang-bang control) and we investigate in
great detail the efficiency of the dynamical decoupling techniques both for
Gaussian and non-Gaussian (slow) noise at qubit pure dephasing and at optimal
point. We demonstrate that control sequences can be successfully implemented as
diagnostic tools to infer spectral proprieties of a few fluctuators interacting
with the qubit. The analysis is extended in order to include the effect of
noise in the pulses and we give upper bounds on the noise levels that can be
tolerated in the pulses while still achieving efficient dynamical decoupling
performance. | cond-mat_mes-hall |
The transmission spectra of the graphene-based Fibonacci superlattice: We consider the gapped graphene superlattice (SL) constructed in accordance
with the Fibonacci rule. Quasi-periodic modulation is due to the difference in
the values of the energy gap in different SL elements. It is shown that the
effective splitting of the allowed bands and thereby forming a series of gaps
is realized under the normal incidence of electrons on the SL as well as under
oblique incidence. Energy spectra reveal periodical character on the whole
energy scale. The splitting of allowed bands is subjected to the inflation
Fibonacci rule. The gap associated with the new Dirac point is formed in every
Fibonacci generation. The location of this gap is robust against the change in
the SL period but at the same time it is sensitive to the ratio of barrier and
well widths; also it is weakly dependent on values of the mass term in the
Hamiltonian. | cond-mat_mes-hall |
Chaos in two-dimensional Kepler problem with spin-orbit coupling: We consider classical two-dimensional Kepler system with spin-orbit coupling
and show that at a sufficiently strong coupling it demonstrates a chaotic
behavior. The chaos emerges since the spin-orbit coupling reduces the number of
the integrals of motion as compared to the number of the degrees of freedom.
This reduction is manifested in the equations of motion as the emergence of the
anomalous velocity determined by the spin orientation. By using analytical and
numerical arguments, we demonstrate that the chaotic behavior, being driven by
this anomalous term, is related to the system energy dependence on the initial
spin orientation. We observe the critical dependence of the dynamics on the
initial conditions, where system can enter and exit a stability domain by very
small changes in the initial spin orientation. Thus, this system can
demonstrate a reentrant order-from-disorder transition driven by very small
variations in the initial conditions. | cond-mat_mes-hall |
Entanglement spectrum and symmetries in non-Hermitian fermionic
non-interacting models: We study the properties of the entanglement spectrum in gapped
non-interacting non-Hermitian systems, and its relation to the topological
properties of the system Hamiltonian. Two different families of entanglement
Hamiltonians can be defined in non-Hermitian systems, depending on whether we
consider only right (or equivalently only left) eigenstates or a combination of
both left and right eigenstates. We show that their entanglement spectra can
still be computed efficiently, as in the Hermitian limit. We discuss how
symmetries of the Hamiltonian map into symmetries of the entanglement spectrum
depending on the choice of the many-body state. Through several examples in one
and two dimensions, we show that the biorthogonal entanglement Hamiltonian
directly inherits the topological properties of the Hamiltonian for line gapped
phases, with characteristic singular and energy zero modes. The right (left)
density matrix carries distinct information on the topological properties of
the many-body right (left) eigenstates themselves. In purely point gapped
phases, when the energy bands are not separable, the relation between the
entanglement Hamiltonian and the system Hamiltonian breaks down. | cond-mat_mes-hall |
Conductance Fluctuations and Domain Depinning in Quasi-2D
Charge-Density-Wave 1T-TaS$_2$ Thin Films: We investigated the temperature dependence of the conductance fluctuations in
thin films of the quasi-two-dimensional 1T-TaS$_2$ van der Waals material. The
conductance fluctuations, determined from the derivative current-voltage
characteristics of two-terminal 1T-TaS$_2$ devices, appear prominently at the
electric fields that correspond to the transitions between various
charge-density-wave macroscopic quantum condensate phases and at the onset of
the depinning of the charge density wave domains. The depinning threshold
field, $E_D$, monotonically increases with decreasing temperature within the
nearly commensurate charge-density-wave phase. The $E_D$ value increases with
the decreasing 1T-TaS$_2$ film thickness, revealing the surface pinning of the
charge density waves. Our analysis suggests that depinning is absent in the
commensurate phase. It is induced by the electric field but facilitated by
local heating. The measured trends for $E_D$ of the domain depinning are
important for understanding the physics of charge density waves in
quasi-two-dimensional crystals and for developing electronic devices based on
this type of quantum materials. | cond-mat_mes-hall |
Formulation of Time-Resolved Counting Statistics Based on a
Positive-Operator-Valued Measure: We propose a derivation of the full counting statistics of electronic current
based on a positive-operator-valued measure. Our approach justifies the
Levitov-Lesovik formula in the long-time limit, but can be generalized to the
detection of finite-frequency noise correlations. The combined action of the
projection postulate and the quantum formula for current noise at high
frequencies imply an additional white noise. Estimates for this additional
noise are in accordance with known experiments. We propose an experimental test
of our conjecture by a simultaneous measurement of high- and low-frequency
noise. | cond-mat_mes-hall |
Calculating interface transport parameters at finite temperatures:
Nonmagnetic interfaces: First-principles scattering calculations are used to investigate spin
transport through interfaces between diffusive nonmagnetic metals where the
symmetry lowering leads to an enhancement of the effect of spin-orbit coupling
(SOC) and to a discontinuity of the spin currents passing through the
interfaces. From the conductance and local spin currents calculated for
nonmagnetic bilayers, we extract values of the room temperature interface
resistance $R_{\rm I}$, of the spin memory loss parameter $\delta$ and of the
interface spin Hall angle $\Theta_{\rm I}$ for nonmagnetic Au$|$Pt and Au$|$Pd
interfaces using a frozen thermal disorder scheme to model finite temperatures.
Substantial values of all three parameters are found with important
consequences for experiments involving nonmagnetic spacer and capping layers.
The temperature dependence of the interface parameters is determined for
Au$|$Pt. | cond-mat_mes-hall |
2D transport and screening in topological insulator surface states: We study disorder effects on the surface states of the topological insulator
Bi$_2$Se$_3$ close to the topologically protected crossing point. Close to
charge neutrality, local fluctuations in carrier density arising from the
random charged disorder in the environment result in electron and hole puddles
that dominate the electronic properties of these materials. By calculating the
polarizability of the surface state using the random phase approximation, and
determining the characteristics of puddles using the self-consistent
approximation, we find that band asymmetry plays a crucial role in determining
experimentally measured quantities including the conductivity and the puddle
autocorrelation length. | cond-mat_mes-hall |
Floquet band structure of a semi-Dirac system: In this work we use Floquet-Bloch theory to study the influence of circularly
and linearly polarized light on two-dimensional band structures with semi-Dirac
band touching points, taking the anisotropic nearest neighbor hopping model on
the honeycomb lattice as an example. We find circularly polarized light opens a
gap and induces a band inversion to create a finite Chern number in the
two-band model. By contrast, linearly polarized light can either open up a gap
(polarized in the quadratically dispersing direction) or split the semi-Dirac
band touching point into two Dirac points (polarized in the linearly dispersing
direction) by an amount that depends on the amplitude of the light. Motivated
by recent pump-probe experiments, we investigated the non-equilibrium spectral
properties and momentum-dependent spin-texture of our model in the Floquet
state following a quench in absence of phonons, and in the presence of phonon
dissipation that leads to a steady-state independent of the pump protocol.
Finally, we make connections to optical measurements by computing the frequency
dependence of the longitudinal and transverse optical conductivity for this
two-band model. We analyze the various contributions from inter-band
transitions and different Floquet modes. Our results suggest strategies for
optically controlling band structures and experimentally measuring topological
Floquet systems. | cond-mat_mes-hall |
Intermediate phase and pseudo phase transition in an artificial spin ice
model: In this paper we conduct Monte Carlo simulations to investigate the
thermodynamic properties of a geometry of artificial spin ice recently proposed
in the literature that had been termed "rewritable" spin ice, for its
experimental realization allows total control over the microstates of the
system. Our results show that in the thermodynamic limit a single phase
transition between a fully magnetized state and a paramagnetic state exists,
whereas for finite systems an intermediate phase also emerges, engendering a
low temperature pseudo phase transition. This intermediate phase is
characterized by large magnetic domains separated by domain walls composed of
monopole-like excitations, resulting in low net magnetization values. We also
show that two types of low energy excitations that behave as magnetic monopoles
emerge in the system, both of which are geometrically constrained to move along
a predefined path. | cond-mat_mes-hall |
Majorana qubit decoherence by quasiparticle poisoning: We consider the problem of quasiparticle poisoning in a nanowire-based
realization of a Majorana qubit, where a spin-orbit-coupled semiconducting wire
is placed on top of a (bulk) superconductor. By making use of recent
experimental data exhibiting evidence of a low-temperature residual
non-equilibrium quasiparticle population in superconductors, we show by means
of analytical and numerical calculations that the dephasing time due to the
tunneling of quasiparticles into the nanowire may be problematically short to
allow for qubit manipulation. | cond-mat_mes-hall |
Diffusion and multifractality at the metal-insulator transition: We review the time evolution of wavepackets at the metal-insulator transition
in two- and three-dimensional disordered systems. The importance of scale
invariance and multifractal eigenfunction fluctuations is stressed. The
implications of the frequency- and wavevector-dependence of the diffusion
coefficient are compared with the results of numerical simulations. We argue
that network models are particularly suited for the investigation of the
dynamics of disordered systems. | cond-mat_mes-hall |
Non-Markovian transients in transport across chiral quantum wires using
space-time non-equilibrium Green functions: We study a system of two non-interacting quantum wires with fermions of
opposite chirality with a point contact junction at the origin across which
tunneling can take place when an arbitrary time-dependent bias between the
wires is applied. We obtain the exact dynamical non-equilibrium Green function
by solving Dyson's equation analytically. Both the space-time dependent two and
four-point functions are written down in a closed form in terms of simple
functions of position and time. This allows us to obtain, among other things,
the I-V characteristics for an arbitrary time-dependent bias. Our method is a
superior alternative to competing approaches to non-equilibrium as we are able
to account for transient phenomena as well as the steady state. We study the
approach to steady state by computing the time evolution of the equal-time
one-particle Green function. Our method can be easily applied to the problem of
a double barrier contact whose internal properties can be adjusted to induce
resonant tunneling leading to a conductance maximum. We then consider the case
of a finite bandwidth in the point contact and calculate the non-equilibrium
transport properties which exhibit non-Markovian behaviour. When a subsequently
constant bias is suddenly switched on, the current shows a transient build up
before approaching its steady state value in contrast to the infinite bandwidth
case. This transient property is consistent with numerical simulations of
lattice systems using time-dependent DMRG (tDMRG) suggesting thereby that this
transient build up is merely due to the presence of a short distance cutoff in
the problem description and not on the other details. | cond-mat_mes-hall |
Measuring electron energy distribution by current fluctuations: A recent concept of local noise sensor is extended to measure the energy
resolved electronic energy distribution $f(\varepsilon)$ at a given location
inside a non-equilibrium normal metal interconnect. A quantitative analysis of
$f(\varepsilon)$ is complicated because of a nonlinear differential resistance
of the noise sensor, represented by a diffusive InAs nanowire. Nevertheless, by
comparing the non-equilibrium results with reference equilibrium measurements,
we conclude that $f(\varepsilon)$ is indistinguishable from the Fermi
distribution. | cond-mat_mes-hall |
Anomalous thermoelectric properties of a Floquet topological insulator
with spin momentum non-orthogonality: The spin momentum non-orthogonality in 3D topological insulators leads to
modification of the spin texture and brings in an out-of-plane spin
polarization component. Apart from spin texture, the anomalous thermoelectric
properties of these materials are worth studying. In this paper, we have
pointed out that the off resonant light used to irradiate the surface states,
induces a gap, which becomes momentum dependent due to the presence of
non-orthogonal terms in the Hamiltonian. Importantly, to maintain the off
resonant condition of light, the momentum value should satisfy a bound.
Furthermore, the momentum dependent gap causes a topological transition at
higher value of momentum, which is important to analyse the unusual double peak
structure of the Nernst and electrical conductivities. | cond-mat_mes-hall |
Nano-beam clamping revisited: Within recent years, the field of nano-mechanics has diversified in a variety
of applications, ranging from quantum information processing to biological
molecules recognition. Among the diversity of devices produced these days, the
simplest (but versatile) element remains the doubly-clamped beam: it can store
very large tensile stresses (producing high resonance frequencies $f_0$ and
quality factors $Q$), is interfaceable with electric setups (by means of
conductive layers), and can be produced easily in clean rooms (with scalable
designs including multiplexing). Besides, its mechanical properties are the
simplest to describe. Resonance frequencies and $Q$s are being modeled, with as
specific achievement the ultra-high quality resonances based on ``soft
clamping'' and ``phonon shields''. Here, we demonstrate that the fabrication
undercut of the clamping regions of basic nano-beams produces a ``natural soft
clamping'', given for free. We present the analytic theory that enables to fit
experimental data, which can be used for $\{ Q , f_0 \}$ design: beyond Finite
Element Modeling validation, the presented expressions provide a profound
understanding of the phenomenon, with both a Q enhancement and a downwards
frequency shift. | cond-mat_mes-hall |
Spectral statistics of disordered metals in the presence of several
Aharonov-Bohm fluxes: The form factor for spectral correlations in a diffusive metal is calculated
in the presence of several Aharonov-Bohm fluxes. When the fluxes $\phi$ are
equal, the correlations are universal functions of $n g^2 \phi$ where $g$ is
the dimensionless conductance and $n$ is the number of applied fluxes. This
explains recent flux dependence of the correlations found numerically at the
metal-insulator transition. | cond-mat_mes-hall |
Magnetopumping current in graphene Corbino pump: We study conductance and adiabatic pumped charge and spin currents in a
graphene quantum pump with Corbino geometry in the presence of an applied
perpendicular magnetic field. The pump is driven by the periodic and out of
phase modulations of the magnetic field and an electrostatic potential applied
to the ring area of the pump. We show that the Zeeman splitting, despite of its
smallness, can suppress the conductance oscillations at the zero doping and in
a threshold value for the flux piercing the ring area which depends on the
inner lead radius and thus on the flux penetrating in it. Moreover, it
generates a considerable spin conductance at infinitesimal nonzero doping and
at the magnetic flux, that charge conductance starts to suppress. We find that
the pumped charge and spin currents increase by the magnetic field with small
oscillations until they start to suppress due to the effect of the nonzero
doping and the Zeeman splitting. In graphene Corbino pumps with small inner
leads the Zeeman splitting shows its effect in a large value of the magnetic
field and thus we can get a considerable pumped charge and spin currents at the
enough small magnetic fields. | cond-mat_mes-hall |
Microscopic analysis of shot-noise suppression in nondegenerate
diffusive conductors: We present a theoretical investigation of shot-noise suppression due to
long-range Coulomb interaction in nondegenerate diffusive conductors.
Calculations make use of an ensemble Monte Carlo simulator self-consistently
coupled with a one-dimensional Poisson solver. We analyze the noise in a
lightly doped active region surrounded by two contacts acting as thermal
reservoirs. By taking the doping of the injecting contacts and the applied
voltage as variable parameters, the influence of elastic and inelastic
scattering in the active region is investigated. The transition from ballistic
to diffusive transport regimes under different contact injecting statistics is
analyzed and discussed. Provided significant space-charge effects take place
inside the active region, long-range Coulomb interaction is found to play an
essential role in suppressing the shot noise at $qU \gg k_BT$. In the elastic
diffusive regime, momentum space dimensionality is found to modify the
suppression factor $\gamma$, which within numerical uncertainty takes values
respectively of about 1/3, 1/2 and 0.7 in the 3D, 2D and 1D cases. In the
inelastic diffusive regime, shot noise is suppressed to the thermal value. | cond-mat_mes-hall |
Prediction of inelastic light scattering spectra from electronic
collective excitations in GaAs/AlGaAs core-multishell nanowires: We predict inelastic light scattering spectra from electron collective
excitations in a coaxial quantum well embedded in a core-multishell GaAs/AlGaAs
nanowire. The complex composition, the hexagonal cross section and the remote
doping of typical samples are explicitly included, and the free electron gas is
obtained by a DFT approach. Inelastic light scattering cross sections due to
charge and spin collective excitations belonging to quasi-1D and quasi-2D
states, which coexist in such radial heterostructures, are predicted in the
non-resonant approximation from a fully three-dimensional multi-subband TDDFT
formalism. We show that collective excitations can be classified in azimuthal,
radial and longitudinal excitations, according to the associated density
fluctuations, and we suggest that their character can be exposed by specific
spectral dispersion of inelastic light scattering along different planes of the
heterostructure. | cond-mat_mes-hall |
55Mn NMR in Mn12 acetate: Hyperfine interaction and magnetic relaxation
of cluster: The 55Mn NMR in oriented powder Mn12Ac has been investigated at 1.4-2.0 K in
zero field and with external fields along the c-axis. Three kinds of 55Mn NMR
composed of five-fold quadrupole-split lines for I=5/2 nuclei have been
interpreted to arise from Mn4+ ion, and two crystallographically-inequivalent
Mn3+ ions, respectively. It is found that the isotropic hyperfine field in the
Mn4+ ion with 3d3 configuration indicates a large amount of reduction (26%) as
compared with the theoretical evaluation. In the analysis for the hyperfine
field of Mn3+ ions with 3d4 configuration, we have taken into account of the
anisotropic dipolar contribution in addition to the Fermi-contact term in order
to explain two kinds of 55Mn NMR frequencies in Mn3+ ions in inequivalent
sites. By using the hyperfine coupling constants of twelve manganese ions in
Mn12Ac, the total hyperfine interaction of the ferrimagnetic ground state of
S=10 has been determined to amount to 0.3 cm-1 in magnitude at most, the
magnitude of which corresponds to the nuclear hyperfine field he(0.32 kG seen
by Mn12 cluster spin. The relaxation of the cluster magnetization was
investigated by observing the recovery of the 55Mn spin-echo intensity in the
fields of 0.20-1.90 T along the c-axis at 2.0 K. It was found that the
magnetization of the cluster exhibits the (t-recovery in the short time regime.
The relaxation time decreases with increasing external field following
significant dips at every 0.45 T. This is interpreted to be due to the effects
of thermally-assisted quantum tunneling between the spin states at magnetic
level crossings. | cond-mat_mes-hall |
Topological superconductivity in lead nanowires: Superconductors with an odd number of bands crossing the Fermi energy have
topologically protected Andreev states at interfaces, including Majorana states
in one dimensional geometries. Superconductivity, a low number of 1D channels,
large spin orbit coupling, and a sizeable Zeeman energy, are present in lead
nanowires produced by nanoindentation of a Pb tip on a Pb substrate, in
magnetic fields higher than the Pb bulk critical field. A number of such
devices have been analyzed. In some of them, the dependence of the critical
current on magnetic field, and the Multiple Andreev Reflections observed at
finite voltages, are compatible with the existence of topological
superconductivity. | cond-mat_mes-hall |
Tunneling through two resonant levels: fixed points and conductances: We study point contact tunneling between two leads of a Tomonaga-Luttinger
liquid through two degenerate resonant levels in parallel. This is one of the
simplest cases of a quantum junction problem where the Fermi statistics of the
electrons plays a non-trivial role through the Klein factors appearing in
bosonization. Using a mapping to a `generalized Coulomb model' studied in the
context of the dissipative Hofstadter model, we find that any asymmetry in the
tunneling amplitudes from the two leads grows at low temperatures, so that
ultimately there is no conductance across the system. For the symmetric case,
we identify a non-trivial fixed point of this model; the conductance at that
point is generally different from the conductance through a single resonant
level. | cond-mat_mes-hall |
Thermal fluctuation field for current-induced domain wall motion: Current-induced domain wall motion in magnetic nanowires is affected by
thermal fluctuation. In order to account for this effect, the
Landau-Lifshitz-Gilbert equation includes a thermal fluctuation field and
literature often utilizes the fluctuation-dissipation theorem to characterize
statistical properties of the thermal fluctuation field. However, the theorem
is not applicable to the system under finite current since it is not in
equilibrium. To examine the effect of finite current on the thermal
fluctuation, we adopt the influence functional formalism developed by Feynman
and Vernon, which is known to be a useful tool to analyze effects of
dissipation and thermal fluctuation. For this purpose, we construct a quantum
mechanical effective Hamiltonian describing current-induced domain wall motion
by generalizing the Caldeira-Leggett description of quantum dissipation. We
find that even for the current-induced domain wall motion, the statistical
properties of the thermal noise is still described by the
fluctuation-dissipation theorem if the current density is sufficiently lower
than the intrinsic critical current density and thus the domain wall tilting
angle is sufficiently lower than pi/4. The relation between our result and a
recent result, which also addresses the thermal fluctuation, is discussed. We
also find interesting physical meanings of the Gilbert damping alpha and the
nonadiabaticy parameter beta; while alpha characterizes the coupling strength
between the magnetization dynamics (the domain wall motion in this paper) and
the thermal reservoir (or environment), beta characterizes the coupling
strength between the spin current and the thermal reservoir. | cond-mat_mes-hall |
A scalar photon theory for near-field radiative heat transfer: We study a one-dimensional model of radiative heat transfer for which the
effect of the electromag- netic field is only from the scalar potential and
thereby ignoring the vector potential contribution. This is a valid assumption
when the distances between objects are of the order of nanometers. Using Lorenz
gauge, the scalar field is quantized with the canonical quantization scheme,
giving rise to scalar photons. In the limit as the speed of light approaches
infinity, the theory reduces to a pure Coulomb interaction governed by the
Poisson equation. The model describes very well parallel plate capacitor
physics, where a new length scale related to its capacitance emerges. Shorter
than this length scale we see greater radiative heat transfer. This differs
markedly from the usual Rytov fluctuational electrodynamics theory in which the
enhancement is due to evanescent modes shorter than the thermal wavelengths.
Our theory may explain recent experiments where charge fluctuations instead of
current fluctuations play a dominant role in radiative heat transfer. Finally,
due to the asymmetric electron-bath couplings, thermal rectification effects
are also observed and reported. | cond-mat_mes-hall |
Quantum non-demolition measurements of a qubit coupled to a harmonic
oscillator: We theoretically describe the weak measurement of a two-level system (qubit)
and quantify the degree to which such a qubit measurement has a quantum
non-demolition (QND) character. The qubit is coupled to a harmonic oscillator
which undergoes a projective measurement. Information on the qubit state is
extracted from the oscillator measurement outcomes, and the QND character of
the measurement is inferred by the result of subsequent measurements of the
oscillator. We use the positive operator valued measure (POVM) formalism to
describe the qubit measurement. Two mechanisms lead to deviations from a
perfect QND measurement: (i) the quantum fluctuations of the oscillator, and
(ii) quantum tunneling between the qubit states $|0>$ and $|1>$ during
measurements. Our theory can be applied to QND measurements performed on
superconducting qubits coupled to a circuit oscillator. | cond-mat_mes-hall |
Extended Hubbard model for mesoscopic transport in donor arrays in
silicon: Arrays of dopants in silicon are promising platforms for the quantum
simulation of the Fermi-Hubbard model. We show that the simplest model with
only on-site interaction is insufficient to describe the physics of an array of
phosphorous donors in silicon due to the strong intersite interaction in the
system. We also study the resonant tunneling transport in the array at low
temperature as a mean of probing the features of the Hubbard physics, such as
the Hubbard bands and the Mott gap. Two mechanisms of localization which
suppresses transport in the array are investigated: The first arises from the
electron-ion core attraction and is significant at low filling; the second is
due to the sharp oscillation in the tunnel coupling caused by the intervalley
interference of the donor electron's wavefunction. This disorder in the tunnel
coupling leads to a steep exponential decay of conductance with channel length
in one-dimensional arrays, but its effect is less prominent in two-dimensional
ones. Hence, it is possible to observe resonant tunneling transport in a
relatively large array in two dimensions. | cond-mat_mes-hall |
Nonreciprocal Emergence of Hybridized Magnons in magnetic thin Films: We investigate the transfer and control of nonreciprocity through magnons
themselves in permalloy thin films deposited on surface oxide silicon
substrate. Evidences of nonreciprocal emergence of hybridized dipole exchange
magnons (spin waves) at two permalloy surfaces are provided by studying magnon
transmission and asymmetry, via Brillouin light scattering measurements. The
dipole dominated spin wave and exchange dominated spin wave are found to be
localized near the top and bottom surfaces, respectively, and traveling along
opposite directions. The nonreciprocity and the localization are intertwined
and ca n be tuned by an in plane magnetic field. The effects are well explained
by the magnetostatic theory and can be quantitatively reproduced by the
micromagnetic simulations. Our findings provide a simple and flexible approach
to nonreciprocal all magnon logi c devices with highly compatible with silicon
based integrated circuit technology. | cond-mat_mes-hall |
Spaser Spectroscopy with Subwavelength Spatial Resolution: A new method for high-sensitivity subwavelength spectromicroscopy is proposed
based on the usage of a spaser (near-field laser) in the form of a scanning
probe microscope tip. The high spatial resolution is defined by the tip's
curvature, as is the case for apertureless scanning near-field optical
microscopy. In contrast to the latter method, we suggest using radiationless
plasmon pumping by neighbouring quantum dots instead of irradiation of the tip
by an external laser beam. The spaser generation spectrum is analyzed. The
plasmon generation is suppressed due to absorption at the transition
frequencies of the neighbouring nano-objects (molecules or clusters) under
study. As a result, narrow dips appear in the wide plasmon generation spectrum.
Further, the highest sensitivity is achieved near the spaser generation
threshold. The sensitivity of the spaser spectromicroscope is estimated. | cond-mat_mes-hall |
Controlled Growth of a Line Defect in Graphene and Implications for
Gate-Tunable Valley Filtering: Atomically precise tailoring of graphene can enable unusual transport
pathways and new nanometer-scale functional devices. Here we describe a recipe
for the controlled production of highly regular "5-5-8" line defects in
graphene by means of simultaneous electron irradiation and Joule heating by
applied electric current. High-resolution transmission electron microscopy
reveals individual steps of the growth process. Extending earlier theoretical
work suggesting valley-discriminating capabilities of a graphene 5-5-8 line
defect, we perform first-principles calculations of transport and find a strong
energy dependence of valley polarization of the charge carriers across the
defect. These findings inspire us to propose a compact electrostatically gated
"valley valve" device, a critical component for valleytronics. | cond-mat_mes-hall |
Topological phase transitions in the non-Abelian honeycomb lattice: Ultracold Fermi gases trapped in honeycomb optical lattices provide an
intriguing scenario, where relativistic quantum electrodynamics can be tested.
Here, we generalize this system to non-Abelian quantum electrodynamics, where
massless Dirac fermions interact with effective non-Abelian gauge fields. We
show how in this setup a variety of topological phase transitions occur, which
arise due to massless fermion pair production events, as well as pair
annihilation events of two kinds: spontaneous and strongly-interacting induced.
Moreover, such phase transitions can be controlled and characterized in optical
lattice experiments. | cond-mat_mes-hall |
Carbon Nanotubes as Nanoelectromechanical Systems: We theoretically study the interplay between electrical and mechanical
properties of suspended, doubly clamped carbon nanotubes in which charging
effects dominate. In this geometry, the capacitance between the nanotube and
the gate(s) depends on the distance between them. This dependence modifies the
usual Coulomb models and we show that it needs to be incorporated to capture
the physics of the problem correctly. We find that the tube position changes in
discrete steps every time an electron tunnels onto it. Edges of Coulomb
diamonds acquire a (small) curvature. We also show that bistability in the tube
position occurs and that tunneling of an electron onto the tube drastically
modifies the quantized eigenmodes of the tube. Experimental verification of
these predictions is possible in suspended tubes of sub-micron length. | cond-mat_mes-hall |
Macroscopic resonant tunneling of magnetic flux: We have developed a quantitative theory of resonant tunneling of magnetic
flux between discrete macroscopically distinct quantum states in SQUID systems.
The theory is based on the standard density-matrix approach. Its new elements
include the discussion of the two different relaxation mechanisms that exist
for the double-well potential, and description of the ``photon-assisted''
tunneling driven by external rf radiation. It is shown that in the case of
coherent flux dynamics, rf radiation should lead to splitting of the peaks of
resonant flux tunneling, indicating that the resonant tunneling is a convenient
tool for studying macroscopic quantum coherence of flux. | cond-mat_mes-hall |
Polariton transport in one-dimensional channels: We study theoretically the transport of linearly polarized exciton-polaritons
in a quasi one-dimensional microcavity channel separating two polariton
condensates generated by optical pumping. The direction and value of mass and
spin currents are controlled by the relative phase and polarisation of two
condensates, as in the stationary Josephson effect. However, due to dissipation
and particle-particle interactions, the current denisty is inhomogeneous: it
strongly depends on the coordinate along the axis of the channel. A stationary
spin domain can be created in the channel, its position would be sensitive to
the phase difference between two bordering condensates. | cond-mat_mes-hall |
Graphene plasmons and retardation: strong light-matter coupling: We study the retardation regime of doped graphene plasmons, given by the
nominal crossing of the unretarded plasmon and light-cone. In addition to
modifications in the plasmon dispersion relation, retardation implies strong
coupling between propagating light and matter, even for homogeneous graphene,
which opens up the possibility of efficient plasmonics in simple graphene
devices. We exemplify this enhancement in a double-layer configuration that
exhibits {\em perfect} (if lossless) light transmissions across a classically
forbidden region, providing a simpler analog of the corresponding phenomenon in
perforated metal sheets. We also show that (broad) Fabry-P\'erot resonances
present without graphene turn into sharply peaked, quasi-discrete modes in the
presence of graphene where graphene's response function is given by the typical
Fano lineshape. | cond-mat_mes-hall |
Landau levels, self-adjoint extensions and Hall conductivity on a cone: In this work we obtain the Landau levels and the Hall conductivity at zero
temperature of a two-dimensional electron gas on a conical surface. We
investigate the integer quantum Hall effect considering two different
approaches. The first one is an extrinsic approach which employs an effective
scalar potential that contains both the Gaussian and the mean curvature of the
surface. The second one, an intrinsic approach where the Gaussian curvature is
the sole term in the scalar curvature potential. From a theoretical point of
view, the singular Gaussian curvature of the cone may affect the wave functions
and the respective Landau levels. Since this problem requests {\it self-adjoint
extensions}, we investigate how the conical tip could influence the integer
quantum Hall effect, comparing with the case were the coupling between the wave
functions and the conical tip is ignored. This last case corresponds to the
so-called {\it Friedrichs extension}. In all cases, the Hall conductivity is
enhanced by the conical geometry depending on the opening angle. There are a
considerable number of theoretical papers concerned with the self-adjoint
extensions on a cone and now we hope the work addressed here inspires
experimental investigation on these questions about quantum dynamics on a cone. | cond-mat_mes-hall |
Engineering Quantum Anomalous Hall Plateaus and Anti-Chiral States with
AC Fields: We investigate the AC electric field induced quantum anomalous Hall effect in
honeycomb lattices and derive the full phase diagram for arbitrary field
amplitude and phase polarization. We show how to induce anti-chiral edge modes
as well as topological phases characterized by a Chern number larger than $1$
by means of suitable drivings. In particular, we find that the Chern number
develops plateaus as a function of the frequency, providing an time-dependent
analogue to the ones in the quantum Hall effect. | cond-mat_mes-hall |
Walker-like Domain Wall breakdown in layered Antiferromagnets driven by
staggered spin-orbit fields: Within linear continuum theory, no magnetic texture can propagate faster than
the maximum group velocity of its spin waves. Here we report a transient regime
due to the appearance of additional antiferromagnetic textures that breaks the
Lorentz translational invariance of the magnetic system by atomistic spin
dynamics simulations. This dynamical regime is akin to domain wall
Walker-breakdown in ferromagnets and involves the nucleation of an
antiferromagnetic domain wall pair. Subsequently, one of the nucleated
180$^{\circ}$ domain wall creates with the original domain wall a 360$^{\circ}$
spin-rotation which remains static even under the action of the spin-orbit
field. The other 180$^{\circ}$ domain wall becomes accelerated to
super-magnonic speeds. Under large spin-orbit fields, multiple domain wall
generation and recombination is obtained which may explain the recently
experimentally observed current pulse induce shattering of large domain
structures into small fragmented domains and the subsequent slow recreation of
large-scale domain formation prior current pulse. | cond-mat_mes-hall |
Dynamical Shiba states by precessing magnetic moments in an s-wave
superconductor: We study theoretically the dynamics of a Shiba state forming around
precessing classical spin in an s-wave superconductor. Utilizing a rotating
wave description for the precessing magnetic impurity, we find the resulting
Shiba bound state quasi-energy and the spatial extension of the Shiba
wavefunction. We show that such a precession pertains to dc charge and spin
currents flowing through a normal STM tip tunnel coupled to the superconductor
in the vicinity of the impurity. We calculate these currents and find that they
strongly depend on the magnetic impurity precession frequency, precession
angle, and on the position of the Shiba energy level in the superconducting
gap. The resulting charge current is found to be proportional to the difference
between the electron and hole wavefunctions of the Shiba state, being a direct
measure for such an asymmetry. By dynamically driving the impurity one can
infer the spin dependence of the Shiba states in the absence of a
spin-polarized STM tip | cond-mat_mes-hall |
Nonlinear coherent transport of waves in disordered media: We present a diagrammatic theory for coherent backscattering from disordered
dilute media in the nonlinear regime. The approach is non-perturbative in the
strength of the nonlinearity. We show that the coherent backscattering
enhancement factor is strongly affected by the nonlinearity, and corroborate
these results by numerical simulations. Our theory can be applied to several
physical scenarios like scattering of light in nonlinear Kerr media, or
propagation of matter waves in disordered potentials. | cond-mat_mes-hall |
Impurity-directed Transport within a Finite Disordered Lattice: We consider a finite, disordered 1D quantum lattice with a side-attached
impurity. We study theoretically the transport of a single electron from the
impurity into the lattice, at zero temperature. The transport is dominated by
Anderson localization and, in general, the electron motion has a random
character due to the lattice disorder. However, we show that by adjusting the
impurity energy the electron can attain quasi-periodic motions, oscillating
between the impurity and a small region of the lattice. This region corresponds
to the center of a localized state in the lattice with an energy matched by
that of the impurity. By precisely tuning the impurity energy, the electron can
be set to oscillate between the impurity and a region far from the impurity,
even distances larger than the Anderson localization length. The electron
oscillations result from the interference of hybridized states, which have some
resemblance to Pendry's necklace states [J. B. Pendry, J. Phys. C: Solid State
Phys. 20, 733-742 (1987)]. The dependence of the electron motion on the
impurity energy gives a potential mechanism for selectively routing an electron
towards different regions of a 1D disordered lattice. | cond-mat_mes-hall |
Spacer-layer-tunable magnetism and high-field topological Hall effect in
topological insulator heterostructures: Controlling magnetic order in magnetic topological insulators (MTIs) is a key
to developing spintronic applications with MTIs, and is commonly achieved by
changing the magnetic doping concentration, which inevitably affects
spin-orbit-coupling strength and the very topological properties. Here, we
demonstrate tunable magnetic properties in topological heterostructures over a
wide range, from a ferromagnetic phase with Curie temperature of around 100 K
all the way to a paramagnetic phase, while keeping the overall chemical
composition the same, by controlling the thickness of non-magnetic spacer
layers between two atomically-thin magnetic layers. This work showcases that
spacer-layer control is a powerful tool to manipulate magneto-topological
functionalities in MTI heterostructures. Furthermore, the interaction between
the MTI and the Cr2O3 buffer layers also led to robust topological Hall effect
surviving up to a record-high 6 T of magnetic field, shedding light on the
critical role of interfacial layers in thin film topological materials. | cond-mat_mes-hall |
Anomalous Finite Size Effects on Surface States in the Topological
Insulator Bi$_2$Se$_3$: We study how the surface states in the strong topological insulator
Bi$_2$Se$_3$ are influenced by finite size effects, and compare our results
with those recently obtained for 2D topological insulator HgTe. We demonstrate
two important distinctions: \textit{(i)} contrary to HgTe, the surface-states
in Bi$_2$Se$_3$ display a remarkable robustness towards decreasing the width
$L$ down to a few nm, thus ensuring that the topological surface states remain
intact, and \textit{(ii)} the gapping due to the hybridization of the surface
states features an oscillating exponential decay as a function of $L$ in
Bi$_2$Se$_3$ in sharp contrast to HgTe. Our findings suggest that Bi$_2$Se$_3$
is suitable for nanoscale applications in quantum computing or spintronics.
Also, we propose a way to experimentally detect both of the predicted effects. | cond-mat_mes-hall |
Phonon Driven Nonlinear Electrical Behavior in Molecular Devices: Electronic transport in a model molecular device coupled to local phonon
modes is theoretically analyzed. The method allows for obtaining an accurate
approximation of the system's quantum state irrespective of the electron and
phonon energy scales. Nonlinear electrical features emerge from the calculated
current-voltage characteristics. The quantum corrections with respect to the
adiabatic limit characterize the transport scenario, and the polaronic
reduction of the effective device-lead coupling plays a fundamental role in the
unusual electrical features. | cond-mat_mes-hall |
High-Chern number phase in the topological insulator multilayer
structures: The high-Chern number phases with a Chern number C>1 have been observed in a
recent experiment that performed on the topological insulator (TI) multilayer
structures, consisting of the alternating magnetic-doped and undoped TI layers.
In this paper, we develop an effective method to determine the Chern numbers in
the TI multilayer structures and then make a systematic study on the Chern
number phase diagrams that are modulated by the magnetic doping and the middle
layer thickness. We point out that in the multilayer structure, the high-C
behavior can be attributed to the band inversion mechanisms. Moreover, we find
that the lowest bands may be multifold degenerate around the Gamma point, and
when they are inverted, the Chern number change will be larger than one.
Besides the TI multilayer structures implemented in the experiment, we also
explore the high-C phase realizations in two other kinds of the TI multilayer
structures. The implications of our results for experiments are discussed. | cond-mat_mes-hall |
Correlated breakdown of carbon nanotubes in an ultra-high density
aligned array: We demonstrate that in a densely packed aligned array of single walled carbon
nanotubes, the breakdown of one nanotube leads to a highly correlated breakdown
of neighboring nanotubes, thereby producing a nano-fissure. We show that the
origin of the correlation is the electrostatic field of the broken nanotubes
that produces locally inhomogeneous current and Joule heating distributions in
the neighboring intact nanotubes triggering their breakdowns in the vicinity of
the broken nanotubes. Our results suggest that the densely aligned array
behaves like a correlated solid. | cond-mat_mes-hall |
Quantum anomalous Hall effect in atomic crystal layers from in-plane
magnetization: We theoretically report that, with \textit{in-plane} magnetization, the
quantum anomalous Hall effect (QAHE) can be realized in two-dimensional atomic
crystal layers with preserved inversion symmetry but broken out-of-plane mirror
reflection symmetry. We take the honeycomb lattice as an example, where we find
that the low-buckled structure, which makes the system satisfy the symmetric
criteria, is crucial to induce QAHE. The topologically nontrivial bulk gap
carrying a Chern number of $\mathcal{C}=\pm1$ opens in the vicinity of the
saddle points $M$, where the band dispersion exhibits strong anisotropy. We
further show that the QAHE with electrically tunable Chern number can be
achieved in Bernal-stacked multilayer systems, and the applied interlayer
potential differences can dramatically decrease the critical magnetization to
make the QAHE experimentally feasible. | cond-mat_mes-hall |
Time-Delay Polaritonics: Non-linearity and finite signal propagation speeds are omnipresent in nature,
technologies, and real-world problems, where efficient ways of describing and
predicting the effects of these elements are in high demand. Advances in
engineering condensed matter systems, such as lattices of trapped condensates,
have enabled studies on non-linear effects in many-body systems where exchange
of particles between lattice nodes is effectively instantaneous. Here, we
demonstrate a regime of macroscopic matter-wave systems, in which ballistically
expanding condensates of microcavity exciton-polaritons act as picosecond,
microscale non-linear oscillators subject to time-delayed interaction. The ease
of optical control and readout of polariton condensates enables us to explore
the phase space of two interacting condensates up to macroscopic distances
highlighting its potential in extended configurations. We demonstrate
deterministic tuning of the coupled-condensate system between fixed point and
limit cycle regimes, which is fully reproduced by time-delayed coupled
equations of motion similar to the Lang-Kobayashi equation. | cond-mat_mes-hall |
Phase transitions on the surface of a carbon nanotube: A suspended carbon nanotube can act as a nanoscale resonator with remarkable
electromechanical properties and the ability to detect adsorption on its
surface at the level of single atoms. Understanding adsorption on nanotubes and
other graphitic materials is key to many sensing and storage applications. Here
we show that nanotube resonators offer a powerful new means of investigating
fundamental aspects of adsorption on carbon, including the collective behaviour
of adsorbed matter and its coupling to the substrate electrons. By monitoring
the vibrational resonance frequency in the presence of noble gases, we observe
the formation of monolayers on the cylindrical surface and phase transitions
within these monolayers, and simultaneous modification of the electrical
conductance. The monolayer observations also demonstrate the possibility of
studying the fundamental behaviour of matter in cylindrical geometry. | cond-mat_mes-hall |
A spin dynamics approach to solitonics: It is spatial dispersion which is exclusively responsible for the emergence
of exchange interaction and magnetic ordering. In contrast, magneto-crystalline
anisotropy present in any realistic material brings in a certain non-linearity
to the equation of motion. Unlike homogeneous ferromagnetic ordering a variety
of non-collinear ground state configurations emerge as a result of competition
among exchange, anisotropy, and dipole-dipole interaction. These particle-like
states, e.g. magnetic soliton, skyrmion, domain wall, form a spatially
localised clot of magnetic energy. In this paper we explore topologically
protected magnetic solitons that might potentially be applied for logical
operations and/or information storage in the rapidly advancing filed of
solitonics (and skyrmionics). An ability to easily create, address, and
manipulate such structures is among the prerequisite forming a basis of -onics
technology, and is investigated in detail here using numerical and analytical
tools. | cond-mat_mes-hall |
Electronic Bloch oscillation in bilayer graphene gradient superlattices: We investigate the electronic Bloch oscillation in bilayer graphene gradient
superlattices using transfer matrix method. By introducing two kinds of
gradient potentials of square barriers along electrons propagation direction,
we find that Bloch oscillations up to terahertz can occur. Wannier-Stark
ladders, as the counterpart of Bloch oscillation, are obtained as a series of
equidistant transmission peaks, and the localization of the electronic wave
function is also signature of Bloch oscillation. Forthermore, the period of
Bloch oscillation decreases linearly with increasing gradient of barrier
potentials. | cond-mat_mes-hall |
Microscopic Theory of Skyrmions in Quantum Hall Ferromagnets: We present a microscopic theory of skyrmions in the monolayer quantum Hall
ferromagnet. It is a peculiar feature of the system that the number density and
the spin density are entangled intrinsically as dictated by the W$%_{\infty}$
algebra. The skyrmion and antiskyrmion states are constructed as W$_{\infty
}$-rotated states of the hole-excited and electron-excited states,
respectively. They are spin textures accompanied with density modulation that
decreases the Coulomb energy. We calculate their excitation energy as a
function of the Zeeman gap and compared the result with experimental data. | cond-mat_mes-hall |
Dynamics and condensation of polaritons in an optical nanocavity coupled
to two-dimensional materials: We present a comprehensive investigation of the light-matter interaction
dynamics in two-dimensional materials coupled with a spectrally isolated cavity
mode in the strong coupling regime. The interaction between light and matter
breaks the translational symmetry of excitons in the two-dimensional lattice
and results in the emergence of a localized polariton state. Employing a novel
approach involving transformation to exciton reaction coordinates, we derive a
Markovian master equation to describe the formation of a macroscopic population
in the localized polariton state. Our study shows that the construction of a
large-scale polariton population is affected by correction terms addressing the
breakdown of translational symmetry. Increasing the spatial width of the cavity
mode increases the Coulomb scattering rates while the correction terms saturate
and affect the system's dynamics progressively less. Tuning the lattice
temperature can induce bistability and hysteresis with different origins than
those recognized for quantum wells in larger microcavities. We identify a limit
temperature $T_{\mathrm{l}}$ as a key factor for stimulated emissions and
forming a macroscopic population, enriching our understanding of strong
coupling dynamics in systems with extreme confinement. | cond-mat_mes-hall |
Hole-doping-induced half-metallic ferromagnetism in highly-air-stable
PdSe2 monolayer under uniaxial stress: Two-dimensional (2D) high-temperature ferromagnetic materials are important
for spintronic application. Fortunately, a highly-air-stable PdSe$_2$ monolayer
semiconductor has been made through exfoliation from the layered bulk material.
It is very highly desirable to realize robust ferromagnetism, even
half-metallic ferromagnetism (100\% spin polarization), in such excellent
nonmagnetic monolayer semiconductors. Here, the first-principles investigation
shows that the PdSe$_2$ monolayer can be made to attain Stoner ferromagnetism
with the maximal Curie temperature reaching to 800K, and the hole concentration
threshold for ferromagnetism decreases with applied uniaxial stress.
Furthermore, half-metallicity can be achieved in some hole concentration
regions. For the strain of 10\% (uniaxial tensile stress of 4.4 N/m), the
monolayer can attain half-metallic ferromagnetism up to 150 K. The magnetic
anisotropic energy is suitable to not only stabilizing the 2D ferromagnetism
but also realizing fast magnetization reversal. The magnetization can be also
controlled by applying a transverse uniaxial stress. The highly-air-stable
PdSe$_2$ monolayer, with these advantages, should be promising for spintronic
applications. | cond-mat_mes-hall |
Selective Conduction of Organic Molecules via Free-Standing Graphene: A race is held between ten species of organic gas molecules on a graphene
substrate driven by thermal gradients via molecular dynamics. Fast conduction
of the molecules is observed with selectivity for aromatic compounds. This
selectivity stems from the fact that the planar structure of the aromatic
molecule helps keep a shorter distance to the substrate, which is the key to
the driving force at the gas-solid interface. The drift velocity monotonically
increases with decreasing molecule density, with no ballistic transport
observable even for a single molecule. A non-linear regime is discovered for
the conduction of benzene molecules under large thermal gradients. At low
temperature, molecules formed aggregation and move collectively along specific
path in the graphene substrate. | cond-mat_mes-hall |
Tunable Sample-wide Electronic Kagome Lattice in Low-angle Twisted
Bilayer Graphene: Overlaying two graphene layers with a small twist angle can create a moire
superlattice to realize exotic phenomena that are entirely absent in graphene
monolayer. A representative example is the predicted formation of localized
pseudo-Landau levels (PLLs) with Kagome lattice in tiny-angle twisted bilayer
graphene (TBG) with theta < 0.3 deg when the graphene layers are subjected to
different electrostatic potentials. However, this was shown only for the model
of rigidly rotated TBG which is not realized in reality due to an interfacial
structural reconstruction. It is believed that the interfacial structural
reconstruction strongly inhibits the formation of the PLLs. Here, we
systematically study electronic properties of the TBG with 0.075 deg < theta <
1.2 deg and demonstrate, unexpectedly, that the PLLs are quite robust for all
the studied TBG. The structural reconstruction suppresses the formation of the
emergent Kagome lattice in the tiny-angle TBG. However, for the TBG around
magic angle, the sample-wide electronic Kagome lattices with tunable lattice
constants are directly imaged by using scanning tunneling microscope. Our
observations open a new direction to explore exotic correlated phases in moire
systems. | cond-mat_mes-hall |
Vacuum Thermal Switch Made of Phase Transition Materials Considering
Thin Film and Substrate Effects: In the present study, we demonstrate a vacuum thermal switch based on
near-field thermal radiation between phase transition materials, i.e., vanadium
dioxide (VO2), whose phase changes from insulator to metal at 341 K. Similar
modulation effect has already been demonstrated and it will be extended to
thin-film structure with substrate in this paper. Strong coupling of surface
phonon polaritons between two insulating VO2 plates significantly enhances the
near-field heat flux, which on the other hand is greatly reduced when the VO2
emitter becomes metallic, resulting strong thermal switching effect.
Fluctuational electrodynamics predicts more than 80% heat transfer reduction at
sub-30-nm vacuum gaps and 50% at vacuum gap of 1 micron. By replacing the bulk
VO2 receiver with a thin film of several tens of nanometers, the switching
effect can be further improved over a broad range of vacuum gaps from 10 nm to
1 um. In addition, for the purpose of more practical setup in experiments and
applications, the SiO2 substrate effect is also considered for the structure
with thin-film emitter or receiver. | cond-mat_mes-hall |
Electronic bandstructure and van der Waals coupling of ReSe2 revealed by
high-resolution angle-resolved photoemission spectroscopy: ReSe2 and ReS2 are unusual compounds amongst the layered transition metal
dichalcogenides as a result of their low symmetry, with a characteristic
in-plane anisotropy due to in-plane rhenium chains. They preserve inversion
symmetry independent of the number of layers and, in contrast to more
well-known transition metal dichalcogenides, bulk and few-monolayer Re-TMD
compounds have been proposed to behave as electronically and vibrational
decoupled layers. Here, we probe for the first time the electronic band
structure of bulk ReSe2 by direct nanoscale angle-resolved photoemission
spectroscopy. We find a highly anisotropic in- and out-of-plane electronic
structure, with the valence band maxima located away from any particular
high-symmetry direction. The effective mass doubles its value perpendicular to
the Re chains and the interlayer van der Waals coupling generates significant
electronic dispersion normal to the layers. Our density functional theory
calculations, including spin-orbit effects, are in excellent agreement with
these experimental findings. | cond-mat_mes-hall |
Fabry-Perot interferometry at the $ν$ = 2/5 fractional quantum Hall
state: Electronic Fabry-P{\'e}rot interferometry is a powerful method to probe
quasiparticle charge and anyonic braiding statistics in the fractional quantum
Hall regime. We extend this technique to the hierarchy $\nu = 2/5$ fractional
quantum Hall state, possessing two edge modes that in our device can be
interfered independently. The outer edge mode exhibits interference similar to
the behavior observed at the $\nu = 1/3$ state, indicating that the outer edge
mode at $\nu = 2/5$ has properties similar to the single mode at $\nu = 1/3$.
The inner mode shows an oscillation pattern with a series of discrete phase
jumps indicative of distinct anyonic braiding statistics. After taking into
account the impact of bulk-edge coupling, we extract an interfering
quasiparticle charge ${e^*} = 0.17 \pm 0.02$ and anyonic braiding phase $\theta
_a = (-0.43 \pm 0.05)\times 2\pi$, which serve as experimental verification of
the theoretically predicted values of $e^* = \frac{1}{5}$ and $\theta _a =
-\frac{4\pi}{5}$. | cond-mat_mes-hall |
Disorder-driven exceptional lines and Fermi ribbons in tilted nodal-line
semimetals: We consider the impact of disorder on the spectrum of three-dimensional
nodal-line semimetals. We show that the combination of disorder and a tilted
spectrum naturally leads to a non-Hermitian self-energy contribution that can
split a nodal line into a pair of exceptional lines. These exceptional lines
form the boundary of an open and orientable bulk Fermi ribbon in reciprocal
space on which the energy gap vanishes. We find that the orientation and shape
of such a disorder-induced bulk Fermi ribbon is controlled by the tilt
direction and the disorder properties, which can also be exploited to realize a
twisted bulk Fermi ribbon with nontrivial winding number. Our results put
forward a paradigm for the exploration of non-Hermitian topological phases of
matter. | cond-mat_mes-hall |
Transport in a Dissipative Luttinger Liquid: We study theoretically the transport through a single impurity in a
one-channel Luttinger liquid coupled to a dissipative (ohmic) bath . For
non-zero dissipation $\eta$ the weak link is always a relevant perturbation
which suppresses transport strongly. At zero temperature the current voltage
relation of the link is $I\sim \exp(-E_0/eV)$ where $E_0\sim\eta/\kappa$ and
$\kappa$ denotes the compressibility. At non-zero temperature $T$ the linear
conductance is proportional to $\exp(-\sqrt{{\cal C}E_0/k_BT})$. The decay of
Friedel oscillation saturates for distance larger than $L_{\eta}\sim 1/\eta $
from the impurity. | cond-mat_mes-hall |
Quantum size phenomena in single-crystalline bismuth nanostructures: Size-dependent quantization of energy spectrum of conducting electrons in
solids leads to oscillating dependence of electronic properties on
corresponding dimension(s). In conventional metals with typical energy Fermi
EF~1 eV and the charge carrier's effective masses m* of the order of free
electron mass m0, the quantum size phenomena provide noticeable impact only at
nanometer scales. Here we experimentally demonstrate that in single-crystalline
semimetal bismuth nanostructures the electronic conductivity non-monotonously
decreases with reduction of the effective diameter. In samples grown along the
particular crystallographic orientation the electronic conductivity abruptly
increases at scales of about 50 nm due to metal-to-insulator transition
mediated by the quantum confinement effect. The experimental findings are in
reasonable agreement with theory predictions. The quantum-size phenomena should
be taken into consideration to optimize operation of the next generation of
ultra-small quantum nanoelectronic circuits. | cond-mat_mes-hall |
The influence of a strong infrared radiation field on the conductance
properties of doped semiconductors: This work presents an analytic angular differential cross section formula for
the electromagnetic radiation field assisted electron scattering by %% was on
impurities in semiconductors. These impurities are approximated with various
model potentials. The scattered electrons are described by the well-known
Volkov wave function, which has been used describe strong laser field matter
interaction for more than half a century, %% I would remove this time reference
for clarity which exactly describes the interaction of the electron with the
external oscillating field. These calculations show that the electron
conductance in a semiconductor could be enhanced by an order of magnitude if an
infrared electromagnetic field is present with $ 10^{11} < I < 10^{13}$
W/cm$^2$ intensity. | cond-mat_mes-hall |
Spin current generation and control in carbon nanotubes by combining
rotation and magnetic field: We study the quantum dynamics of ballistic electrons in rotating carbon
nanotubes in the presence of a uniform magnetic field. When the field is
parallel to the nanotube axis, the rotation-induced electric field brings about
the spin-orbit interaction which, together with the kinetic, inertial, and
Zeeman terms, compose the Schr\"odinger-Pauli Hamiltonian of the system. Full
diagonalization of this Hamiltonian yields the eigenstates and eigenenergies
leading to the calculation of the charge and spin currents. Our main result is
the demonstration that, by suitably combining the applied magnetic field
intensity and rotation speed, one can tune one of the currents to zero while
keeping the other one finite, giving rise to a spin current generator. | cond-mat_mes-hall |
Atomistic Simulation of Phonon and Magnon Thermal Transport across the
Ferro-Paramagnetic Transition: A temperature-dependent approach involving Green-Kubo equilibrium atomic and
spin dynamics (GKEASD) is reported to assess phonon and magnon thermal
transport processes accounting for phonon-magnon interactions. Using
body-center cubic (BCC) iron as a case study, GKEASD successfully reproduces
its characteristic temperature-dependent spiral and lattice thermal
conductivities. The non-electronic thermal conductivity, i.e., the sum of
phonon and magnon thermal conductivities, calculated using GKEASD for BCC Fe
agrees well with experimental measurements. Spectral energy analysis reveals
that high-frequency phonon-magnon scattering rates are one order of magnitude
larger than those at low frequencies due to energy scattering conservation
rules and high densities of states. Higher temperatures further accentuate this
phenomenon. This new framework fills existing gaps in simulating thermal
transport across the ferro- to para-magnetic transition. Future application of
this methodology to phonon- and magnon-dominant insulators and semiconductors
will enhance understanding of emerging thermoelectric, spin caloritronic and
superconducting materials. | cond-mat_mes-hall |
Theoretical methods for excitonic physics in two-dimensional materials: In this tutorial we introduce the reader to several theoretical methods of
determining the exciton wave functions and the corresponding eigenenergies. The
methods covered are either analytical, semi-analytical, or numeric. We make
explicit all the details associated with the different methods, thus allowing
newcomers to do research on their own, without experiencing a steep learning
curve. The tutorial starts with a variational method and ends with a simple
semi-analytical approach to solve the Bethe-Salpeter equation in
two-dimensional (2D) gapped materials. For the first methods addressed in this
tutorial, we focus on a single layer of hexagonal Boron Nitride (hBN) and of
transition metal dichalcogenide (TMD), as these are exemplary materials in the
field of 2D excitons. For explaining the Bethe- Salpeter method we choose the
biased bilayer graphene, which presents a tunnable band gap. The system has the
right amount of complexity (without being excessive). This allows the
presentation of the solution of the Bethe-Salpeter equation in a context that
can be easily generalized to more complex systems or to apply it to simpler
models. | cond-mat_mes-hall |
Superlattice of resonators on monolayer graphene created by intercalated
gold nanoclusters: Here we report on a "new" type of ordering which allows to modify the
electronic structure of a graphene monolayer (ML). We have intercalated small
gold clusters between the top monolayer graphene and the buffer layer of
epitaxial graphene. We show that these clusters perturb the quasiparticles on
the ML graphene, and act as quantum dots creating a superlattice of resonators
on the graphene ML, as revealed by a strong pattern of standing waves. A
detailed analysis of the standing wave patterns using Fourier Transform
Scanning Tunneling Spectroscopy strongly indicates that this phenomenon can
arise from a strong modification of the band structure of graphene and (or)
from Charge Density Waves (CDW)where a large extension of Van Hove
singularities are involved. | cond-mat_mes-hall |
QED with magnetic textures: Coherent exchange between photons and different matter excitations (like
qubits, acoustic surface waves or spins) allows for the entanglement of light
and matter and provides a toolbox for performing fundamental quantum physics.
On top of that, coherent exchange is a basic ingredient in the majority of
quantum information processors. In this work, we develop the theory for
coupling between magnetic textures (vortices and skyrmions) stabilized in
ferromagnetic nanodiscs and photons generated in a circuit. In particular, we
show how to perform broadband spectroscopy of the magnetic textures by sending
photons through a transmission line and recording the transmission. We also
discuss the possibility of reaching the strong coupling regime between these
texture excitations and a single photon residing in a cavity. | cond-mat_mes-hall |
Self-consistent multi-mode lasing theory for complex or random lasing
media: A semiclassical theory of single and multi-mode lasing is derived for open
complex or random media using a self-consistent linear response formulation.
Unlike standard approaches which use closed cavity solutions to describe the
lasing modes, we introduce an appropriate discrete basis of functions which
describe also the intensity and angular emission pattern outside the cavity.
This constant flux (CF) basis is dictated by the Green function which arises
when formulating the steady state Maxwell-Bloch equations as a self-consistent
linear response problem. This basis is similar to the quasi-bound state basis
which is familiar in resonator theory and it obeys biorthogonality relations
with a set of dual functions. Within a single-pole approximation for the Green
function the lasing modes are proportional to these CF states and their
intensities and lasing frequencies are determined by a set of non-linear
equations. When a near threshold approximation is made to these equations a
generalized version of the Haken-Sauermann equations for multi-mode lasing is
obtained, appropriate for open cavities. Illustrative results from these
equations are given for single and few mode lasing states, for the case of
dielectric cavity lasers. The standard near threshold approximation is found to
be unreliable. Applications to wave-chaotic cavities and random lasers are
discussed. | cond-mat_mes-hall |
Transport measurements on van der Waals heterostructures under pressure: The interlayer coupling, which has a strong influence on the properties of
van der Waals heterostructures, strongly depends on the interlayer distance.
Although considerable theoretical interest has been demonstrated, experiments
exploiting a variable interlayer coupling on nanocircuits are scarce due to the
experimental difficulties. Here, we demonstrate a novel method to tune the
interlayer coupling using hydrostatic pressure by incorporating van der Waals
heterostructure based nanocircuits in piston-cylinder hydrostatic pressure
cells with a dedicated sample holder design. This technique opens the way to
conduct transport measurements on nanodevices under pressure using up to 12
contacts without constraints on the sample at fabrication level. Using
transport measurements, we demonstrate that hexagonal boron nitride capping
layer provides a good protection of van der Waals heterostructures from the
influence of the pressure medium, and we show experimental evidence of the
influence of pressure on the interlayer coupling using weak localization
measurements on a TMDC/graphene heterostructure. | cond-mat_mes-hall |
$\mathcal{PT}$-symmetric non-Hermitian Dirac semimetals: Parity-time ($\mathcal{PT}$) symmetry plays an important role both in
non-Hermitian and topological systems. In non-Hermitian systems $\mathcal{PT}$
symmetry can lead to an entirely real energy spectrum, while in topological
systems $\mathcal{PT}$ symmetry gives rise to stable and protected Dirac
points. Here, we study a $\mathcal{PT}$-symmetric system which is both
non-Hermitian and topological, namely a $\mathcal{PT}$-symmetric Dirac
semimetal with non-Hermitian perturbations in three dimensions. We find that,
in general, there are only two types of symmetry allowed non-Hermitian
perturbations, namely non-Hermitian kinetic potentials, and non-Hermitian
anti-commuting potentials. For both of these non-Hermitian potentials we
investigate the band topology for open and periodic boundary conditions,
determine the exceptional points, and compute the surface states. We find that
with periodic boundary conditions, the non-Hermitian kinetic potential leads to
exceptional rings, while the non-Hermitian anti-commuting potential generates
exceptional spheres, which separate regions with broken $\mathcal{PT}$ symmetry
from regions with unbroken $\mathcal{PT}$ symmetry. With open boundary
conditions, the non-Hermitian kinetic potential induces a non-Hermitian skin
effect which is localized on both sides of the sample due to symmetry, while
the non-Hermitian anticommuting potential leads to Fermi ribbon surface states. | cond-mat_mes-hall |
Inelastic Cotunneling Resonances in the Coulomb-Blockade Transport in
Donor-Atom Transistors: We report finite-bias characteristics of electrical transport through
phosphorus donors in silicon nanoscale transistors, in which we observe
inelastic-cotunneling current in the Coulomb blockade region. The cotunneling
current appears like a resonant-tunneling current peak emerging from the
excited state at the crossover between blockade and non-blockade regions. These
cotunneling features are unique, since the inelastic-cotunneling currents have
so far been reported either as a broader hump or as a continuous increment of
current. This finding is ascribed purely due to excitation-related inelastic
cotunneling involving the ground and excited states. Theoretical calculations
were performed for a two-level quantum dot, supporting our experimental
observation. | cond-mat_mes-hall |
Unconventional transformation of spin Dirac phase across a topological
quantum phase transition: The topology of a topological material can be encoded in its surface states.
These surface states can only be removed by a bulk topological quantum phase
transition into a trivial phase. Here we use photoemission spectroscopy to
image the formation of protected surface states in a topological insulator as
we chemically tune the system through a topological transition. Surprisingly,
we discover an exotic spin-momentum locked, gapped surface state in the trivial
phase that shares many important properties with the actual topological surface
state in anticipation of the change of topology. Using a spin-resolved
measurement, we show that apart from a surface band-gap these states develop
spin textures similar to the topological surface states well-before the
transition. Our results offer a general paradigm for understanding how surface
states in topological phases arise and are suggestive for future realizing Weyl
arcs, condensed matter supersymmetry and other fascinating phenomena in the
vicinity of topological quantum criticality. | cond-mat_mes-hall |
Spin-polarized quantum transport through a T-shape quantum dot-array: a
model of spin splitter: We in this paper study theoretically the spin-polarized quantum transport
through a T-shape quantum dot-array by means of transfer-matrix method along
with the Green^{,}s function technique. Multi-magnetic fields are used to
produce the spin-polarized transmission probabilities and therefore the spin
currents, which are shown to be tunable in a wide range by adjusting the
energy, and the direction-angle of magnetic fields as well. Particularly the
opposite- spin- polarization currents separately flowing out to two electrodes
can be generated and thus the system acts as a spin splitter. | cond-mat_mes-hall |
Spontaneous spin polarization in quantum point contacts: We use spatial spin separation by a magnetic focusing technique to probe the
polarization of quantum point contacts. The point contacts are fabricated from
p-type GaAs/AlGaAs heterostructures. A finite polarization is measured in the
low-density regime, when the conductance of a point contact is tuned to
<2e^2/h. Polarization is stronger in samples with a well defined ``0.7
structure'' | cond-mat_mes-hall |
Effects of bonding type and interface geometry on coherent transport
through the single-molecule magnet Mn12: We examine theoretically coherent electron transport through the
single-molecule magnet Mn$_{12}$, bridged between Au(111) electrodes, using the
non-equilibrium Green's function method and the density-functional theory. We
analyze the effects of bonding type, molecular orientation, and geometry
relaxation on the electronic properties and charge and spin transport across
the single-molecule junction. We consider nine interface geometries leading to
five bonding mechanisms and two molecular orientations: (i) Au-C bonding, (ii)
Au-Au bonding, (iii) Au-S bonding, (iv) Au-H bonding, and (v) physisorption via
van der Waals forces. The two molecular orientations of Mn$_{12}$ correspond to
the magnetic easy axis of the molecule aligned perpendicular [hereafter denoted
as orientation (1)] or parallel [orientation (2)] to the direction of electron
transport. We find that the electron transport is carried by the lowest
unoccupied molecular orbital (LUMO) level in all the cases that we have
simulated. Relaxation of the junction geometries mainly shifts the relevant
occupied molecular levels toward the Fermi energy as well as slightly reduces
the broadening of the LUMO level. As a result, the current slightly decreases
at low bias voltage. Our calculations also show that placing the molecule in
the orientation (1) broadens the LUMO level much more than in the orientation
(2), due to the internal structure of the Mn$_{12}$. Consequently, junctions
with the former orientation yield a higher current than those with the latter.
Among all of the bonding types considered, the Au-C bonding gives rise to the
highest current (about one order of magnitude higher than the Au-S bonding),
for a given distance between the electrodes. The current through the junction
with other bonding types decreases in the order of Au-Au, Au-S, and Au-H.
Importantly, the spin-filtering effect in all the nine geometries stays robust
and their ratios of the majority-spin to the minority-spin transmission
coefficients are in the range of 10$^3$ to 10$^8$. The general trend in
transport among the different bonding types and molecular orientations obtained
from this study may be applied to other single-molecular magnets. | cond-mat_mes-hall |
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