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Controlling surface charge and spin density oscillations by Dirac
plasmon interaction in thin topological insulators: We study the selective excitation at infrared and THz frequencies of optical
and acoustic plasmonic modes supported by thin topological insulators. These
modes are characterized by effective net charge or net spin density,
respectively, and we study their excitation by combining many-body and
electromagnetic calculations. We first show that non-locality can significantly
modify the plasmonic response: it changes the energy of propagating plasmons up
to tens of percent. We then discuss how, by changing the distance between a
dipolar source and a semi-infinite 10 nm thin film, it is possible to control
the excitation of acoustic and optical propagating plasmons, which can
propagate over a distance of several plasmonic wavelengths. Furthermore, we
consider 10 nm thin TI nanodisks and study the excitation of acoustic and
optical localized plasmon modes by a point dipole source and plane wave
illumination, respectively. The resonant plasmonic modes appear at frequencies
that strongly depends on the size of the disk, and that can be potentially
tuned by applying electrostatic gating to modify the Fermi Energy of the
conductive 2-dimensional layer that forms at the interfaces of the TI. We
observe a spectral shift from ~29 $\mu$m to ~34 $\mu$m by changing the Fermi
Energy from 250meV to 350meV. Last, the electromagnetic energy of these
plasmonics modes can be confined to very small regions, of effective volume
~(120 nm)^3 for the smaller disk considered, much less than the free-space
wavelength cubed $\lambda$^3 ~(35000 nm)^3. The strong confinement is desirable
for achieving very efficient coupling with nearby systems. Our detailed study
thus shows that thin topological insulators are a promising system to control
both the spin and charge oscillations associated with the plasmonic resonances,
with possible applications to fast, compact and electrically-controlled
spintronics devices. | cond-mat_mes-hall |
Spin-flip transitions between Zeeman sublevels in semiconductor quantum
dots: We have studied spin-flip transitions between Zeeman sublevels in GaAs
electron quantum dots. Several different mechanisms which originate from
spin-orbit coupling are shown to be responsible for such processes.
It is shown that spin-lattice relaxation for the electron localized in a
quantum dot is much less effective than for the free electron. The spin-flip
rates due to several other mechanisms not related to the spin-orbit interaction
are also estimated. | cond-mat_mes-hall |
Disentangling electron- and electric field-induced ring-closing
reactions in a diarylethene derivative on Ag(111): Using scanning tunneling microscopy and spectroscopy we investigate the
adsorption properties and ring-closing reaction of a diarylethene derivative
(C5F-4Py) on a Ag(111) surface. We identify an electron-induced reaction
mechanism, with a quantum yield varying from $10^{-14}-10^{-9}$ per electron
upon variation of the bias voltage from $1-2$ V. We ascribe the drastic
increase in switching efficiency to a resonant enhancement upon tunneling
through molecular orbitals. Additionally, we resolve the ring-closing reaction
even in the absence of a current passing through the molecule. In this case the
electric-field can modify the reaction barrier, leading to a finite switching
probability at 4.8 K. A detailed analysis of the switching events shows that a
simple plate-capacitor model for the tip-surface junction is insufficient to
explain the distance dependence of the switching voltage. Instead, describing
the tip as a sphere is in agreement with the findings. We resolve small
differences in the adsorption configuration of the closed isomer, when
comparing the electron- and field-induced switching product. | cond-mat_mes-hall |
Index theorems, generalized Hall currents and topology for gapless
defect fermions: We show how the index of the fermion operator from the Euclidean action can
be used to uncover the existence of gapless modes living on defects (such as
edges and vortices) in topological insulators and superconductors. The 1-loop
Feynman diagram that computes the index reveals an analog of the Quantum Hall
current flowing on and off the defect -- even in systems without conserved
currents or chiral anomalies -- and makes explicit the interplay between
topology in momentum and coordinate space. We provide several explicit
examples. | cond-mat_mes-hall |
Tunneling Conductance in a Two-dimensional Dirac Semimetal Protected by
Non-symmorphic Symmetry: We theoretically study a tunneling effect in a two-dimensional Dirac
semimetal with two Dirac points protected by non-symmorphic symmetries. The
tunnel barrier can be arranged by a magnetic exchange potential which opens a
gap at the Dirac points which can be induced by a magnetic proximity effect of
a ferromagnetic insulator. We found that the tunnel decay length increases with
a decrease in the strength of the spin-orbit coupling, and moreover the
dependence is attributed to the correlation of sublattice and spin degree of
freedoms which lead to symmetry-protected Dirac points. The tunnel probability
is quite different in two Dirac points, and thus the tunnel effect can be
applied to the highly-selective valley filter. | cond-mat_mes-hall |
Spontaneous interlayer exciton coherence in quantum Hall bilayers at
nu=1 and nu=2: a tutorial: This tutorial paper reviews some of the physics of quantum Hall bilayers with
a focus on the case where there is low or zero tunnelling between the two
layers. We describe the interlayer coherent states at filling factors nu=1 and
nu=2 as exciton condensates and discuss some of the theory associated with
these states. | cond-mat_mes-hall |
Effective medium theory for disordered two-dimensional graphene: We develop an Effective Medium Theory to study the electrical transport
properties of disordered graphene. The theory includes non-linear screening and
exchange-correlation effects allowing us to consider experimentally relevant
strengths of the Coulomb interaction. Assuming random Coulomb impurities, we
calculate the electrical conductivity as a function of gate voltage describing
quantitatively the full cross-over from the fluctuations dominated regime
around the Dirac point to the large doping regime at high gate voltages. We
find that the conductivity at the Dirac point is strongly affected by exchange
correlation effects. | cond-mat_mes-hall |
Layered Opposite Rashba Spin-Orbit Coupling in Bilayer Graphene: Loss of
Spin Chirality, Symmetry Breaking and Topological Transition: Inversion symmetry in bilayer graphene allows for layered opposite Rashba
spin-orbit coupling (LO-RSOC) -- the situation when the RSOC has the same
magnitude but the opposite sign in two coupled spatially separated layers. We
show that the LO-RSOC results in the loss of spin chirality in the momentum
space, in contrast to the common uniform RSOC. This chirality loss makes it
difficult to experimentally establish whether the LO-RSOC (on the scale of 10
meV) exists, because the band structure is insensitive to it. To solve this
problem, we propose to identify the LO-RSOC either by gating to break the
inversion symmetry or by magnetic field to break the time-reversal symmetry.
Remarkably, we observe the transition between trivial and non-trivial band
topology as the system deviates from the LO Rashba state. Ab inito calculations
suggest that bilayer graphene encapsulated by two monolayers of Au is a
candidate to be a LO Rashba system. | cond-mat_mes-hall |
Prediction of the Magnetotoroidic Effect from Atomistic Simulations: An effective Hamiltonian technique is used to investigate the effect of
applying curled electric fields on physical properties of stress-free BiFeO3
dots being under open-circuit electrical boundary conditions. It is discovered
that such fields can lead to a control of not only the magnitude but also the
direction of the magnetization. On a microscopic point of view, such control
originates from the field-induced transformation or switching of electrical
vortices and their couplings with oxygen octahedral tilts and magnetic dipoles.
This control involves striking intermediate states, and constitutes a novel
phenomenon that can be termed as "magnetotoroidic" effect. | cond-mat_mes-hall |
Partition Functions of Non-Abelian Quantum Hall States: Partition functions of edge excitations are obtained for non-Abelian Hall
states in the second Landau level, such as the anti-Read-Rezayi state, the
Bonderson-Slingerland hierarchy and the Wen non-Abelian fluid, as well as for
the non-Abelian spin-singlet state. The derivation is straightforward and
unique starting from the non-Abelian conformal field theory data and solving
the modular invariance conditions. The partition functions provide a complete
account of the excitation spectrum and are used to describe experiments of
Coulomb blockade and thermopower. | cond-mat_mes-hall |
A fast, sensitive, room-temperature graphene nanomechanical bolometer: Bolometers are a powerful and vital means of detecting light in the IR to THz
frequencies, and they have been adopted for a range of uses from astronomical
observation to thermal imaging. As uses diversify, there is an increasing
demand for faster, more sensitive room-temperature bolometers. To this end,
graphene has generated interest because of its miniscule heat capacity and its
intrinsic ultra-broadband absorption, properties that would allow it to quickly
detect low levels of light of nearly any wavelength. Yet, graphene has
disappointed its expectations in traditional electrical bolometry at room
temperature, because of its weakly temperature-dependent resistivity and
exceptionally high thermal conductivity. Here, we overcome these challenges
with a new approach that detects light by tracking the resonance frequency of a
graphene nanomechanical resonator. The absorbed light heats up and thermally
tensions the resonator, thereby changing its frequency. Using this approach, we
achieve a room-temperature noise-equivalent power of 7 pW/Hz^1/2, a value 100
times more sensitive than electrical graphene bolometers, and speeds (1.3 MHz)
that greatly surpass state-of-the-art microbolometers. | cond-mat_mes-hall |
Magnetization damping in a local-density approximation: The linear response of itinerant transition metal ferromagnets to transverse
magnetic fields is studied in a self-consistent adiabatic local-density
approximation. The susceptibility is calculated from a microscopic Hamiltonian,
including spin-conserving impurities, impurity induced spin-orbit interaction
and magnetic impurities using the Keldysh formalism. The Gilbert damping
constant in the Landau-Lifshitz-Gilbert equation is identified, parametrized by
an effective transverse spin dephasing rate, and is found to be inversely
proportional to the exchange splitting. Our result justify the phenomenological
treatment of transverse spin dephasing in the study of current-induced
magnetization dynamics in weak, itinerant ferromagnets by Tserkovnyak
\textit{et al.}. We show that neglect of gradient corrections in the
quasiclassical transport equations leads to incorrect results when the exchange
potential becomes of the order of the Fermi energy. | cond-mat_mes-hall |
Interaction of phonons with discrete breather in strained graphene: We numerically analyze the interaction of small-amplitude phonon waves with
standing gap discrete breather (DB) in strained graphene. To make the system
support gap DB, strain is applied to create a gap in the phonon spectrum. We
only focus on the in-plane phonons and DB, so the issue is investigated under a
quasi-one-dimensional setup. It is found that, for the longitudinal sound waves
having frequencies below 6 THz, DB is transparent and thus no radiation of
energy from DB takes place; whereas for those sound waves with higher
frequencies within the acoustic (optical) phonon band, phonon is mainly
transmitted (reflected) by DB, and concomitantly, DB radiates its energy when
interacting with phonons. The latter case is supported by the fact that, the
sum of the transmitted and reflected phonon energy densities is noticeably
higher than that of the incident wave. Our results here may provide insight
into energy transport in graphene when the spatially localized nonlinear
vibration modes are presented. | cond-mat_mes-hall |
Electronic transport in Si nanowires: Role of bulk and surface disorder: We calculate the resistance and mean free path in long metallic and
semiconducting silicon nanowires (SiNWs) using two different numerical
approaches: A real space Kubo method and a recursive Green's function method.
We compare the two approaches and find that they are complementary: depending
on the situation a preferable method can be identified. Several numerical
results are presented to illustrate the relative merits of the two methods. Our
calculations of relaxed atomic structures and their conductance properties are
based on density functional theory without introducing adjustable parameters.
Two specific models of disorder are considered: Un-passivated, surface
reconstructed SiNWs are perturbed by random on-site (Anderson) disorder whereas
defects in hydrogen passivated wires are introduced by randomly removed H
atoms. The un-passivated wires are very sensitive to disorder in the surface
whereas bulk disorder has almost no influence. For the passivated wires, the
scattering by the hydrogen vacancies is strongly energy dependent and for
relatively long SiNWs (L>200 nm) the resistance changes from the Ohmic to the
localization regime within a 0.1 eV shift of the Fermi energy. This high
sensitivity might be used for sensor applications. | cond-mat_mes-hall |
Topological electronic states and thermoelectric transport at phase
boundaries in single-layer WSe$_2$: An effective Hamiltonian theory: Monolayer transition metal dichalcogenides in the distorted octahedral
1T$^\prime$ phase exhibit a large bulk bandgap and gapless boundary states,
which is an asset in the ongoing quest for topological electronics. In
single-layer tungsten diselenide (WSe$_2$), the boundary states have been
observed at well ordered interfaces between 1T$^\prime$ and semiconducting (1H)
phases. This paper proposes an effective 4-band theory for the boundary states
in single-layer WSe$_2$,describing a Kramers pair of in-gap states as well as
the behaviour at the spectrum termination points on the conduction and valence
bands of the 1T$^\prime$ phase. The spectrum termination points determine the
temperature and chemical potential dependences of the ballistic conductance and
thermopower at the phase boundary. Notably, the thermopower shows an ambipolar
behaviour, changing the sign in the bandgap of the 1T$^\prime$ - WSe$_2$ and
reflecting its particle-hole asymmetry. The theory establishes a link between
the bulk band structure and ballistic boundary transport in single-layer
WSe$_2$ and is applicable to a range of related topological materials. | cond-mat_mes-hall |
Spin-wave diode and circulator based on unidirectional coupling: In magnonics, an emerging branch of wave physics characterized by low-energy
consumption, it is highly desirable to realize circuit elements within the
scope of spin-wave computing. Here, based on numerical simulations, we
demonstrate the functionality of the spin-wave diode and the circulator to
steer and manipulate spin waves over a wide range of frequency in the GHz
regime. They take advantage of the unidirectional coupling induced by the
interfacial Dzyaloshinskii-Moriya interaction to transfer the spin wave between
thin ferromagnetic layers in only one direction of propagation. Using the
multilayered structure consisting of Py and Co in direct contact with Pt, we
obtain sub-micrometer-size devices of high efficiency. In the diode, the power
loss ratio between forward and reverse direction reaches 22 dB, while in the
four-port circulator, the efficiency exceeds 13 dB. Thus, our work contributes
to the emerging branch of energy-efficient magnonic logic devices, where,
thanks to short wavelength of spin waves, it is possible to realize nanoscale
devices. | cond-mat_mes-hall |
Two-dimensional group delay in graphene probed by spin precession
measurements: We take graphene as an example to demonstrate that the present widely adopted
expression is only the scattering component of a true 2D group delay in the
condensed matter context, in which the spatial Goos-H\"{a}nchen (GH) shift
along an interface contributes an intrinsic component. We relate the dwell time
to spin precession and derive a relation between the 2D group delay and dwell
time, whereby we for the first time reveal that, the group delay for 2D
ballistic electronic systems can be directly observed by measuring a
conductance difference in a weak-field spin precession experiment. This
physical observable not only implies the group delay being a relevant quantity
even in the condensed matter context, but also provides an experimental
evidence for the intrinsic effect of the GH shift. Finally, we revisit the 2D
Hartman effect, a central issue of the group delay, by analytically solving it
via the vested relation and calculating the proposed observable at the Dirac
point. | cond-mat_mes-hall |
Unconventional transport properties in systems with triply degenerate
quadratic band crossings: A quadratic band crossing (QBC) is a crossing of two bands with quadratic
dispersion, which has been intensively investigated due to its appearance in
Bernal-stacked bilayer graphene. Here, we study an extension of QBCs, the
triply degenerate quadratic band crossing (TQBC), which is a three-band
crossing node containing two quadratic dispersing bands and a flat band. We
focus on two types of TQBCs. The first type contains a symmetry-protected QBC
and a free-electron band, the prototype of which is the AA-stacked bilayer
squareoctagon lattice. In a magnetic field, such a TQBC exhibits an anomalous
Landau level structure, leading to a distinctive quantum Hall effect which
displays an infinite ladder of Hall plateaus when the chemical potential
approaches zero. The other type of TQBC can be viewed as a pseudospin-1
extension of the bilayer-graphene QBC. Under perturbations, this type of TQBCs
may split into linear pseudospin-1 Dirac-Weyl fermions. When tunneling through
a potential barrier, the transmission probability of the first type decays
exponentially with the barrier width for any incident angle, similar to the
free-electron case, while the second type hosts an all-angle perfect reflection
when the energy of the incident particles is equal to half the barrier height. | cond-mat_mes-hall |
Intervalley scattering of graphene massless Dirac fermions at 3-periodic
grain boundaries: We study how low-energy charge carriers scatter off periodic and linear
graphene grain boundaries oriented along the zigzag direction with a
periodicity three times greater than that of pristine graphene. These defects
map the two Dirac points into the same position, and thus allow for intervalley
scattering to occur. Starting from graphene's first-neighbor tight-binding
model we show how can we compute the boundary condition seen by graphene's
massless Dirac fermions at such grain boundaries. We illustrate this procedure
for the 3-periodic pentagon-only grain boundary, and then work out the
low-energy electronic scattering off this linear defect. We also compute the
effective generalized potential seen by the Dirac fermions at the grain
boundary region. | cond-mat_mes-hall |
Resistance of a Molecule: In recent years, several experimental groups have reported measurements of
the current-voltage (I-V) characteristics of individual or small numbers of
molecules. Our purpose in this chapter is to provide an intuitive explanation
for the observed I-V characteristics using simple models to illustrate the
basic physics. In contrast to the MOSFET, whose I-V is largely dominated by
classical electrostatics, the I-V characteristics of molecules is determined by
a more interesting interplay between nineteenth century physics
(electrostatics) and twentieth century physics (quantum transport) and it is
important to do justice to both aspects.
We start with a qualitative discussion of the main factors affecting the I-V
characteristics of molecular conductors, using a simple toy model to illustrate
their role. Including the effects of: (1) Shift in the energy level due to
charging effects and (2) broadening of the energy levels due to the coupling to
the two contacts we obtain realistic I-V plots, even though the toy model
assumes that conduction takes place independently through individual molecular
levels. In general, however, the full non-equilibrium Green's function (NEGF)
formalism is needed. Here, we describe the NEGF formalism as a generalized
version of the one-level model. This formalism provides a convenient framework
for describing quantum transport and can be used in conjunction with ab initio
or semi-empirical Hamiltonians. A simple semi-empirical model for a gold wire
serves to illustrate the full NEGF formalism. This example is particularly
instructive because it shows the lowest possible "Resistance of a 'Molecule'"
per channel. | cond-mat_mes-hall |
Noise-created bistability and stochastic resonance of impurities
diffusing in a semiconductor layer: We investigate the dynamics of impurities walking along a semiconductor layer
assisted by thermal noise of strength $D$ and external harmonic potential
$V(x)$. Applying a nonhomogeneous hot temperature in the vicinity of the
potential minimum may modify the external potential into a bistable effective
potential.
We propose the ways of mobilizing and eradicating the unwanted impurities
along the semiconductor layer. Furthermore, the thermally activated rate of
hopping for the impurities as a function of the model parameters is studied in
high barrier limit. Via two state approximation, we also study the stochastic
resonance (SR) of the impurities dynamics where the same noise source that
induces the dynamics also induces the transition from mono-stable to bistable
state which leads to SR in the presence of time varying field. | cond-mat_mes-hall |
Valley-Spin Polarization in the Magneto-Optical Response of Silicene and
Other Similar 2D Crystals: We calculate the magneto-optical conductivity and electronic density of
states for silicene, the silicon equivalent of graphene, and similar crystals
such as germanene. In the presence of a perpendicular magnetic field and
electric field gating, we note that four spin- and valley-polarized levels can
be seen in the density of states and transitions between these levels lead to
similarly polarized absorption lines in the longitudinal, transverse Hall, and
circularly polarized dynamic conductivity. While previous spin and
valley-polarization predicted for the conductivity is only present in the
response to circularly polarized light, we show that distinct spin- and
valley-polarization can also be seen in the longitudinal magneto-optical
conductivity at experimentally attainable energies. The frequency of the
absorption lines may be tuned by the electric and magnetic field to onset in a
range varying from THz to the infrared. This potential to isolate charge
carriers of definite spin and valley label may make silicene a promising
candidate for spin- and valleytronic devices. | cond-mat_mes-hall |
Metal-insulator transition and tunable Dirac-cone surface state in the
topological insulator TlBi1-xSbxTe2 studied by angle-resolved photoemission: We report a systematic angle-resolved photoemission spectroscopy on
topological insulator (TI) TlBi1-xSbxTe2 which is bulk insulating at 0.5 < x <
0.9 and undergoes a metal-insulator-metal transition with the Sb content x. We
found that this transition is characterized by a systematic hole doping with
increasing x, which results in the Fermi-level crossings of the bulk conduction
and valence bands at x~ 0 and x~1, respectively. The Dirac point of the
topological surface state is gradually isolated from the valence-band edge,
accompanied by a sign reversal of Dirac carriers. We also found that the Dirac
velocity is the largest among known solid-solution TI systems. The
TlBi1-xSbxTe2 system thus provides an excellent platform for Dirac-cone
engineering and device applications of TIs. | cond-mat_mes-hall |
Canted ground state in artificial molecules at high magnetic fields: We analyze the transitions that a magnetic field provokes in the ground state
of an artificial homonuclear diatomic molecule. For that purpose, we have
performed numerical diagonalizations for a double quantum dot around the regime
of filling factor 2. We present phase diagrams in terms of tunneling and Zeeman
couplings, and confinement strength. We identify a series of transitions from
ferromagnetic to symmetric states through a set of canted states with
antiferromagnetic couping between the two quantum dots. | cond-mat_mes-hall |
Renormalization of the dephasing by zero point fluctuations: We study the role of zero-point-fluctuations (ZPF) in dephasing at low
temperature. Unlike the Caldeira-Leggett model where the interaction is with an
homogeneous fluctuating field of force, here we consider the effect of short
range scattering by localized bath modes. We find that in presence of ZPF the
inelastic cross-section gets renormalized. Thus indirectly ZPF might contribute
to the dephasing at low temperature. | cond-mat_mes-hall |
Current-induced skyrmion motion on magnetic nanotubes: Magnetic skyrmions are believed to be the promising candidate of information
carriers in spintronics. However, the skyrmion Hall effect due to the
nontrivial topology of skyrmions can induce a skyrmion accumulation or even
annihilation at the edge of the devices, which hinders the real-world
applications of skyrmions. In this work, we theoretically investigate the
current-driven skyrmion motion on magnetic nanotubes which can be regarded as
"edgeless" in the tangential direction. By performing micromagnetic
simulations, we find that the skyrmion motion exhibits a helical trajectory on
the nanotube, with its axial propagation velocity proportional to the current
density. Interestingly, the skyrmion's annular speed increases with the
increase of the thickness of the nanotube. A simple explanation is presented.
Since the tube is edgeless for the tangential skyrmion motion, a stable
skyrmion propagation can survive in the presence of a very large current
density without any annihilation or accumulation. Our results provide a new
route to overcome the edge effect in planar geometries. | cond-mat_mes-hall |
Magneto-intersubband resistance oscillations in GaAs quantum wells
placed in a tilted magnetic field: The magnetotransport of highly mobile 2D electrons in wide GaAs single
quantum wells with three populated subbands placed in titled magnetic fields is
studied. The bottoms of the lower two subbands have nearly the same energy
while the bottom of the third subband has a much higher energy ($E_1\approx
E_2<<E_3$). At zero in-plane magnetic fields magneto-intersubband oscillations
(MISO) between the $i^{th}$ and $j^{th}$ subbands are observed and obey the
relation $\Delta_{ij}=E_j-E_i=k\cdot\hbar\omega_c$, where $\omega_c$ is the
cyclotron frequency and $k$ is an integer. An application of in-plane magnetic
field produces dramatic changes in MISO and the corresponding electron
spectrum. Three regimes are identified. At $\hbar\omega_c \ll \Delta_{12}$ the
in-plane magnetic field increases considerably the gap $\Delta_{12}$, which is
consistent with the semi-classical regime of electron propagation. In contrast
at strong magnetic fields $\hbar\omega_c \gg \Delta_{12}$ relatively weak
oscillating variations of the electron spectrum with the in-plane magnetic
field are observed. At $\hbar\omega_c \approx \Delta_{12}$ the electron
spectrum undergoes a transition between these two regimes through magnetic
breakdown. In this transition regime MISO with odd quantum number $k$
terminate, while MISO corresponding to even $k$ evolve $continuously$ into the
high field regime corresponding to $\hbar\omega_c \gg \Delta_{12}$ | cond-mat_mes-hall |
Quasiparticle Interference Studies of Quantum Materials: Exotic electronic states are realized in novel quantum materials. This field
is revolutionized by the topological classification of materials. Such
compounds necessarily host unique states on their boundaries. Scanning
tunneling microscopy studies of these surface states have provided a wealth of
spectroscopic characterization, with the successful cooperation of ab initio
calculations. The method of quasiparticle interference imaging proves to be
particularly useful for probing the dispersion relation of the surface bands.
Herein, how a variety of additional fundamental electronic properties can be
probed via this method is reviewed. It is demonstrated how quasiparticle
interference measurements entail mesoscopic size quantization and the
electronic phase coherence in semiconducting nanowires; helical spin protection
and energy-momentum fluctuations in a topological insulator; and the structure
of the Bloch wave function and the relative insusceptibility of topological
electronic states to surface potential in a topological Weyl semimetal. | cond-mat_mes-hall |
Graphene Terahertz Absorption: The unique terahertz properties of graphene has been identified for novel
optoelectronic applications. In a graphene sample with bias voltage added,
there is an enhanced absorption in the far infrared region and a diminished
absorption in the infrared region. The strength of enhancement(diminishment)
increases with the gate voltage, and the enhancement compensates the
diminishment. We find that it is the coherence length of electrons in graphene
that allows pure electronic transitions between states differing by small
momentums and makes intraband transition possible, is responsible for the far
infrared enhancement. Phonon assisted processes are not necessary and would not
in any case contribute to a sum rule. This naturally leads to results obeying
the general sum-rule in optical absorptions. Our prediction of the strength of
enhancement(diminishment) in terms of the bias agrees with experiments. This is
the first direct calculation we are aware of, since the prior phonon assisted
model for indirect transition should not obey a sum rule. | cond-mat_mes-hall |
Suppression of the Persistent Spin Hall Current by Defect Scattering: We study the linear response spin Hall conductivity of a two-dimensional
electron gas (2DEG) in the presence of the Rashba spin orbit interaction in the
diffusive transport regime. When defect scattering is modeled by isotropic
short-range potential scatterers the spin Hall conductivity vanishes due to the
vertex correction. A non-vanishing spin Hall effect may be recovered for
dominantly forward defect scattering. | cond-mat_mes-hall |
Topological phases of quantized light: Topological photonics is an emerging research area that focuses on the
topological states of classical light. Here we reveal the topological phases
that are intrinsic to the particle nature of light, i.e., solely related to the
quantized Fock states and the inhomogeneous coupling between them. The
Hamiltonian of two cavities coupled with a two-level atom is an intrinsic
one-dimensional Su-Schriefer-Heeger model of Fock states. By adding another
cavity, the Fock-state lattice is extended to two dimensions with a honeycomb
structure, where the strain due to the inhomogeneity of the coupling strengths
induces a Lifshitz topological phase transition between a semimetal and a band
insulator. In the semimetallic phase, the strain is equivalent to a
pseudomagnetic field, which results in the quantization of the Landau levels
and the valley Hall effect. We further construct a Haldane model where the
topological phases can be characterized by the topological markers. This study
demonstrates a fundamental distinction between the topological phases of bosons
and fermions and provides a novel platform for studying topological physics in
dimensions higher than three. | cond-mat_mes-hall |
Field induced phase segregation and collective excitations of a trapped
spinor Bose-Einstein condensate: A hydrodynamic description is used to study the zero-temperature properties
of a trapped spinor Bose-Einstein condensate in the presence of a uniform
magnetic field. We show that, in the case of antiferromagnetic spin-spin
interaction, the polar and ferromagnetic configurations of the ground state can
coexist in the trap. These two phases are spatially segregated in such a way
that the polar state occupies the inner part while the ferromagnetic state
occupies the outer part of the atomic cloud. We also derive a set of coupled
hydrodynamic equations for the number density and spin density excitations of
the system. It is shown that these equations can be analytically solved for the
system in an isotropic harmonic trap and a constant magnetic field. Remarkably,
the related low lying excitation spectra are completely determined by the
solutions in the region occupied by the polar state. We find that, within the
Thomas-Fermi approximation, the presence of a constant magnetic field does not
change the excitation spectra which still possess the similar form of that
obtained by Stringari. | cond-mat_mes-hall |
Band structure and magnetotransport of a two-dimensional electron gas in
the presence of spin-orbit interaction: The band structure and magnetotransport of a two-dimensional electron gas
(2DEG), in the presence of the Rashba (RSOI) and Dresselhaus (DSOI) terms of
the spin-orbit interaction and of a perpendicular magnetic field, is
investigated. Exact and approximate analytical expressions for the band
structure are obtained and used to calculate the density of states (DOS) and
the longitudinal magnetoresitivity assuming a Gaussian type of level
broadening. The interplay between the Zeeman coupling and the two terms of the
SOI is discussed. If the strengths $\alpha$ and $ \beta$, of the RSOI and DSOI,
respectively, are equal and the $g$ factor vanishes, the two spin states are
degenerate and a shifted Landau-level structure appears. With the increase of
the difference $\alpha- \beta$, a novel beating pattern of the DOS and of the
Shubnikov-de Haas (SdH) oscillations appears distinctly different from that
occurring when one of these strengths vanishes. | cond-mat_mes-hall |
Commensuration torques and lubricity in double moire systems: We study the commensuration torques and layer sliding energetics of
alternating twist trilayer graphene (t3G) and twisted bilayer graphene on
hexagonal boron nitride (t2G/BN) that have two superposed moire interfaces.
Lattice relaxations for typical graphene twist angles of $\sim 1^{\circ}$ in
t3G or t2G/BN are found to break the out-of-plane layer mirror symmetry, give
rise to layer rotation energy local minima dips of the order of $\sim 10^{-1}$
meV/atom at double moire alignment angles, and have sliding energy landscape
minima between top-bottom layers of comparable magnitude. Moire superlubricity
is restored for twist angles as small as $\sim 0.03^\circ$ away from alignment
resulting in suppression of sliding energies by several orders of magnitude of
typically $\sim 10^{-4}$ meV/atom, hence indicating the precedence of rotation
over sliding in the double moire commensuration process. | cond-mat_mes-hall |
Magnetic field enhancement of organic photovoltaic cells performance: Charge separation is a critical process for achieving high efficiencies in
organic photovoltaic cells. The initial tightly bound excitonic electron-hole
pair has to dissociate fast enough in order to avoid photocurrent generation
and thus power conversion efficiency loss via geminate recombination. Such
process takes place assisted by transitional states that lie between the
initial exciton and the free charge state. Due to spin conservation rules these
intermediate charge transfer states typically have singlet character. Here we
propose a donor-acceptor model for a generic organic photovoltaic cell in which
the process of charge separation is modulated by a magnetic field which tunes
the energy levels. The impact of a magnetic field is to intensify the
generation of charge transfer states with triplet character via inter-system
crossing. As the ground state of the system has singlet character, triplet
states are recombination-protected, thus leading to a higher probability of
successful charge separation. Using the open quantum systems formalism we
demonstrate that not only the population of triplet charge transfer states
grows in the presence of a magnetic field, but also how the power outcome of an
organic photovoltaic cell is in that way increased. | cond-mat_mes-hall |
Theory of magnetoelectric photocurrent generated by direct interband
transitions in semiconductor quantum well: A linearly polarized light normally incident on a semiconductor quantum well
with spin-orbit coupling may generate pure spin current via direct interband
optical transition. An electric photocurrent can be extracted from the pure
spin current when an in-plane magnetic field is applied, which has been
recently observed in the InGaAs/InAlAs quantum well [Dai et al., Phys. Rev.
Lett. 104, 246601 (2010)]. Here we present a theoretical study of this
magnetoelectric photocurrent effect associated with the interband transition.
By employing the density matrix formalism, we show that the photoexcited
carrier density has an anisotropic distribution in k space, strongly dependent
on the orientation of the electron wavevector and the polarization of the
light. This anisotropy provides an intuitive picture of the observed dependence
of the photocurrent on the magnetic field and the polarization of the light. We
also show that the ratio of the pure spin photocurrent to the magnetoelectric
photocurrent is approximately equal to the ratio of the kinetic energy to the
Zeeman energy, which enables us to estimate the magnitude of the pure spin
photocurrent. The photocurrent density calculated with the help of an
anisotropic Rashba model and the Kohn-Luttinger model can produce all three
terms in the fitting formula for measured current, with comparable order of
magnitude, but discrepancies are still present and further investigation is
needed. | cond-mat_mes-hall |
Circular edge states in photonic crystals with a Dirac node: Edge states are studied for the two-dimensional Dirac equation in a circular
geometry. The properties of the two-component electromagnetic field are
discussed in terms of the three-component polarization field, which can form a
vortex structure near the Dirac node with a vorticity changing with the sign of
the Dirac mass. The Berry curvature of the polarization field is related to the
Berry curvature of the Dirac spinor state. This quantity is sensitive to a
change of boundary conditions. In particular, it vanishes for a geometry with a
single boundary but not for a geometry with two boundaries. This effect is
robust against the creation of a step-like edge inside the sample. | cond-mat_mes-hall |
Imaging ferroelectric domains with a single-spin scanning quantum sensor: The ability to sensitively image electric fields is important for
understanding many nanoelectronic phenomena, including charge accumulation at
surfaces and interfaces and field distributions in active electronic devices. A
particularly exciting application is the visualization of domain patterns in
ferroelectric and nanoferroic materials owing to their potential in computing
and data storage. Here, we use a scanning nitrogen-vacancy (NV) microscope,
well known for its use in magnetometry, to image domain patterns in
piezoelectric (Pb[Zr$_{x}$Ti$_{1-x}$]O$_{3}$) and improper ferroelectric
(YMnO$_{3}$) materials through their electric fields. Electric field detection
is enabled by measuring the Stark shift of the NV spin using a gradiometric
detection scheme. Analysis of the electric field maps allows us to discriminate
between different types of surface charge distributions, as well as to
reconstruct maps of the three-dimensional electric field vector and charge
density. The ability to measure both stray electric and magnetic fields under
ambient conditions opens exciting opportunities for the study of multiferroic
and multifunctional materials and devices. | cond-mat_mes-hall |
Interference of magnetointersubband and phonon-induced resistance
oscillations in single GaAs quantum wells with two populated subbands: Low-temperature electron magnetotransport in single GaAs quantum wells with
two populated subbands is studied at large filling factors.
Magneto-inter-subband (MIS) and acoustic-phonon induced oscillations of the
dissipative resistance are found to be coexisting but interfering substantially
with each other. The experiments show that amplitude of the MIS-oscillations
enhances significantly by phonons, indicating "constructive interference"
between the phonon scattering and the intersubband electron transitions.
Temperature damping of the quantum oscillations is found to be related to
broadening of Landau levels caused by considerable electron-electron
scattering. | cond-mat_mes-hall |
Insights on magnon topology and valley-polarization in 2D bilayer
quantum magnets: The rich and unconventional physics in layered 2D magnets can open new
avenues for topological magnonics and magnon valleytronics. In particular,
two-dimensional (2D) bilayer quantum magnets are gaining increasing attention
due to their intriguing stacking-dependent magnetism, controllable ground
states, and topological excitations induced by magnetic spin-orbit couplings
(SOCs). Despite the substantial research on these materials, their topological
features remain widely unexplored to date. The present study comprehensively
investigates the magnon topology and magnon valley-polarization in honeycomb
bilayers with collinear magnetic order. We elucidate the separate and combined
effects of the SOC, magnetic ground-states, stacking order, and inversion
symmetry breaking on the topological phases, magnon valley transport, and the
Hall and Nernst effects. The comprehensive analysis suggests clues to determine
the SOC's nature and predicts unconventional Hall and Nernst conductivities in
topologically trivial phases. We further report on novel bandgap closures in
layered antiferromagnets and detail their topological implications. We believe
the present study provides important insights into the fundamental physics and
technological potentials of topological 2D magnons. | cond-mat_mes-hall |
Signatures of neutral quantum Hall modes in transport through
low-density constrictions: Constrictions in fractional quantum Hall (FQH) systems not only facilitate
backscattering between counter-propagating edge modes, but also may reduce the
constriction filling fraction $\nu_c$ with respect to the bulk filling fraction
$\nu_b$. If both $\nu_b$ and $\nu_c$ correspond to incompressible FQH states,
at least part of the constriction region is surrounded by composite edges,
whose low energy dynamics is characterized by a charge mode and one or several
neutral modes. In the incoherent regime, decay of neutral modes describes the
equilibration of composite FQH edges, while in the limit of coherent transport,
the presence of neutral modes gives rise to universal conductance fluctuations.
In addition, neutral modes renormalize the strength of scattering across the
constriction, and thus can determine the relative strength of forward and
backwards scattering. | cond-mat_mes-hall |
Macroscopic Resonant Tunneling through Andreev Interferometers: We investigate the conductance through and the spectrum of ballistic chaotic
quantum dots attached to two s-wave superconductors, as a function of the phase
difference $\phi$ between the two order parameters. A combination of analytical
techniques -- random matrix theory, Nazarov's circuit theory and the
trajectory-based semiclassical theory -- allows us to explore the
quantum-to-classical crossover in detail. When the superconductors are not
phase-biased, $\phi=0$, we recover known results that the spectrum of the
quantum dot exhibits an excitation gap, while the conductance across two normal
leads carrying $N_{\rm N}$ channels and connected to the dot via tunnel
contacts of transparency $\Gamma_{\rm N}$ is $\propto \Gamma_{\rm N}^2 N_{\rm
N}$. In contrast, when $\phi=\pi$, the excitation gap closes and the
conductance becomes $G \propto \Gamma_{\rm N} N_{\rm N}$ in the universal
regime. For $\Gamma_{\rm N} \ll 1$, we observe an order-of-magnitude
enhancement of the conductance towards $G \propto N_{\rm N}$ in the
short-wavelength limit. We relate this enhancement to resonant tunneling
through a macroscopic number of levels close to the Fermi energy. Our
predictions are corroborated by numerical simulations. | cond-mat_mes-hall |
Synthesis of Graphene on Gold: Here we report chemical vapor deposition of graphene on gold surface at
ambient pressure. We studied effects of the growth temperature, pressure and
cooling process on the grown graphene layers. The Raman spectroscopy of the
samples reveals the essential properties of the graphene grown on gold surface.
In order to characterize the electrical properties of the grown graphene
layers, we have transferred them on insulating substrates and fabricated field
effect transistors. Owing to distinctive properties of gold, the ability to
grow graphene layers on gold surface could open new applications of graphene in
electrochemistry and spectroscopy. | cond-mat_mes-hall |
Effects of substrate corrugation during helium adsorption on graphene in
the grand canonical ensemble: Adsorption of 4He on atomically flat substrates such as graphene provides a
route towards the engineering of low dimensional quantum phases including
superfluids and strongly interacting insulators. In this study, we explore the
effects of graphene corrugation on the helium adsorption process via quantum
Monte Carlo simulations in the grand canonical ensemble. We utilize an
empirical adsorption potential based on the superposition of individual
helium-carbon interactions and systematically control corrugation, from a
smooth membrane to the fully rough potential, via the implementation of a
cutoff in reciprocal space. The results highlight the importance of using a
fully corrugated potential to understand the plethora of commensurate and
incommensurate phases in the first adsorbed layer. Surprisingly, some residual
effects of corrugation are still present before and during the promotion and
onset of a second adsorbed layer, where a smooth adsorption can lead to
enhanced particle fluctuations in a helium-interaction dominated regime. | cond-mat_mes-hall |
Gibbs phenomenon and the emergence of the steady-state in quantum
transport: Simulations are increasingly employing explicit reservoirs - internal, finite
regions - to drive electronic or particle transport. This naturally occurs in
simulations of transport via ultracold atomic gases. Whether the simulation is
numerical or physical, these approaches rely on the rapid development of the
steady state. We demonstrate that steady state formation is a manifestation of
the Gibbs phenomenon well-known in signal processing and in truncated discrete
Fourier expansions. Each particle separately develops into an individual steady
state due to the spreading of its wave packet in energy. The rise to the steady
state for an individual particle depends on the particle energy - and thus can
be slow - and ringing oscillations appear due to filtering of the response
through the electronic bandwidth. However, the rise to the total steady state -
the one from all particles - is rapid, with timescale $\pi/W$, where $W$ is the
bandwidth. Ringing oscillations are now also filtered through the bias window,
and they decay with a higher power. The Gibbs constant - the overshoot of the
first ring - can appear in the simulation error. These results shed light on
the formation of the steady state and support the practical use of explicit
reservoirs to simulate transport at the nanoscale or using ultracold atomic
lattices. | cond-mat_mes-hall |
Subgap dynamics of double quantum dot coupled between superconducting
and normal leads: Dynamical processes induced by the external time-dependent fields can provide
valuable insight into the characteristic energy scales of a given physical
system. We investigate them here in a nanoscopic heterostructure, consisting of
the double quantum dot coupled in series to the superconducting and the
metallic reservoirs, analyzing its response to (i)~abrupt bias voltage applied
across the junction, (ii) sudden change of the energy levels, and imposed by
(iii)~their periodic driving. We explore subgap properties of this setup which
are strictly related to the in-gap quasiparticles and discuss their signatures
manifested in the time-dependent charge currents. The characteristic multi-mode
oscillations, their beating patters and photon-assisted harmonics reveal a rich
spectrum of dynamical features that might be important for designing the
superconducting qubits. | cond-mat_mes-hall |
Parametric symmetry breaking in a nonlinear resonator: Much of the physical world around us can be described in terms of harmonic
oscillators in thermodynamic equilibrium. At the same time, the far from
equilibrium behavior of oscillators is important in many aspects of modern
physics. Here, we investigate a resonating system subject to a fundamental
interplay between intrinsic nonlinearities and a combination of several driving
forces. We have constructed a controllable and robust realization of such a
system using a macroscopic doubly clamped string. We experimentally observe a
hitherto unseen double hysteresis in both the amplitude and the phase of the
resonator's response function and present a theoretical model that is in
excellent agreement with the experiment. Our work provides a thorough
understanding of the double-hysteretic response through a symmetry breaking of
parametric phase states that elucidates the selection criteria governing
transitions between stable solutions. Our study motivates applications ranging
from ultrasensitive force detection to low-energy computing memory units. | cond-mat_mes-hall |
Time-Loop Formalism for Irreversible Quantum Problems: Steady State
Transport in Junctions with Asymmetric Dynamics: Non-unitary quantum mechanics has been used in the past to study
irreversibility, dissipation and decay in a variety of physical systems. In
this letter, we propose a general scheme to deal with systems governed by
non-Hermitian Hamiltonians. We argue that the Schwinger-Keldysh formalism gives
a natural description for those problems. To elucidate the method, we study a
simple model inspired by mesoscopic physics --an asymmetric junction. The
system is governed by a non-Hermitian Hamiltonian which captures essential
aspects of irreversibility. | cond-mat_mes-hall |
Charge puddles in the bulk and on the surface of the topological
insulator BiSbTeSe$_2$ studied by scanning tunneling microscopy and optical
spectroscopy: The topological insulator BiSbTeSe$_2$ corresponds to a compensated
semiconductor in which strong Coulomb disorder gives rise to the formation of
charge puddles, i.e., local accumulations of charge carriers, both in the bulk
and on the surface. Bulk puddles are formed if the fluctuations of the Coulomb
potential are as large as half of the band gap. The gapless surface, in
contrast, is sensitive to small fluctuations but the potential is strongly
suppressed due to the additional screening channel provided by metallic surface
carriers. To study the quantitative relationship between the properties of bulk
puddles and surface puddles, we performed infrared transmittance measurements
as well as scanning tunneling microscopy measurements on the same sample of
BiSbTeSe$_2$, which is close to perfect compensation. At 5.5 K, we find surface
potential fluctuations occurring on a length scale $r_s = 40-50$ nm with
amplitude $\Gamma = 8-14$ meV which is much smaller than in the bulk, where
optical measurements detect the formation of bulk puddles. In this nominally
undoped compound, the value of $\Gamma$ is smaller than expected for pure
screening by surface carriers, and we argue that this arises most likely from a
cooperative effect of bulk screening and surface screening. | cond-mat_mes-hall |
Magnetic-field asymmetry of nonlinear mesoscopic transport: We investigate departures of the Onsager relations in the nonlinear regime of
electronic transport through mesoscopic systems. We show that the nonlinear
current--voltage characteristic is not an even function of the magnetic field
due only to the magnetic-field dependence of the screening potential within the
conductor. We illustrate this result for two types of conductors: A quantum
Hall bar with an antidot and a chaotic cavity connected to quantum point
contacts. For the chaotic cavity we obtain through random matrix theory an
asymmetry in the fluctuations of the nonlinear conductance that vanishes
rapidly with the size of the contacts. | cond-mat_mes-hall |
Direct Bandgap Emission from Hexagonal Ge and SiGe Alloys: Silicon crystallized in the usual cubic (diamond) lattice structure has
dominated the electronics industry for more than half a century. However, cubic
silicon (Si), germanium (Ge) and SiGe-alloys are all indirect bandgap
semiconductors that cannot emit light efficiently. Accordingly, achieving
efficient light emission from group-IV materials has been a holy grail in
silicon technology for decades and, despite tremendous efforts, it has remained
elusive. Here, we demonstrate efficient light emission from direct bandgap
hexagonal Ge and SiGe alloys. We measure a subnanosecond,
temperature-insensitive radiative recombination lifetime and observe a similar
emission yield to direct bandgap III-V semiconductors. Moreover, we demonstrate
how by controlling the composition of the hexagonal SiGe alloy, the emission
wavelength can be continuously tuned in a broad range, while preserving a
direct bandgap. Our experimental findings are shown to be in excellent
quantitative agreement with the ab initio theory. Hexagonal SiGe embodies an
ideal material system to fully unite electronic and optoelectronic
functionalities on a single chip, opening the way towards novel device concepts
and information processing technologies. | cond-mat_mes-hall |
Particle tunneling through a polarizable insulator: The tunneling probability between two leads connected by a molecule, a chain,
a film, or a bulk polarizable insulator is investigated within a model of an
electron tunneling from lead A to a state higher in energy, describing the
barrier, and from there to lead B. To describe the possibility of energy
exchange with excitations of the molecule or the insulator we couple the
intermediate state to a single oscillator or to a spectrum of these,
respectively. In the single-oscillator case we find for weak coupling that the
tunneling is weakly suppressed by a Debye-Waller-type factor. For stronger
coupling the oscillator gets 'stiff' and we observe a suppression of tunneling
since the effective barrier is increased. The probability for the electron to
excite the oscillator increases with the coupling. In the case of a film, or a
bulk barrier the behavior is qualitatively the same as in the single oscillator
case. An insulating chain, as opposed to a film or a bulk connecting the two
leads,shows an 'orthogonality catastrophe' similar to that of an electronic
transition in a Fermi gas. | cond-mat_mes-hall |
Influence of Landau level mixing on the properties of elementary
excitations in graphene in strong magnetic field: Massless Dirac electrons in graphene fill Landau levels with energies scaled
as square roots of their numbers. Coulomb interaction between electrons leads
to mixing of different Landau levels. The relative strength of this interaction
depends only on dielectric susceptibility of surrounding medium and can be
large in suspended graphene. We consider influence of Landau level mixing on
the properties of magnetoexcitons and magnetoplasmons - elementary
electron-hole excitations in graphene in quantizing magnetic field. We show
that, at small enough background dielectric screening, the mixing leads to very
essential change of magnetoexciton and magnetoplasmon dispersion laws in
comparison with the lowest Landau level approximation. | cond-mat_mes-hall |
Colossal orbital Zeeman effect driven by tunable spin-Berry curvature in
a kagome metal: Berry phase and the related concept of Berry curvature can give rise to many
unconventional phenomena in solids. In this work, we discover colossal orbital
Zeeman effect of topological origin in a newly synthesized bilayer kagome metal
TbV6Sn6. We use spectroscopic-imaging scanning tunneling microscopy to study
the magnetic field induced renormalization of the electronic band structure.
The nonmagnetic vanadium d-orbitals form Dirac crossings at the K point with a
small mass gap and strong Berry curvature induced by the spin-orbit coupling.
We reveal that the magnetic field leads to the splitting of gapped Dirac
dispersion into two branches with giant momentum-dependent g factors, resulting
in the substantial renormalization of the Dirac band. These measurements
provide a direct observation of the magnetic field controlled orbital Zeeman
coupling to the enormous orbital magnetic moments of up to 200 Bohr magnetons
near the gapped Dirac points. Interestingly, the effect is increasingly
non-linear, and becomes gradually suppressed at higher magnetic fields.
Theoretical modeling further confirms the existence of orbital magnetic moments
in TbV6Sn6 produced by the non-trivial spin-Berry curvature of the Bloch wave
functions. Our work provides the first direct insight into the
momentum-dependent nature of topological orbital moments and their tunability
by magnetic field concomitant with the evolution of the spin-Berry curvature.
Significantly large orbital magnetic moments driven by the Berry curvature can
also be generated by other quantum numbers beyond spin, such as the valley in
certain graphene-based structures, which may be unveiled using the same tools
highlighted in our work. | cond-mat_mes-hall |
Reversible Single Spin Control of Individual Magnetic Molecule by
Hydrogen Atom Adsorption: The reversible control of a single spin of an atom or a molecule is of great
interest in Kondo physics and a potential application in spin based
electronics.Here we demonstrate that the Kondo resonance of manganese
phthalocyanine molecules on an Au(111) substrate have been reversibly switched
off and on via a robust route through attachment and detachment of single
hydrogen atom to the magnetic core of the molecule. As further revealed by
density functional theory calculations, even though the total number of
electrons of the Mn ion remains almost the same in the process, gaining one
single hydrogen atom leads to redistribution of charges within 3d orbitals with
a reduction of the molecular spin state from S = 3/2 to S = 1 that directly
contributes to the Kondo resonance disappearance. This process is reversed by a
local voltage pulse or thermal annealing to desorb the hydrogen atom. | cond-mat_mes-hall |
Non-Markovian electron dynamics in nanostructures coupled to dissipative
contacts: In quasiballistic semiconductor nanostructures, carrier exchange between the
active region and dissipative contacts is the mechanism that governs
relaxation. In this paper, we present a theoretical treatment of transient
quantum transport in quasiballistic semiconductor nanostructures, which is
based on the open system theory and valid on timescales much longer than the
characteristic relaxation time in the contacts. The approach relies on a model
interaction between the current-limiting active region and the contacts, given
in the scattering-state basis. We derive a non-Markovian master equation for
the irreversible evolution of the active region's many-body statistical
operator by coarse-graining the exact dynamical map over the contact relaxation
time. In order to obtain the response quantities of a nanostructure under bias,
such as the potential and the charge and current densities, the non-Markovian
master equation must be solved numerically together with the Schr\"{o}dinger,
Poisson, and continuity equations. We discuss how to numerically solve this
coupled system of equations and illustrate the approach on the example of a
silicon nin diode. | cond-mat_mes-hall |
Spin-splitting in the quantum Hall effect of disordered GaAs layers with
strong overlap of the spin subbands: With minima in the diagonal conductance G_{xx} and in the absolute value of
the derivative |dG_{xy}/dB| at the Hall conductance value G_{xy}=e^{2}/h,
spin-splitting is observed in the quantum Hall effect of heavily Si-doped GaAs
layers with low electron mobility 2000 cm^2/Vs in spite of the fact that the
spin-splitting is much smaller than the level broadening. Experimental results
can be explained in the frame of the scaling theory of the quantum Hall effect,
applied independently to each of the two spin subbands. | cond-mat_mes-hall |
Realization of bulk insulating property and carrier manipulation in
reversible spin current regime of the ideal topological insulator TlBiSe2: The surfaces of three-dimensional topological insulators (TIs) characterized
by a spin-helical Dirac fermion provide a fertile ground for realizing exotic
phenomena as well as having potential for wide-ranging applications. To realize
most of their special properties, the Dirac point (DP) is required to be
located near the Fermi energy with a bulk insulating property while it is
hardly achieved in most of the discovered TIs. It has been recently found that
TlBiSe2 features an in-gap DP, where upper and lower parts of surface Dirac
cone are both utilized. Nevertheless, investigations of the surface transport
properties of this material are limited due to the lack of bulk insulating
characteristics. Here, we present the first realization of bulk insulating
property by tuning the composition of Tl1-xBi1+xSe2-d without introducing guest
atoms that can bring the novel properties into the reality. This result
promises to shed light on new exotic topological phenomena on the surface. | cond-mat_mes-hall |
Efficient Computation of Kubo Conductivity for Incommensurate 2D
Heterostructures: Here we introduce a numerical method for computing conductivity via the Kubo
Formula for incommensurate 2D bilayer heterostructures using a tight-binding
framework. We begin with deriving the momentum space formulation and Kubo
Formula from the real space tight-binding model using the appropriate Bloch
transformation operator. We further discuss the resulting algorithm along with
its convergence rate and computation cost in terms of parameters such as
relaxation time and temperature. In particular, we show that for low
frequencies, low temperature, and long relaxation times conductivity can be
computed very efficiently using momentum space for a wide class of materials.
We then demonstrate our method by computing conductivity for twisted bilayer
graphene (tBLG) for small twist angles. | cond-mat_mes-hall |
Universal quantum computation on a semiconductor quantum wire network: Universal quantum computation (UQC) using Majorana fermions on a 2D
topological superconducting (TS) medium remains an outstanding open problem.
This is because the quantum gate set that can be generated by braiding of the
Majorana fermions does not include \emph{any} two-qubit gate and also the
single-qubit $\pi/8$ phase gate. In principle, it is possible to create these
crucial extra gates using quantum interference of Majorana fermion currents.
However, it is not clear if the motion of the various order parameter defects
(vortices, domain walls, \emph{etc.}), to which the Majorana fermions are bound
in a TS medium, can be quantum coherent. We show that these obstacles can be
overcome using a semiconductor quantum wire network in the vicinity of an
$s$-wave superconductor, by constructing topologically protected two-qubit
gates and any arbitrary single-qubit phase gate in a topologically unprotected
manner, which can be error corrected using magic state distillation. Thus our
strategy, using a judicious combination of topologically protected and
unprotected gate operations, realizes UQC on a quantum wire network with a
remarkably high error threshold of $0.14$ as compared to $10^{-3}$ to $10^{-4}$
in ordinary unprotected quantum computation. | cond-mat_mes-hall |
Controlling Hot Electron Spatial and Momentum Distributions in
Nanoplasmonic Systems: Volume versus Surface Effects: Hot carrier spatial and momentum distributions in nanoplasmonic systems
depend sensitively on the optical excitation parameters and nanoscale geometry,
which therefore determine the efficiency and functionality of plasmon-enhanced
catalysts, photovoltaics, and nanocathodes. A growing appreciation over the
past decade for the distinction between volume- and surface-mediated
photoexcitation and electron emission from such systems has underscored the
need for direct mechanistic insight and quantification of these two processes.
Toward this end, we use angle-resolved photoelectron velocity mapping to
directly distinguish volume and surface contributions to nanoplasmonic hot
electron emission from gold nanorods as a function of aspect ratio, down to the
spherical limit. Nanorods excited along their longitudinal surface plasmon axis
exhibit surprising transverse photoemission distributions due to the dominant
volume excitation mechanisms, as reproduced via ballistic Monte Carlo
modelling. We further demonstrate a screening-induced transition from volume
(transverse) to surface (longitudinal) photoemission with red detuning of the
excitation laser and determine the relative cross-sections of the two
mechanisms via combined volume and surface multiphoton photoemission modelling.
Based on these results, we are able to identify geometry- and material-specific
contributions to the photoemission cross-sections and offer general principles
for designing nanoplasmonic systems to control hot electron excitation and
emission distributions. | cond-mat_mes-hall |
Theory of defect dynamics in graphene: defect groupings and their
stability: We use our theory of periodized discrete elasticity to characterize defects
in graphene as the cores of dislocations or groups of dislocations. Earlier
numerical implementations of the theory predicted some of the simpler defect
groupings observed in subsequent Transmission Electron Microscope experiments.
Here we derive the more complicated defect groupings of three or four defect
pairs from our theory, show that they correspond to the cores of two pairs of
dislocation dipoles and ascertain their stability. | cond-mat_mes-hall |
Understanding the structure of the first atomic contact in Gold: We have studied experimentally the phenomena of jump-to-contact (JC) and
jump-out-of-contact (JOC) in gold electrodes. JC can be observed at the first
contact when the two metals approach each other while JOC occurs in the last
contact before breaking. When the indentation depth between the electrodes is
limited to a certain value of conductance, a highly reproducible behaviour in
the evolution of the conductance can be obtained for hundreds of cycles of
formation and rupture. Molecular dynamics simulations of this process show how
the two metallic electrodes are shaped into tips of a well-defined
crystallographic structure formed through a mechanical annealing mechanism. We
report a detailed analysis of the atomic configurations obtained before contact
and rupture of these stable structures and obtained their conductance using
first-principlesquantum transport calculations. These results help us
understand the values of conductance obtained experimentally in the JC and JOC
phenomena and improve our understanding of atomic-sized contacts and the
evolution of their structural characteristics. | cond-mat_mes-hall |
Correlation and dephasing effects on the non-radiative coherence between
bright excitons in an InAs QD ensemble measured with 2D spectroscopy: Exchange-mediated fine-structure splitting of bright excitons in an ensemble
of InAs quantum dots is studied using optical two-dimensional Fourier-transform
spectroscopy. By monitoring the non-radiative coherence between the bright
states, we find that the fine-structure splitting decreases with increasing
exciton emission energy at a rate of 0.1 $\mu$eV/meV. Dephasing rates are
compared to population decay rates to reveal that pure dephasing causes the
exciton optical coherences to decay faster than the radiative limit at low
temperature, independent of excitation density. Fluctuations of the bright
state transition energies are nearly perfectly-correlated, protecting the
non-radiative coherence from interband dephasing mechanisms. | cond-mat_mes-hall |
Surface twist instabilities and skyrmion states in chiral ferromagnets: In epitaxial MnSi/Si(111) films, the in-plane magnetization saturation is
never reached due to the formation of specific surface chiral modulations with
the propagation direction perpendicular to the film surfaces [Wilson et al.
Phys. Rev. B 88, 214420 (2013)]. In this paper we show that the occurrence of
such chiral surface twists is a general effect attributed to all bulk and con-
fined magnetic crystals lacking inversion symmetry. We present experimental
investigations of this phenomenon in nanolayers of MnSi/Si(111) supported by
detailed theoretical analysis within the standard phenomenological model. In
magnetic nanolayers with intrinsic or induced chirality, such surface induced
instabilities become sizeable effects and play a crucial role in the formation
of skyrmion lattices and other nontrivial chiral modulations. | cond-mat_mes-hall |
The origin of the core-level binding energy shifts in nanoclusters: We investigate the shifts of the core-level binding energies in small gold
nanoclusters by using {\it ab initio} density functional theory calculations.
The shift of the 4$f$ states is calculated for magic number nanoclusters in a
wide range of sizes and morphologies. We find a non-monotonous behavior of the
core-level shift in nanoclusters depending on the size. We demonstrate that
there are three main contributions to the Au 4$f$ shifts, which depend
sensitively on the interatomic distances, coordination and quantum confinement.
They are identified and explained by the change of the on-site electrostatic
potential. | cond-mat_mes-hall |
Long-range Kitaev Chains via Planar Josephson Junctions: We show how a recently proposed solid state Majorana platform comprising a
planar Josephson junction proximitized to a 2D electron gas (2DEG) with Rashba
spin-orbit coupling and Zeeman field can be viewed as an effectively one
dimensional (1D) Kitaev chain with long-range pairing and hopping terms. We
highlight how the couplings of the 1D system may be tuned by changing
experimentally realistic parameters. We also show that the mapping is robust to
disorder by computing the Clifford pseudospectrum index in real space for the
long-range Kitaev chain across several topological phases. This mapping opens
up the possibility of using current experimental setups to explore 1D
topological superconductors with non-standard, and tunable couplings. | cond-mat_mes-hall |
Temperature evolution of the quantum Hall effect in the FISDW state:
Theory vs Experiment: We discuss the temperature dependence of the Hall conductivity $\sigma_{xy}$
in the magnetic-field-induced spin-density-wave (FISDW) state of the
quasi-one-dimensional Bechgaard salts (TMTSF)_2X. Electronic thermal
excitations across the FISDW energy gap progressively destroy the quantum Hall
effect, so $\sigma_{xy}(T)$ interpolates between the quantized value at zero
temperature and zero value at the transition temperature T_c, where FISDW
disappears. This temperature dependence is similar to that of the superfluid
density in the BCS theory of superconductivity. More precisely, it is the same
as the temperature dependence of the Fr\"ohlich condensate density of a regular
CDW/SDW. This suggests a two-fluid picture of the quantum Hall effect, where
the Hall conductivity of the condensate is quantized, but the condensate
fraction of the total electron density decreases with increasing temperature.
The theory appears to agree with the experimental results obtained by measuring
all three components of the resistivity tensor simultaneously on a
(TMTSF)_2PF_6 sample and then reconstructing the conductivity tensor. | cond-mat_mes-hall |
Accessing the anisotropic non-thermal phonon populations in black
phosphorus: We combine femtosecond electron diffuse scattering experiments and
first-principles calculations of the coupled electron-phonon dynamics to
provide a detailed momentum-resolved picture of the ultrafast lattice
thermalization in a thin film of black phosphorus. The measurements reveal the
emergence of highly anisotropic non-thermal phonon populations which persist
for several picoseconds following excitation of the electrons with a light
pulse. Combining ultrafast dynamics simulations based on the time-dependent
Boltzmann formalism and calculations of the structure factor, we reproduce the
experimental data and identify the vibrational modes primarily responsible for
the carrier relaxation via electron-phonon coupling and the subsequent lattice
thermalization via phonon-phonon scattering. In particular, we attribute the
non-equilibrium lattice dynamics of black phosphorus to highly-anisotropic
phonon-assisted scattering processes, which are primarily mediated by
high-energy optical phonons. Our approach paves the way towards unravelling and
controlling microscopic energy-flow pathways in two-dimensional materials and
van der Waals heterostructures, and may also be extended to other
non-equilibrium phenomena involving coupled electron-phonon dynamics such as
superconductivity, phase transitions or polaron physics. | cond-mat_mes-hall |
Electrostatic quantum dot confinement in phosphorene: We consider states localized by electrostatic potentials in phosphorene using
an atomistic tight binding approach. From the results of the tight-binding
calculations of the confined states we extract effective masses for the
conduction band electrons in the armchair and zigzag directions. The masses
derived in this way are used for a simple single-band effective mass model
which, as we find, reproduces very well the tight-binding energy spectrum in
external magnetic field, the probability densities and the interaction effects.
Both methods produce Wigner crystallization for the ground-state of the
electron pair with the single-electron islands separated in the armchair
direction already for small quantum dots. | cond-mat_mes-hall |
Asymmetric power dissipation in electronic transport through a quantum
point contact: We investigate the power dissipated by an electronic current flowing through
a quantum point contact in a two-dimensional electron gas. Based on the
Landauer-B\"uttiker approach to quantum transport, we evaluate the power that
is dissipated on the two sides of the constriction as a function of the Fermi
energy, temperature, and applied voltage. We demonstrate that an asymmetry
appears in the dissipation, which is most pronounced when the quantum point
contact is tuned to a conductance step where the transmission strongly depends
on energy. At low temperatures, the asymmetry is enhanced when the temperature
increases. An estimation for the position of the maximum dissipation is
provided. | cond-mat_mes-hall |
Electron states in the quantum wire with periodic serial structure: A model quantum wire embedded in a matrix permeable to electron waves is
investigated in terms of electronic states. The wire is assumed to have a 1D
crystal structure. Through electron waves propagating in its surroundings,
lateral modes are coupled with Bloch waves propagating along the wire axis,
which results in modes splitting into multiplets. The results presented in this
study have been obtained by direct solution of the Schrodinger equation in the
effective mass approximation. | cond-mat_mes-hall |
Subgap features due to quasiparticle tunneling in quantum dots coupled
to superconducting leads: We present a microscopic theory of transport through quantum dot set-ups
coupled to superconducting leads. We derive a master equation for the reduced
density matrix to lowest order in the tunneling Hamiltonian and focus on
quasiparticle tunneling. For high enough temperatures transport occurs in the
subgap region due to thermally excited quasiparticles, which can be used to
observe excited states of the system for low bias voltages. On the example of a
double quantum dot we show how subgap transport spectroscopy can be done.
Moreover, we use the single level quantum dot coupled to a normal and a
superconducting lead to give a possible explanation for the subgap features
observed in the experiments published in Appl. Phys. Lett. 95, 192103 (2009). | cond-mat_mes-hall |
Atomic relaxation and electronic structure in twisted bilayer MoS2 with
rotation angle of 5.09 degrees: It is now well established theoretically and experimentally that a moir\'e
pattern, due to a rotation of two atomic layers with respect to each other,
creates low-energy flat bands. First discovered in twisted bilayer graphene,
these new electronic states are at the origin of strong electronic correlations
and even of unconventional superconductivity. Twisted bilayers (tb) of
transition metal dichalcogenides (TMDs) also exhibit flat bands around their
semiconductor gap at small rotation angles. In this paper, we present a DFT
study to analyze the effect of the atomic relaxation on the low-energy bands of
tb-MoS2 with a rotation angle of 5.09 degrees. We show that in-plane atomic
relaxation is not essential here, while out-of-plane relaxation dominates the
electronic structure. We propose a simple and efficient atomic model to predict
this relaxation. | cond-mat_mes-hall |
Quantum Entanglement in Nanocavity Arrays: We show theoretically how quantum interference between linearly coupled modes
with weak local nonlinearity allows the generation of continuous variable
entanglement. By solving the quantum master equation for the density matrix, we
show how the entanglement survives realistic levels of pure dephasing. The
generation mechanism forms a new paradigm for entanglement generation in arrays
of coupled quantum modes. | cond-mat_mes-hall |
Disorder and localization effects on the local spectroscopic and
infrared-optical properties of $\mbox{Ga}_{1-x}\mbox{Mn}_x\mbox{As}$: We study numerically the influence of disorder and localization effects on
the local spectroscopic characteristics and infrared optical properties of
$\mbox{Ga}_{1-x}\mbox{Mn}_x\mbox{As}$. We treat the band structure and disorder
effects at an equal level by using exact diagonalization supercell simulation
method. This method accurately describes the low doping limit and gives a clear
picture of the transition to higher dopings, which captures the localization
effects inaccessible to other theoretical methods commonly used. Our
simulations capture the rich mid-gap localized states observed in scanning
tunneling microscopy studies and reproduce the observed features of the
infrared optical absorption experiments. We show clear evidence of a disordered
valence band model for metallic samples in which (i) there is no impurity band
detached from the valence band, (ii) the disorder tends to localize and pull
states near the top of the valence band into the gap region, and (iii) the
Fermi energy is located deep in the delocalized region away from the mobility
edge. We identify localized states deep in the gap region by visualizing the
probability distribution of the quasiparticles and connecting it to their
respective participation ratios. The analysis of the infrared-optical
absorption data indicates that it does not have a direct relation to the nature
of the states at the Fermi energy. | cond-mat_mes-hall |
Limitations of the kinetic theory to describe the near-field heat
exchanges in many-body systems: We investigate the radiative heat transfer along a chain of nanoparticles
using both a purely kinetic approach based on the solution of a Boltzmann
transport equation and an exact method (Landauer's approach) based on
fluctuational electrodynamics. We show that the kinetic theory generally fails
to predict properly the heat flux transported along the chain both at close
(near-field regime) and large separation (far-field regime) distances. We
report a deviation of a factor two between the heat fluxes predicted by the two
approaches in the diffusive regime of heat transport and we show that this
difference becomes even greater than two orders of magnitude in the ballistic
regime. | cond-mat_mes-hall |
2D massless QED Hall half-integer conductivity and graphene: Starting from the photon self-energy tensor in a magnetized medium, the 3D
complete antisymmetric form of the conductivity tensor is found in the static
limit of a fermion system $C$ non-invariant under fermion-antifermion exchange.
The massless relativistic 2D fermion limit in QED is derived by using the
compactification along the dimension parallel to the magnetic field. In the
static limit and at zero temperature the main features of quantum Hall effect
(QHE) are obtained: the half-integer QHE and the minimum value proportional to
$e^2/h$ for the Hall conductivity . For typical values of graphene the plateaus
of the Hall conductivity are also reproduced. | cond-mat_mes-hall |
Conductance Quantization and Magnetoresistance in Magnetic Point
Contacts: We theoretically study the electron transport through a magnetic point
contact (PC) with special attention to the effect of an atomic scale domain
wall (DW). The spin precession of a conduction electron is forbidden in such an
atomic scale DW and the sequence of quantized conductances depends on the
relative orientation of magnetizations between left and right electrodes. The
magnetoresistance is strongly enhanced for the narrow PC and oscillates with
the conductance. | cond-mat_mes-hall |
Spin-thermopower in interacting quantum dots: Using analytical arguments and the numerical renormalization group method we
investigate the spin-thermopower of a quantum dot in a magnetic field. In the
particle-hole symmetric situation the temperature difference applied across the
dot drives a pure spin current without accompanying charge current. For
temperatures and fields at or above the Kondo temperature, but of the same
order of magnitude, the spin-Seebeck coefficient is large, of the order of
k_B/e. Via a mapping, we relate the spin-Seebeck coefficient to the
charge-Seebeck coefficient of a negative-U quantum dot where the corresponding
result was recently reported by Andergassen et al. in Phys. Rev. B 84, 241107
(2011). For several regimes we provide simplified analytical expressions. In
the Kondo regime, the dependence of the spin-Seebeck coefficient on the
temperature and the magnetic field is explained in terms of the shift of the
Kondo resonance due to the field and its broadening with the temperature and
the field. We also consider the influence of breaking the particle-hole
symmetry and show that a pure spin current can still be realized provided a
suitable electric voltage is applied across the dot. Then, except for large
asymmetries, the behavior of the spin-Seebeck coefficient remains similar to
that found in the particle-hole symmetric point. | cond-mat_mes-hall |
A new kind of 2D topological insulators BiCN with a giant gap and its
substrate effects: Based on DFT calculation, we predict that BiCN, i.e., bilayer Bi films
passivated with -CN group, is a novel 2D Bi-based material with highly
thermodynamic stability, and demonstrate that it is also a new kind of 2D TI
with a giant SOC gap (? 1 eV) by direct calculation of the topological
invariant Z2 and obvious exhibition of the helical edge states. Monolayer h-BN
and MoS2 are identified as good candidate substrates for supporting the
nontrivial topological insulating phase of the 2D TI films, since the two
substrates can stabilize and weakly interact with BiCN via van derWaals
interaction and thus hardly affect the electronic properties, especially the
band topology. The topological properties are robust against the strain and
electric field. This may provide a promising platform for realization of novel
topological phases. | cond-mat_mes-hall |
Discovery of highly spin-polarized conducting surface states in the
strong spin-orbit coupling semiconductor Sb$_2$Se$_3$: Majority of the A$_2$B$_3$ type chalcogenide systems with strong spin-orbit
coupling, like Bi$_2$Se$_3$, Bi$_2$Te$_3$ and Sb$_2$Te$_3$ etc., are
topological insulators. One important exception is Sb$_2$Se$_3$, where a
topological non-trivial phase was argued to be possible under ambient
conditions, but such a phase could be detected to exist only under pressure. In
this Letter, we show that like Bi$_2$Se$_3$, Sb$_2$Se$_3$, displays generation
of highly spin-polarized current under mesoscopic superconducting point
contacts as measured by point contact Andreev reflection spectroscopy. In
addition, we observe a large negative and anisotropic magnetoresistance in
Sb$_2$Se$_3$, when the field is rotated in the basal plane. However, unlike in
Bi$_2$Se$_3$, in case of Sb$_2$Se$_3$ a prominent quasiparticle interference
(QPI) pattern around the defects could be obtained in STM conductance imaging.
Thus, our experiments indicate that Sb$_2$Se$_3$ is a regular band insulator
under ambient conditions, but due to it's high spin-orbit coupling, non-trivial
spin-texture exists on the surface and the system could be on the verge of a
topological insulator phase. | cond-mat_mes-hall |
Non-Hermitian boundary and interface states in nonreciprocal
higher-order topological metals and electrical circuits: Non-Hermitian skin-edge states emerge only at one edge in one-dimensional
nonreciprocal chains, where all states are localized at the edge irrespective
of eigenvalues. The bulk topological number is the winding number associated
with the complex energy spectrum, which is well defined for metals. We study
non-Hermitian nonreciprocal systems in higher dimensions, and propose to
realize them with the use of electric diode circuits. We first investigate
one-dimensional interface states between two domains carrying different
topological numbers, where all states are localized at the interface. They are
a generalization of the skin-edge states. Then we generalize them into higher
dimensions. We show that there emerge a rich variety of boundary states and
interface states including surface, line and point states in three-dimensional
systems. They emerge at boundaries of several domains carrying different
topological numbers. The resulting systems are the first-order, second-order
and third-order topological metals. Such states may well be observed by
measuring the two-point impedance in diode circuits. | cond-mat_mes-hall |
Graphene wormholes: A condensed matter illustration of Dirac fermions in
curved space: We study the properties of graphene wormholes in which a short nanotube acts
as a bridge between two graphene sheets, where the honeycomb carbon lattice is
curved from the presence of 12 heptagonal defects. By taking the nanotube
bridge with very small length compared to the radius, we develop an effective
theory of Dirac fermions to account for the low-energy electronic properties of
the wormholes in the continuum limit, where the frustration induced by the
heptagonal defects is mimicked by a line of fictitious gauge flux attached to
each of them. We find in particular that, when the effective gauge flux from
the topological defects becomes maximal, the zero-energy modes of the Dirac
equation can be arranged into two triplets, that can be thought as the
counterpart of the two triplets of zero modes that arise in the dual instance
of the continuum limit of large spherical fullerenes. We further investigate
the graphene wormhole spectra by performing a numerical diagonalization of
tight-binding hamiltonians for very large lattices realizing the wormhole
geometry. The correspondence between the number of localized electronic states
observed in the numerical approach and the effective gauge flux predicted in
the continuum limit shows that graphene wormholes can be consistently described
by an effective theory of two Dirac fermion fields in the curved geometry of
the wormhole, opening the possibility of using real samples of the carbon
material as a playground to experiment with the interaction between the
background curvature and the Dirac fields. | cond-mat_mes-hall |
Ab initio study of structural stability of small 3$d$ late transition
metal clusters : Interplay of magnetization and hybridization: Using first-principles density functional theory based calculations, we
analyze the structural stability of small clusters of 3$d$ late transition
metals. We consider the relative stability of the two structures - layer-like
structure with hexagonal closed packed stacking and more compact structure of
icosahedral symmetry. We find that the Co clusters show an unusual stability in
hexagonal symmetry compared to the small clusters of other members which are
found to stabilize in icosahedral symmetry based structure. Our study reveals
that this is driven by the interplay between the magnetic energy gain and the
gain in covalency through $s$-$d$ hybridization effect. Although we have
focused our study primarily on clusters of size 19 atoms, we find that this
behavior to be general for clusters having sizes between 15 and 20. | cond-mat_mes-hall |
A Mechanical Implementation and Diagrammatic Calculation of Entangled
Basis States: We give for the first time a diagrammatic calculational tool of quantum
entanglement. We present a pedagogical and simple mechanical implementation of
quantum entanglement or "spooky action at a distance" to give a tangible
realization of this weird quantum mechanical concept alien to classical
physics. When two or more particles are correlated in a certain way, no matter
how far apart they are in space, their states remain correlated. Their
correlation, which is instantaneous, does not seem to involve any communication
which is limited by the speed of light. The same mechanical implementation
demonstrates the fundamental physical limits of any computational processes.
The analytical derivations of calculational entangled basis states are given
and their corresponding diagrammatic representations give an efficient aid in
determining the calculational entangled basis states. A quantum Fourier
transform for the two-state diagrams representing entangled basis states
('renormalized qubits') can also be formulated. Our results seem to advocate
the idea that quantum entanglement generates the extra dimensions of the
gravitational theory, indeed quantum entanglement is related to deep issues in
the unification of general relativity and quantum mechanics. This extra
dimensions of spacetime entanglement are currently being speculated in the
literature. | cond-mat_mes-hall |
"Reservoir model" for shallow modulation-doped digital magnetic quantum
wells: Digital Magnetic Heterostructures (DMH) are semiconductor structures with
magnetic monolayers. Here we study electronic and magneto-transport properties
of shallow modulation-doped (ZnSe/ZnCdSe) DMHs with spin-5/2 Mn impurities. We
compare the "reservoir" model, possibly relevant to shallow geometries, to the
usual "constant-density" model. Our results are obtained by solving the
Kohn-Sham equations within the Local Spin Density Approximation (LSDA). In the
presence of a magnetic field, we show that both models exhibit characteristic
behaviors for the electronic structure, two-dimensional carrier density, Fermi
level and transport properties. Our results illustrate the relevance of
exchange and correlation effects in the study shallow heterostructures of the
group II-VI. | cond-mat_mes-hall |
Analysis of the exciton-exciton interaction in semiconductor quantum
wells: The exciton-exciton interaction is investigated for quasi-two-dimensional
quantum structures. A bosonization scheme is applied including the full spin
structure. For generating the effective interaction potentials, the
Hartree-Fock and Heitler-London approaches are improved by a full two-exciton
calculation which includes the van der Waals effect. With these potentials the
biexciton formation in bilayer systems is investigated. For coupled quantum
wells the two-body scattering matrix is calculated and employed to give a
modified relation between exciton density and blue shift. Such a relation is of
central importance for gauging exciton densities in experiments which pave the
way toward Bose-Einstein condensation of excitons. | cond-mat_mes-hall |
Probing surface states exposed by crystal terminations at arbitrary
orientations of three-dimensional topological insulators: The topological properties of the bulk band structure of a three-dimensional
topological insulator (TI) manifest themselves in the form of metallic surface
states. In this paper, we propose a probe which directly couples to an exotic
property of these surface states, namely the spin-momentum locking. We show
that the information regarding the spin textures, so extracted, for different
surfaces can be put together to reconstruct the parameters characterizing the
bulk band structure of the material, hence acting as a hologram. For specific
TI materials like, $\text{Bi}_2\text{Se}_3, \text{Bi}_2\text{Te}_3 \text{and
Sb}_2\text{Te}_3$, the planar surface states are distinct from one another with
regard to their spectrum and the associated spin texture for each angle
($\theta$), which the normal to the surface makes with the crystal growth axis.
We develop a tunnel Hamiltonian between such arbitrary surfaces and a spin
polarized STM which provides a unique fingerprint of the dispersion and the
associated spin texture corresponding to each $\theta$. Additionally, the
theory presented in this article can be used to extract value of $\theta$ for a
given arbitrary planar surface from the STM spectra itself hence effectively
mimicking X-ray spectroscopy. | cond-mat_mes-hall |
Contamination of polymethylmethacrylate by organic quantum emitters: We report the observation of ubiquitous contamination of
polymethylmethacrylate by organic molecules with optical activity in the
visible spectral range. Contamination sites of individual solvent-specific
fluorophores in thin films of polymethylmethacrylate constitute fluorescence
hot-spots with quantum emission statistics and quantum yields approaching 30%
at cryogenic temperatures. Our findings not only resolve prevalent puzzles in
the assignment of spectral features to various nanoemitters in polymer
matrices, they also identify means for simple and cost-efficient realization of
single-photon sources in the visible spectral range. | cond-mat_mes-hall |
Effect of electron-phonon scattering on shot noise in nanoscale
junctions: We investigate the effect of electron-phonon inelastic scattering on shot
noise in nanoscale junctions in the regime of quasi-ballistic transport. We
predict that when the local temperature of the junction is larger than its
lowest vibrational mode energy $eV_c$, the inelastic contribution to shot noise
(conductance) increases (decreases) with bias as $V$ ($\sqrt{V}$). The
corresponding Fano factor thus increases as $\sqrt{V}$. We also show that the
inelastic contribution to the Fano factor saturates with increasing thermal
current exchanged between the junction and the bulk electrodes to a value
which, for $V>>V_c$, is independent of bias. A measurement of shot noise may
thus provide information about the local temperature and heat dissipation in
nanoscale conductors. | cond-mat_mes-hall |
Quantum heat transfer: A Born Oppenheimer method: We develop a Born-Oppenheimer type formalism for the description of quantum
thermal transport along hybrid nanoscale objects. Our formalism is suitable for
treating heat transfer in the off-resonant regime, where e.g., the relevant
vibrational modes of the interlocated molecule are high relative to typical
bath frequencies, and at low temperatures when tunneling effects dominate. A
general expression for the thermal energy current is accomplished, in the form
of a generalized Landauer formula. In the harmonic limit this expression
reduces to the standard Landauer result for heat transfer, while in the
presence of nonlinearities multiphonon tunneling effects are realized. | cond-mat_mes-hall |
Topological Wannier cycles for the bulk and edges: Topological materials are often characterized by unique edge states which are
in turn used to detect different topological phases in experiments. Recently,
with the discovery of various higher-order topological insulators, such
spectral topological characteristics are extended from edge states to corner
states. However, the chiral symmetry protecting the corner states is often
broken in genuine materials, leading to vulnerable corner states even when the
higher-order topological numbers remain quantized and invariant. Here, we show
that a local artificial gauge flux can serve as a robust probe of the Wannier
type higher-order topological insulators which is effective even when the
chiral symmetry is broken. The resultant observable signature is the emergence
of the cyclic spectral flows traversing one or multiple band gaps. These
spectral flows are associated with the local modes bound to the artificial
gauge flux. This phenomenon is essentially due to the cyclic transformation of
the Wannier orbitals when the local gauge flux acts on them. We extend
topological Wannier cycles to systems with C2 and C3 symmetries and show that
they can probe both the bulk and the edge Wannier centers, yielding rich
topological phenomena. | cond-mat_mes-hall |
Relation between unidirectional spin Hall magnetoresistance and spin
current-driven magnon generation: We perform electronic measurements of unidirectional spin Hall
magnetoresistance (USMR) in a Permalloy/Pt bilayer, in conjunction with
magneto-optical Brillouin light spectroscopy of spin current-driven magnon
population. We show that the current dependence of USMR closely follows the
dipolar magnon density, and that both dependencies exhibit the same scaling
over a large temperature range of 80-400 K. These findings demonstrate a close
relationship between spin current-driven magnon generation and USMR, and
indicate that the latter is likely dominated by the dipolar magnons. | cond-mat_mes-hall |
Fano-Josephson effect of Majorana bound states: We investigate the Josephson current in a Fano-Josephson junction formed by
the direct coupling between two topological superconducting wires and their
indirect coupling via a quantum dot. It is found that when two Majorana zero
modes respectively appear in the wires, the Fano interference causes abundant
Josephson phase transition processes. What is notable is that in the presence
of appropriate direct and indirect inter-wire couplings, the fractional
Josephson effect disappears and then such a structure transforms into a
$0$-phase normal Josephson junction. On the other hand, if finite coupling
occurs between the Majorana bound states at the ends of each wire, the normal
Josepshon current is robustly in the $0$ phase, weakly dependent on the Fano
effect. We believe that the results in this work are helpful for describing the
Fano-modified Josephson effect. | cond-mat_mes-hall |
Steady-State Entanglement in the Nuclear Spin Dynamics of a Double
Quantum Dot: We propose a scheme for the deterministic generation of steady-state
entanglement between the two nuclear spin ensembles in an electrically defined
double quantum dot. Due to quantum interference in the collective coupling to
the electronic degrees of freedom, the nuclear system is actively driven into a
two-mode squeezed-like target state. The entanglement build-up is accompanied
by a self-polarization of the nuclear spins towards large Overhauser field
gradients. Moreover, the feedback between the electronic and nuclear dynamics
leads to multi-stability and criticality in the steady-state solutions. | cond-mat_mes-hall |
Modulating spin relaxation in nanowires with infrared light at room
temperature: Spintronic devices usually rely on long spin relaxation times and/or lengths
for optimum performance. Therefore, the ability to modulate these quantities
with an external agent offers unique possibilities. The dominant spin
relaxation mechanism in most technologically important semiconductors is the
D'yakonov-Perel' (DP) mechanism which vanishes if the spin carriers (electrons)
are confined to a single conduction subband in a quantum wire grown in certain
crystallographic directions, or polycrystalline quantum wires. Here, we report
modulating the DP spin relaxation rate (and hence the spin relaxation length)
in self assembled 50-nm diameter InSb nanowires with infrared light at room
temperature. In the dark, almost all the electrons in the nanowires are in the
lowest conduction subband at room temperature, resulting in near-complete
absence of DP relaxation. This allows observation of spin-sensitive effects in
the magnetoresistance. Under infrared illumination, electrons are photoexcited
to higher subbands and the DP spin relaxation mechanism is revived, leading to
a three-fold decrease in the spin relaxation length. Consequently, the spin
sensitive effects are no longer observable under illumination. This phenomenon
may have applications in spintronic room-temperature infrared photodetection. | cond-mat_mes-hall |
Thermally-Assisted Spin-Transfer Torque Magnetization Reversal of
Uniaxial Nanomagnets in Energy Space: The asymptotic behavior of switching time as a function of current for a
uniaxial macrospin under the effects of both spin-torque and thermal noise is
explored analytically by focusing on its diffusive energy space dynamics. The
scaling dependence ($I\rightarrow 0$, $<\tau\propto\exp(-\xi(1-I)^2)$) is shown
to confirm recent literature results. The analysis shows the mean switching
time to be functionally independent of the angle between the spin current and
magnet's uniaxial axes. These results have important implications for modeling
the energetics of thermally assisted magnetization reversal of spin transfer
magnetic random access memory bit cells. | cond-mat_mes-hall |
Chirality from interfacial spin-orbit coupling effects in magnetic
bilayers: As nanomagnetic devices scale to smaller sizes, spin-orbit coupling due to
the broken structural inversion symmetry at interfaces becomes increasingly
important. Here we study interfacial spin-orbit coupling effects in magnetic
bilayers using a simple Rashba model. The spin-orbit coupling introduces
chirality into the behavior of the electrons and through them into the
energetics of the magnetization. In the derived form of the magnetization
dynamics, all of the contributions that are linear in the spin-orbit coupling
follow from this chirality, considerably simplifying the analysis. For these
systems, an important consequence is a correlation between the
Dzyaloshinskii-Moriya interaction and the spin-orbit torque. We use this
correlation to analyze recent experiments. | cond-mat_mes-hall |
Excitons in atomically thin black phosphorus: Raman scattering and photoluminescence spectroscopy are used to investigate
the optical properties of single layer black phosphorus obtained by mechanical
exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy,
performed in situ on the same flake as the photoluminescence measurements,
demonstrates the single layer character of the investigated samples. The
emission spectra, dominated by excitonic effects, display the expected in plane
anisotropy. The emission energy depends on the type of substrate on which the
flake is placed due to the different dielectric screening. Finally, the blue
shift of the emission with increasing temperature is well described using a two
oscillator model for the temperature dependence of the band gap. | cond-mat_mes-hall |
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