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Magically strained bilayer graphene with flat bands: Twist bilayer graphenes with magical angle have nearly flat band, which
become strongly correlated electron systems. Herein, we propose another system
based on strained bilayer graphene that have flat band at the intrinsic Fermi
level. The top and bottom layers are uniaxially stretched along different
directions. When the strength and directions of the strain satisfy certain
condition, the periodical lattices of the two layers are commensurate to each
other. The regions with AA, AB and BA stacking arrange in a triangular lattice.
With magical strain, the bands around the intrinsic Fermi level are nearly flat
and have large gap from the other bands. This system could provide more
feasible platform for graphene-based integrated electronic system with
superconductivity. | cond-mat_mes-hall |
Maxwell's demon in a double quantum dot with continuous charge detection: Converting information into work has during the last decade gained renewed
interest as it gives insight into the relation between information theory and
thermodynamics. Here we theoretically investigate an implementation of
Maxwell's demon in a double quantum dot and demonstrate how heat can be
converted into work using only information. This is accomplished by
continuously monitoring the charge state of the quantum dots and transferring
electrons against a voltage bias using a feedback scheme. We investigate the
electrical work produced by the demon and find a non-Gaussian work
distribution. To illustrate the effect of a realistic charge detection scheme,
we develop a model taking into account noise as well as a finite delay time,
and show that an experimental realization is feasible with present day
technology. Depending on the accuracy of the measurement, the system is
operated as an implementation of Maxwell's demon or a single-electron pump. | cond-mat_mes-hall |
Spin-Orbit gauge and quantum spin Hall effect: We have shown that the non-Abelian spin-orbit gauge field strength of the
Rashba and Dresselhaus interactions, when split into two Abelian field
strengths, the Hamiltonian of the system can be re-expressed as a Landau level
problem with a particular relation between the two coupling parameters. The
quantum levels are created with up and down spins with opposite chirality and
leads to the quantum spin Hall effect. | cond-mat_mes-hall |
Detecting magneto-optical interactions in nanostructures: Effects due to magneto-optical interactions are responsible for most of the
phenomena discovered in optoelectronics and spintronics. Magneto-optical
interactions can generate elementary excitations of the order of light-magnetic
matter, which can flow under certain conditions. Here, we observe the
intensities of magneto-optical interactions in hexagonal arrays of magnetic
nanowires using experimental measurements and simulations. Nanowires of three
materials (cobalt-Co, iron-Fe, and nickel-Ni) were electrodeposited on alumina
membranes by the AC electrodeposition method. Our results reveal that the
magneto-optical behavior can produce, under certain conditions, a kind of
avalanche of magneto-optical interactions, which is dynamic. Such an
observation shows the possibility of generating a magneto-optical current
(spin-opto current). | cond-mat_mes-hall |
Delta-T noise for fractional quantum Hall states at different filling
factor: The current fluctuations due to a temperature bias, i.e. the delta-$T$ noise,
allow one to access properties of strongly interacting systems which cannot be
addressed by the usual voltage-induced noise. In this work, we study the full
delta-$T$ noise between two different fractional quantum Hall edge states, with
filling factors $(\nu_L,\nu_R)$ in the Laughlin sequence, coupled through a
quantum point contact and connected to two reservoirs at different
temperatures. We are able to solve exactly the problem for all couplings and
for any set of temperatures in the specific case of an hybrid junction
$(1/3,1)$. Moreover, we derive a universal analytical expression which connects
the delta-$T$ noise to the equilibrium one valid for all generic pairs
$(\nu_L,\nu_R)$ up to the first order in the temperature mismatch. We expect
that the linear term can be accessible in nowadays experimental set-ups. We
describe the two opposite coupling regimes focusing on the strong one which
correspond to a non-trivial situation. Our analysis on delta-$T$ noise allows
us to better understand the transport properties of strongly interacting
systems and to move toward more involved investigation concerning the
statistics and scaling dimension of their emergent excitations. | cond-mat_mes-hall |
The spin Hall effect: In metallic systems with spin-orbit coupling a longitudinal charge current
may generate a transverse pure spin current; vice-versa an injected pure spin
current may result in a transverse charge current. Such direct and inverse spin
Hall effects share the same microscopic origin: intrinsic band/device structure
properties, external factors such as impurities, or a combination of both. They
allow all-electrical manipulation of the electronic spin degrees of
freedom,i.e. without magnetic elements, and their transverse nature makes them
potentially dissipationless. It is customary to talk of spin Hall effects in
plural form, referring to a group of related phenomena typical of spin-orbit
coupled systems of lowered symmetry. | cond-mat_mes-hall |
Quantum many-body simulation using monolayer exciton-polaritons in
coupled-cavities: Quantum simulation is a promising approach to understand complex strongly
correlated many-body systems using relatively simple and tractable systems.
Photon-based quantum simulators have great advantages due to the possibility of
direct measurements of multi-particle correlations and ease of simulating
non-equilibrium physics. However, interparticle interaction in existing
photonic systems is often too weak limiting the potential of quantum
simulation. Here we propose an approach to enhance the interparticle
interaction using exciton-polaritons in MoS$_2$ monolayer quantum-dots embedded
in 2D photonic crystal microcavities. Realistic calculation yields optimal
repulsive interaction in the range of $1$-$10$~meV --- more than an order of
magnitude greater than the state-of-art value. Such strong repulsive
interaction is found to emerge neither in the photon-blockade regime for small
quantum dot nor in the polariton-blockade regime for large quantum dot, but in
the crossover between the two regimes with a moderate quantum-dot radius around
20~nm. The optimal repulsive interaction is found to be largest in MoS$_2$
among commonly used optoelectronic materials. Quantum simulation of strongly
correlated many-body systems in a finite chain of coupled cavities and its
experimental signature are studied via exact diagonalization of the many-body
Hamiltonian. A method to simulate 1D superlattices for interacting
exciton-polariton gases in serially coupled cavities is also proposed.
Realistic considerations on experimental realizations reveal advantages of
transition metal dichalcogenide monolayer quantum-dots over conventional
semiconductor quantum-emitters. | cond-mat_mes-hall |
Analytic approach to the edge state of the Kane-Mele Model: We investigate the edge state of a two-dimensional topological insulator
based on the Kane-Mele model. Using complex wave numbers of the Bloch wave
function, we derive an analytical expression for the edge state localized near
the edge of a semi-infinite honeycomb lattice with a straight edge. For the
comparison of the edge type effects, two types of the edges are considered in
this calculation; one is a zigzag edge and the other is an armchair edge. The
complex wave numbers and the boundary condition give the analytic equations for
the energies and the wave functions of the edge states. The numerical solutions
of the equations reveal the intriguing spatial behaviors of the edge state. We
define an edge-state width for analyzing the spatial variation of the
edge-state wave function. Our results show that the edge-state width can be
easily controlled by a couple of parameters such as the spin-orbit coupling and
the sublattice potential. The parameter dependences of the edge-state width
show substantial differences depending on the edge types. These demonstrate
that, even if the edge states are protected by the topological property of the
bulk, their detailed properties are still discriminated by their edges. This
edge dependence can be crucial in manufacturing small-sized devices since the
length scale of the edge state is highly subject to the edges. | cond-mat_mes-hall |
Unravelling the electrical properties of epitaxial Graphene nanoribbons: The size-dependent electrical resistivity of single-layer graphene ribbons
has been studied experimentally for ribbon widths from 16 nm to 320 nm. The
experimental findings are that the resistivity follows a more dramatic trend
than that seen for metallic nanowires of similar dimensions, due to a
combination of surface scattering from the edges, band-gap related effects and
shifts in the Fermi level due to edge effects. We show that the Charge
Neutrality point switches polarity below a ribbon width of around 50 nm, and
that at this point, the thermal coefficient of resistance is a maximum. The
majority doping type therefore can be controlled by altering ribbon width. We
also demonstrate that an alumina passivation layer has a significant effect on
the mean free path of the charge carriers within the graphene, which can be
probed directly via measurements of the width-dependent resistivity. We propose
a model for conduction that takes edge and confinement effects into account. | cond-mat_mes-hall |
Optical manipulation of nuclear spin by a two-dimensional electron gas: Conduction electrons are used to optically polarize, detect and manipulate
nuclear spin in a (110) GaAs quantum well. Using optical Larmor magnetometry,
we find that nuclear spin can be polarized along or against the applied
magnetic field, depending on field polarity and tilting of the sample with
respect to the optical pump beam. Periodic optical excitation of the
quantum-confined electron spin reveals a complete spectrum of optically-induced
and quadrupolar-split nuclear resonances, as well as evidence for delta m = 2
transitions. | cond-mat_mes-hall |
Classification of Exceptional Nodal Topologies Protected by
$\mathcal{PT}$ Symmetry: Exceptional degeneracies, at which both eigenvalues and eigenvectors
coalesce, and parity-time ($\mathcal{PT}$) symmetry, reflecting balanced gain
and loss in photonic systems, are paramount concepts in non-Hermitian systems.
We here complete the topological classification of exceptional nodal
degeneracies protected by $\mathcal{PT}$ symmetry in up to three dimensions and
provide simple example models whose exceptional nodal topologies include
previously overlooked possibilities such as second-order knotted surfaces of
arbitrary genus, third-order knots and fourth-order points. | cond-mat_mes-hall |
Dynamical characterization of Weyl nodes in Floquet Weyl semimetal
phases: Due to studies in nonequilibrium (periodically-driven) topological matter, it
is now understood that some topological invariants used to classify equilibrium
states of matter do not suffice to describe their nonequilibrium counterparts.
Indeed, in Floquet systems the additional gap arising from the periodicity of
the quasienergy Brillouin zone often leads to unique topological phenomena
without equilibrium analogues. In the context of Floquet Weyl semimetal, Weyl
points may be induced at both quasienergy zero and $\pi/T$ ($T$ being the
driving period) and these two types of Weyl points can be very close to each
other in the momentum space. Because of their momentum-space proximity, the
chirality of each individual Weyl point may become hard to characterize in both
theory and experiments, thus making it challenging to determine the system's
overall topology. In this work, inspired by the construction of dynamical
winding numbers in Floquet Chern insulators, we propose a dynamical invariant
capable of characterizing and distinguishing between Weyl points at different
quasienergy values, thus advancing one step further in the topological
characterization of Floquet Weyl semimetals. To demonstrate the usefulness of
such a dynamical topological invariant, we consider a variant of the
periodically kicked Harper model (the very first model in studies of Floquet
topological phases) that exhibits many Weyl points, with the number of Weyl
points rising unlimitedly with the strength of some system parameters.
Furthermore, we investigate the two-terminal transport signature associated
with the Weyl points. Theoretical findings of this work pave the way for
experimentally probing the rich topological band structures of some seemingly
simple Floquet semimetal systems. | cond-mat_mes-hall |
Electron transport through molecular bridge systems: Electron transport characteristics are investigated through some molecular
chains attached to two non-superconducting electrodes by the use of Green's
function method. Here we do parametric calculations based on the tight-binding
formulation to characterize the electron transport through such bridge systems.
The transport properties are significantly influenced by (a) the length of the
molecular chain and (b) the molecule-to-electrodes coupling strength and here
we focus are results in these aspects. In this context we also discuss the
steady state current fluctuations, the so-called shot noise, which is a
consequence of the quantization of charge and is not directly available through
conductance measurements. | cond-mat_mes-hall |
Nonlocal Response and Anamorphosis: the Case of Few-Layer Black
Phosphorus: Few-layer black phosphorus was recently rediscovered as a narrow-bandgap
atomically thin semiconductor and has already attracted unprecedented attention
due to its interesting properties. One feature of this material that sets it
apart from other atomically thin crystals is its structural in-plane anisotropy
which manifests in strongly anisotropic transport characteristics. However,
traditional angle-resolved conductance measurements present a challenge for
nanoscale systems such as black phosphorus, calling for new approaches in
precision studies of transport anisotropy. Here we show that the nonlocal
response, being exponentially sensitive to the anisotropy value, provides a
powerful tool for determining the anisotropy. This is established by combining
measurements of the orientation-dependent nonlocal resistance response with the
analysis based on the anamorphosis relations. We demonstrate that the nonlocal
response can differ by orders of magnitude for different crystallographic
directions even when the anisotropy is at most order-one, allowing us to
extract accurate anisotropy values. | cond-mat_mes-hall |
Magnonic bending, phase shifting and interferometry in a 2D
reconfigurable nanodisk crystal: Strongly-interacting nanomagnetic systems are pivotal across next-generation
technologies including reconfigurable magnonics and neuromorphic computation.
Controlling magnetisation state and local coupling between neighbouring
nanoelements allows vast reconfigurable functionality and a host of associated
functionalities. However, existing designs typically suffer from an inability
to tailor inter-element coupling post-fabrication and nanoelements restricted
to a pair of Ising-like magnetisation states. Here, we propose a new class of
reconfigurable magnonic crystal incorporating nanodisks as the functional
element. Magnetic nanodisks are crucially bistable in macrospin and vortex
states, allowing inter-element coupling to be selectively activated (macrospin)
or deactivated (vortex). Through microstate engineering, we leverage the
distinct coupling behaviours and magnonic band structures of bistable nanodisks
to achieve reprogrammable magnonic waveguiding, bending, gating and
phase-shifting across a 2D network. The potential of nanodisk-based magnonics
for wave-based computation is demonstrated via an all-magnon interferometer
exhibiting XNOR logic functionality. Local microstate control is achieved here
via topological magnetic writing using a magnetic force microscope tip. | cond-mat_mes-hall |
Continuum model for chiral induced spin selectivity in helical molecules: A minimal model is exactly solved for electron spin transport on a helix.
Electron transport is assumed to be supported by well oriented $p_z$ type
orbitals on base molecules forming a staircase of definite chirality. In a
tight binding interpretation, the SOC opens up an effective $\pi_z-\pi_z$
coupling via interbase $p_{x,y}-p_z$ hopping, introducing spin coupled
transport. The resulting continuum model spectrum shows two Kramers doublet
transport channels with a gap proportional to the SOC. Each doubly degenerate
channel satisfies time reversal symmetry, nevertheless, a bias chooses a
transport direction and thus selects for spin orientation. The model predicts
which spin orientation is selected depending on chirality and bias, changes in
spin preference as a function of input Fermi level and scattering suppression
protected by the SO gap. We compute the spin current with a definite helicity
and find it to be proportional to the torsion of the chiral structure and the
non-adiabatic Aharonov- Anandan phase. To describe room temperature transport
we assume that the total transmission is the result of a product of coherent
steps limited by the coherence length. | cond-mat_mes-hall |
Pauli spin blockade in weakly coupled quantum dots: In a two-level system, constituted by two serially coupled single level
quantum dots, coupled to external leads we find that the current is suppressed
in one direction of biasing caused by a fully occupied two-electron triplet
state in the interacting region. The efficiency of the current suppression is
governed by the ratio between the interdot tunnelling rate and the level
off-set. In the opposite bias direction, the occupation of the two-electron
triplet is lifted which allows a larger current to flow through the system,
where the conductance is provided by transitions between one-electron states
and two-electron singlet states. Is is also shown that a finite ferromagnetic
interdot exchange interaction provides an extended range of the current
suppression, while an anti-ferromagnetic exchange leads to a decreased range of
the blockade regime. | cond-mat_mes-hall |
Charge Relaxation and Dephasing in Coulomb Coupled Conductors: The dephasing time in coupled mesoscopic conductors is caused by the
fluctuations of the dipolar charge permitted by the long range Coulomb
interaction. We relate the phase breaking time to elementary transport
coefficients which describe the dynamics of this dipole: the capacitance, an
equilibrium charge relaxation resistance and in the presence of transport
through one of the conductors a non-equilibrium charge relaxation resistance.
The discussion is illustrated for a quantum point contact in a high magnetic
field in proximity to a quantum dot. | cond-mat_mes-hall |
Spatial patterns of dissipative polariton solitons in semiconductor
microcavities: Semiconductor microcavities operating in the polaritonic regime are highly
non-linear, high speed systems due to the unique half-light, half-matter nature
of polaritons. Here, we report for the first time the observation of
propagating multi-soliton polariton patterns consisting of multi-peak
structures either along (x) or perpendicular to (y) the direction of
propagation. Soliton arrays of up to 5 solitons are observed, with the number
of solitons controlled by the size or power of the triggering laser pulse. The
break-up along the x direction occurs due to interplay of bistability, negative
effective mass and polariton-polariton scattering, while in the y direction the
break-up results from nonlinear phase-dependent interactions of propagating
fronts. We show the experimental results are in good agreement with numerical
modelling. Our observations are a step towards ultrafast all-optical signal
processing using sequences of solitons as bits of information. | cond-mat_mes-hall |
Reversible edge spin currents in antiferromagnetically proximitized
dichalcogenides: We explore proximity effects on transition metal dichalcogenide ribbons
deposited on antiferromagnetic (AFM) insulating substrates. We model these
hybrid heterostructures using a tight-binding model that incorporates exchange
and Rashba fields induced by proximity to the AFM material. The robust edge
states that disperse in the midgap of the dichalcogenide are strongly affected
by induced exchange fields that reflect different AFM ordering in the
substrate. This results in enhanced spin-orbit coupling effects and complex
spin projection content for states on zigzag ribbon edges. Gated systems that
shift the Fermi level in the midgap range are also shown to exhibit spin
polarized currents on these edges. Antiparallel exchange fields along the edge
results in spin currents that can reverse polarization with the applied field.
The added functionality of these hybrid structures can provide spintronic
devices and versatile platforms to further exploit proximity effects in diverse
material systems. | cond-mat_mes-hall |
Arbitrary qubit transformations on tuneable Rashba rings: An exact solution is presented for the time-dependent wavefunction of a
Kramers doublet which propagates around a quantum ring with tuneable Rashba
spin-orbit interaction. By propagating in segments it is shown that
Kramers-doublet qubits may be defined for which transformations on the Bloch
sphere may be performed for an integral number of revolutions around the ring.
The conditions for full coverage of the Bloch sphere are determined and
explained in terms of sequential qubit rotations due to electron motion along
the segments, with change of rotation axes between segments due to adiabatic
changes in the Rashba spin-orbit interaction. Prospects and challenges for
possible realizations are discussed for which rings based on InAs quantum wires
are promising candidates. | cond-mat_mes-hall |
Current-induced switching in transport through anisotropic magnetic
molecules: Anisotropic single-molecule magnets may be thought of as molecular switches,
with possible applications to molecular spintronics. In this paper, we consider
current-induced switching in single-molecule junctions containing an
anisotropic magnetic molecule. We assume that the carriers interact with the
magnetic molecule through the exchange interaction and focus on the regime of
high currents in which the molecular spin dynamics is slow compared to the time
which the electrons spend on the molecule. In this limit, the molecular spin
obeys a non-equilibrium Langevin equation which takes the form of a generalized
Landau-Lifshitz-Gilbert equation and which we derive microscopically by means
of a non-equilibrium Born-Oppenheimer approximation. We exploit this Langevin
equation to identify the relevant switching mechanisms and to derive the
current-induced switching rates. As a byproduct, we also derive S-matrix
expressions for the various torques entering into the Landau-Lifshitz-Gilbert
equation which generalize previous expressions in the literature to
non-equilibrium situations. | cond-mat_mes-hall |
Control of nonlocal magnon spin transport via magnon drift currents: Spin transport via magnon diffusion in magnetic insulators is important for a
broad range of spin-based phenomena and devices. However, the absence of the
magnon equivalent of an electric force is a bottleneck. In this work, we
demonstrate the controlled generation of magnon drift currents in yttrium iron
garnet/platinum heterostructures. By performing electrical injection and
detection of incoherent magnons, we find magnon drift currents that stem from
the interfacial Dzyaloshinskii-Moriya interaction. We can further control the
magnon drift by the orientation of the magnetic field. The drift current
changes the magnon propagation length by up to $\pm$ 6 % relative to diffusion.
We generalize the magnonic spin transport theory to include a finite drift
velocity resulting from any inversion asymmetric interaction, and obtain
results consistent with our experiments. | cond-mat_mes-hall |
Dephasing by extremely dilute magnetic impurities revealed by
Aharonov-Bohm oscillations: We have probed the magnetic field dependence of the electron phase coherence
time $\tau_\phi$ by measuring the Aharonov-Bohm conductance oscillations of
mesoscopic Cu rings. Whereas $\tau_\phi$ determined from the low-field
magnetoresistance saturates below 1 K, the amplitude of Aharonov-Bohm $h/e$
oscillations increases strongly on a magnetic field scale proportional to the
temperature. This provides strong evidence that a likely explanation for the
frequently observed saturation of $\tau_\phi$ at low temperature in weakly
disordered metallic thin films is the presence of extremely dilute magnetic
impurities. | cond-mat_mes-hall |
States near Dirac points of rectangular graphene dot in a magnetic field: In neutral graphene dots the Fermi level coincides with the Dirac points. We
have investigated in the presence of a magnetic field several unusual
properties of single electron states near the Fermi level of such a
rectangular-shaped graphene dot with two zigzag and two armchair edges. We find
that a quasi-degenerate level forms near zero energy and the number of states
in this level can be tuned by the magnetic field. The wavefunctions of states
in this level are all peaked on the zigzag edges with or without some weight
inside the dot. Some of these states are magnetic field-independent surface
states while the others are field-dependent. We have found a scaling result
from which the number of magnetic field-dependent states of large dots can be
inferred from those of smaller dots. | cond-mat_mes-hall |
Adiabatic and local approximations for the Kohn-Sham potential in
time-dependent Hubbard chains: We obtain the exact Kohn-Sham potentials $V_{\mathrm{KS}}$ of time-dependent
density-functional theory for 1D Hubbard chains, driven by a d.c.\ external
field, using the time-dependent electron density and current density obtained
from exact many-body time-evolution. The exact $V_{\mathrm{xc}}$ is compared to
the adiabatically-exact $V_{\mathrm{xc}}^{\mathrm{ad}}$ and the "instantaneous
ground state" $V_{\mathrm{xc}}^{\mathrm{igs}}$. The latter is shown to work
effectively in some cases when the former fails. Approximations for the
exchange-correlation potential $V_{\mathrm{xc}}$ and its gradient, based on the
local density and on the local current density, are also considered and both
physical quantities are observed to be far outside the reach of any possible
local approximation. Insight into the respective roles of ground-state and
excited-state correlation in the time-dependent system, as reflected in the
potentials, is provided by the pair correlation function. | cond-mat_mes-hall |
Ultrafast Quantum-path Interferometry Revealing the Generation Process
of Coherent Phonons: Optical dual-pulse pumping actively creates quantum-mechanical superposition
of the electronic and phononic states in a bulk solid. We here made transient
reflectivity measurements in an n-GaAs using a pair of relative-phase-locked
femtosecond pulses and found characteristic interference fringes. This is a
result of quantum-path interference peculiar to the dual-pulse excitation as
indicated by theoretical calculation. Our observation reveals that the pathway
of coherent phonon generation in the n-GaAs is impulsive stimulated Raman
scattering at the displaced potential due to the surface-charge field, even
though the photon energy lies in the opaque region. | cond-mat_mes-hall |
Carbon nanotube: a low-loss spin-current waveguide: We demonstrate with a quantum-mechanical approach that carbon nanotubes are
excellent spin-current waveguides and are able to carry information stored in a
precessing magnetic moment for long distances with very little dispersion and
with tunable degrees of attenuation. Pulsed magnetic excitations are predicted
to travel with the nanotube Fermi velocity and are able to induce similar
excitations in remote locations. Such an efficient way of transporting magnetic
information suggests that nanotubes are promising candidates for memory devices
with fast magnetization switchings. | cond-mat_mes-hall |
Imaging coherent electron wave flow in a two-dimensional electron gas: We measure the energy distribution of electrons passing through a
two-dimensional electron gas using a scanning probe microscope. We present
direct spatial images of coherent electron wave flow from a quantum point
contact formed in a GaAs/AlGaAs two-dimensional electron gas using a liquid He
cooled SPM. A negative voltage is placed on the tip, which creates a small
region of depleted electrons that backscatters electron waves. Oscillating the
voltage on the tip and locking into this frequency gives the spatial derivative
of electron flow perpendicular to the direction of current flow. We show images
of electron flow using this method. By measuring the amount of electrons
backscattered as a function of the voltage applied to the tip, the energy
distribution of electrons is measured. | cond-mat_mes-hall |
Magneto-resistance quantum oscillations in a magnetic two-dimensional
electron gas: Magneto-transport measurements of Shubnikov-de Haas (SdH) oscillations have
been performed on two-dimensional electron gases (2DEGs) confined in CdTe and
CdMnTe quantum wells. The quantum oscillations in CdMnTe, where the 2DEG
interacts with magnetic Mn ions, can be described by incorporating the
electron-Mn exchange interaction into the traditional Lifshitz-Kosevich
formalism. The modified spin splitting leads to characteristic beating pattern
in the SdH oscillations, the study of which indicates the formation of Mn
clusters resulting in direct anti-ferromagnetic Mn-Mn interaction. The Landau
level broadening in this system shows a peculiar decrease with increasing
temperature, which could be related to statistical fluctuations of the Mn
concentration. | cond-mat_mes-hall |
Minigap and Andreev bound states in ballistic graphene: A finite-size normal conductor, proximity-coupled to a superconductor has
been predicted to exhibit a so-called minigap, in which quasiparticle
excitations are prohibited. Here, we report on the direct observation of such a
minigap in ballistic graphene, coupled to superconducting MoRe leads. The
minigap is probed by finite bias spectroscopy through a weakly coupled junction
in the graphene region and its value is given by the dimensions of the device.
Besides the minigap, we observe a distinct peak in the differential resistance,
which we attribute to weakly coupled Andreev bound states (ABS) located near
the superconductor-graphene interface. For weak magnetic fields, the phase
accumulated in the normal-conducting region shifts the ABS in quantitative
agreement with predictions from tight-binding calculations based on the
Bogolioubov-de Gennes equation as well as with an analytical semiclassical
model. | cond-mat_mes-hall |
Quantum Plasmonic Nanoantennas: We study plasmonic excitations in the limit of few electrons, in one-atom
thick sodium chains, and characterize them based on collectivity. We also
compare the excitations to classical localised plasmon modes and find for the
longitudinal mode a quantum-classical transition around 10 atoms. The
transverse mode appears at much higher energies than predicted classically for
all chain lengths. The electric field enhancement is also considered which is
made possible by considering the effects of electron-phonon coupling on the
broadening of the electronic spectra. Large field enhancements are possible on
the molecular level allowing us to consider the validity of using molecules as
the ultimate small size limit of plasmonic antennas. Additionally, we consider
the case of a dimer system of two sodium chains, where the gap can be
considered as a picocavity, and we analyse the charge-transfer states and their
dependence on the gap size as well as chain size. Our results and methods are
useful for understanding and developing ultra-small, tunable and novel
plasmonic devices that utilise quantum effects that could have applications in
quantum optics, quantum metamaterials, cavity-quantum electrodynamics and
controlling chemical reactions, as well as deepening our understanding of
localised plasmons in low dimensional molecular systems. | cond-mat_mes-hall |
Spin-singlet hierarchy in the fractional quantum Hall effect: We show that the so-called permanent quantum Hall states are formed by the
integer quantum Hall effects on the Haldane-Rezayi quantum Hall state. Novel
conformal field theory description along with this picture is deduced. The odd
denominator plateaux observed around $\nu=5/2$ are the permanent states if the
$\nu=5/2$ plateau is the Haldane-Rezayi state. We point out that there is no
such hierarchy on other candidate states for $\nu=5/2$. We propose experiments
to test our prediction. | cond-mat_mes-hall |
Graphene Transistor as a Probe for Streaming Potential: We explore the dependence of electrical transport in a graphene field effect
transistor (GraFET) on the flow of the liquid within the immediate vicinity of
that transistor. We find large and reproducible shifts in the charge neutrality
point of GraFETs that are dependent on the fluid velocity and the ionic
concentration. We show that these shifts are consistent with the variation of
the local electrochemical potential of the liquid next to graphene that are
caused by the fluid flow (streaming potential). Furthermore, we utilize the
sensitivity of electrical transport in GraFETs to the parameters of the fluid
flow to demonstrate graphene-based mass flow and ionic concentration sensing.
We successfully detect a flow as small as~70nL/min, and detect a change in the
ionic concentration as small as ~40nM. | cond-mat_mes-hall |
Photovoltaic performances in a cavity-coupled double quantum dots
photocell: Revealing the quantum regime of photovoltaics is crucial to enhancing the
internal quantum efficiency of a double quantum dots (DQDs) photocell housed in
a cavity. In this study, the performance of a quantum photovoltaic is evaluated
based on the current-voltage and power-voltage characteristics in a
cavity-coupled DQDs photocell. The results show that the cavity-DQDs coupling
coefficient plays a dissipative role in the photovoltaic performance, and the
cavity has a limited size for the photovoltaic performance. Additionally, more
low-energy photons are easily absorbed by this cavity-coupled DQDs photocell
compared with the case without cavity. These results may provide some
strategies for improving the photoelectric conversion efficiency and internal
quantum efficiency of cavity-coupled DQDs photocells. | cond-mat_mes-hall |
Superoperator nonequilibrium Green's function theory of many-body
systems; Applications to charge transfer and transport in open junctions: Nonequilibrium Green's functions provide a powerful tool for computing the
dynamical response and particle exchange statistics of coupled quantum systems.
We formulate the theory in terms of the density matrix in Liouville space and
introduce superoperator algebra that greatly simplifies the derivation and the
physical interpretation of all quantities. Expressions for various observables
are derived directly in real time in terms of superoperator nonequilibrium
Green's functions (SNGF), rather than the artificial time-loop required in
Schwinger's Hilbert-space formulation. Applications for computing interaction
energies, charge densities, average currents, current induced fluorescence,
electroluminescence and current fluctuation (electron counting) statistics are
discussed. | cond-mat_mes-hall |
Bichromatic Rabi control of semiconductor qubits: Electrically-driven spin resonance is a powerful technique for controlling
semiconductor spin qubits. However, it faces challenges in qubit addressability
and off-resonance driving in larger systems. We demonstrate coherent
bichromatic Rabi control of quantum dot hole spin qubits, offering a
spatially-selective approach for large qubit arrays. By applying simultaneous
microwave bursts to different gate electrodes, we observe multichromatic
resonance lines and resonance anticrossings that are caused by the ac Stark
shift. Our theoretical framework aligns with experimental data, highlighting
interdot motion as the dominant mechanism for bichromatic driving. | cond-mat_mes-hall |
Thermally activated intersubband scattering and oscillating
magnetoresistance in quantum wells: Experimental studies of magnetoresistance in high-mobility wide quantum wells
reveal oscillations which appear with an increase in temperature to 10 K and
whose period is close to that of Shubnikov-de Haas oscillations. The observed
phenomenon is identified as magnetointersubband oscillations caused by the
scattering of electrons between two occupied subbands and the third subband
which becomes occupied as a result of thermal activation. These small-period
oscillations are less sensitive to thermal suppression than the largeperiod
magnetointersubband oscillations caused by the scattering between the first and
the second subbands. Theoretical study, based on consideration of electron
scattering near the edge of the third subband, gives a reasonable explanation
of our experimental findings. | cond-mat_mes-hall |
Dirac-Harper Theory for One Dimensional Moiré Superlattices: We study a Dirac Harper model for moir\'e bilayer superlattices where layer
antisymmetric strain periodically modulates the interlayer coupling between two
honeycomb lattices in one spatial dimension. Discrete and continuum
formulations of this model are analyzed. For sufficiently long moir\'e period
the we find low energy spectra that host a manifold of weakly dispersive bands
arising from a hierarchy of momentum and position dependent mass inversions. We
analyze their charge distributions, mode count and valley-coherence using exact
symmetries of the lattice model and approximate symmetries of a four-flavor
version of the Jackiw-Rebbi one dimensional solution. | cond-mat_mes-hall |
Opto-Electronic Characterization of Three Dimensional Topological
Insulators: We demonstrate that the terahertz/infrared radiation induced photogalvanic
effect, which is sensitive to the surface symmetry and scattering details, can
be applied to study the high frequency conductivity of the surface states in
(Bi1-xSbx)2Te3 based three dimensional (3D) topological insulators (TI). In
particular, measuring the polarization dependence of the photogalvanic current
and scanning with a micrometre sized beam spot across the sample, provides
access to (i) topographical inhomogeneity's in the electronic properties of the
surface states and (ii) the local domain orientation. An important advantage of
the proposed method is that it can be applied to study TIs at room temperature
and even in materials with a high electron density of bulk carriers. | cond-mat_mes-hall |
Ground-state quantum geometry in superconductor-quantum dot chains: Multiterminal Josephson junctions constitute engineered topological systems
in arbitrary synthetic dimensions defined by the superconducting phases.
Microwave spectroscopy enables the measurement of the quantum geometric tensor,
a fundamental quantity describing both the quantum geometry and the topology of
the emergent Andreev bound states in a unified manner. In this work we propose
an experimentally feasible multiterminal setup of $N$ quantum dots connected to
$N+1$ superconducting leads to study nontrivial topology in terms of the
many-body Chern number of the ground state. Moreover, we generalize the
microwave spectroscopy scheme to the multiband case and show that the elements
of the quantum geometric tensor of the noninteracting ground state can be
experimentally accessed from the measurable oscillator strengths at low
temperature. | cond-mat_mes-hall |
Hole Spin Coherence in a Ge/Si Heterostructure Nanowire: Relaxation and dephasing of hole spins are measured in a gate-defined Ge/Si
nanowire double quantum dot using a fast pulsed-gate method and dispersive
readout. An inhomogeneous dephasing time $T_2^* \sim 0.18~\mathrm{\mu s}$
exceeds corresponding measurements in III-V semiconductors by more than an
order of magnitude, as expected for predominately nuclear-spin-free materials.
Dephasing is observed to be exponential in time, indicating the presence of a
broadband noise source, rather than Gaussian, previously seen in systems with
nuclear-spin-dominated dephasing. | cond-mat_mes-hall |
Orbital Magnetism of Graphene Nanostructures: Bulk and Confinement
Effects: We consider the orbital magnetic properties of non-interacting charge
carriers in graphene-based nanostructures in the low-energy regime. The
magnetic response of such systems results both, frombulk contributions and from
confinement effects that can be particularly strong in ballistic quantum dots.
First we provide a comprehensive study of the magnetic susceptibility $\chi$ of
bulk graphene in a magnetic field for the different regimes arising from the
relative magnitudes of the energy scales involved, i.e. temperature, Landau
level spacing and chemical potential. We show that for finite temperature or
chemical potential, $\chi$ is not divergent although the diamagnetic
contribution $\chi_{0}$ from the filled valance band exhibits the well-known
$-B^{-1/2}$ dependence. We further derive oscillatory modulations of $\chi$,
corresponding to de Haas-van Alphen oscillations of conventional
two-dimensional electron gases. These oscillations can be large in graphene,
thereby compensating the diamagnetic contribution $\chi_{0}$ and yielding a net
paramagnetic susceptibility for certain energy and magnetic field regimes.
Second, we predict and analyze corresponding strong, confinement-induced
susceptibility oscillations in graphene-based quantum dots with amplitudes
distincly exceeding the corresponding bulk susceptibility. Within a
semiclassical approach we derive generic expressions for orbital magnetism of
graphene quantum dots with regular classical dynamics. Graphene-specific
features can be traced back to pseudospin interference along the underlying
periodic orbits. We demonstrate the quality of the semiclassical approximation
by comparison with quantum mechanical results for two exemplary mesoscopic
systems, a graphene disk with infinite mass-type edges and a rectangular
graphene structure with armchair and zigzag edges, using numerical
tight-binding calculations in the latter case. | cond-mat_mes-hall |
Spin-orbit interactions mediated negative differential resistance in a
quasi-two-dimensional electron gas with finite thickness: Effects of the spin-orbit interactions on the energy spectrum, Fermi surface
and spin dynamics are studied in structural- and bulk-inversion asymmetric
quasi-two-dimensional structures with a finite thickness in the presence of a
parabolic transverse confining potential. One-particle quantum mechanical
problem in the presence of an in-plane magnetic field is solved numerically
exact. Interplay of the spin-orbit interactions, orbital- and Zeeman-effects of
the in-plane magnetic field yields a multi-valley subband structure, typical
for realization of the Gunn effect. A possible Gunn-effect-mediated spin
accumulation is discussed. | cond-mat_mes-hall |
Modulation theory of quantum tunneling into a Calogero-Sutherland fluid: Quantum hydrodynamics of interacting electrons with a parabolic single
particle spectrum is studied using the Calogero-Sutherland model. The effective
action and modulation equations, describing evolution of periodic excitations
in the fluid, are derived. Applications to the problem of a single electron
tunneling into the FQHE edge state are discussed. | cond-mat_mes-hall |
Purcell effect at metal-insulator transitions: We investigate the spontaneous emission rate of a two-level quantum emitter
next to a composite medium made of randomly distributed metallic inclusions
embedded in a dielectric host matrix. In the near-field, the Purcell factor can
be enhanced by two-orders of magnitude relative to the case of an homogeneous
metallic medium, and reaches its maximum precisely at the insulator-metal
transition. By unveiling the role of the decay pathways on the emitter's
lifetime, we demonstrate that, close to the percolation threshold, the
radiation emission process is dictated by electromagnetic absorption in the
heterogeneous medium. We show that our findings are robust against change in
material properties, shape of inclusions, and apply for different effective
medium theories as well as for a wide range of transition frequencies. | cond-mat_mes-hall |
Correlated Insulator Behaviour at Half-Filling in Magic Angle Graphene
Superlattices: Van der Waals (vdW) heterostructures are an emergent class of metamaterials
comprised of vertically stacked two-dimensional (2D) building blocks, which
provide us with a vast tool set to engineer their properties on top of the
already rich tunability of 2D materials. One of the knobs, the twist angle
between different layers, plays a crucial role in the ultimate electronic
properties of a vdW heterostructure and does not have a direct analog in other
systems such as MBE-grown semiconductor heterostructures. For small twist
angles, the moir\'e pattern produced by the lattice misorientation creates a
long-range modulation. So far, the study of the effect of twist angles in vdW
heterostructures has been mostly concentrated in graphene/hexagonal boron
nitride (h-BN) twisted structures, which exhibit relatively weak interlayer
interaction due to the presence of a large bandgap in h-BN. Here we show that
when two graphene sheets are twisted by an angle close to the theoretically
predicted 'magic angle', the resulting flat band structure near charge
neutrality gives rise to a strongly-correlated electronic system. These flat
bands exhibit half-filling insulating phases at zero magnetic field, which we
show to be a Mott-like insulator arising from electrons localized in the
moir\'e superlattice. These unique properties of magic-angle twisted bilayer
graphene (TwBLG) open up a new playground for exotic many-body quantum phases
in a 2D platform made of pure carbon and without magnetic field. The easy
accessibility of the flat bands, the electrical tunability, and the bandwidth
tunability though twist angle may pave the way towards more exotic correlated
systems, such as unconventional superconductors or quantum spin liquids. | cond-mat_mes-hall |
Enhanced thermoelectric response in the fractional quantum Hall effect: We study the linear thermoelectric response of a quantum dot embedded in a
constriction of a quantum Hall bar with fractional filling factors nu=1/m
within Laughlin series. We calculate the figure of merit ZT for the maximum
efficiency at a fixed temperature difference. We find a significant enhancement
of this quantity in the fractional filling in relation to the integer-filling
case, which is a direct consequence of the fractionalization of the electron in
the fractional quantum Hall state. We present simple theoretical expressions
for the Onsager coefficients at low temperatures, which explicitly show that ZT
and the Seebeck coefficient increase with m. | cond-mat_mes-hall |
Spin soliton of Holstein model with spin-orbit coupling in
one-dimensional conjugated polymers: For Holstein model with Rashba spin-orbit coupling (SOC) we establish the
nonlinear Schr\"odinger equations and obtain exact soliton solution
analytically. It is found that the soliton is spin polarized determined both by
the SOC and the electron-phonon (e-ph) interaction. The soliton can be used to
describe the spin transport or spin current in organic semiconductors. | cond-mat_mes-hall |
Transport signatures of symmetry protection in 1D Floquet topological
insulators: Time-periodic external drives have emerged as a powerful tool to artificially
create topological phases of matter. Prime examples are Floquet topological
insulators (FTIs), where a gapped bulk supports in-gap edge states, protected
against symmetry-preserving local perturbations. Similar to an ordinary static
topological insulator, the robustness of an edge state in a one-dimensional
(1D) FTI shows up as a pinning of its quasienergy level, but now inside one of
two distinct bulk gaps. Here we propose a scheme for probing this unique
feature by observing transport characteristics of a 1D finite-sized FTI
attached to external leads. We present predictions for transmission spectra
using a nonequilibrium Green's function approach. Our analysis covers FTIs with
time-independent and periodically driven boundary perturbations which either
preserve or break the protecting chiral symmetry. | cond-mat_mes-hall |
Effective Hamiltonians in the Quantum Rabi Problem: We revisit the theoretical description of the ultrastrong light-matter
interaction in terms of exactly solvable effective Hamiltonians. A perturbative
approach based on polaronic and spin-dependent squeezing transformations
provides an effective Hamiltonian for the quantum Rabi model up to the second
order in the expansion parameter. The model consistently includes both rotating
and counter-rotating terms, going therefore beyond the rotating wave
approximation. Analytical and numerical results show that the proposed
Hamiltonian performs better than the Bloch-Siegert model when calculating
operator averages (e.g.\, the mean photon number and number of excitations).
This improvement is due to a refined calculation of the dressed states within
the present model. Regarding the frequency shift induced by the qubit-photon
interaction, we find a different sign from the Bloch-Siegert value. This
influences the eigenstates structure in a non-trivial way and ensures the
correct calculation of the number of excitations associated to a given dressed
state. As a consistency check, we show that the exactly solvable independent
boson model is reproduced as a special limit case of the perturbative
Hamiltonian. | cond-mat_mes-hall |
All-electron GW calculation for molecules: Ionization energy and
electron affinity of conjugated molecules: An efficient all-electron G$^0$W$^0$ method and a quasiparticle
selfconsistent GW (QSGW) method for molecules are proposed in the molecular
orbital space with the full random phase approximation. The convergence with
basis set is examined. As an application, the ionization energy ($I$) and
electron affinity ($A$) of a series of conjugated molecules (up to 32 atoms)
are calculated and compared to experiment. The QSGW result improves the
G$^0$W$^0$ result and both of them are in significantly better agreement with
experimental data than those from Hartree-Fock (HF) and hybrid density
functional calculations, especially for $A$. The nearly correct energy gap and
suppressed self-interaction error by the HF exchange make our method a good
candidate for investigating electronic and transport properties of molecular
systems. | cond-mat_mes-hall |
Through-membrane electron-beam lithography for ultrathin membrane
applications: We present a technique to fabricate ultrathin (down to 20 nm) uniform
electron transparent windows at dedicated locations in a SiN membrane for in
situ transmission electron microscopy experiments. An electron-beam (e-beam)
resist is spray-coated on the backside of the membrane in a KOH- etched cavity
in silicon which is patterned using through-membrane electron-beam lithography.
This is a controlled way to make transparent windows in membranes, whilst the
topside of the membrane remains undamaged and retains its flatness. Our
approach was optimized for MEMS-based heating chips but can be applied to any
chip design. We show two different applications of this technique for (1)
fabrication of a nanogap electrode by means of electromigration in thin
free-standing metal films and (2) making low-noise graphene nanopore devices. | cond-mat_mes-hall |
Tunable optical nonlinearity for TMD polaritons dressed by a Fermi sea: We study a system of a transition metal dichalcogenide (TMD) monolayer placed
in an optical resonator, where strong light-matter coupling between excitons
and photons is achieved. We present quantitative theory of the nonlinear
optical response for exciton-polaritons for the case of doped TMD monolayer,
and analyze in detail two sources of nonlinearity. The first nonlinear response
contribution stems from the Coulomb exchange interaction between excitons. The
second contribution comes from the reduction of Rabi splitting that originates
from phase space filling at increased exciton concentration and the composite
nature of excitons. We demonstrate that both nonlinear contributions are
enhanced in the presence of free electrons. As free electron concentration can
be routinely controlled by an externally applied gate voltage, this opens a way
of electrical tuning of the nonlinear optical response. | cond-mat_mes-hall |
New Multi-Scale Simulation Framework for Next-Generation Electronic
Design Automation with Application to the Junctionless Transistor: In this paper we present a new multi-scale simulation scheme for
next-generation electronic design automation for nano-electronics. The scheme
features a combination of the first-principles quantum mechanical calculation,
semi-classical semiconductor device simulation, compact model generation and
circuit simulation. To demonstrate the feasibility of the proposed scheme, we
apply our newly developed quantum mechanics/electromagnetics method to simulate
the junctionless transistors. The simulation results are consistent with the
experimental measurements and provide new insights on the depletion effect of
the hetero-doped gate on the drain current. Based on the calculated I-V curves,
a compact model is then constructed for the junctionless transistors. The
validity of the compact model is further verified by the transient circuit
simulation of an inverter. | cond-mat_mes-hall |
Dual-gated hBN/bilayer-graphene superlattices and the transitions
between the insulating phases at the charge neutrality point: We report on transport properties in dual-gated hexagonal boron nitride
(hBN)/bilayer-graphene (BLG) superlattices. Here, BLG is nontwisted, i.e.,
plain. This paper focuses on the charge neutrality point (CNP) for a plain BLG.
Under a perpendicular magnetic field, transitions between two insulating phases
at the CNP are detected by varying a displacement field with the study on the
resistance-temperature characteristics and the magnetoresistance. This work
opens avenues for exploring the global phase diagram of the hBN/BLG
superlattices beyond the CNP. | cond-mat_mes-hall |
Organic single-photon switch: The recent progress in nanotechnology [1,2] and single-molecule spectroscopy
[3-5] paves the way for cost-effective organic quantum optical technologies
emergent with a promise to real-life devices operating at ambient conditions.
In this letter, we harness $\pi$-conjugated segments of an organic ladder-type
polymer strongly coupled to a microcavity forming correlated collective dressed
states of light, so-called of exciton-polariton condensates. We explore an
efficient way for all-optical ultra-fast control over the macroscopic
condensate wavefunction via a single photon. Obeying Bose statistics,
exciton-polaritons exhibit an extreme nonlinearity undergoing bosonic
stimulation [6] which we have managed to trigger at the single-photon level.
Relying on the nature of organic matter to sustain stable excitons dressed with
high energy molecular vibrations we have developed a principle that allows for
single-photon nonlinearity operation at ambient conditions opening the door for
practical implementations like sub-picosecond switching, amplification and
all-optical logic at the fundamental limit of single light quanta. | cond-mat_mes-hall |
Optical nanoscopy via quantum control: We present a scheme for nanoscopic imaging of a quantum mechanical two-level
system using an optical probe in the far-field. Existing super-resolution
schemes require more than two-levels and depend on an incoherent response to
the lasers. Here, quantum control of the two states proceeds via rapid
adiabatic passage. We implement this scheme on an array of semiconductor
self-assembled quantum dots. Each quantum dot results in a bright spot in the
image with extents down to 30 nm ({\lambda}/31). Rapid adiabatic passage is
established as a versatile tool in the super-resolution toolbox. | cond-mat_mes-hall |
Strain-induced pseudomagnetic and scalar fields in symmetry-enforced
Dirac nodes: It is known that Dirac nodes can be present at high-symmetry points of
Brillouin zone only for certain space groups. For these cases, the effect of
strain is treated by symmetry considerations. The dependence of strain-induced
potentials on the strain tensor is found. In all but two cases, the
pseudomagnetic field potential is present. It can be used to control valley
currents. | cond-mat_mes-hall |
Chargeless spin current for switching and coupling of domain walls in
magnetic nanowires: The demonstration of the generation and control of a pure spin current
(without net charge flow) by electric fields and/or temperature gradient has
been an essential leap in the quest for low-power consumption electronics. The
key issue of whether and how such a current can be utilized to drive and
control information stored in magnetic domain walls (DWs) is still outstanding
and is addressed here. We demonstrate that pure spin current acts on DWs in a
magnetic stripe with an effective spin-transfer torque resulting in a mutual
DWs separation dynamics and picosecond magnetization reversal. In addition,
long-range ($\sim$ mm) antiferromagnetic DWs coupling emerges. If one DW is
pinned by geometric constriction, the spin current induces a dynamical spin
orbital interaction that triggers an internal electric field determined by
$\vec{E} \sim \hat{e}_{x} \cdot (\vec{n}_{1} \times \vec{n}_{2})$ where
$\vec{n}_{1/2}$ are the effective DWs orientations and $\hat{e}_{x} $ is their
spatial separation vector. This leads to charge accumulation or persistent
electric current in the wire. As DWs are routinely realizable and tuneable, the
predicted effects bear genuine potential for power-saving spintronics devices. | cond-mat_mes-hall |
Single-Particle-Picture Breakdown in laterally weakly confining GaAs
Quantum Dots: We present a detailed investigation of different excitonic states weakly
confined in single GaAs/AlGaAs quantum dots obtained by the Al droplet-etching
method. For our analysis we make use of temperature-, polarization- and
magnetic field-dependent $\mu$-photoluminescence measurements, which allow us
to identify different excited states of the quantum dot system. Besides that,
we present a comprehensive analysis of g-factors and diamagnetic coefficients
of charged and neutral excitonic states in Voigt and Faraday configuration.
Supported by theoretical calculations by the Configuration interaction method,
we show that the widely used single-particle Zeeman Hamiltonian cannot be used
to extract reliable values of the g-factors of the constituent particles from
excitonic transition measurements. | cond-mat_mes-hall |
Magnetic Structure of Nano-Graphite Moebius Ribbon: We consider the electronic and magnetic properties of nanographite ribbon
with zigzag edges under the periodic or Moebius boundary conditions. The zigzag
nano-graphite ribbons possess edge localized states at the Fermi level which
cause a ferrimagnetic spin polarization localized at the edge sites even in the
very weak Coulomb interaction. The imposition of the Moebius boundary condition
makes the system non-AB-bipartite lattice, and depress the spin polarization,
resulting in the formation of a magnetic domain wall. The width of the magnetic
domain depends on the Coulomb interaction and narrows with increasing U/t. | cond-mat_mes-hall |
Valley-Polarized Quantum Anomalous Hall State in Moiré
MoTe$_2$/WSe$_2$ Heterobilayers: Moir\'e heterobilayer transition metal dichalcogenides (TMDs) emerge as an
ideal system for simulating the single-band Hubbard model and interesting
correlated phases have been observed in these systems. Nevertheless, the
moir\'e bands in heterobilayer TMDs were believed to be topologically trivial.
Recently, it was reported that both a quantum valley Hall insulating state at
filling $\nu=2$ (two holes per moir\'e unit cell) and a valley-polarized
quantum anomalous Hall state at filling $\nu=1$ were observed in AB stacked
moir\'e MoTe$_2$/WSe$_2$ heterobilayers. However, how the topologically
nontrivial states emerge is not known. In this work, we propose that the
pseudo-magnetic fields induced by lattice relaxation in moir\'e
MoTe$_2$/WSe$_2$ heterobilayers could naturally give rise to moir\'e bands with
finite Chern numbers. We show that a time-reversal invariant quantum valley
Hall insulator is formed at full-filing $\nu=2$, when two moir\'e bands with
opposite Chern numbers are filled. At half-filling $\nu=1$, Coulomb interaction
lifts the valley degeneracy and results in a valley-polarized quantum anomalous
Hall state, as observed in the experiment. Our theory identifies a new way to
achieve topologically non-trivial states in heterobilayer TMD materials. | cond-mat_mes-hall |
Theoretical Description of Scanning Tunneling Potentiometry: A theoretical description of scanning tunneling potentoimetry (STP)
measurement is presented to address the increasing need for a basis to
interpret experiments on macrscopic samples. Based on a heuristic understanding
of STP provided to facilitate theoretical understanding, the total tunneling
current related to the density matrix of the sample is derived within the
general framework of quantum transport. The measured potentiometric voltage is
determined implicitly as the voltage necessary to null the tunneling current.
Explicit expressions of measured voltages are presented under certain
assumptions, and limiting cases are discussed to connect to previous results.
The need to go forward and formulate the theory in terms of a local density
matrix is also discussed. | cond-mat_mes-hall |
Anisotropic thermoelectric effect in helimagnetic tunnel junctions: Thermoelectric transport across
normal-metal/helical-multiferroic/ferromagnetic heterojunctions is
theoretically investigated. We find a anisotropic charge and spin thermopower
with a $C_{2v}$ symmetry. The angular dependence on the magnetization
orientation of the ferromagnetic layer is substantiated by a phenomenological
theory based on the symmetry of the effective spin-orbit interaction induced by
the topology of the spiral magnetic order in the multiferroic barrier. | cond-mat_mes-hall |
Magnetoresistance in the in-plane magnetic field induced semi-metallic
phase of inverted HgTe quantum wells: In this study we have measured the magnetoresistance response of inverted
HgTe quantum wells in the presence of a large parallel magnetic field up to 33
T is applied. We show that in quantum wells with inverted band structure a
monotonically decreasing magnetoresistance is observed when a magnetic field up
to order 10 T is applied parallel to the quantum well plane. This feature is
accompanied by a vanishing of non-locality and is consistent with a predicted
modification of the energy spectrum that becomes gapless at a critical in-plane
field $B_{c}$. Magnetic fields in excess of $B_c$ allow us to investigate the
evolution of the magnetoresistance in this field-induced semi-metallic region
beyond the known regime. After an initial saturation phase in the presumably
gapless phase, we observe a strong upturn of the longitudinal resistance. A
small residual Hall signal picked up in non-local measurements suggests that
this feature is likely a bulk phenomenon and caused by the semi-metallicity of
the sample. Theoretical calculations indeed support that the origin of these
features is classical and a power law upturn of the resistance can be expected
due to the specifics of two-carrier transport in thin (semi-)metallic samples
subjected to large magnetic fields. | cond-mat_mes-hall |
Electronic structure of graphene on single crystal copper substrates: The electronic structure of graphene on Cu(111) and Cu(100) single crystals
is investigated using low energy electron microscopy, low energy electron
diffraction and angle resolved photoemission spectroscopy. On both substrates
the graphene is rotationally disordered and interactions between the graphene
and substrate lead to a shift in the Dirac crossing of $\sim$ -0.3 eV and the
opening of a $\sim$ 250 meV gap. Exposure of the samples to air resulted in
intercalation of oxygen under the graphene on Cu(100), which formed a
($\sqrt{2} \times 2\sqrt{2}$)R45$^{\rm o}$ superstructure. The effect of this
intercalation on the graphene $\pi$ bands is to increase the offset of the
Dirac crossing ($\sim$ -0.6 eV) and enlarge the gap ($\sim$ 350 meV). No such
effect is observed for the graphene on Cu(111) sample, with the surface state
at $\Gamma$ not showing the gap associated with a surface superstructure. The
graphene film is found to protect the surface state from air exposure, with no
change in the effective mass observed. | cond-mat_mes-hall |
Semiclassical Theory for Decay and Fragmentation Processes in Chaotic
Quantum Systems: We consider quantum decay and photofragmentation processes in open chaotic
systems in the semiclassical limit. We devise a semiclassical approach which
allows us to consistently calculate quantum corrections to the classical decay
to high order in an expansion in the inverse Heisenberg time. We present
results for systems with and without time reversal symmetry and also for the
symplectic case, as well as extending recent results to non-localized initial
states. We further analyze related photodissociation and photoionization
phenomena and semiclassically compute cross-section correlations, including
their Ehrenfest time dependence. | cond-mat_mes-hall |
Midinfrared Third Harmonic Generation from Macroscopically Aligned
Ultralong Single-Wall Carbon Nanotubes: We report the observation of strong third harmonic generation from a
macroscopic array of aligned ultralong single-wall carbon nanotubes (SWCNTs)
with intense midinfrared radiation. Through power-dependent experiments, we
determined the absolute value of the third-order nonlinear optical
susceptibility, $\chi^{(3)}$, of our SWCNT film to be 5.53 $\times$ 10$^{-12}$
esu, three orders of magnitude larger than that of the fused silica reference
we used. Taking account of the filling factor of 8.75% for our SWCNT film, we
estimate a $\chi^{(3)}$ of 6.32 $\times$ 10$^{-11}$ esu for a fully dense film.
Furthermore, through polarization-dependent experiments, we extracted all the
nonzero elements of the $\chi^{(3)}$ tensor, determining the magnitude of the
weaker tensor elements to be $\sim$1/6 of that of the dominant
$\chi^{(3)}_{zzzz}$ component. | cond-mat_mes-hall |
Do the Size Effects Exist?: In this short paper we review a series of publications, some of which are our
own, where various aspects of size effects were examined. By analyzing a series
of examples we show that various intensive macroscopic characteristics of
nanoobjects exhibit non-trivial size dependencies on the scale of 200 to 40 A.
Drastic variations take place for sizes in the region 50-60 A for ordinary
systems, and 60-200 A in the case of magnetic systems. We argue that X-ray and
neutron scattering gives an excellent metrological support in the domain from
100 A to 10 A. | cond-mat_mes-hall |
Superconductor-semiconductor magnetic microswitch: A hybrid superconductor--two-dimensional electron gas microdevice is
presented. Its working principle is based on the suppression of Andreev
reflection at the superconductor-semiconductor interface caused by a magnetic
barrier generated by a ferromagnetic strip placed on top of the structure.
Device switching is predicted with fields up to some mT and working frequencies
of several GHz, making it promising for applications ranging from microswitches
and storage cells to magnetic field discriminators. | cond-mat_mes-hall |
Influence of vibrational modes on the electronic properties of DNA: We investigate the electron (hole) transport through short double-stranded
DNA wires in which the electrons are strongly coupled to the specific
vibrational modes (vibrons) of the DNA. We analyze the problem starting from a
tight-binding model of DNA, with parameters derived from ab-initio
calculations, and describe the dissipative transport by equation-of-motion
techniques. For homogeneous DNA sequences like Poly- (Guanine-Cytosine) we find
the transport to be quasi-ballistic with an effective density of states which
is modified by the electron-vibron coupling. At low temperatures the linear
conductance is strongly enhanced, but above the `semiconducting' gap it is
affected much less. In contrast, for inhomogeneous (`natural') sequences almost
all states are strongly localized, and transport is dominated by dissipative
processes. In this case, a non-local electron-vibron coupling influences the
conductance in a qualitative and sequence-dependent way. | cond-mat_mes-hall |
Giant Microwave Sensitivity of Magnetic Array by Long-Range Chiral
Interaction Driven Skin Effect: Non-Hermitian skin effect was observed in one-dimensional systems with
short-range chiral interaction. Long-range chiral interaction mediated by
traveling waves also favors the accumulation of energy, but has not yet showed
non-Hermitian topology. Here we find that the strong interference brought by
the wave propagation is detrimental for accumulation. By suppression of
interference via the damping of traveling waves, we predict the non-Hermitian
skin effect of magnetic excitation in a periodic array of magnetic nanowires
that are coupled chirally via spin waves of thin magnetic films. The local
excitation of a wire at one edge by weak microwaves of magnitude $\sim \mu{\rm
T}$ leads to a considerable spin-wave amplitude at the other edge, i.e. a
remarkable functionality useful for sensitive, non-local, and non-reciprocal
detection of microwaves. | cond-mat_mes-hall |
Generating indistinguishable photons from a quantum dot in a noisy
environment: Single photons from semiconductor quantum dots are promising resources for
linear optical quantum computing, or, when coupled to spin states, quantum
repeaters. To realize such schemes, the photons must exhibit a high degree of
indistinguishability. However, the solid-state environment presents inherent
obstacles for this requirement as intrinsic semiconductor fluctuations can
destroy the photon indistinguishability. Here we use resonance fluorescence to
generate indistinguishable photons from a single quantum dot in an environment
filled with many charge-fluctuating traps. Over long time-scales ($>50$
$\mu$s), flickering of the emission due to significant spectral fluctuations
reduce the count rates. Nevertheless, due to the specificity of resonance
fluorescence, high-visibility two-photon interference is achieved. | cond-mat_mes-hall |
Cavity optomechanical transduction sensing of single molecules: We report narrow linewidth optomechanical oscillation of a silica microsphere
immersed in a buffer solution. Through a novel optomechanical transduction
sensing approach, single 10-nm-radius silica beads and Bovine serum albumin
(BSA) protein molecules with a molecular weight of 66 kDalton were detected.
This approach predicts the detection of 3.9 kDalton single molecules at a
signal-to-noise ration above unity. | cond-mat_mes-hall |
Non-Hermitian Lindhard function and Friedel oscillations: The Lindhard function represents the basic building block of many-body
physics and accounts for charge response, plasmons, screening, Friedel
oscillation, RKKY interaction etc. Here we study its non-Hermitian version in
one dimension, where quantum effects are traditionally enhanced due to spatial
confinement, and analyze its behavior in various limits of interest. Most
importantly, we find that the static limit of the non-Hermitian Lindhard
function has no divergence at twice the Fermi wavenumber and vanishes
identically for all other wavenumbers at zero temperature. Consequently, no
Friedel oscillations are induced by a non-Hermitian, imaginary impurity to
lowest order in the impurity potential at zero temperature. Our findings are
corroborated numerically on a tight-binding ring by switching on a weak real or
imaginary potential. We identify conventional Friedel oscillations or heavily
suppressed density response, respectively. | cond-mat_mes-hall |
The effects of a magnetic barrier and a nonmagnetic spacer in tunnel
structures: The spin-polarized transport is investigated in a new type of magnetic tunnel
junction which consists of two ferromagnetic electrodes separated by a magnetic
barrier and a nonmagnetic metallic spacer. Based on the transfer matrix method
and the nearly-free-electron-approximation the dependence of the tunnel
magnetoresistance (TMR) and electron-spin polarization on the nonmagnetic layer
thickness and the applied bias voltage are studied theoretically. The TMR and
spin polarization show an oscillatory behavior as a function of the spacer
thickness and the bias voltage. The oscillations originate from the quantum
well states in the spacer, while the existence of the magnetic barrier gives
rise to a strong spin polarization and high values of the TMR. Our results may
be useful for the development of spin electronic devices based on coherent
transport. | cond-mat_mes-hall |
Mode- and size-dependent Landau-Lifshitz damping in magnetic
nanostructures: Evidence for non-local damping: We demonstrate a strong dependence of the effective damping on the nanomagnet
size and the particular spin-wave mode that can be explained by the theory of
intralayer transverse-spin-pumping. The effective Landau-Lifshitz damping is
measured optically in individual, isolated nanomagnets as small as 100 nm. The
measurements are accomplished by use of a novel heterodyne magneto-optical
microwave microscope with unprecedented sensitivity. Experimental data reveal
multiple standing spin-wave modes that we identify by use of micromagnetic
modeling as having either localized or delocalized character, described
generically as end- and center-modes. The damping parameter of the two modes
depends on both the size of the nanomagnet as well as the particular spin-wave
mode that is excited, with values that are enhanced by as much as 40% relative
to that measured for an extended film. Contrary to expectations based on the ad
hoc consideration of lithography-induced edge damage, the damping for the
end-mode decreases as the size of the nanomagnet decreases. The data agree with
the theory for damping caused by the flow of intralayer transverse
spin-currents driven by the magnetization curvature. These results have serious
implications for the performance of nanoscale spintronic devices such as
spin-torque-transfer magnetic random access memory. | cond-mat_mes-hall |
Resistivity of Graphene Nanoribbon Interconnects: Graphene nanoribbon interconnects are fabricated, and the extracted
resistivity is compared to that of Cu. It is found that the average resistivity
at a given line-width (18nm<W<52nm) is about 3X that of a Cu wire, whereas the
best GNR has a resistivity comparable to that of Cu. The conductivity is found
to be limited by impurity scattering as well as LER scattering; as a result,
the best reported GNR resistivity is 3X the limit imposed by substrate phonon
scattering. This study reveals that even moderate-quality graphene nanowires
have the potential to outperform Cu for use as on-chip interconnects. | cond-mat_mes-hall |
Resistivity anisotropy of quantum Hall stripe phases: Quantum Hall stripe phases near half-integer filling factors $\nu \ge 9/2$
were predicted by Hartree-Fock (HF) theory and confirmed by discoveries of
giant resistance anisotropies in high-mobility two-dimensional electron gases.
A theory of such anisotropy was proposed by MacDonald and Fisher, although they
used parameters whose dependencies on the filling factor, electron density, and
mobility remained unspecified. Here, we fill this void by calculating the
hard-to-easy resistivity ratio as a function of these three variables.
Quantitative comparison with experiment yields very good agreement which we
view as evidence for the "plain vanilla" smectic stripe HF phases. | cond-mat_mes-hall |
Dephasing of a particle in a dissipative environment: The motion of a particle in a ring of length L is influenced by a dirty metal
environment whose fluctuations are characterized by a short correlation
distance $\ell << L$. We analyze the induced decoherence process, and compare
the results with those obtained in the opposing Caldeira-Leggett limit ($\ell
>> L$). A proper definition of the dephasing factor that does not depend on a
vague semiclassical picture is employed. Some recent Monte-Carlo results about
the effect of finite temperatures on "mass renormalization" in this system are
illuminated. | cond-mat_mes-hall |
Intercalated Rare-Earth Metals under Graphene on SiC: Intercalation of rare earth metals ($RE$ = Eu, Dy, and Gd) is achieved by
depositing the $RE$ metal on graphene that is grown on silicon-carbide (SiC)
and by subsequent annealing at high temperatures to promote intercalation. STM
images of the films reveal that the graphene layer is defect free and that each
of the intercalated metals has a distinct nucleation pattern. Intercalated Eu
forms nano-clusters that are situated on the vertices of a Moir{\`e} pattern,
while Dy and Gd form randomly distributed nano-clusters. X-ray magnetic
circular dichroism (XMCD) measurements of intercalated films reveal the
magnetic properties of these $RE$'s nano-clusters. Furthermore, field
dependence and temperature dependence of the magnetic moments extracted from
the XMCD show paramagnetic-like behaviors with moments that are generally
smaller than those predicted by the Brillouin function. XMCD measurements of
$RE$-oxides compared with those of the intercalated $RE$'s under graphene after
exposure to air for months indicate that the graphene membranes protect these
intercalants against oxidation. | cond-mat_mes-hall |
Hot carriers in a bipolar graphene: Hot carriers in a doped graphene under dc electric field is described taking
into account the intraband energy relaxation due to acoustic phonon scattering
and the interband generation-recombination transitions caused by thermal
radiation. The consideration is performed for the case when the intercarrier
scattering effectively establishes the quasiequilibrium electron-hole
distributions, with effective temperature and concentrations of carriers. The
concentration and energy balance equations are solved taking into account an
interplay between weak energy relaxation and generation-recombination
processes. The nonlinear conductivity is calculated for the momentum relaxation
caused by the elastic scattering. The current-voltage characteristics, and the
transition between bipolar and monopolar regimes of conductivity are obtained
and analyzed, for different temperatures and gate voltages. | cond-mat_mes-hall |
Magnetic-Field-Dependent Thermodynamic Properties of Square and
Quadrupolar Artificial Spin Ice: Applied magnetic fields are an important tuning parameter for artificial spin
ice (ASI) systems, as they can drive phase transitions between different
magnetic ground states, or tune through regimes with high populations of
emergent magnetic excitations (e.g., monopole-like quasiparticles). Here, using
simulations supported by experiments, we investigate the thermodynamic
properties and magnetic phases of square and quadrupolar ASI as a function of
applied in-plane magnetic fields. Monte Carlo simulations are used to generate
field-dependent maps of the magnetization, the magnetic specific heat, the
thermodynamic magnetization fluctuations, and the magnetic order parameters,
all under equilibrium conditions. These maps reveal the diversity of magnetic
orderings and the phase transitions that occur in different regions of the
phase diagrams of these ASIs, and are experimentally supported by
magneto-optical measurements of the equilibrium "magnetization noise" in
thermally-active ASIs. | cond-mat_mes-hall |
Effective interfacial Dzyaloshinskii-Moriya interaction and skyrmion
stabilization in ferromagnet/paramagnet and ferromagnet/superconductor hybrid
systems: It is shown that a term in the form of Dzyaloshinskii-Moriya interaction
(DMI) contributes to the free energy of a ferromagnetic (FM) film on a
paramagnetic (PM) (an FM above the critical temperature, Tc) or superconducting
(SC) substrate occurring in the London limit. This contribution results from
magnetostatic interaction between the film and substrate under which the
substrate affects FM magnetization back via its magnetic field produced by
magnetization inhomogeneity in the film. Strikingly, in the FM/PM system this
effective DMI stabilizes chiral magnetic textures, e.g., magnetic skyrmions
(MSk's) of the Neel-type, which is in contrast to that in the FM/SC one. A
strong temperature sensitivity of the effective DMI allows for tuning the
coupling between the FM film and PM or SC substrate and thus controlling the
MSk radius in FM/PM. | cond-mat_mes-hall |
A Movable Valley Switch Driven by Berry Phase in Bilayer Graphene
Resonators: Since its discovery, Berry phase has been demonstrated to play an important
role in many quantum systems. In gapped Bernal bilayer graphene, the Berry
phase can be continuously tuned from zero to 2pi, which offers a unique
opportunity to explore the tunable Berry phase on the physical phenomena. Here,
we report experimental observation of Berry phases-induced valley splitting and
crossing in moveable bilayer graphene p-n junction resonators. In our
experiment, the bilayer graphene resonators are generated by combining the
electric field of scanning tunneling microscope tip with the gap of bilayer
graphene. A perpendicular magnetic field changes the Berry phase of the
confined bound states in the resonators from zero to 2pi continuously and leads
to the Berry phase difference for the two inequivalent valleys in the bilayer
graphene. As a consequence, we observe giant valley splitting and unusual
valley crossing of the lowest bound states. Our results indicate that the
bilayer graphene resonators can be used to manipulate the valley degree of
freedom in valleytronics. | cond-mat_mes-hall |
Theory of the plasma-wave photoresponse of a gated graphene sheet: The photoresponse of graphene has recently received considerable attention.
The main mechanisms yielding a finite dc response to an oscillating radiation
field which have been investigated include responses of photovoltaic,
photo-thermoelectric, and bolometric origin. In this Article we present a fully
analytical theory of a photoresponse mechanism which is based on the excitation
of plasma waves in a gated graphene sheet. By employing the theory of
relativistic hydrodynamics, we demonstrate that plasma-wave photodetection is
substantially influenced by the massless Dirac fermion character of carriers in
graphene and that the efficiency of photodetection can be improved with respect
to that of ordinary parabolic-band electron fluids in semiconductor
heterostructures. | cond-mat_mes-hall |
Active feedback of a Fabry-Perot cavity to the emission of a single
InAs/GaAs quantum dot: We present a detailed study of the use of Fabry-Perot (FP) cavities for the
spectroscopy of single InAs quantum dots (QDs). We derive optimal cavity
characteristics and resolution limits, and measure photoluminescence linewidths
as low as 0.9 GHz. By embedding the QDs in a planar cavity, we obtain a
sufficiently large signal to actively feed back on the length of the FP to lock
to the emission of a single QD with a stability below 2% of the QD linewidth.
An integration time of approximately two seconds is found to yield an optimum
compromise between shot noise and cavity length fluctuations. | cond-mat_mes-hall |
Fermionic and bosonic ac conductivities at strong disorder: We study the ac conduction in a system of fermions or bosons strongly
localised in a disordered array of sites with short-range interactions at
frequencies larger than the intersite tunnelling but smaller than the
characteristic fluctuation of the on-site energy. While the main contribution
$\sigma_0(\omega)$ to the conductivity comes from local dipole-type excitations
on close pairs of sites, coherent processes on three or more sites lead to an
interference correction $\sigma_1(\omega)$, which depends on the statistics of
the charge carriers and can be suppressed by magnetic field. For bosons the
correction is always positive, while for fermions it can be positive or
negative depending on whether the conduction is dominated by effective
single-particle or single-hole processes. We calculate the conductivity
explicitly assuming a constant density of states of single-site excitations.
Independently of the statistics, $\sigma_0(\omega)=const$. For bosons
$\sigma_1(\omega)\propto \log(C/\omega)$. For fermions
$\sigma_1(\omega)\propto\log[\max(A,\omega)/\omega]-\log[\max(B,\omega)/\omega]$,
where the first and the second term are respectively the particle and hole
contributions, $A$ and $B$ being the particle and hole energy cutoffs. The ac
magnetoresistance has the same sign as $\sigma_1(\omega)$. | cond-mat_mes-hall |
Absence of nonlocal resistance in microstructures of PbTe quantum wells: We report on experiments allowing to set an upper limit on the magnitude of
the spin Hall effect and the conductance by edge channels in quantum wells of
PbTe embedded between PbEuTe barriers. We reexamine previous data obtained for
epitaxial microstructures of n-type PbSe and PbTe, in which pronounced nonlocal
effects and reproducible magnetoresistance oscillations were found. Here we
show that these effects are brought about by a quasi-periodic network of
threading dislocations adjacent to the BaF$_2$ substrate, which give rise to a
p-type interfacial layer and an associated parasitic parallel conductance. We
then present results of transport measurements on microstructures of modulation
doped PbTe/(Pb,Eu)Te:Bi heterostructures for which the influence of parasitic
parallel conductance is minimized, and for which quantum Hall transport had
been observed, on similar samples, previously. These structures are of H-shaped
geometry and they are patterned of 12 nm thick strained PbTe quantum wells
embedded between Pb$_{0.92}$Eu$_{0.08}$Te barriers. The structures have
different lateral sizes corresponding to both diffusive and ballistic electron
transport in non-equivalent L valleys. For these structures no nonlocal
resistance is detected confirming that PbTe is a trivial insulator. The
magnitude of spin Hall angle gamma is estimated to be smaller than 0.02 for
PbTe/PbEuTe microstructures in the diffusive regime. | cond-mat_mes-hall |
Extraction of many-body configurations from nonlinear absorption in
semiconductor quantum wells: Detailed electronic many-body configurations are extracted from
quantitatively measured timeresolved nonlinear absorption spectra of resonantly
excited GaAs quantum wells. The microscopic theory assigns the observed
spectral changes to a unique mixture of electron-hole plasma, exciton, and
polarization effects. Strong transient gain is observed only under co-circular
pump-probe conditions and is attributed to the transfer of pump-induced
coherences to the probe. | cond-mat_mes-hall |
Long-time coherence in fourth-order spin correlation functions: We study the long-time decay of fourth-order electron spin correlation
functions for an isolated singly charged semi-conductor quantum dot. The
electron spin dynamics is governed by the applied external magnetic field as
well as the hyperfine interaction. While the long-time coherent oscillations in
the correlation functions can be understood within an semi-classical approach
treating the Overhauser field as frozen, the field dependent decay of its
amplitude reported in different experiments cannot be explained by the
central-spin model indicating the insufficiency of such a description. By
incorporating the nuclear Zeeman splitting and the strain induced
nuclear-electric quadrupolar interaction, we find the correct crossover from a
fast decay in small magnetic fields to a slow exponential asymptotic in large
magnetic fields. It originates from a competition between the quadrupolar
interaction inducing an enhanced spin decay and the nuclear Zeeman term that
suppressed the spin-flip processes. We are able to explain the magnetic field
dependency of the characteristic long-time decay time $T_2$ depending on the
experimental setups. The calculated asymptotic values of $T_2 = 3 -4\,\mu$s
agree qualitatively well with the experimental data. | cond-mat_mes-hall |
Wide range electrical tunability of single photon emission from
chromium-based colour centres in diamond: We demonstrate electrical control of the single photon emission spectrum from
chromium-based colour centres implanted in monolithic diamond. Under an
external electric field the tunability range is typically three orders of
magnitude larger than the radiative linewidth and at least one order of
magnitude larger than the observed linewidth. The electric and magnetic field
dependence of luminescence gives indications on the inherent symmetry and we
propose Cr-X or X-Cr-Y type noncentrosymmetric atomic configurations as most
probable candidates for these centres. | cond-mat_mes-hall |
Electronic Transport and Thermopower in 2D and 3D Heterostructures--A
Theory Perspective: In this review, we discuss the impact of interfaces and heterojuctions on the
electronic and thermoelectric transport properties of materials. We review
recent progress in understanding electronic transport in two-dimensional (2D)
materials ranging from graphene to transition metal dichalcogenides (TMDs),
their homojunctions (grain boundaries), lateral heterojunctions (such as
graphene/MoS$_2$ lateral interfaces), and vertical van der Waals (vdW)
heterostructures. We also review work in thermoelectric properties of 2D
heterojunctions, as well as their applications in creating devices such as
resonant tunneling diodes (RTDs). Lastly, we turn our focus to work in
three-dimensional (3D) heterostructures. While transport in 3D heterostructures
has been researched for several decades, here we review recent progress in
theory and simulation of quantum effects on transport via the Wigner and
non-equilibrium Green's functions (NEGF) approaches. These simulation
techniques have been successfully applied toward understanding the impact of
heterojunctions on the thermoelectric properties, with applications in energy
harvesting, and electron resonant tunneling, with applications in RTDs. We
conclude that tremendous progress has been made in both simulation and
experiments toward the goal of understanding transport in heterostructures and
this progress will soon be parlayed into improved energy converters and quantum
nanoelectronic devices. | cond-mat_mes-hall |
Stretching graphene using polymeric micro-muscles: The control of strain in two-dimensional materials opens exciting
perspectives for the engineering of their electronic properties. While this
expectation has been validated by artificial-lattice studies, it remains
elusive in the case of atomic lattices. Remarkable results were obtained on
nanobubbles and nano-wrinkles, or using scanning probes; microscale strain
devices were implemented exploiting deformable substrates or external loads.
These devices lack, however, the flexibility required to fully control and
investigate arbitrary strain profiles. Here, we demonstrate a novel approach
making it possible to induce strain in graphene using polymeric micrometric
artificial muscles (MAMs) that contract in a controllable and reversible way
under an electronic stimulus. Our method exploits the mechanical response of
poly-methyl-methacrylate (PMMA) to electron-beam irradiation. Inhomogeneous
anisotropic strain and out-of-plane deformation are demonstrated and studied by
Raman, scanning-electron and atomic-force microscopy. These can all be easily
combined with the present device architecture. The flexibility of the present
method opens new opportunities for the investigation of strain and
nanomechanics in two-dimensional materials. | cond-mat_mes-hall |
Electrical and thermal transport through $α-T_3$ NIS junction: We investigate the electrical and thermal transport properties of the
$\alpha-T_3$ based normal metal-insulator-superconductor (NIS) junction using
Blonder-Tinkham-Klapwijk (BTK) theory. We show that the tunneling conductance
of the NIS junction is an oscillatory function of the effective barrier
potential ($\chi$) of the insulating region upto a thin barrier limit. The
periodicity and the amplitudes of the oscillations largely depend on the values
of $\alpha$ and the gate voltage of the superconducting region, namely, $U_0$.
Further, the periodicity of the oscillation changes from $\pi$ to $\pi/2$ as we
increase $U_0$. To assess the thermoelectric performance of such a junction, we
have computed the Seebeck coefficient, the thermoelectric figure of merit,
maximum power output, efficiency at the maximum output power of the system, and
the thermoelectric cooling of the NIS junction as a self-cooling device. Our
results on the thermoelectric cooling indicate practical realizability and
usefulness for using our system as efficient cooling detectors, sensors, etc.,
and hence could be crucial to the experimental success of the thermoelectric
applications of such junction devices. Furthermore, for an $\alpha-T_3$
lattice, whose limiting cases denote a graphene or a dice lattice, it is
interesting to ascertain which one is more suitable as a thermoelectric device
and the answer seems to depend on the $U_0$. We observe that for an
$\alpha-T_3$ lattice corresponding to $U_0=0$, graphene ($\alpha=0$) is more
feasible for constructing a thermoelectric device, whereas for $U_0 \gg E_F$,
the dice lattice ($\alpha=1$) has a larger utility. | cond-mat_mes-hall |
Ab initio theory of electron-phonon mediated ultrafast spin relaxation
of laser-excited hot electrons in transition-metal ferromagnets: We report a computational theoretical investigation of electron spin-flip
scattering induced by the electron-phonon interaction in the transition-metal
ferromagnets bcc Fe, fcc Co and fcc Ni. The Elliott-Yafet electron-phonon
spin-flip scattering is computed from first-principles, employing a generalized
spin-flip Eliashberg function as well as ab initio computed phonon dispersions.
Aiming at investigating the amount of electron-phonon mediated demagnetization
in femtosecond laser-excited ferromagnets, the formalism is extended to treat
laser-created thermalized as well as nonequilibrium, nonthermal hot electron
distributions. Using the developed formalism we compute the phonon-induced spin
lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated
demagnetization rate is evaluated for laser-created thermalized and
nonequilibrium electron distributions. Nonthermal distributions are found to
lead to a stronger demagnetization rate than hot, thermalized distributions,
yet their demagnetizing effect is not enough to explain the experimentally
observed demagnetization occurring in the subpicosecond regime. | cond-mat_mes-hall |
Thermoelectric and Seebeck coefficients of granular metals: In this work we present a detailed study and derivation of the thermopower
and thermoelectric coefficient of nano-granular metals at large tunneling
conductance between the grains, g_T>> 1. An important criterion for the
performance of a thermoelectric device is the thermodynamic figure of merit
which is derived using the kinetic coefficients of granular metals. All results
are valid at intermediate temperatures, E_c>>T/g_T>\delta, where \delta is the
mean energy level spacing for a single grain and E_c its charging energy. We
show that the electron-electron interaction leads to an increase of the
thermopower with decreasing grain size and discuss our results in the light of
future generation thermoelectric materials for low temperature applications.
The behavior of the figure of merit depending on system parameters like grain
size, tunneling conductance, and temperature is presented. | cond-mat_mes-hall |
Nanoelectromechanics of shuttle devices: A single-electron tunneling (SET) device with a nanoscale central island that
can move with respect to the bulk source- and drain electrodes allows for a
nanoelectromechanical (NEM) coupling between the electrical current through the
device and mechanical vibrations of the island. Although an electromechanical
"shuttle" instability and the associated phenomenon of single-electron
shuttling were predicted more than 15 years ago, both theoretical and
experimental studies of NEM-SET structures are still carried out. New
functionalities based on quantum coherence, Coulomb correlations and coherent
electron-spin dynamics are of particular current interest. In this article we
present a short review of recent activities in this area. | cond-mat_mes-hall |
Geometrical meaning of winding number and its characterization of
topological phases in one-dimensional chiral non-Hermitian systems: We unveil the geometrical meaning of winding number and utilize it to
characterize the topological phases in one-dimensional chiral non-Hermitian
systems. While chiral symmetry ensures the winding number of Hermitian systems
being integers, it can take half integers for non-Hermitian systems. We give a
geometrical interpretation of the half integers by demonstrating that the
winding number $\nu$ of a non-Hermitian system is equal to half of the
summation of two winding numbers $\nu_1$ and $\nu_2$ associated with two
exceptional points respectively. The winding numbers $\nu_1$ and $\nu_2$
represent the times of real part of the Hamiltonian in momentum space
encircling the exceptional points and can only take integers. We further find
that the difference of $\nu_1$ and $\nu_2$ is related to the second winding
number or energy vorticity. By applying our scheme to a non-Hermitian
Su-Schrieffer-Heeger model and an extended version of it, we show that the
topologically different phases can be well characterized by winding numbers.
Furthermore, we demonstrate that the existence of left and right zero-mode edge
states is closely related to the winding number $\nu_1$ and $\nu_2$. | cond-mat_mes-hall |
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