text
stringlengths 89
2.49k
| category
stringclasses 19
values |
|---|---|
Towards Substrate Engineering of Graphene-Silicon Schottky Diode
Photodetectors: Graphene-Silicon Schottky diode photodetectors possess beneficial properties
such as high responsivities and detectivities, broad spectral wavelength
operation and high operating speeds. Various routes and architectures have been
employed in the past to fabricate devices. Devices are commonly based on the
removal of the silicon-oxide layer on the surface of silicon by wet-etching
before deposition of graphene on top of silicon to form the graphene-silicon
Schottky junction. In this work, we systematically investigate the influence of
the interfacial oxide layer, the fabrication technique employed and the silicon
substrate on the light detection capabilities of graphene-silicon Schottky
diode photodetectors. The properties of devices are investigated over a broad
wavelength range from near-UV to short-/mid-infrared radiation, radiation
intensities covering over five orders of magnitude as well as the suitability
of devices for high speed operation. Results show that the interfacial layer,
depending on the required application, is in fact beneficial to enhance the
photodetection properties of such devices. Further, we demonstrate the
influence of the silicon substrate on the spectral response and operating
speed. Fabricated devices operate over a broad spectral wavelength range from
the near-UV to the short-/mid-infrared (thermal) wavelength regime, exhibit
high photovoltage responses approaching 10$^6$ V/W and short rise- and
fall-times of tens of nanoseconds. | cond-mat_mes-hall |
Interference Effects on Kondo-Assisted Transport through Double Quantum
Dots: We systematically investigate electron transport through double quantum dots
with particular emphasis on interference induced via multiple paths of electron
propagation. By means of the slave-boson mean-field approximation, we calculate
the conductance, the local density of states, the transmission probability in
the Kondo regime at zero temperature. It is clarified how the Kondo-assisted
transport changes its properties when the system is continuously changed among
the serial, parallel and T-shaped double dots. The obtained results for the
conductance are explained in terms of the Kondo resonances influenced by
interference effects. We also discuss the impacts due to the spin-polarization
of ferromagnetic leads. | cond-mat_mes-hall |
Quantum transport through single and multilayer icosahedral fullerenes: We use a tight-binding Hamiltonian and Green functions methods to calculate
the quantum transmission through single-wall fullerenes and bilayered and
trilayered onions of icosahedral symmetry attached to metallic leads. The
electronic structure of the onion-like fullerenes takes into account the
curvature and finite size of the fullerenes layers as well as the strength of
the intershell interactions depending on to the number of interacting atom
pairs belonging to adjacent shells. Misalignment of the symmetry axes of the
concentric icosahedral shells produces breaking of the level degeneracies of
the individual shells, giving rise some narrow quasi-continuum bands instead of
the localized discrete peaks of the individual fullerenes. As a result, the
transmission function for non symmetrical onions are rapidly varying functions
of the Fermi energy. Furthermore, we found that most of the features of the
transmission through the onions are due to the electronic structure of the
outer shell with additional Fano-like antiresonances arising from coupling with
or between the inner shells. | cond-mat_mes-hall |
Low conductance of the nickel atomic junctions in hydrogen atmosphere: The low conductance of nickel atomic junctions in the hydrogen environment is
studied using the nonequilibrium Green's function theory combined with
first-principles calculations. The Ni junction bridged by a $H_2$ molecule has
a conductance of approximately 0.7 $G_0$. This conductance is contributed by
the anti-bonding state of the $H_2$ molecule, which forms a bonding state with
the $3d$ orbitals of the nearby Ni atoms. In contrast, the Ni junction bridged
by the two single H atoms has a conductance of approximately 1 $G_0$, which is
weakly spin-polarized. The spin-up channels were found to contribute mostly to
the conductance at a small junction gap, while the spin-down channels play a
dominant role at a larger junction gap. | cond-mat_mes-hall |
A scattering model of 1D quantum wire regular polygons: We calculate the quantum states of regular polygons made of 1D quantum wires
treating each polygon vertex as a scatterer. The vertex scattering matrix is
analytically obtained from the model of a circular bend of a given angle of a
2D nanowire. In the single mode limit the spectrum is classified in doublets of
vanishing circulation, twofold split by the small vertex reflection, and
singlets with circulation degeneracy. Simple analytic expressions of the energy
eigenvalues are given. It is shown how each polygon is characterized by a
specific spectrum. | cond-mat_mes-hall |
Controlling transport properties of graphene nanoribbons by
codoping-induced edge distortions: One notable manifestation of the peculiar edge-localized states in zigzag
graphene nanoribbons (zGNRs) is the p-type (n-type) characteristics of nitrogen
(boron) edge-doped GNRs, and such behavior was so far considered to be
exclusive for zGNRs. Carrying out first-principles electronic structure and
quantum transport calculations, we herein show that the donor-acceptor
transition behavior can also arise in the B/N edge-doped armchair GNRs (aGNRs)
by introducing a bipolar P codopant atom into the energetically most favorable
nearest neighbor edge sites. The n-type (p-type) transport properties of B,P
(N,P) co-doped aGNRs are also shown to be superior to those of reference single
N (B) doped aGNRs in that the valence (conduction) band edge conductance
spectra are better preserved. Disentangling the chemical doping and structural
distortion effects, we will demonstrate that the latter plays an important role
in determining the transport type and explains the donor-acceptor transition
feature as well as the bipolar character of P-doped aGNRs. We thus propose the
systematic modification of GNR edge atomic structures via co-doping as a novel
approach to control charge transport characteristics of aGNRs. | cond-mat_mes-hall |
Current-induced mechanical torque in chiral molecular rotors: A great endeavor has been undertaken to engineer molecular rotors operated by
an electrical current. A frequently met operation principle is the transfer of
angular momentum taken from the incident flux. In this paper we present an
alternative driving agent that works also in situations where angular momentum
of the incoming flux is conserved. This situation arises typically with
molecular rotors that exhibit an easy axis of rotation. For quantitative
analysis we investigate here a classical model, where molecule and wires are
represented by a rigid curved path. We demonstrate that in the presence of
chirality the rotor generically undergoes a directed motion, provided that the
incident current exceeds a threshold value. Above threshold, the corresponding
rotation frequency (per incoming particle current) for helical geometries turns
out to be $2\pi m/M_1$, where $m/M_1$ is the ratio of the mass of an incident
charge carrier and the mass of the helix per winding number. | cond-mat_mes-hall |
Low-power photothermal self-oscillation of bimetallic nanowires: We investigate the nonlinear mechanics of a bimetallic, optically absorbing
SiN-Nb nanowire in the presence of incident laser light and a reflecting Si
mirror. Situated in a standing wave of optical intensity and subject to
photothermal forces, the nanowire undergoes self-induced oscillations at low
incident light thresholds of $<1\, \rm{\mu W}$ due to engineered strong
temperature-position ($T$-$z$) coupling. Along with inducing self-oscillation,
laser light causes large changes to the mechanical resonant frequency
$\omega_0$ and equilibrium position $z_0$ that cannot be neglected. We present
experimental results and a theoretical model for the motion under laser
illumination. In the model, we solve the governing nonlinear differential
equations by perturbative means to show that self-oscillation amplitude is set
by the competing effects of direct $T$-$z$ coupling and $2\omega_0$ parametric
excitation due to $T$-$\omega_0$ coupling. We then study the linearized
equations of motion to show that the optimal thermal time constant $\tau$ for
photothermal feedback is $\tau \to \infty$ rather than the widely reported
$\omega_0 \tau = 1$. Lastly, we demonstrate photothermal quality factor ($Q$)
enhancement of driven motion as a means to counteract air damping.
Understanding photothermal effects on micromechanical devices, as well as
nonlinear aspects of optics-based motion detection, can enable new device
applications as oscillators or other electronic elements with smaller device
footprints and less stringent ambient vacuum requirements. | cond-mat_mes-hall |
Non-Hermitian Boundary Modes: We consider conditions for the existence of boundary modes in non-Hermitian
systems with edges of arbitrary co-dimension. Through a universal formulation
of formation criteria for boundary modes in terms of local Green functions, we
outline a generic perspective on the appearance of such modes and generate
corresponding dispersion relations. In the process, we explain the skin effect
in both topological and non-topological systems, exhaustively generalizing
bulk-boundary correspondence in the presence of non-Hermiticity. This is
accomplished via a doubled Green's function, inspired by doubled Hamiltonian
methods used to classify Floquet and, more recently, non-Hermitian topological
phases. Our work constitutes a general tool, as well as, a unifying perspective
for this rapidly evolving field. Indeed, as a concrete application we find that
our method can expose novel non-Hermitian topological regimes beyond the reach
of previous methods. | cond-mat_mes-hall |
Fast and anomalous exciton diffusion in two-dimensional hybrid
perovskites: Two-dimensional hybrid perovskites are currently in the spotlight of
condensed matter and nanotechnology research due to their intriguing
optoelectronic and vibrational properties with emerging potential for
light-harvesting and -emitting applications. While it is known that these
natural quantum wells host tightly bound excitons, the mobilities of these
fundamental optical excitations at the heart of the optoelectronic applications
are still largely unexplored. Here, we directly monitor the diffusion of
excitons through ultrafast emission microscopy from liquid helium to room
temperature in hBN-encapsulated two-dimensional hybrid perovskites. We find
very fast diffusion with characteristic hallmarks of free exciton propagation
for all temperatures above 50 K. In the cryogenic regime we observe nonlinear,
anomalous behavior with an exceptionally rapid expansion of the exciton cloud
followed by a very slow and even negative effective diffusion. We discuss our
findings in view of efficient exciton-phonon coupling, highlighting
two-dimensional hybrids as promising platforms for many-body physics research
and optoelectronic applications on the nanoscale. | cond-mat_mes-hall |
Delocalisation of Majorana quasiparticles in plaquette--nanowire hybrid
system: Interplay between superconductivity, spin-orbit coupling and magnetic field
can lead to realisation of the topologically non--trivial states which in
finite one dimensional nanowires are manifested by emergence of a pair of
zero-energy Majorana bound states. On the other hand, in two dimensional
systems spin current contributed by the edge states might appear. We
investigate novel properties of the bound states in a system of mixed
dimensionality, composed of one-dimensional nanowire connected with
two-dimensional plaquette. This setup could be patterned epitaxially, e.g.
using heterostructure analogous to what has been reported recently by F.
Nichele et al., Phys. Rev. Lett. 119, 136803 (2017). We study this system,
assuming either its part or the entire structure to be in topologically
non--trivial superconducting state. Our results predict delocalisation of the
Majorana modes, upon leaking from the nanowire to the nanocluster with some
tendency towards its corners. | cond-mat_mes-hall |
Waiting time distributions in Quantum spin hall based heterostructures: For the distinction of the Andreev bound states and Majorana bound states, we
study the waiting time distributions (WTDs) for heterostructures, based on one
dimensional edge states of a two dimensional topological insulators (TI) in
combination with an proximitized s-wave superconductor (SC) and an applied
magnetic field. We show for the time reversal symmetric (TRS) situation of a
Josephson junction details of the WTD. This includes different transport
processes, different numbers of Andreev bound states and the phase difference
of the SC. We further consider a Zeeman field in the normal part of the
junctions revealing novel features in the WTD along the phase transition
between trivial bound states and Majorana bound states. We finally discuss
clear signatures to discriminate between them. | cond-mat_mes-hall |
High-speed metamagnetic resistive switching of FeRh through Joule
heating: Due to its proximity to room temperature and demonstrated high degree of
temperature tunability, the metamagnetic ordering transition in FeRh is
attractive for novel high-performance computing devices seeking to use
magnetism as the state variable. We demonstrate electrical control of the
transition via Joule heating in FeRh wires. Finite element simulations based on
abrupt state transition within each domain result in a globally smooth
transition that agrees with the experimental findings and provides insight into
the thermodynamics involved. We measure a 150 K decrease in transition
temperature with currents up to 60 mA, limited only by the dimensions of the
device. The sizeable shift in transition temperature scales with current
density and wire length, suggesting the absolute resistance and heat
dissipation of the substrate are also important. The FeRh phase change is
evaluated by pulsed I-V using a variety of bias conditions. We demonstrate high
speed (~ ns) memristor-like behavior and report device performance parameters
such as switching speed and power consumption that compare favorably with
state-of-the-art phase change memristive technologies. | cond-mat_mes-hall |
Contact-induced negative differential resistance in short-channel
graphene FETs: In this work, we clarify the physical mechanism for the phenomenon of
negative output differential resistance (NDR) in short-channel graphene FETs
(GFETs) through non-equilibrium Green's function (NEGF) simulations and a
simpler semianalytical ballistic model that captures the essential physics.
This NDR phenomenon is due to a transport mode bottleneck effect induced by the
graphene Dirac point in the different device regions, including the contacts.
NDR is found to occur only when the gate biasing produces an n-p-n or p-n-p
polarity configuration along the channel, for both positive and negative
drain-source voltage sweep. In addition, we also explore the impact on the NDR
effect of contact-induced energy broadening in the source and drain regions and
a finite contact resistance. | cond-mat_mes-hall |
Control of a spin qubit in a lateral GaAs quantum dot based on symmetry
of gating potential: We study the influence of quantum dot symmetry on the Rabi frequency and
phonon induced spin relaxation rate in a single electron GaAs spin qubit. We
find that anisotropic dependence on the magnetic field direction is independent
of the choice of the gating potential. Also, we discover that relative
orientation of the quantum dot, with respect to the crystallographic frame, is
relevant in systems with ${\bf C}_{1{\rm v}}$, ${\bf C}_{2{\rm v}}$, or ${\bf
C}_{n}$ ($n\neq4r$) symmetry. To demonstrate the important impact of the gating
potential shape on the spin qubit lifetime, we compare the effects of an
equilateral triangle, square, and rectangular confinement with the known
results for the harmonic potential. In the studied cases, enhanced spin qubit
lifetime is revealed, reaching almost six orders of magnitude increase for the
equilateral triangle gating. | cond-mat_mes-hall |
Coherent radiation by magnets with exchange interactions: A wide class of materials acquires magnetic properties due to particle
interactions through exchange forces. These can be atoms and molecules
composing the system itself, as in the case of numerous magnetic substances. Or
these could be different defects, as in the case of graphene, graphite, carbon
nanotubes, and related materials. The theory is suggested describing fast
magnetization reversal in magnetic systems, whose magnetism is caused by
exchange interactions. The effect is based on the coupling of a magnetic sample
with an electric circuit producing a feedback magnetic field. This method can
find various applications in spintronics. The magnetization reversal can be
self-organized, producing spin superradiance. A part of radiation is absorbed
by a resonator magnetic coil. But an essential part of radiation can also be
emitted through the coil sides. | cond-mat_mes-hall |
Second-order polaron resonances in self assembled quantum dots: We theoretically study the optical properties of an InAs/GaAs quantum dot
(QD) near the area of the second-order resonance between an electron confined
in the QD and two longitudinal optical phonons. We present the absorption
spectra of an inhomogeneously broadened QD ensemble and show that the minimal
model needed for an accurate description of such a system needs to account for
3-phonon states. We study also the influence of the QD height to width ratio on
the optical properties of the polaron system. The dependence of the width of
the resonance and the position of the second-order resonant feature on the
height to width ratio is presented. | cond-mat_mes-hall |
Polarization Of Quantum Hall States, Skyrmions and Berry Phase: We have discussed here the polarization of quantum Hall states in the
framework of the hierarchical analysis of IQHE and FQHE in terms of Berry
phase. It is observed that we have fully polarized states for the filling
factor $\nu=1$ as well as $\nu=\frac{1}{2m+1}$, $m$ being an integer. However,
for $\nu=p$ as well as $\nu =\frac{p}{q}$, with $p>1$ and odd, $q$ odd we have
partially polarized states and for $\nu=\frac{p}{q}$, $p$ even, $q$ odd we have
unpolarized states. It has been found that skyrmion excitations exist only for
fully polarized states and for partially polarized and unpolarized states
skyrmionic excitations do not exist. | cond-mat_mes-hall |
Magnetization dynamics and spin pumping induced by standing elastic
waves: The magnetization dynamics induced by standing elastic waves excited in a
thin ferromagnetic film is described with the aid of micromagnetic simulations
taking into account the magnetoelastic coupling between spins and lattice
strains. The simulations have been performed for the 2 nm thick Fe81Ga19 film
dynamically strained by longitudinal and transverse standing waves with various
frequencies, which span a wide range around the resonance frequency nu_res of
coherent magnetization precession in unstrained Fe81Ga19 film. It is found that
standing elastic waves give rise to complex local magnetization dynamics and
spatially inhomogeneous dynamic magnetic patterns. The spatio-temporal
distributions of the magnetization oscillations in standing elastic waves have
the form of standing spin waves with the same wavelength. Remarkably, the
amplitude of magnetization precession does not go to zero at the nodes of these
spin waves, which cannot be precisely described by simple analytical formulae.
In the steady-state regime, the magnetization oscillates with the frequency of
elastic wave, except for the case of longitudinal waves with frequencies well
below nu_res, where the magnetization precesses with a variable frequency
strongly exceeding the wave frequency. The precession amplitude at the
antinodes of standing spin waves strongly increases when the frequency of
elastic wave becomes close to nu_res. The results obtained for the
magnetization dynamics driven by elastic waves are used to calculate the spin
current pumped from the dynamically strained ferromagnet into adjacent
paramagnetic metal. Importantly, the transverse charge current created by the
spin current via the inverse spin Hall effect is high enough to be measured
experimentally. | cond-mat_mes-hall |
Magnetic anisotropy in surface-supported single-ion lanthanide complexes: Single-ion lanthanide-organic complexes can provide stable magnetic moments
with well-defined orientation for spintronic applications on the atomic level.
Here, we show by a combined experimental approach of scanning tunneling
microscopy and X-ray absorption spectroscopy that
dysprosium-tris(1,1,1-trifluoro-4-(2-thienyl)-2,4butanedionate) (Dy(tta)$_3$)
complexes deposited on a Au(111) surface undergo a molecular distortion,
resulting in distinct crystal field symmetry imposed on the Dy ion. This leads
to an easy-axis magnetization direction in the ligand plane. Furthermore, we
show that tunneling electrons hardly couple to the spin excitations, which we
ascribe to the shielded nature of the $4f$ electrons. | cond-mat_mes-hall |
Light-Driven Nanoscale Vectorial Currents: Controlled charge flows are fundamental to many areas of science and
technology, serving as carriers of energy and information, as probes of
material properties and dynamics, and as a means of revealing or even inducing
broken symmetries. Emerging methods for light-based current control offer
promising routes beyond the speed and adaptability limitations of conventional
voltage-driven systems. However, optical generation and manipulation of
currents at nanometer spatial scales remains a basic challenge and a crucial
step towards scalable optoelectronic systems for microelectronics and
information science. Here, we introduce vectorial optoelectronic metasurfaces
in which ultrafast light pulses induce local directional charge flows around
symmetry-broken plasmonic nanostructures, with tunable responses and arbitrary
patterning down to sub-diffractive nanometer scales. Local symmetries and
vectorial current distributions are revealed by polarization- and
wavelength-sensitive electrical readout and terahertz (THz) emission, while
spatially-tailored global currents are demonstrated in the direct generation of
elusive broadband THz vector beams. We show that in graphene, a detailed
interplay between electrodynamic, thermodynamic, and hydrodynamic degrees of
freedom gives rise to rapidly-evolving nanoscale driving forces and charge
flows under extreme temporal and spatial confinement. These results set the
stage for versatile patterning and optical control over nanoscale currents in
materials diagnostics, THz spectroscopies, nano-magnetism, and ultrafast
information processing. | cond-mat_mes-hall |
Surface Percolation and Growth. An alternative scheme for breaking the
diffraction limit in optical patterning: A nanopatterning scheme is presented by which the structure height can be
controlled in the tens of nanometers range and the lateral resolution is a
factor at least three times better than the point spread function of the
writing beam. The method relies on the initiation of the polymerization
mediated by a very inefficient energy transfer from a fluorescent dye molecule
after single photon absorption. The mechanism has the following distinctive
steps: the dye adsorbs on the substrate surface with a higher concentration
than in the bulk, upon illumination it triggers the polymerization, then
isolated islands develop and merge into a uniform structure (percolation),
which subsequently grows until the illumination is interrupted. This
percolation mechanism has a threshold that introduces the needed nonlinearity
for the fabrication of structures beyond the diffraction limit. | cond-mat_mes-hall |
Electron-Hole Liquid in Monolayer Transition Metal Dichalcogenide
Heterostructures: Monolayer films of transition metal dichalcogenides (in particular, MoS2,
MoSe2, WS2, and WSe2) can be considered as ideal systems for the studies of
high-temperature electron-hole liquids. The quasi-two-dimensional nature of
electrons and holes ensures their stronger interaction as compared to that in
bulk semiconductors. The screening of the Coulomb interaction in monolayer
heterostructures is significantly reduced, since it is determined by the
permittivities of the environment (e.g., vacuum and substrate), which are much
lower than those characteristic of the films of transition metal
dichalcogenides. The multivalley structure of the energy spectrum of charge
carriers in transition metal dichalcogenides significantly reduces the kinetic
energy, resulting in the increase in the equilibrium density and binding energy
of the electron-hole liquid. The binding energy of the electron-hole liquid and
its equilibrium density are determined. It is shown that the two-dimensional
Coulomb potential should be used in the calculations for the electron-hole
liquid. | cond-mat_mes-hall |
The specific heat and the radial thermal expansion of bundles of
single-walled carbon nanotubes: The specific heat at constant pressure of bundles of single-walled carbon
nanotubes closed at their ends has been investigated in a temperature interval
of 2-120 K. It is found that the curve of heat capacity has features near 5,
36, 80, and 100 K. The experimental results on the heat capacity and the radial
thermal expansion coefficient of bundles of SWNTs oriented perpendicular to the
sample axis have been compared. It is found that the curves of the heat
capacity and the radial thermal expansion coefficient exhibit a similar
temperature behavior above 10 K. The temperature dependence of the Gruneisen
coefficient has been calculated. The curve of the Gruneisen coefficient also
has a feature near 36 K. Above 36 K the Gruneisen coefficient is practically
independent of temperature. Below 36 K the Gruneisen coefficient decreases
monotonically with lowering temperature and becomes negative below 6 K. | cond-mat_mes-hall |
Optical orientation of excitons in a longitudinal magnetic field in
indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment: The exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots
(QDs) with indirect band gap and type-I band alignment are studied. The
negligible (less than $0.2~\mu$eV) value of the anisotropic exchange
interaction in these QDs prevents a mixing of the excitonic basis states with
pure spin and allows for the formation of spin polarized bright excitons for
quasi-resonant circularly polarized excitation. In a longitudinal magnetic
field, the recombination and spin dynamics of the excitons are controlled by
the hyperfine interaction between the electron and nuclear spins. A QD blockade
by dark excitons is observed in magnetic field eliminating the impact of the
nuclear spin fluctuations. A kinetic equation model, which accounts for the
population dynamics of the bright and dark exciton states as well as for the
spin dynamics, has been developed, which allows for a quantitative description
of the experimental data. | cond-mat_mes-hall |
Spectral and transport properties of the two-dimensional Lieb lattice: The specific topology of the line centered square lattice (known also as the
Lieb lattice) induces remarkable spectral properties as the macroscopically
degenerated zero energy flat band, the Dirac cone in the low energy spectrum,
and the peculiar Hofstadter-type spectrum in magnetic field. We study here the
properties of the finite Lieb lattice with periodic and vanishing boundary
conditions. We find out the behavior of the flat band induced by disorder and
external magnetic and electric fields. We show that in the confined Lieb
plaquette threaded by a perpendicular magnetic flux there are edge states with
nontrivial behavior. The specific class of twisted edge states, which have
alternating chirality, are sensitive to disorder and do not support IQHE, but
contribute to the longitudinal resistance. The symmetry of the transmittance
matrix in the energy range where these states are located is revealed. The
diamagnetic moments of the bulk and edge states in the Dirac-Landau domain, and
also of the flat states in crossed magnetic and electric fields are shown. | cond-mat_mes-hall |
Microwave amplification with nanomechanical resonators: Sensitive measurement of electrical signals is at the heart of modern science
and technology. According to quantum mechanics, any detector or amplifier is
required to add a certain amount of noise to the signal, equaling at best the
energy of quantum fluctuations. The quantum limit of added noise has nearly
been reached with superconducting devices which take advantage of
nonlinearities in Josephson junctions. Here, we introduce a new paradigm of
amplification of microwave signals with the help of a mechanical oscillator. By
relying on the radiation pressure force on a nanomechanical resonator, we
provide an experimental demonstration and an analytical description of how the
injection of microwaves induces coherent stimulated emission and signal
amplification. This scheme, based on two linear oscillators, has the advantage
of being conceptually and practically simpler than the Josephson junction
devices, and, at the same time, has a high potential to reach quantum limited
operation. With a measured signal amplification of 25 decibels and the addition
of 20 quanta of noise, we anticipate near quantum-limited mechanical microwave
amplification is feasible in various applications involving integrated
electrical circuits. | cond-mat_mes-hall |
Second-Harmonic Generation in Nano-Structured Metamaterials: We conduct a theoretical and numerical study on the second-harmonic (SH)
optical response of a nano-structured metamaterial composed of a periodic array
of inclusions. Both the inclusions and their surrounding matrix are made of
centrosymmetrical materials, for which SH is strongly suppressed, but by
appropriately choosing the shape of the inclusions, we may produce a
geometrically non-centrosymmetric system which does allow efficient SH
generation. Variations in the geometrical configuration allows tuning the
linear and quadratic spectra of the optical response of the system. We develop
an efficient scheme for calculating the nonlinear polarization, extending a
formalism for the calculation of the macroscopic dielectric function using
Haydock's recursion method. We apply the formalism developed here to an array
of holes within an Ag matrix, but it can be readily applied to any metamaterial
made of arbitrary materials and for inclusions of any geometry within the
long-wavelength regime. | cond-mat_mes-hall |
Terahertz-induced resistance oscillations in high mobility
two-dimensional electron systems: We report on a theoretical work on magnetotransport under terahertz radiation
with high mobility two-dimensional electron systems. We focus on the
interaction between the obtained radiation-induced magnetoresistance
oscillations (RIRO) and the Shubnikov-de Haas (SdHO) oscillations. We study two
effects experimentally obtained with this radiation. First, the observed
disappearance of the SdHO oscillations simultaneously with the vanishing
resistance at the zero resistance states region. And secondly the strong
modulation of the SdHO oscillations at sufficient terahertz radiation power. We
conclude that both effects share the same physical origin, the interference
between the average advanced distance by the scattered electron between
irradiated Landau states, (RIRO), and the available initial density of states
at a certain magnetic field, (SdHO). Thus, from a physical standpoint, what the
terahertz experiments and theoretical simulations reveal is, on the one hand,
the oscillating nature of the Landau states subjected to radiation and, on the
other hand, how they behave in the presence of scattering. | cond-mat_mes-hall |
Are Microwave Induced Zero Resistance States Necessarily Static?: We study the effect of inhomogeneities in Hall conductivity on the nature of
the Zero Resistance States seen in the microwave irradiated two-dimensional
electron systems in weak perpendicular magnetic fields, and we show that
time-dependent domain patterns may emerge in some situations. For an annular
Corbino geometry, with an equilibrium charge density that varies linearly with
radius, we find a time-periodic non-equilibrium solution, which might be
detected by a charge sensor, such as an SET. For a model on a torus, in
addition to static domain patterns seen at high and low values of the
equilibrium charge inhomogeneity, we find that, in the intermediate regime, a
variety of nonstationary states can also exist. We catalog the possibilities we
have seen in our simulations. Within a particular phenomenological model, we
show that linearizing the nonlinear charge continuity equation about a
particularly simple domain wall configuration and analyzing the eigenmodes
allows us to estimate the periods of the solutions to the full nonlinear
equation. | cond-mat_mes-hall |
Exchange interaction effects in the thermodynamic properties of quantum
dots: We study electron-electron interaction effects in the thermodynamic
properties of quantum-dot systems. We obtain the direct and exchange
contributions to the specific heat C_v in the self-consistent Hartree-Fock
approximation at finite temperatures. An exchange-induced phase transition is
observed and the transition temperature is shown to be inversely proportional
to the size of the system. The exchange contribution to C_v dominates over the
direct and kinetic contributions in the intermediate regime of interaction
strength (r_s ~ 1). Furthermore, the electron-electron interaction modifies
both the amplitude and the period of magnetic field induced oscillations in
C_v. | cond-mat_mes-hall |
Scattering by linear defects in graphene: a tight-binding approach: We develop an analytical scattering formalism for computing the transmittance
through periodic defect lines within the tight-binding model of graphene. We
first illustrate the method with a relatively simple case, the pentagon-only
defect line. Afterwards, more complex defect lines are treated, namely the
zz(558) and the zz(5757) ones. The formalism developed, only uses simple
tight-binding concepts, reducing the problem to matrix manipulations which can
be easily worked out by any computational algebraic calculator. | cond-mat_mes-hall |
Screening of a Luttinger liquid wire by a scanning tunneling microscope
tip: II. Transport properties: We study the effect of an electrostatic coupling between a scanning tunneling
microscope tip and a Luttinger liquid wire on the tunneling current and noise
between the two. Solving the Dyson equations non perturbatively for a local
interaction potential, we derive the Green's functions associated to the wire
and to the tip. Interestingly, the electrostatic coupling leads to the
existence of new correlators, which we call mixed Green's functions, which are
correlators between the bosonic fields of the wire and the tip. Next, we
calculate the transport properties up to second order with the amplitude of the
tunnel transfer: the tunnel current is strongly reduced by the presence of
screening. The zero-frequency noise is modified in a similar way, but the Fano
factor remains unchanged. We also consider the effect of the screening on the
asymmetry of the finite-frequency non-symmetrized noise and on the conductance. | cond-mat_mes-hall |
Oxygen vacancies dynamics in redox-based interfaces: Tailoring the
memristive response: Redox-based memristive devices are among the alternatives for the next
generation of non volatile memories, but also candidates to emulate the
behavior of synapses in neuromorphic computing devices. It is nowadays well
established that the motion of oxygen vacancies (OV) at the nanoscale is the
key mechanism to reversibly switch metal/insulator/metal structures from
insulating to conducting, i.e. to accomplish the resistive switching effect.
The control of OV dynamics has a direct effect on the resistance changes, and
therefore on different figures of memristive devices, such as switching speed,
retention, endurance or energy consumption. Advances in this direction demand
not only experimental techniques that allow for measurements of OV dynamics,
but also of theoretical studies that shed light on the involved mechanisms.
Along this goal, we analize the OV dynamics in redox interfaces formed when an
oxidizable metallic electrode is in contact with the insulating oxide. We show
how the transfer of OV can be manipulated by using different electrical stimuli
protocols to optimize device figures such as the ON/OFF ratio or the energy
dissipation linked to the writing process. Analytical expressions for attained
resistance values, including the high and low resistance states are derived in
terms of total transferred OV in a nanoscale region of the interface. Our
predictions are validated with experiments performed in
Ti/La$_{1/3}$Ca$_{2/3}$MnO$_{3}$ redox memristive devices. | cond-mat_mes-hall |
Ramsey Interference in a Multi-level Quantum System: We report Ramsey interference in the excitonic population of a negatively
charged quantum dot revealing the coherence of the state in the limit where
radiative decay is dominant. Our experiments show that the decay time of the
Ramsey interference is limited by the spectral width of the transition.
Applying a vertical magnetic field induces Zeeman split transitions that can be
addressed by changing the laser detuning to reveal 2, 3 and 4 level system
behaviour. We show that under finite field the phase-sensitive control of two
optical pulses from a single laser can be used to prepare both population and
spin qubits simultaneously. | cond-mat_mes-hall |
Quantum information processing with large nuclear spins in GaAs
semiconductors: We propose an implementation for quantum information processing based on
coherent manipulations of nuclear spins I=3/2 in GaAs semiconductors. We
describe theoretically an NMR method which involves multiphoton transitions and
which exploits the non-equidistance of nuclear spin levels due to quadrupolar
splittings. Starting from known spin anisotropies we derive effective
Hamiltonians in a generalized rotating frame, valid for arbitrary I, which
allow us to describe the non-perturbative time evolution of spin states
generated by magnetic rf fields. We identify an experimentally accessible
regime where multiphoton Rabi oscillations are observable. In the nonlinear
regime, we find Berry phase interference effects. | cond-mat_mes-hall |
Effect of the electromagnetic environment on current fluctuations in
driven tunnel junctions: We examine current fluctuations in tunnel junctions driven by a superposition
of a constant and a sinusoidal voltage source. In standard setups the external
voltage is applied to the tunneling element via an impedance providing an
electromagnetic environment of the junction. The modes of this environment are
excited by the time-dependent voltage and are the source of Johnson-Nyquist
noise. We determine the autocorrelation function of the current flowing in the
leads of the junction in the weak tunneling limit up to terms of second order
in the tunneling Hamiltonian. The driven modes of the electromagnetic
environment are treated exactly by means of a unitary transformation introduced
recently. Particular emphasis is placed on the spectral function of the current
fluctuations. The spectrum is found to comprise three contributions: a term
arising from the Johnson-Nyquist noise of the environmental impedance, a part
due to the shot noise of the tunneling element and a third contribution which
comes from the cross-correlation between fluctuations caused by the
electromagnetic environment and fluctuations of the tunneling current. All
three parts of the spectral function occur already for devices under dc bias.
The spectral function of ac driven tunneling elements can be determined from
the result for a dc bias by means of a photo-assisted tunneling relation of the
Tien-Gordon type. Specific results are given for an Ohmic environment and for a
junction driven through a resonator. | cond-mat_mes-hall |
Electronics and photonics: two sciences in the benefit of solar energy
conversion: This paper gives a personal global point of view on two sciences: electronics
and photonics towards plasmonics and solar energy conversion. The new research
directions in these two sciences are pointed out by comparison and in the
perspective of future new solar devices. A parallel and the equivalence between
electronics and photonics are presented. Starting from electron in electronics,
photon, solitons and plasmons in photonics, electrical cables - optical fibers,
plasmonic wave guides, electrical circuits - optical circuits, electrical
transistors - optical transistors, plasmonster, electrical generators - pulsed
lasers and spasers, photonics gets step by step all the tools already existing
in electronics. Solar energy could be converted in many ways, the most known is
the conversion in electricity. Today we need that the energy is in form of
electricity because most of the apparatus that we use are based on electricity:
informatics, motors, etc. However, the progress in photonics with optical
circuits, optical transistors etc., shows that the photonics informatics could
be possible. Also the optical manipulation and optical engines concept were
already demonstrated experimentally. If the laser propulsion will be achieved,
and the optical engines will work, the question that will rise tomorrow is:
"Shall we still use the electricity in the future? What will be the solar
devices tomorrow?" | cond-mat_mes-hall |
Neck Barrier Engineering in Quantum Dot Dimer Molecules via
Intra-Particle Ripening: Coupled colloidal quantum dot (CQD) dimers represent a new class of
artificial molecules composed of fused core/shell semiconductor nanocrystals.
The electronic coupling and wavefunction hybridization is enabled by the
formation of an epitaxial connection with a coherent lattice between the shells
of the two neighboring quantum dots where the shell material and its dimensions
dictate the quantum barrier characteristics for the charge carriers. Herein we
introduce a colloidal approach to control the neck formation at the interface
between the two CQDs in such artificial molecular constructs. This allows the
tailoring of the neck barrier in pre-linked homodimers formed via fusion of
multifaceted wurtzite CdSe/CdS CQDs. The effects of reaction time, temperature
and excess ligands is studied. The neck filling process follows an
intraparticle ripening mechanism at relatively mild reaction conditions while
avoiding inter-particle ripening. The degree of surface ligand passivation
plays a key role in activating the surface atom diffusion to the neck region.
The degree of neck filling strongly depends also on the initial relative
orientation of the two CQDs, where homonymous plane attachment allows for
facile neck growth, unlike the case of heteronymous plane attachment. Upon
neck-filling, the observed red-shift of the absorption and fluorescence
measured both for ensemble and single dimers, is assigned to enhanced
hybridization of the confined wavefunction in CQD dimer molecules, as supported
by quantum calculations. The fine tuning of the particle interface introduced
herein provides therefore a powerful tool to further control the extent of
hybridization and coupling in CQD molecules. | cond-mat_mes-hall |
Van Hove scenario of anisotropic transport in a two-dimensional
spin-orbit coupled electron gas in an in-plane magnetic field: We study electronic transport in two-dimensional spin-orbit coupled electron
gas subjected to an in-plane magnetic field. The interplay of the spin-orbit
interaction and the magnetic field leads to the Van Hove singularity of the
density of states and strong anisotropy of Fermi contours. We develop a method
that allows one to exactly calculate the nonequilibrium distribution function
for these conditions within the framework of the semiclassical Boltzmann
equation without using the scattering time approximation. The method is applied
to calculate the conductivity tensor and the tensor of spin polarization
induced by the electric field (Aronov-Lyanda-Geller-Edelstein effect). It is
found that both the conductivity and the spin polarization have a sharp
singularity as functions of the Fermi level or magnetic field, which occurs
when the Fermi level passes through the Van Hove singularity. In addition, the
transport anisotropy dramatically changes near the singularity. | cond-mat_mes-hall |
Quantum theory of an electron waiting time clock: The electron waiting time is the time that passes between two subsequent
charge transfers in an electronic conductor. Recently, theories of electron
waiting times have been devised for quantum transport in Coulomb-blockade
structures and for mesoscopic conductors, however, so far a proper description
of a detector has been missing. Here we develop a quantum theory of a waiting
time clock capable of measuring the distribution of waiting times between
electrons above the Fermi sea in a mesoscopic conductor. The detector consists
of a mesoscopic capacitor coupled to a quantum two-level system whose coherent
precession we monitor. Under ideal operating conditions our waiting time clock
recovers the results of earlier theories without a detector. We investigate
possible deviations due to an imperfect waiting time clock. As specific
applications we consider a quantum point contact with a constant voltage and
lorentzian voltage pulses applied to an electrode. | cond-mat_mes-hall |
Effect of charged line defects on conductivity in graphene: numerical
Kubo and analytical Boltzmann approaches: Charge carrier transport in single-layer graphene with one-dimensional
charged defects is studied theoretically. Extended charged defects, considered
an important factor for mobility degradation in chemically-vapor-deposited
graphene, are described by a self-consistent Thomas-Fermi potential. A
numerical study of electronic transport is performed by means of a
time-dependent real-space Kubo approach in honeycomb lattices containing
millions of carbon atoms, capturing the linear response of realistic size
systems in the highly disordered regime. Our numerical calculations are
complemented with a kinetic transport theory describing charge transport in the
weak scattering limit. The semiclassical transport lifetimes are obtained by
computing scattered amplitudes within the second Born approximation. The
transport electron-hole asymmetry found in the semiclassical approach is
consistent with the Kubo calculations. In the strong scattering regime, the
conductivity is found to be a sublinear function of electronic density and
weakly dependent on the Thomas-Fermi screening wavelength. We attribute this
atypical behavior to the extended nature of one-dimensional charged defects.
Our results are consistent with recent experimental reports. | cond-mat_mes-hall |
Unexpected Behavior of the Local Compressibility Near the B=0
Metal-Insulator Transition: We have measured the local electronic compressibility of a two-dimensional
hole gas as it crosses the B=0 Metal-Insulator Transition. In the metallic
phase, the compressibility follows the mean-field Hartree-Fock (HF) theory and
is found to be spatially homogeneous. In the insulating phase it deviates by
more than an order of magnitude from the HF predictions and is spatially
inhomogeneous. The crossover density between the two types of behavior, agrees
quantitatively with the transport critical density, suggesting that the system
undergoes a thermodynamic change at the transition. | cond-mat_mes-hall |
Evolution of the Spin Hall Magnetoresistance in Cr$_2$O$_3$/Pt bilayers
close to the Néel temperature: We study the evolution of magnetoresistance with temperature in thin film
bilayers consisting of platinum and the antiferromagnet Cr$_2$O$_3$ with its
easy axis out of the plane. We vary the temperature from 20 - 60{\deg}C, close
to the N\'eel temperature of Cr$_2$O$_3$ of approximately 37{\deg}C. The
magnetoresistive response is recorded during rotations of the external magnetic
field in three mutually orthogonal planes. A large magnetoresistance having a
symmetry consistent with a positive spin Hall magnetoresistance is observed in
the paramagnetic phase of the Cr$_2$O$_3$, which however vanishes when cooling
to below the N\'eel temperature. Comparing to analogous experiments in a
Gd$_3$Ga$_5$O$_{12}$/Pt heterostructure, we conclude that a paramagnetic field
induced magnetization in the insulator is not sufficient to explain the
observed magnetoresistance. We speculate that the type of magnetic moments at
the interface qualitatively impacts the spin angular momentum transfer, with
the $3d$ moments of Cr sinking angular momentum much more efficiently as
compared to the more localized $4f$ moments of Gd. | cond-mat_mes-hall |
Localized States and Quantum Spin Hall Effect in Si-Doped InAs/GaSb
Quantum Wells: We study localized in-gap states and quantum spin Hall effect in Si-doped
InAs/GaSb quantum wells. We propose a model describing donor and/or acceptor
impurities to describe Si dopants. This model shows in-gap bound states and
wide conductance plateau with the quantized value $2e^2/h$ in light dopant
concentration, consistent with recent experiments by Du et al. We predict a
conductance dip structure due to backward scattering in the region where the
localization length $\xi$ is comparable with the sample width $L_y$ but much
smaller than the sample length $L_x$. | cond-mat_mes-hall |
Escape from a zero current state in a one dimensional array of Josephson
junctions: A long one dimensional array of small Josephson junctions exhibits Coulomb
blockade of Cooper pair tunneling. This zero current state exists up to a
switching voltage, Vsw, where there is a sudden onset of current. In this paper
we present histograms showing how Vsw changes with temperature for a long array
and calculations of the corresponding escape rates. Our analysis of the problem
is based on the existence of a voltage dependent energy barrier and we do not
make any assumptions about its shape. The data divides up into two temperature
regimes, the higher of which can be explained with Kramers thermal escape
model. At low temperatures the escape becomes independent of temperature. | cond-mat_mes-hall |
Topological classification of the single-wall carbon nanotube: The single-wall carbon nanotube (SWNT) can be a one-dimensional topological
insulator, which is characterized by a $\mathbb{Z}$-topological invariant,
winding number. Using the analytical expression for the winding number, we
classify the topology for all possible chiralities of SWNTs in the absence and
presence of a magnetic field, which belongs to the topological categories of
BDI and AIII, respectively. We find that the majority of SWNTs are nontrivial
topological insulators in the absence of a magnetic field. In addition, the
topological phase transition takes place when the band gap is closed by
applying a magnetic field along the tube axis, in all the SWNTs except armchair
nanotubes. The winding number determines the number of edge states localized at
the tube ends by the bulk-edge correspondence, the proof of which is given for
SWNTs in general. This enables the identification of the topology in
experiments. | cond-mat_mes-hall |
Tuning the magnetic configuration of bilayer graphene quantum dot by
twisting: Twistronic has recently attracted tremendous attention because the twisting
can engineer the bilayer graphene-like materials into varying types of strongly
correlated phases. In this paper, we study the twisting of bilayer graphene
(BLG) quantum dots (QDs) with hexagonal shape and zigzag edges. In the
untwisted BLG-QDs, the zigzag edges of graphene host spontaneous magnetism with
varying magnetic configurations. As a BLG-QD being adiabatically twisted, the
quantum state evolves as a function of the twisting angle. If the twisting
angle changes across certain critical value, the magnetic configuration of the
quantum state sharply changes. For the twisting process with increasing or
decreasing twisting angle, the number and value of the critical twisting angles
are different. Thus, the twisting process with the twisting angle increasing
and decreasing back and forth could enter a hysteresis loop. The twisting of
BLG QDs with adatom is also investigated. The tuning features of the magnetic
configuration of the twisted BLG-QDs could be applied for graphene-based
quantum memory devices. | cond-mat_mes-hall |
Photoluminescence in PbS nanocrystal thin films: Nanocrystal density,
film morphology and energy transfer: We show that photoluminescence properties of PbS nanocrystal thin films are
directly related to film morphology and nanocrystal density. In densely packed
PbS nanocrystal films, low-temperature donor-to-acceptor energy transfer is
mainly responsible for the photoluminescence spectra narrowing and shift toward
longer wavelengths. At elevated temperatures, back energy transfer is proposed
to be responsible for an unusual photoluminescence intensity temperature
dependence. In thin films with a low PbS nanocrystal density, the energy
transfer is suppressed, and the effect is dramatically reduced. | cond-mat_mes-hall |
Gate-Controlled Magnetoresistance of a Paramagnetic Insulator|Platinum
Interface: We report an electric field-induced in-plane magnetoresistance of an
atomically flat paramagnetic insulator|platinum (Pt) interface at low
temperatures with an ionic liquid gate. Transport experiments as a function of
applied magnetic field strength and direction obey the spin Hall
magnetoresistance phenomenology with perpendicular magnetic anisotropy. Our
results establish the utility of ionic gating as an alternative method to
control spintronic devices without using ferromagnets. | cond-mat_mes-hall |
Deterministic Weak Localization in Periodic Structures: The weak localization is found for perfect periodic structures exhibiting
deterministic classical diffusion. In particular, the velocity autocorrelation
function develops a universal quantum power law decay at 4 times Ehrenfest
time, following the classical stretched-exponential type decay. Such
deterministic weak localization is robust against weak enough randomness (e.g.,
quantum impurities). In the 1D and 2D cases, we argue that at the quantum limit
states localized in the Bravis cell are turned into Bloch states by quantum
tunnelling. | cond-mat_mes-hall |
Electrical transport in deformed nanostrips: electrical signature of
reversible mechanical failure: We calculate the electrical conductivity of a thin crystalline strip of atoms
confined within a quasi one dimensional channel of fixed width. The
conductivity shows anomalous behavior as the strip is deformed under tensile
loading. Beyond a critical strain, the solid fails by the nucleation of
alternating bands of solid and {\em smectic} like phases accompanied by a jump
in the conductivity. Since the failure of the strip in this system is known to
be reversible, the onductivity anomaly may have practical use as a sensitive
strain transducer. | cond-mat_mes-hall |
Spatially dispersive dynamical response of hot carriers in doped
graphene: We study theoretically wave-vector and frequency dispersion of the complex
dynamic conductivity tensor (DCT), $\sigma_{lm}(\mathbf{k}, \omega)$, of doped
monolayer graphene under a strong dc electric field. For a general analysis, we
consider the weak ac field of arbitrary configuration given by two independent
vectors, the ac field polarization and the wave vector $\mathbf{k}$. The
high-field transport and linear response to the ac field are described on the
base of the Boltzmann kinetic equation. We show that the real part of DCT,
calculated in the collisionless regime, is not zero due to dissipation of the
ac wave, whose energy is absorbed by the resonant Dirac quasiparticles
effectively interacting with the wave. The role of the kinematic resonance at
$\omega = v_F |{\bf k}|$ ($v_{F}$ is the Fermi velocity) is studied in detail
taking into account deviation from the linear energy spectrum and screening by
the charge carriers. The isopower-density curves and distributions of angle
between the ac current density and field vectors are presented as a map which
provides clear graphic representation of the DCT anisotropy. Also, the map
shows certain ac field configurations corresponding to a negative power
density, thereby it indicates regions of terahertz frequency for possible
electrical (drift) instability in the graphene system. | cond-mat_mes-hall |
Material Parameters for Faster Ballistic Switching of an In-plane
Magnetized Nanomagnet: High-speed magnetization switching of a nanomagnet is necessary for faster
information processing. The ballistic switching by a pulsed magnetic filed is a
promising candidate for the high-speed switching. It is known that the
switching speed of the ballistic switching can be increased by increasing the
magnitude of the pulsed magnetic field. However it is difficult to generate a
strong and short magnetic field pulse in a small device. Here we explore
another direction to achieve the high-speed ballistic switching by designing
material parameters such as anisotropy constant, saturation magnetization, and
the Gilbert damping constant. We perform the macrospin simulations for the
ballistic switching of in-plane magnetized nano magnets with varying material
parameters. The results are analyzed based on the switching dynamics on the
energy density contour. We show that the pulse width required for the ballistic
switching can be reduced by increasing the magnetic anisotropy constant or by
decreasing the saturation magnetization. We also show that there exists an
optimal value of the Gilbert damping constant that minimizes the pulse width
required for the ballistic switching. | cond-mat_mes-hall |
Temperature-linear Resistivity in Twisted Double Bilayer Graphene: We report an experimental study of carrier density (n), displacement field
(D) and twist angle ({\theta}) dependence of temperature (T)-linear resistivity
in twisted double bilayer graphene (TDBG). For a large twist angle
({\theta}>1.5{\deg}) where correlated insulating states are absent, we observe
a T-linear resistivity (with the slope of the order ~10{\Omega}/K) over a wide
range of carrier density and its slope decreases with increasing of n, in
agreement with acoustic phonon scattering model semi-quantitatively. The slope
of T-linear resistivity is non-monotonically dependent on the displacement
field with a single peak structure. For device with {\theta}~1.23{\deg} at
which correlated states emerge, the slope of T-linear resistivity is found
maximum (~100{\Omega}/K) at the boundary of the halo structure where phase
transition occurs, with signatures of continuous phase transition, Planckian
dissipation, and the diverging effective mass; these observations are in line
with quantum critical behaviors, which might be due to the symmetry-breaking
instability at the critical points. Our results shed new light on correlated
physics in TDBG and other twisted moir\'e systems. | cond-mat_mes-hall |
Electric field effects on the band gap and edge states of monolayer
1T'-WTe2: Monolayer 1T'-WTe2 is a quantum spin Hall insulator with a gapped bulk and
gapless helical edge states persisting to temperatures around 100 K. Recent
studies have revealed a topological-to-trivial phase transition as well the
emergence of an unconventional, potentially topological superconducting state
upon tuning the carrier concentration with gating. However, despite extensive
studies, the effects of gating on the band structure and the helical edge
states have not yet been established. In this work we present a combined
low-temperature STM and first principles study of back-gated monolayer 1T'-WTe2
films grown on graphene. Consistent with a quantum spin Hall system, the films
show well-defined bulk gaps and clear edge states that span the gap. By
directly measuring the density of states with STM spectroscopy, we show that
the bulk band gap magnitude shows substantial changes with applied gate
voltage, which is contrary to the na\"ive expectation that a gate would rigidly
shift the bands relative to the Fermi level. To explain our data, we carry out
density functional theory and model Hamiltonian calculations which show that a
gate electric field causes doping and inversion symmetry breaking which
polarizes and spin-splits the bulk bands. Interestingly, the calculated spin
splitting from the effective Rashba-like spin-orbit coupling can be in the tens
of meV for the electric fields in the experiment, which may be useful for
spintronics applications. Our work reveals the strong effect of electric fields
on the bulk band structure of monolayer 1T'-WTe2, which will play a critical
role in our understanding of gate-induced phenomena in this system. | cond-mat_mes-hall |
Valley filters using graphene blister defects from first principles: Valleytronics, which makes use of the two valleys in graphenes, attracts
considerable attention and a valley filter is expected to be the central
component in valleytronics. We propose the application of the graphene valley
filter using blister defects to the investigation of the valley-dependent
transport properties of the Stone--Wales and blister defects of graphenes by
density functional theory calculations. It is found that the intervalley
transition from the $\mathbf{K}$ valley to the $\mathbf{K}^\prime$ valleys is
completely suppressed in some defects. Using a large bipartite honeycomb cell
including several carbon atoms in a cell and replacing atomic orbitals with
molecular orbitals in the tight-binding model, we demonstrate analytically and
numerically that the symmetry between the A and B sites of the bipartite
honeycomb cell contributes to the suppression of the intervalley transition. In
addition, the universal rule for the atomic structures of the blisters
suppressing the intervalley transition is derived. Furthermore, by introducing
additional carbon atoms to graphenes to form blister defects, we can split the
energies of the states at which resonant scattering occurs on the $\mathrm{K}$
and $\mathrm{K}^\prime$ channel electrons. Because of this split, the fully
valley-polarized current will be achieved by the local application of a gate
voltage. | cond-mat_mes-hall |
Mesoscopic to universal crossover of transmission phase of multi-level
quantum dots: Transmission phase \alpha measurements of many-electron quantum dots (small
mean level spacing \delta) revealed universal phase lapses by \pi between
consecutive resonances. In contrast, for dots with only a few electrons (large
\delta), the appearance or not of a phase lapse depends on the dot parameters.
We show that a model of a multi-level quantum dot with local Coulomb
interactions and arbitrary level-lead couplings reproduces the generic features
of the observed behavior. The universal behavior of \alpha for small \delta
follows from Fano-type antiresonances of the renormalized single-particle
levels. | cond-mat_mes-hall |
Magnetic field induces giant nonlinear optical response in Weyl
semimetals: We study the second-order optical response of Weyl semimetals in the presence
of a magnetic field. We consider an idealized model of a perfectly linear Weyl
node and use the Kubo formula at zero temperature to calculate the intrinsic
contribution to photocurrent and second harmonic generation conductivity
components. We obtain exact analytical expressions applicable at arbitrary
values of frequency, chemical potential, and magnetic field. Our results show
that finite magnetic field significantly enhances the nonlinear optical
response in semimetals, while magnetic resonances lead to divergences in
nonlinear conductivity. In realistic systems, these singularities are
regularized by a finite scattering rate, but result in pronounced peaks which
can be detected experimentally, provided the system is clean and interactions
are weak. We also perform a semiclassical calculation that complements and
confirms our microscopic results at small magnetic fields and frequencies. | cond-mat_mes-hall |
Low-field magnetotransport in graphene cavity devices: Confinement and edge structures are known to play significant roles in
electronic and transport properties of two-dimensional materials. Here, we
report on low-temperature magnetotransport measurements of lithographically
patterned graphene cavity nanodevices. It is found that the evolution of the
low-field magnetoconductance characteristics with varying carrier density
exhibits different behaviors in graphene cavity and bulk graphene devices. In
the graphene cavity devices, we have observed that intravalley scattering
becomes dominant as the Fermi level gets close to the Dirac point. We associate
this enhanced intravalley scattering to the effect of charge inhomogeneities
and edge disorder in the confined graphene nanostructures. We have also
observed that the dephasing rate of carriers in the cavity devices follows a
parabolic temperature dependence, indicating that the direct Coulomb
interaction scattering mechanism governs the dephasing at low temperatures. Our
results demonstrate the importance of confinement in carrier transport in
graphene nanostructure devices. | cond-mat_mes-hall |
Skyrmion Brownian circuit implemented in a continuous ferromagnetic thin
film: The fabrication of a skyrmion circuit which stabilizes skyrmions is important
to realize micro- to nano-sized skyrmion devices. One example of promising
skyrmion-based device is Brownian computers, which have been theoretically
proposed, but not realized. It would require a skyrmion circuit in which the
skyrmion is stabilized and easily movable. However, the usual skyrmion circuits
fabricated by etching of the ferromagnetic film decrease the demagnetization
field stabilizing the skyrmions, and thus prevent their formation. In this
study, a skyrmion Brownian circuit implemented in a continuous ferromagnetic
film with patterned SiO$_2$ capping to stabilize the skyrmion formation. The
patterned SiO$_2$ capping controls the saturation field of the ferromagnetic
layer and forms a wire-shaped skyrmion potential well, which stabilizes
skyrmion formation in the circuit. Moreover, we implement a hub (Y-junction)
circuit without pinning sites at the junction by patterned SiO$_2$ capping.
This technique enables the efficient control of skyrmion-based memory and logic
devices, as well as Brownian computers. | cond-mat_mes-hall |
Thickness and electric field dependent polarizability and dielectric
constant in phosphorene: Based on extensive first principle calculations, we explore the thickness
dependent effective di- electric constant and slab polarizability of few layer
black phosphorene. We find that the dielectric constant in ultra-thin
phosphorene is thickness dependent and it can be further tuned by applying an
out of plane electric field. The decreasing dielectric constant with reducing
number of layers of phosphorene, is a direct consequence of the lower
permittivity of the surface layers and the in- creasing surface to volume
ratio. We also show that the slab polarizability depends linearly on the number
of layers, implying a nearly constant polarizability per phosphorus atom. Our
calculation of the thickness and electric field dependent dielectric properties
will be useful for designing and interpreting transport experiments in gated
phosphorene devices, wherever electrostatic effects such as capacitance, charge
screening etc. are important. | cond-mat_mes-hall |
Tunable spin and transport in porphyrin-graphene nanoribbon hybrids: Recently, porphyrin units have been attached to graphene nanoribbons
(Por-GNR) enabling a multitude of possible structures. Here we report first
principles calculations of two prototypical, experimentally feasible, Por-GNR
hybrids, one of which displays a small band gap relevant for its use as
electrode in a device. Embedding a Fe atom in the porphyrin causes spin
polarization with a spin ground state $S=1$. We employ density functional
theory and nonequilibrium Green's function transport calculations to examine a
2-terminal setup involving one Fe-Por-GNR between two metal-free, small band
gap, Por-GNR electrodes. The coupling between the Fe-$d$ and GNR band states
results in a Fano anti-resonance feature in the spin transport close to the
Fermi energy. This feature makes transport highly sensitive to the Fe spin
state. We demonstrate how mechanical strain or chemical adsorption on the Fe
give rise to a spin-crossover to $S=1$ and $S=0$, respectively, directly
reflected in a change in transport. Our theoretical results provide a clue for
the on-surface synthesis of Por-GNRs hybrids, which can open a new avenue for
carbon-based spintronics and chemical sensing. | cond-mat_mes-hall |
Control of charging in resonant tunneling through InAs nanocrystal
quantum dots: Tunneling spectroscopy of InAs nanocrystals deposited on graphite was
measured using scanning tunneling microscopy, in a double-barrier
tunnel-junction configuration. The effect of the junction symmetry on the
tunneling spectra is studied experimentally and modeled theoretically. When the
tip is retracted, we observe resonant tunneling through the nanocrystal states
without charging. This is in contrast to previous measurements on similar
nanocrystals anchored to gold by linker molecules, where charging took place.
Charging is regained upon reducing the tip-nanocrystal distance, making the
junctions more symmetric. The effect of voltage distribution between the
junctions on the measured spectra is also discussed. | cond-mat_mes-hall |
Quantum transport in flat bands and super-metallicity: Quantum physics in flat-band (FB) systems embodies a variety of exotic
phenomenon and even counter intuitive features. The quantum transport in
several graphene based compounds that exhibit a flat band and a tunable gap is
investigated. Despite the localized nature of the FB states and a zero group
velocity, a super-metallic (SM) phase at the FB energy is revealed. The SM
phase is robust against the inelastic scattering strength and controlled only
by the inter-band transitions between the FB and the dispersive bands. The SM
phase appears insensitive and quasi independent of the gap amplitude and nature
of the lattice (disordered or nano-patterned). The universal nature of the
unconventional FB transport is illustrated with the case of electrons in the
Lieb lattice. | cond-mat_mes-hall |
Cotunneling effects in GaAs vertical double quantum dot: We observed lifting of Coulomb blockade in GaAs vertical double quantum dot
with low potential barriers, induced by cotunneling mechanisms at dilution
fridge temperature of 10 mK. Several distinct features were observed, compared
to single dot case, and appropriate explanation for them was given | cond-mat_mes-hall |
Spin dynamics and spin-dependent recombination of a polaron pair under a
strong ac drive: We study theoretically the recombination within a pair of two polarons in
magnetic field subject to a strong linearly polarized ac drive. Strong drive
implies that the Zeeman frequencies of the pair-partners are much smaller than
the Rabi frequency, so that the rotating wave approximation does not apply.
What makes the recombination dynamics nontrivial, is that the partners
recombine only when they form a singlet, S. By admixing singlet to triplets,
the drive induces the triplet recombination as well. We calculate the effective
decay rate of all four spin modes. Our main finding is that, under the strong
drive, the major contribution to the decay of the modes comes from short time
intervals when the driving field passes through zero. When the recombination
time in the absence of drive is short, fast recombination from S leads to
anomalously slow recombination from the other spin states of the pair. We show
that, with strong drive, this recombination becomes even slower. The
corresponding decay rate falls off as a power law with the amplitude of the
drive. | cond-mat_mes-hall |
Fast and slow edges in bilayer graphene nanoribbons: Tuning the
transition from band- to Mott-insulator: We show that gated bilayer graphene zigzag ribbons possess a fast and a slow
edge, characterized by edge state velocities that differ due to non-negligible
next-nearest-neighbor hopping elements. By applying bosonization and
renormalization group methods, we find that the slow edge can acquire a sizable
interaction-induced gap, which is tunable via an external gate voltage V_{g}.
In contrast to the gate-induced gap in the bulk, the interaction-induced gap
depends non-monotonously on the on-site potential V. | cond-mat_mes-hall |
Designer quantum states of matter created atom-by-atom: With the advances in high resolution and spin-resolved scanning tunneling
microscopy as well as atomic-scale manipulation, it has become possible to
create and characterize quantum states of matter bottom-up, atom-by-atom. This
is largely based on controlling the particle- or wave-like nature of electrons,
as well as the interactions between spins, electrons, and orbitals and their
interplay with structure and dimensionality. We review the recent advances in
creating artificial electronic and spin lattices that lead to various exotic
quantum phases of matter, ranging from topological Dirac dispersion to complex
magnetic order. We also project future perspectives in non-equilibrium
dynamics, prototype technologies, engineered quantum phase transitions and
topology, as well as the evolution of complexity from simplicity in this newly
developing field. | cond-mat_mes-hall |
Reaction diffusion dynamics and the Schryer-Walker solution for domain
walls of the Landau-Lifshitz-Gilbert equation: We study the dynamics of the equation obtained by Schryer and Walker for the
motion of domain walls. The reduced equation is a reaction diffusion equation
for the angle between the applied field and the magnetization vector. If the
hard axis anisotropy $K_d$ is much larger than the easy axis anisotropy $K_u$,
there is a range of applied fields where the dynamics does not select the
Schryer-Walker solution. We give analytic expressions for the speed of the
domain wall in this regime and the conditions for its existence. | cond-mat_mes-hall |
Formation of image-potential states at the graphene/metal interface: The formation of image-potential states at the interface between a graphene
layer and a metal surface is studied by means of model calculations. An
analytical one-dimensional model-potential for the combined system is
constructed and used to calculate energies and wave functions of the
image-potential states at the Gamma-point as a function of the graphene-metal
distance. It is demonstrated how the double series of image-potential states of
free-standing graphene evolves into interfacial states that interact with both
surfaces at intermediate distances and finally into a single series of states
resembling those of a clean metal surface covered by a monoatomic spacer layer.
The model quantitatively reproduces experimental data available for
graphene/Ir(111) and graphene/Ru(0001), systems which strongly differ in
interaction strength and therefore adsorption distance. Moreover, it provides a
clear physical explanation for the different binding energy and lifetime of the
first (n=1) image-potential state in the valley and hill areas of the strongly
corrugated moire superlattice of graphene/Ru(0001). | cond-mat_mes-hall |
Full counting statistics of information content and particle number: We consider a bipartite quantum conductor and discuss the joint probability
distribution of particle number in a subsystem and the self-information
associated with the reduced density matrix of the subsystem. By extending the
multi-contour Keldysh Green function technique, we calculate the R\'enyi
entropy of a positive integer order $M$ subjected to the particle number
constraint, from which we derive the joint probability distribution. For
energy-independent transmission, we derive the time dependence of the
accessible entanglement entropy, or the conditional entropy. We analyze the
joint probability distribution for energy-dependent transmission probability at
the steady state under the coherent resonant tunneling and the incoherent
sequential tunneling conditions. We also discuss the probability distribution
of the efficiency, which measures the information content transfered by a
single electron. | cond-mat_mes-hall |
Ultrafast excitation and topological soliton formation in incommensurate
charge density wave states: Topological soliton is a nonperturbative excitation in commensurate density
wave states and connects degenerate ground states. In incommensurate density
wave states, ground states are continuously degenerate and topological soliton
is reckoned to be smoothly connected to the perturbative phason excitation. We
study the ultrafast nonequilibrium dynamics due to photoexcited electron-hole
pair in a one-dimensional chain with an incommensurate charge density wave
ground state. Time-resolved evolution reveals both perturbative excitation of
collective modes and nonperturbative topological phase transition due to
creating novel topological solitons, where the continuous complex order
parameter with amplitude and phase is essential. We identify the nontrivial
phase-winding solitons in the complex plane unique to this nonequilibrium state
and capture it by a low-energy effective model. The perturbative temporal gap
oscillation and the solitonic in-gap states enter the optical conductivity
absorption edge and the spectral density related to spectroscopic measurement,
providing concrete connections to real experiments. | cond-mat_mes-hall |
Controlling the conductance of molecular wires by defect engineering: a
divide et impera approach: Understanding of charge transport mechanisms in nanoscale structures is
essential for the development of molecular electronic devices. Charge transport
through 1D molecular systems connected between two contacts is influenced by
several parameters such as the electronic structure of the molecule and the
presence of disorder and defects. In this work, we have modeled 1D molecular
wires connected between electrodes and systematically investigated the
influence of both soliton formation and the presence of defects on properties
such as the conductance and the density of states. Our numerical calculations
have shown that the transport properties are highly sensitive to the position
of both solitons and defects. Interestingly, the introduction of a single
defect in the molecular wire which divides it into two fragments both
consisting of an odd number of sites creates a new conduction channel in the
center of the band gap resulting in higher zero-bias conductance than for
defect free systems. This phenomenon suggests alternative routes toward
engineering molecular wires with enhanced conductance. | cond-mat_mes-hall |
Thermodynamic Properties of Graphene in Magnetic Field and Rashba
Coupling: We study the thermodynamic properties of massless Dirac fermions in graphene
subjected to a uniform magnetic field $B$ together with Rashba coupling
parameter $\lambda_R$. The thermodynamic functions such as the Helmholtz free
energy, total energy, entropy and heat capacity are obtained in the high
temperature regime using an approach based on the zeta function. These
functions will be numerically examined by considering two cases related to
$\lambda_R$ smaller or greater than $B$. In particular, we show that the
Dulong-Petit law is verified for both cases. | cond-mat_mes-hall |
Driven Andreev molecule: We study the three terminal S-QD-S-QD-S Josephson junction biased with
commensurate voltages. In the absence of an applied voltage, the Andreev bound
states on each quantum dot hybridize forming an `Andreev molecule'. However,
understanding of this system in a non-equilibrium setup is lacking. Applying a
dc voltage on the bijunction makes the system time-periodic, and the
equilibrium Andreev bound states evolve into a ladder of resonances with a
finite lifetime due to multiple Andreev reflections (MAR). Starting from the
time-periodic Bogoliubov-de Gennes equations we map the problem to a
tight-binding chain in the (infinite) Floquet space. The resolvent of this
non-Hermitian block matrix is obtained via a continued fraction method. We
numerically calculate the Floquet-Andreev spectra which could be probed by
local tunneling spectroscopy on the dots. We also consider the subgap current,
and show that the Floquet resonances determine the position of the MAR steps.
Proximity of the two dots causes splitting of the steps, while at large
distances we observe interference effects which cause oscillations in the I-V
curves. The latter effect should persist at very long distances. | cond-mat_mes-hall |
Bloch-point-mediated topological transformations of magnetic domain
walls in cylindrical nanowires: Cylindrical nanowires made of soft magnetic materials, in contrast to thin
strips, may host domain walls of two distinct topologies. Unexpectedly, we
evidence experimentally the dynamic transformation of topology upon wall motion
above a field threshold. Micromagnetic simulations highlight the underlying
precessional dynamics for one way of the transformation, involving the
nucleation of a Bloch-point singularity, however, fail to reproduce the reverse
process. This rare discrepancy between micromagnetic simulations and
experiments raises fascinating questions in material and computer science. | cond-mat_mes-hall |
Excited-state spectroscopy on a quantum dot side-coupled to a quantum
wire: We report excited-state spectroscopy on a quantum dot side-coupled to a
quantum wire with accurate energy estimation. Our method utilizes periodic
voltage pulses on the dot, and the energy calibration is performed with
reference to the bias voltage across the wire. We demonstrate the observation
of the orbital excited state and the Zeeman splitting in a single dot. | cond-mat_mes-hall |
Fragility of spectral flow for topological phases in non-Wigner-Dyson
classes: Topological insulators and superconductors support extended surface states
protected against the otherwise localizing effects of static disorder.
Specifically, in the Wigner-Dyson insulators belonging to the symmetry classes
A, AI, and AII, a band of extended surface states is continuously connected to
a likewise extended set of bulk states forming a ``bridge'' between different
surfaces via the mechanism of spectral flow. In this work we show that this
principle becomes \emph{fragile} in the majority of non-Wigner-Dyson
topological superconductors and chiral topological insulators. In these
systems, there is precisely one point with granted extended states, the center
of the band, $E=0$. Away from it, states are spatially localized, or can be
made so by the addition of spatially local potentials. Considering the
three-dimensional insulator in class AIII and winding number $\nu=1$ as a
paradigmatic case study, we discuss the physical principles behind this
phenomenon, and its methodological and applied consequences. In particular, we
show that low-energy Dirac approximations in the description of surface states
can be treacherous in that they tend to conceal the localizability phenomenon.
We also identify markers defined in terms of Berry curvature as measures for
the degree of state localization in lattice models, and back our analytical
predictions by extensive numerical simulations. A main conclusion of this work
is that the surface phenomenology of non-Wigner-Dyson topological insulators is
a lot richer than that of their Wigner-Dyson siblings, extreme limits being
spectrum wide quantum critical delocalization of all states vs. full
localization except at the $E=0$ critical point. As part of our study we
identify possible experimental signatures distinguishing between these
different alternatives in transport or tunnel spectroscopy. | cond-mat_mes-hall |
Spin-orbit coupled transport and spin torque in a ferromagnetic
heterostructure: Ferromagnetic heterostructures provide an ideal platform to explore the
nature of spin-orbit torques arising from the interplay mediated by itinerant
electrons between a Rashba-type spin-orbit coupling and a ferromagnetic
exchange interaction. For such a prototypic system, we develop a set of coupled
diffusion equations to describe the diffusive spin dynamics and spin-orbit
torques. We characterize the spin torque and its two prominent--out-of-plane
and in-plane--components for a wide range of relative strength between the
Rashba coupling and ferromagnetic exchange. The symmetry and angular dependence
of the spin torque emerging from our simple Rashba model is in an agreement
with experiments. The spin diffusion equation can be generalized to incorporate
dynamic effect such as spin pumping and magnetic damping. | cond-mat_mes-hall |
Cotunneling thermopower of single electron transistors: We study the thermopower of a quantum dot weakly coupled to two reservoirs by
tunnel junctions. At low temperatures the transport through the dot is
suppressed by charging effects (Coulomb blockade). As a result the thermopower
shows an oscillatory dependence on the gate voltage. We study this dependence
in the limit of low temperatures where the transport through the dot is
dominated by the processes of inelastic cotunneling. We also obtain a crossover
formula for intermediate temperatures which connects our cotunneling results to
the known sawtooth behavior in the sequential tunneling regime. As the
temperature is lowered, the amplitude of thermopower oscillations increases,
and their shape changes qualitatively. | cond-mat_mes-hall |
Atomically thin quantum light emitting diodes: Transition metal dichalcogenides (TMDs) are optically active layered
materials providing potential for fast optoelectronics and on-chip photonics.
We demonstrate electrically driven single-photon emission from localised sites
in tungsten diselenide (WSe2) and tungsten disulphide (WS2). To achieve this,
we fabricate a light emitting diode structure comprising single layer graphene,
thin hexagonal boron nitride and TMD mono- and bi-layers. Photon correlation
measurements are used to confirm the single-photon nature of the spectrally
sharp emission. These results present the TMD family as a platform for hybrid,
broadband, atomically precise quantum photonics devices. | cond-mat_mes-hall |
The Parallel Magnetoconductance of Interacting Electrons in a Two
Dimensional Disordered System: The transport properties of interacting electrons for which the spin degree
of freedom is taken into account are numerically studied for small two
dimensional diffusive clusters. On-site electron-electron interactions tend to
delocalize the electrons, while long-range interactions enhance localization.
On careful examination of the transport properties, we reach the conclusion
that it does not show a two dimensional metal insulator transition driven by
interactions. A parallel magnetic field leads to enhanced resistivity, which
saturates once the electrons become fully spin polarized. The strength of the
magnetic field for which the resistivity saturates decreases as electron
density goes down. Thus, the numerical calculations capture some of the
features seen in recent experimental measurements of parallel
magnetoconductance. | cond-mat_mes-hall |
Strong gate-tunability of flat bands in bilayer graphene due to moiré
encapsulation between hBN monolayers: When using hexagonal boron-nitride (hBN) as a substrate for graphene, the
resulting moir\'e pattern creates secondary Dirac points. By encapsulating a
multilayer graphene within aligned hBN sheets the controlled moir\'e stacking
may offer even richer benefits. Using advanced tight-binding simulations on
atomistically-relaxed heterostructures, here we show that the gap at the
secondary Dirac point can be opened in selected moir\'e-stacking
configurations, and is independent of any additional vertical gating of the
heterostructure. On the other hand, gating can broadly tune the gap at the
principal Dirac point, and may thereby strongly compress the first moir\'e
mini-band in width against the moir\'e-induced gap at the secondary Dirac
point. We reveal that in hBN-encapsulated bilayer graphene this novel mechanism
can lead to isolated bands flatter than 10~meV under moderate gating, hence
presenting a convenient pathway towards electronically-controlled
strongly-correlated states on demand. | cond-mat_mes-hall |
The Edge-State Theory of Integer-Quantum-Hall-Effect to Insulator
Transition: Direct transitions, driven by disorder, from several integral quantum Hall
states to an insulator have been observed in experiment. This finding is
enigmatic in light of a theoretical phase diagram, based on rather general
considerations, that predicts a sequence of transitions in which the integer
$n$ characterizing the Hall conductivity is reduced successively by unity,
eventually going from $n=1$ into an insulator. In this work, we suggest that
the direct transition occurs because, in certain parameter regime, the edge
states of different Landau levels are strongly coupled and behave as a single
edge state. It is indicated under what conditions successive transitions may be
seen. | cond-mat_mes-hall |
Quantum Fluctuations along Symmetry Crossover in Kondo-correlated
Quantum Dot: Universal properties of entangled many-body states are controlled by their
symmetry and quantum fluctuations. By magnetic-field tuning of the spin-orbital
degeneracy in a Kondo-correlated quantum dot, we have modified quantum
fluctuations to directly measure their influence on the many-body properties
along the crossover from $SU(4)$ to $SU(2)$ symmetry of the ground state.
High-sensitive current noise measurements combined with the non-equilibrium
Fermi liquid theory clarify that the Kondo resonance and electron correlations
are enhanced as the fluctuations, measured by the Wilson ratio, increase along
the symmetry crossover. Our achievement demonstrates that non-linear noise
constitutes a measure of quantum fluctuations that can be used to tackle
quantum phase transitions. | cond-mat_mes-hall |
Bundling dynamics regulates the active mechanics and transport in carbon
nanotube networks: High-density carbon nanotube networks (CNNs) continue to attract interest as
active elements in nanoelectronic devices, nanoelectromechanical systems (NEMS)
and multifunctional nanocomposites. The interplay between the network
nanostructure and the its properties is crucial, yet current understanding
remains limited to the passive response. Here, we employ a novel superstructure
consisting of millimeter-long vertically aligned singe walled carbon nanotubes
(SWCNTs) sandwiched between polydimethylsiloxane (PDMS) layers to quantify the
effect of two classes of mechanical stimuli, film densification and stretching,
on the electronic and thermal transport across the network. The network deforms
easily with increase in electrical and thermal conductivities suggestive of
floppy yet highly reconfigurable network. Insight from atomistically informed
coarse-grained simulations uncover an interplay between the extent of lateral
assembly of the bundles, modulated by surface zipping/unzipping, and the
elastic energy associated with the bent conformations of the nanotubes/bundles.
During densification, the network becomes highly interconnected yet we observe
a modest increase in bundling primarily due to the reduced spacing between the
SWCNTs. The stretching, on the other hand, is characterized by an initial
debundling regime as the strain accommodation occurs via unzipping of the
branched interconnects, followed by rapid re-bundling as the strain transfers
to the increasingly aligned bundles. In both cases, the increase in the
electrical and thermal conductivity is primarily due to the increase in bundle
size; the changes in network connectivity have a minor effect on the transport.
Our results have broad implications for filamentous networks of inorganic
nanoassemblies composed of interacting tubes, wires and ribbons/belts. | cond-mat_mes-hall |
Theoretical Investigation of Local Electron Temperature in Quantum Hall
Systems: In this work we solve thermo-hydrodynamical equations considering a two
dimensional electron system in the integer quantum Hall regime, to calculate
the spatial distribution of the local electron temperature. We start from the
self-consistently calculated electrostatic and electrochemical potentials in
equilibrium. Next, by imposing an external current, we investigate the
variations of the electron temperature in the linear-response regime. Here a
local relation between the electron density and conductivity tensor elements is
assumed. Following the Ohm's law we obtain local current densities and by
implementing the results of the thermo-hydrodynamical theory, calculate the
local electron temperature. We observe that the local electron temperature
strongly depends on the formation of compressible and incompressible strips. | cond-mat_mes-hall |
Time scales for Majorana manipulation using Coulomb blockade in
gate-controlled superconducting nanowires: We numerically compute the low-energy spectrum of a gate-controlled
superconducting topological nanowire segmented into two islands, each
Josephson-coupled to a bulk superconductor. This device may host two pairs of
Majorana bound states and could provide a platform for testing Majorana fusion
rules. We analyze the crossover between (i) a charge-dominated regime
utilizable for initialization and readout of Majorana bound states, (ii) a
single-island regime for dominating inter-island Majorana coupling, (iii) a
Josephson-plasmon regime for large coupling to the bulk superconductors, and
(iv) a regime of four Majorana bound states allowing for topologically
protected Majorana manipulations. From the energy spectrum, we derive
conservative estimates for the time scales of a fusion-rule testing protocol
proposed recently [arXiv:1511.05153]. We also analyze the steps needed for
basic Majorana braiding operations in branched nanowire structures. | cond-mat_mes-hall |
Cooling of suspended nanostructures with tunnel junctions: We have investigated electronic cooling of suspended nanowires with SINIS
tunnel junction coolers. The suspended samples consist of a free standing
nanowire suspended by four narrow ($\sim$ 200 nm) bridges. We have compared two
different cooler designs for cooling the suspended nanowire. We demonstrate
that cooling of the nanowire is possible with a proper SINIS cooler design. | cond-mat_mes-hall |
Anomalous levitation and annihilation in Floquet topological insulators: Anderson localization in two-dimensional topological insulators takes place
via the so-called levitation and pair annihilation process. As disorder is
increased, extended bulk states carrying opposite topological invariants move
towards each other in energy, reducing the size of the topological gap,
eventually meeting and localizing. This results in a topologically trivial
Anderson insulator. Here, we introduce the anomalous levitation and pair
annihilation, a process unique to periodically-driven, or Floquet systems. Due
to the periodicity of the quasienergy spectrum, we find it is possible for the
topological gap to increase as a function of disorder strength. Thus, after all
bulk states have localized, the system remains topologically nontrivial,
forming an anomalous Floquet Anderson insulator (AFAI) phase. We show a
concrete example for this process, adding disorder via onsite potential "kicks"
to a Chern insulator model. By changing the period between kicks, we can tune
which type of (conventional or anomalous) levitation-and-annihilation occurs in
the system. We expect our results to be applicable to generic Floquet
topological systems and to provide an accessible way to realize AFAIs
experimentally, without the need for multi-step driving schemes. | cond-mat_mes-hall |
Spin Polarization and Transport of Surface States in the Topological
Insulators Bi2Se3 and Bi2Te3 from First Principles: We investigate the band dispersion and the spin texture of topologically
protected surface states in the bulk topological insulators Bi2Se3 and Bi2Te3
by first-principles methods. Strong spin-orbit entanglement in these materials
reduces the spin-polarization of the surface states to ~50% in both cases; this
reduction is absent in simple models but of important implications to
essentially any spintronic application. We propose a way of controlling the
magnitude of spin polarization associated with a charge current in thin films
of topological insulators by means of an external electric field. The proposed
dual-gate device configuration provides new possibilities for electrical
control of spin. | cond-mat_mes-hall |
Few-layer graphene patterned bottom gates for van der Waals
heterostructures: We introduce a method of local gating for van der Waals heterostructures,
employing a few-layer graphene patterned bottom gate. Being a member of the 2D
material family, few-layer graphene adapts perfectly to the commonly used
stacking method. Its versatility regarding patterning as well as its flatness
make it an ideal candidate for experiments on locally gated 2D materials.
Moreover, in combination with ultra-thin hexagonal boron nitride as an
insulating layer, sharp potential steps can be created and the quality of the
investigated 2D material can be sustained. To underline the good feasibility
and performance, we show results on transport experiments in periodically
modulated graphene- boron nitride heterostructures, where the charge carrier
density is tuned via locally acting patterned few layer graphene bottom gates
and a global back gate. | cond-mat_mes-hall |
Interplay of quantum spin Hall effect and spontaneous time-reversal
symmetry breaking in electron-hole bilayers I: Transport properties: The band-inverted electron-hole bilayers, such as InAs/GaSb, are an
interesting playground for the interplay of quantum spin Hall effect and
correlation effects because of the small density of electrons and holes and the
relatively small hybridization between the electron and hole bands. It has been
proposed that Coulomb interactions lead to a time-reversal symmetry broken
phase when the electron and hole densities are tuned from the trivial to the
quantum spin Hall insulator regime. We show that the transport properties of
the system in the time-reversal symmetry broken phase are consistent with the
recent experimental observations in InAs/GaSb. Moreover, we carry out a quantum
transport study on a Corbino disc where the bulk and edge contributions to the
conductance can be separated. We show that the edge becomes smoothly conducting
and the bulk is always insulating when one tunes the system from the trivial to
the quantum spin Hall insulator phase, providing unambiguous transport
signatures of the time-reversal symmetry broken phase. | cond-mat_mes-hall |
Deviation from the normal mode expansion in a coupled
graphene-nanomechanical system: We optomechanically measure the vibrations of a nanomechanical system made of
a graphene membrane suspended on a silicon nitride nanoresonator. When probing
the thermal noise of the coupled nanomechanical device, we observe a
significant deviation from the normal mode expansion. It originates from the
heterogeneous character of mechanical dissipation over the spatial extension of
coupled eigenmodes, which violates one of the fundamental prerequisite for
employing this commonly used description of the nanoresonators' thermal noise.
We subsequently measure the local mechanical susceptibility and demonstrate
that the fluctuation-dissipation theorem still holds and permits a proper
evaluation of the thermal noise of the nanomechanical system. Since it
naturally becomes delicate to ensure a good spatial homogeneity at the
nanoscale, this approach is fundamental to correctly describe the thermal noise
of nanomechanical systems which ultimately impact their sensing capacity. | cond-mat_mes-hall |
Transport properties of overheated electrons trapped on a Helium surface: An ultra-strong photovoltaic effect has recently been reported for electrons
trapped on a liquid Helium surface under a microwave excitation tuned at
intersubband resonance [D. Konstantinov et. al. : J. Phys. Soc. Jpn. 81, 093601
(2012) ]. In this article, we analyze theoretically the redistribution of the
electron density induced by an overheating of the surface electrons under
irradiation, and obtain quantitative predictions for the photocurrent
dependence on the effective electron temperature and confinement voltages. We
show that the photo-current can change sign as a function of the parameters of
the electrostatic confinement potential on the surface, while the photocurrent
measurements reported so far have been performed only at a fixed confinement
potential. The experimental observation of this sign reversal could provide a
reliable estimation of the electron effective temperature in this new out of
equilibrium state. Finally, we have also considered the effect of the
temperature on the outcome of capacitive transport measurement techniques.
These investigations led us to develop, numerical and analytical methods for
solving the Poisson-Boltzmann equation in the limit of very low temperatures
which could be useful for other systems. | cond-mat_mes-hall |
Dirac fermion quantization on graphene edges: Isospin-orbit coupling,
zero modes and spontaneous valley polarization: The paper addresses boundary electronic properties of graphene with a complex
edge structure of the armchair/zigzag/armchair type. It is shown that the
finite zigzag region supports edge bound states with discrete equidistant
spectrum obtained from the Green's function of the continuum Dirac equation.
The energy levels exhibit the coupling between the valley degree of freedom and
the orbital quantum number, analogous to a spin-orbit interaction. The
characteristic feature of the spectrum is the presence of a zero mode, the
bound state of vanishing energy. It resides only in one of the graphene
valleys, breaking spontaneously Kramers' symmetry of the edge states. This
implies the spontaneous valley polarization characterized by the valley isospin
$\pm 1/2$. The polarization is manifested by a zero-magnetic field anomaly in
the local tunneling density of states, and is directly related to the local
electric Hall conductivity. | cond-mat_mes-hall |
Strongly bound excitons in monolayer MoSi$_2$Z$_4$ (Z = pnictogen): Reduced dielectric screening in two-dimensional materials enables bound
excitons, which modifies their optical absorption and optoelectronic response
even at room temperature. Here, we demonstrate the existence of excitons in the
bandgap of the monolayer family of the newly discovered synthetic MoSi$_2$Z$_4$
(Z = N, P, and As) series of materials. All three monolayers support several
bright and strongly bound excitons with binding energies varying from 1 eV to
1.35 eV for the lowest energy exciton resonances. On increasing the pump
fluence, the exciton binding energies get renormalized, leading to a
redshift-blueshift crossover. Our study shows that the MoSi$_2$Z$_4$ series of
monolayers offer an exciting test-bed for exploring the physics of strongly
bound excitons and their non-equilibrium dynamics. | cond-mat_mes-hall |
Zero-field magnetometry using hyperfine-biased nitrogen-vacancy centers
near diamond surfaces: Shallow nitrogen-vacancy (NV) centers in diamond are promising for
nano-magnetometry for they can be placed proximate to targets. To study the
intrinsic magnetic properties, zero-field magnetometry is desirable. However,
for shallow NV centers under zero field, the strain near diamond surfaces would
cause level anti-crossing between the spin states, leading to clock transitions
whose frequencies are insensitive to magnetic signals. Furthermore, the charge
noises from the surfaces would induce extra spin decoherence and hence reduce
the magnetic sensitivity. Here we demonstrate that the relatively strong
hyperfine coupling (130 MHz) from a first-shell 13C nuclear spin can provide an
effective bias field to an NV center spin so that the clock-transition
condition is broken and the charge noises are suppressed. The hyperfine bias
enhances the dc magnetic sensitivity by a factor of 22 in our setup. With the
charge noises suppressed by the strong hyperfine field, the ac magnetometry
under zero field also reaches the limit set by decoherence due to the nuclear
spin bath. In addition, the 130 MHz splitting of the NV center spin transitions
allows relaxometry of magnetic noises simultaneously at two well-separated
frequencies (~2.870 +/- 0.065 GHz), providing (low-resolution) spectral
information of high-frequency noises under zero field. The hyperfine-bias
enhanced zero-field magnetometry can be combined with dynamical decoupling to
enhance single-molecule magnetic resonance spectroscopy and to improve the
frequency resolution in nanoscale magnetic resonance imaging. | cond-mat_mes-hall |
Nonlinear dynamics of a functionally graded piezoelectric
micro-resonator in the vicinity of the primary resonance: This research is on the nonlinear dynamics of a two-sided electrostatically
actuated capacitive micro-beam. The microresonator is composed of silicon and
PZT as a piezoelectric material. PZT is functionally distributed along the
height of the micro-beam according to the power law distribution. The
micro-resonator is simultaneously subjected to DC piezoelectric and two-sided
electrostatic actuations. The DC piezoelectric actuation leads to the
generation of an axial force along the length of the micro-beam and this is
used as a tuning tool to shift the primary resonance of the micro-resonator.
The governing equation of the motion is derived by the minimization of the
Hamiltonian and generalized to the viscously damped systems. The periodic
solutions in the vicinity of the primary resonance are detected by means of the
shooting method and their stability is investigated by determining the
so-called Floquet exponents of the perturbed motions. The basins of attraction
corresponding to three individual periodic orbits are determined. The results
depict that the higher the amplitude of the periodic orbit, the smaller is the
area of the attractor | cond-mat_mes-hall |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.