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Localization of Two Interacting Particles in One-Dimensional Random
Potential: We investigate the localization of two interacting particles in
one-dimensional random potential. Our definition of the two-particle
localization length, $\xi$, is the same as that of v. Oppen et al. [Phys. Rev.
Lett. 76, 491 (1996)] and $\xi$'s for chains of finite lengths are calculated
numerically using the recursive Green's function method for several values of
the strength of the disorder, $W$, and the strength of interaction, $U$. When
U=0, $\xi$ approaches a value larger than half the single-particle localization
length as the system size tends to infinity and behaves as $\xi \sim
W^{-\nu_0}$ for small $W$ with $\nu_0 = 2.1 \pm 0.1$. When $U\neq 0$, we use
the finite size scaling ansatz and find the relation $\xi \sim W^{-\nu}$ with
$\nu = 2.9 \pm 0.2$. Moreover, data show the scaling behavior $\xi \sim
W^{-\nu_0} g(|U|/W^\Delta)$ with $\Delta = 4.0 \pm 0.5$. | cond-mat_mes-hall |
Floquet engineering of lattice structure and dimensionality in twisted
moiré heterobilayers: We present an experimental proposal to tune the effective lattice structure
in twisted transition metal dichalcogenide (TMD) heterobilayers with
time-periodic Floquet drive. We show that elliptically polarized light with
sub-terahertz frequencies $\hbar\omega\sim 1$ meV and moderate electric fields
$E\sim0.2$~MV/cm allows tuning between the native triangular lattice and a
square lattice, while linearly polarized light enables dimensional reduction to
a quasi-one-dimensional geometry. Without drive, these twisted TMDs simulate
the single band Fermi-Hubbard model; we show that this approximation still
holds in the presence of drive. This control opens the door to explore a rich
variety of correlated phases of matter, such as spin liquids and d-wave
superconductivity. | cond-mat_mes-hall |
Signatures of Majorana fermions in hybrid superconductor-semiconductor
nanowire devices: Majorana fermions are particles identical to their own antiparticles. They
have been theoretically predicted to exist in topological superconductors. We
report electrical measurements on InSb nanowires contacted with one normal (Au)
and one superconducting electrode (NbTiN). Gate voltages vary electron density
and define a tunnel barrier between normal and superconducting contacts. In the
presence of magnetic fields of order 100 mT we observe bound, mid-gap states at
zero bias voltage. These bound states remain fixed to zero bias even when
magnetic fields and gate voltages are changed over considerable ranges. Our
observations support the hypothesis of Majorana fermions in nanowires coupled
to superconductors. | cond-mat_mes-hall |
Spin dynamics in InAs-nanowire quantum-dots coupled to a transmission
line: We study theoretically electron spins in nanowire quantum dots placed inside
a transmission line resonator. Because of the spin-orbit interaction, the spins
couple to the electric component of the resonator electromagnetic field and
enable coherent manipulation, storage, and read-out of quantum information in
an all-electrical fashion. Coupling between distant quantum-dot spins, in one
and the same or different nanowires, can be efficiently performed via the
resonator mode either in real time or through virtual processes. For the latter
case we derive an effective spin-entangling interaction and suggest means to
turn it on and off. We consider both transverse and longitudinal types of
nanowire quantum-dots and compare their manipulation timescales against the
spin relaxation times. For this, we evaluate the rates for spin relaxation
induced by the nanowire vibrations (phonons) and show that, as a result of
phonon confinement in the nanowire, this rate is a strongly varying function of
the spin operation frequency and thus can be drastically reduced compared to
lateral quantum dots in GaAs. Our scheme is a step forward to the formation of
hybrid structures where qubits of different nature can be integrated in a
single device. | cond-mat_mes-hall |
Spin superfluidity and long-range transport in thin-film ferromagnets: In ferromagnets, magnons may condense into a single quantum state. Analogous
to superconductors, this quantum state may support transport without
dissipation. Recent works suggest that longitudinal spin transport through a
thin-film ferromagnet is an example of spin superfluidity. Although intriguing,
this tantalizing picture ignores long-range dipole interactions; we demonstrate
that such interactions dramatically affect spin transport. In single-film
ferromagnets, "spin superfluidity" only exists at length scales (a few hundred
nanometers in yttrium iron garnet) somewhat larger than the exchange length.
Over longer distances, dipolar interactions destroy spin superfluidity.
Nevertheless, we predict re-emergence of spin superfluidity in tri-layer
ferromagnet--normal metal--ferromagnet films of $\sim 1\, \mu$m in size. Such
systems also exhibit other types of long-range spin transport in samples
several micrometers in size. | cond-mat_mes-hall |
Gate-control of spin-motive force and spin-torque in Rashba SOC systems: The introduction of a strong Rashba spin orbit coupling (SOC) had been
predicted to enhance the spin motive force (SMF) [see Phys. Rev. Lett. {\bf
108}, 217202 (2012)]. In this work, we predict further enhancement of the SMF
by time modulation of the Rashba coupling $\alpha_R$, which induces an
additional electric field $E^R_d={\dot \alpha_R} m_e/e\hbar({\hat z}\times
{\mathbf m})$. When the modulation frequency is higher than the magnetization
precessing frequency, the amplitude of this field is significantly larger than
previously predicted results. Correspondingly, the spin torque on the
magnetization is also effectively enhanced. Additionally, the nature of SOC
induced spin torque in the system can be transformed from damping to
antidamping-like by modulating ${\dot \alpha_R}$. We also suggest a biasing
scheme to achieve rectification of SMF, {\it i.e.}, by application of a square
wave voltage at the resonant frequency. Finally, we numerically estimate the
resulting spin torque field arising from a Gaussian pulse time modulation of
$\alpha_R$. | cond-mat_mes-hall |
Spiral orientational order in quantum Hall skyrmion lattices: We investigate the existence of spiral ordering in the planar spin
orientation of skyrmions localised on a face centered rectangular lattice
(FCRL). We use the non-linear sigma model (NLSM) to numerically calculate the
minimum energy configurations of this lattice around the $\nu=1$ quantum Hall
ground state. Our variational ansatz contains an angle $\theta$, characterising
the FCRL and an angle $q$, characterising the orientational order. As $\nu$ is
increased towards one, there is a smooth transition from the triangular lattice
(TL) characterised by $(\theta,q) = (120^o,120^o)$ to FCRLs with spiral
orientational order. The novel feature we find is that these phases are
characterised by $\theta, q)$ values such that $\theta+q = 240^o$ (same as the
TL phase). As $\nu$ incresaes further towards one, there is a sharp transition
from the FCRLs to the square lattice (SL), characterised by
$(\theta,q)=(90^o,180^o)$. Consequently, the parameter $\theta+q$ jumps sharply
at the FCRL-SL transition and can serve as an order parameter to characterise
it. | cond-mat_mes-hall |
Low frequency Raman studies of multi-wall carbon nanotubes: experiments
and theory: In this paper, we investigate the low frequency Raman spectra of multi-wall
carbon nanotubes (MWNT) prepared by the electric arc method. Low frequency
Raman modes are unambiguously identified on purified samples thanks to the
small internal diameter of the MWNT. We propose a model to describe these
modes. They originate from the radial breathing vibrations of the individual
walls coupled through the Van der Waals interaction between adjacent concentric
walls. The intensity of the modes is described in the framework of bond
polarization theory. Using this model and the structural characteristics of the
nanotubes obtained from transmission electron microscopy allows to simulate the
experimental low frequency Raman spectra with an excellent agreement. It
suggests that Raman spectroscopy can be as useful regarding the
characterization of MWNT as it is in the case of single-wall nanotubes. | cond-mat_mes-hall |
Optical properties of the Hofstadter butterfly in the Moiré
superlattice: We investigate the optical absorption spectrum and the selection rule for the
Hofstadter butterfly in twisted bilayer graphene under magnetic fields. We
demonstrate that the absorption spectrum exhibits a self-similar recursive
pattern reflecting the fractal nature of the energy spectrum. We find that the
optical selection rule has a nested self-similar structure as well, and it is
governed by the conservation of the total angular momentum summed over
different hierarchies. | cond-mat_mes-hall |
Two components of donor-acceptor recombination in compensated
semiconductors. Analytical model of spectra in presence of electrostatic
fluctuations: We report numerical and analytical studies of the donor-acceptor
recombination in compensated semiconductors. Our calculations take into account
random electric fields of charged impurities which are important in non zero
compensation case. We show that the donor-acceptor optical spectrum can be
described as a sum of two components: monomolecular and bimolecular. In the low
compensation limit we develop two analytical models for both types of the
recombination. Also our numerical simulation predicts that these two components
of the photoluminescence spectra can be resolved under certain experimental
conditions. | cond-mat_mes-hall |
Asymptotic Expressions for Charge Matrix Elements of the Fluxonium
Circuit: In charge-coupled circuit QED systems, transition amplitudes and dispersive
shifts are governed by the matrix elements of the charge operator. For the
fluxonium circuit, these matrix elements are not limited to nearest-neighbor
energy levels and are conveniently tunable by magnetic flux. Previously, their
values were largely obtained numerically. Here, we present analytical
expressions for the fluxonium charge matrix elements. We show that new
selection rules emerge in the asymptotic limit of large Josephson energy and
small inductive energy. We illustrate the usefulness of our expressions for the
qualitative understanding of charge matrix elements in the parameter regime
probed by previous experiments. | cond-mat_mes-hall |
Scanning Tunneling Microscopy and Spectroscopy of Graphene on Insulating
Substrates: Graphene is a truly two-dimensional material with exceptional electronic,
mechanical, and optical properties. As such, it consists of surface only and
can be probed by the well developed surface-science techniques as, e.g.,
scanning tunneling microscopy. This method bridges the gap between the surface
science community and the electronic device community and might lead to novel
combined approaches. Here, I review some of the scanning tunneling microscopy
(STM) and spectroscopy (STS) experiments on monolayer graphene samples. I will
concentrate on graphene samples deposited on insulating substrates, since these
are related to graphene device concepts. In particular, I will discuss the
morphology of graphene on SiO$_2$ and other emerging substrates, some
nanomechanical manipulation experiments using STM, and spectroscopic results.
The latter can map the disorder potentials as well as the interaction of the
electrons with the disorder which is most pronounced in the quantum Hall
regime. | cond-mat_mes-hall |
Rectification in mesoscopic AC-gated semiconductor devices: We measure the rectified dc currents resulting when a 3-terminal
semiconductor device with gate-dependent conductance is driven with an ac gate
voltage. The rectified currents exhibit surprisingly complex behaviour as the
dc source-drain bias voltage, the dc gate voltage and the amplitude of the ac
gate voltage are varied. We obtain good agreement between our data and a model
based on simple assumptions about the stray impedances on the sample chip, over
a wide frequency range. This method is applicable to many types of experiment
which involve ac gating of a non-linear device, and where an undesireable
rectified contribution to the measured signal is present. Finally, we evaluate
the small rectified currents flowing in tunable-barrier electron pumps operated
in the pinched-off regime. These currents are at most $10^{-12}$ of the pumped
current for a pump current of 100 pA. This result is encouraging for the
development of tunable-barrier pumps as metrological current standards. | cond-mat_mes-hall |
Nonequilibrium edge transport in quantum Hall based Josephson junctions: We study the transport properties of a voltage-biased Josephson junction
where the BCS superconducting leads are coupled via the edges of a quantum Hall
sample. In this scenario, an out of equilibrium Josephson current develops,
which is numerically studied within the Floquet-Keldysh Green's function
formalism. We particularly focus on the time-averaged current as a function of
both the bias voltage and the magnetic flux threading the sample and analyze
the resonant multiple Andreev reflection processes that lead to an enhancement
of the quasiparticle transmission. We find that a full tomography of the dc
current in the voltage-flux plane allows for a complete spectroscopy of the
one-way edge modes and could be used as a hallmark of chiral edge mediated
transport in these hybrid devices. | cond-mat_mes-hall |
Josephson Current in Ballistic Graphene Corbino Disk: We solve Dirac-Bogoliubov-De-Gennes (DBdG) equation in a
superconductor-normal graphene superconductor (SGS) junction with Corbino disk
structure to investigate the Josephson current through this junction. We find
that the critical current $I_c$ has a nonzero value at Dirac point in which the
concentration of the carriers is zero. We show this nonzero critical current
depends on the system geometry and it decreases monotonically to zero by
increasing the ratio of the outer to inner radii of the Corbino disk
($R_2/R_1$), while in the limit of $R_2/R_1 \rightarrow 1$ it scales like a
diffusive Corbino disk. The product of the critical current and the
normal-state resistance $I_cR_N$ attains the same value for the planar
structure at zero doping. These results reveals the pseudodiffusive behavior of
the graphene Corbino Josephson junction similar to the planar structure. | cond-mat_mes-hall |
Charge to Magnetic Flux Ratios: It is shown that if the carriers in the fractional quantum Hall effect are
taken as geometrical excitations with quanta of charge e and magnetic flux
h/2e, as proposed in a previous publication, the calculated results are
compatible with the series of fractions obtained experimentally. | cond-mat_mes-hall |
Homogenization of Rough Surfaces: Effective Surface Stress and
Superficial Elasticity: Relating microstructure to properties, electromagnetic, mechanical, thermal
and their couplings has been a major focus of mechanics, physics and materials
science. The majority of the literature focuses on deriving homogenized
constitutive responses for macroscopic composites relating effective properties
to various microstructural details. Due to large surface to volume ratio,
phenomena at the nanoscale require consideration of surface energy effects and
the latter are frequently used to interpret size-effects in material behavior.
Elucidation of the effect of surface roughness on the surface stress and
elastic behavior is relatively under-studied and quite relevant to the behavior
of nanostructures. In this work, we present derivations that relate both
periodic and random roughness to the effective surface elastic behavior. We
find that the residual surface stress is hardly affected by roughness while the
superficial elasticity properties are dramatically altered and, importantly,
may also result in a change in its sign - this has ramifications in
interpretation of sensing based on frequency measurement changes due to surface
elasticity. We show that the square of resonance frequency of a cantilever beam
with rough surface decreases as much as three times of its value for flat
surface. | cond-mat_mes-hall |
Magnetic catalysis and axionic charge-density-wave in Weyl semimetals: Three-dimensional Weyl and Dirac semimetals can support a
chiral-symmetry-breaking, fully gapped, charge-density-wave order even for
sufficiently weak repulsive electron-electron interactions, when placed in
strong magnetic fields. In the former systems, due to the natural momentum
space separation of Weyl nodes the ordered phase lacks the translational
symmetry and represents an axionic phase of matter, while that in a Dirac
semimetal (neglecting the Zeeman coupling) is only a trivial insulator. We
present the scaling of this spectral gap for a wide range of subcritical (weak)
interactions as well as that of the diamagnetic susceptibility with the
magnetic field. A similar mechanism for charge-density-wave ordering at weak
coupling is shown to be operative in double and triple-Weyl semimetals, where
the dispersion is linear (quadratic and cubic, respectively) for the z (planar)
component(s) of the momentum. We here also address the competition between the
charge-density-wave and a spin-density-wave orders, both of which breaks the
chiral symmetry and leads to gapped spectrum, and show that at least in the
weak coupling regime the former is energetically favored. The anomalous surface
Hall conductivity, role of topological defects such as axion strings, existence
of one-dimensional gapless dispersive modes along the core of such defects, and
anomaly cancellation through the Callan-Harvey mechanism are discussed. | cond-mat_mes-hall |
Nearly flat bands in twisted triple bilayer graphene: We investigate the electronic structure of alternating-twist triple
Bernal-stacked bilayer graphene (t3BG) as a function of interlayer coupling
$\omega$, twist angle $\theta$, interlayer potential difference $\Delta$, and
top-bottom bilayers sliding vector $\boldsymbol{\tau}$ for three possible
configurations AB/AB/AB, AB/BA/AB, and AB/AB/BA. The parabolic low-energy band
dispersions in a Bernal-stacked bilayer and gap-opening through a finite
interlayer potential difference $\Delta$ allows the flattening of bands in t3BG
down to $\sim 20$~meV for twist angles $\theta \lesssim 2^{\circ}$ regardless
of the stacking types. The easier isolation of the flat bands and associated
reduction of Coulomb screening thanks to the intrinsic gaps of bilayer graphene
for finite $\Delta$ facilitate the formation of correlation-driven gaps when it
is compared to the metallic phases of twisted trilayer graphene under electric
fields. We obtain the stacking dependent Coulomb energy versus bandwidth $U/W
\gtrsim 1$ ratios in the $\theta$ and $\Delta$ parameter space. We also present
the expected $K$-valley Chern numbers for the lowest-energy nearly flat bands. | cond-mat_mes-hall |
Current induced and interaction driven Dirac-point drag of massless
quasi-relativistic fermions: We study the quasiparticle properties of two-dimensional massless Dirac
Fermions when the many-body states possess a finite momentum density in the
clean limit. The lack of Galilean invariance endows the many-body states at
finite momentum density with qualitative differences from those of the system
at rest. At finite carrier densities we demonstrate the appearance of a
current-induced distortion of the pseudospin texture in momentum space that can
be viewed as a drag of the Dirac point and the origin of which lies entirely in
electron-electron interactions. We discuss the potential observation of this
effect in graphene. | cond-mat_mes-hall |
Acoustic phonon dynamics in thin-films of the topological insulator
Bi2Se3: Transient reflectivity traces measured for nanometer-sized films of the
topological insulator Bi2Se3 revealed GHz-range oscillations driven within the
relaxation of hot carriers photoexcited with ultrashort laser pulses of 1.51 eV
photon energy. These oscillations have been suggested to result from acoustic
phonon dynamics, including coherent longitudinal acoustic phonons in the form
of standing acoustic waves. An increase of oscillation frequency from ~35 to
~70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the
interplay between two different regimes employing traveling-acoustic-waves for
films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR)
modes for films thinner than 40 nm. The amplitude of oscillations decays
rapidly for films below 15 nm thick when the indirect intersurface coupling in
Bi2Se3 films switches the FBAWR regime to that of the Lamb wave excitation. The
frequency range of coherent longitudinal acoustic phonons is in good agreement
with elastic properties of Bi2Se3. | cond-mat_mes-hall |
Strain impacts on commensurate bilayer graphene superlattices: distorted
trigonal warping, emergence of bandgap and direct-indirect bandgap transition: Due to low dimensionality, the controlled stacking of the graphene films and
their electronic properties are susceptible to environmental changes including
strain. The strain-induced modification of the electronic properties such as
the emergence and modulation of bandgaps crucially depends on the stacking of
the graphene films. However, to date, only the impact of strain on electronic
properties of Bernal and AA-stacked bilayer graphene has been extensively
investigated in theoretical studies. Exploiting density functional theory and
tight-binding calculation, we investigate the impacts of in-plane strain on two
different class of commensurate twisted bilayer graphene (TBG) which are
even/odd under sublattice exchange (SE) parity. We find that the SE odd TBG
remains gapless whereas the bandgap increases for the SE even TBG when applying
equibiaxial tensile strain. Moreover, we observe that for extremely large mixed
strains both investigated TBG superstructures demonstrate direct-indirect
bandgap transition. | cond-mat_mes-hall |
Electromagnetic properties of a double layer graphene system with
electron-hole pairing: We study electromagnetic properties of a double layer graphene system in
which electrons from one layer are coupled with holes from the other layer. The
gauge invariant linear response functions are obtained. The frequency
dependences of the transmission, reflection and absorption coefficients are
computed. We predict a peak in the reflection and absorption at the frequency
equals to the gap in the quasiparticle spectrum. It is shown that the
electron-hole pairing results in an essential modification of the spectrum of
surface TM plasmons. We find that the optical TM mode splits into a low
frequency undamped branch and a high frequency damped branch. At zero
temperature the lower branch disappears. It is established that the pairing
does not influence the acoustic TM mode. It is also shown that the pairing
opens the frequency window in the subgap range for the surface TE wave. | cond-mat_mes-hall |
Direct surface charging and alkali-metal doping for tuning the
interlayer magnetic order in planar nanostructures: The continuous reduction of magnetic units to ultra small length scales
inspires efforts to look for a suitable means of controlling magnetic states.
In this study we show two surface charge alteration techniques for tuning the
interlayer exchange coupling (IEC) of ferromagnetic layers separated by
paramagnetic spacers. Our study reveals that already a modest amount of extra
charge can switch the mutual alignment of the magnetization from
anti-ferromagnetic to ferromagnetic or vice verse. We also propose adsorption
of alkali metals as an alternative way of varying the electronic and chemical
properties of magnetic surfaces. Clear evidence is found that the interlayer
magnetic order can be reversed by adsorbing alkali metals on the magnetic
layer. Moreover, alkali metal overlayers strongly enhance the perpendicular
magnetic anisotropy in FePt thin films. These findings combined with atomistic
spin model calculations suggest that electronic or ionic way of surface
charging can have a crucial role for magnetic hardening and spin state control. | cond-mat_mes-hall |
Fast preparation of single hole spin in InAs/GaAs quantum dot in Voigt
geometry magnetic field: The preparation of a coherent heavy-hole spin via ionization of a
spin-polarized electron-hole pair in an InAs/GaAs quantum dot in a Voigt
geometry magnetic field is investigated. For a dot with a 17 ueV bright-exciton
fine-structure splitting, the fidelity of the spin preparation is limited to
0.75, with optimum preparation occurring when the effective fine-structure of
the bright-exciton matches the in-plane hole Zeeman energy. In principle,
higher fidelities can be achieved by minimizing the bright-exciton
fine-structure splitting. | cond-mat_mes-hall |
Evaluation of Spin Waves and Ferromagnetic Resonance Contribution to the
Spin Pumping in Ta/CoFeB Structure: The spin waves and ferromagnetic resonance (FMR) contribution to the spin
pumping signal is studied in the Ta/CoFeB interface under different excitation
bias fields. Ferromagnetic resonance is excited utilizing a coplanar waveguide
and a microwave generator. Using a narrow waveguide of about 3 {\mu}m,
magnetostatic surface spin waves with large wavevector (k) of about 0.81
{\mu}m^-1 are excited. A large k value results in dissociation of spin waves
and FMR frequencies according to the surface spin wave dispersion relation.
Spin waves and FMR contribution to the spin pumping are calculated based on the
area under the Lorentzian curve fitting over experimental results. It is found
that the FMR over spin waves contribution is about 1 at large bias fields in
Ta/CoFeB structure. Based on our spin pumping results, we propose a method to
characterize the spin wave decay constant which is found to be about 5.5 {\mu}m
in the Ta/CoFeB structure at a bias field of 600 Oe. | cond-mat_mes-hall |
Continuous microwave hole burning and population oscillations in a
diamond spin ensemble: Continuous spectral hole burning and spin-level population oscillations are
studied in an inhomogeneously broadened diamond-based spin ensemble composed of
substitutional nitrogen and nitrogen-vacancy centres created through neutron
irradiation and annealing. The burnt spectral features highlight a
detuning-dependent homogeneous hole linewidth that is up to three orders of
magnitude narrower than the total inhomogeneous ensemble linewidth. Continuous
population oscillations are observed to quickly decay beyond a pump and probe
detuning of 5 Hz, and are numerically modelled using a five-level system of
coupled rate equations. Fourier analysis of these oscillations highlight
discrete $^{13}$C hyperfine interactions, with energies within the
inhomogeneous ensemble linewidth, as well as suspected nuclear $3/2$-spin
coupled signatures likely related to the $^7$Li byproduct of neutron
irradiation. | cond-mat_mes-hall |
Interface traps in graphene field effect devices: extraction methods and
influence on characteristics: We study impact of the near-interfacial oxide traps on the C-V and I-V
characteristics of graphene gated structures. Methods of extraction of
interface trap level density in graphene field effect devices from the
capacitance-voltage measurements are described and discussed. It has been found
that the effects of electron-electron or hole-hole interactions and
electron-hole puddles can be mixed in C-V characteristics putting obstacles in
the way of uniquely determined extraction of the interface trap density in
graphene. Influence of the interface traps on DC and AC capacitance and
conductance characteristics of graphene field-effect structures is described.
It has been shown that variety of widths of resistivity peaks in various
samples could be explained by different interface trap capacitance values. | cond-mat_mes-hall |
Pseudospin, velocity and Berry phase in a bilayer graphene: Hamiltonian and eigenstate problem is formulated for a bilayer graphene in
terms of Clifford's geometric algebra \textit{Cl}$_{3,1}$. It is shown that
such approach allows to perform analytical calculations in a simple way if
geometrical algebra rotors are used. The measured quantities are express
through spectrum and rotation half-angle of the pseudospin that appears in
geometric algebra rotors. Properties of free charge carriers -- pseudospin,
velocity and Berry phase -- in a bilayer graphene are investigated in the
presence of the external voltage applied between the two layers. | cond-mat_mes-hall |
Chiral magnetic effect at finite temperature in a field-theoretic
approach: We investigate the existence (or lack thereof) of the chiral magnetic effect
in the framework of finite temperature field theory, studied through the path
integral approach and regularized via the zeta function technique. We show
that, independently of the temperature, gauge invariance implies the absence of
the effect, a fact proved, at zero temperature and in a Hamiltonian approach,
by N. Yamamoto. Indeed, the effect only appears when the manifold is finite in
the direction of the magnetic field and gauge-invariance breaking boundary
conditions are imposed. We present an explicit calculation for antiperiodic and
periodic boundary conditions, which do allow for a CME, since only large gauge
transformations are, then, an invariance of the theory. In both cases, the
associated current does depend on the temperature, a well as on the size of the
sample in the direction of the magnetic field, even for a
temperature-independent chiral chemical potential. In particular, for
antiperiodic boundary conditions, the value of this current only agrees with
the result usually quoted in the literature on the subject in the
zero-temperature limit, while it decreases with the temperature in a
well-determined way. | cond-mat_mes-hall |
Relation between spin Hall effect and anomalous Hall effect in 3$d$
ferromagnetic metals: We study the mechanisms of the spin Hall effect (SHE) and anomalous Hall
effect (AHE) in 3$d$ ferromagnetic metals (Fe, Co, permalloy
(Ni$_{81}$Fe$_{19}$; Py), and Ni) by varying their resistivities and
temperature. At low temperatures where the phonon scattering is negligible, the
skew scattering coefficients of the SHE and AHE in Py are related to its spin
polarization. However, this simple relation breaks down for Py at higher
temperatures as well as for the other ferromagnetic metals at any temperature.
We find that, in general, the relation between the SHE and AHE is more complex,
with the temperature dependence of the SHE being much stronger than that of
AHE. | cond-mat_mes-hall |
Minimal models for nonreciprocal amplification using biharmonic drives: We present a generic system of three harmonic modes coupled parametrically
with a time-varying coupling modulated by a combination of two pump harmonics,
and show how this system provides the minimal platform to realize nonreciprocal
couplings that can lead to gainless photon circulation, and phase-preserving or
phase-sensitive directional amplification. Explicit frequency-dependent
calculations within this minimal paradigm highlight the separation of
amplification and directionality bandwidths, universal in such schemes. We also
study the influence of counter-rotating interactions that can adversely affect
directionality and associated bandwidth; we find that these effects can be
mitigated by suitably designing the properties of the auxiliary mode that plays
the role of an engineered reservoir to the amplification mode space. | cond-mat_mes-hall |
Microwave-induced magnetoresistance of two-dimensional electrons
interacting with acoustic phonons: The influence of electron-phonon interaction on magnetotransport in
two-dimensional electron systems under microwave irradiation is studied
theoretically. Apart from the phonon-induced resistance oscillations which
exist in the absence of microwaves, the magnetoresistance of irradiated samples
contains oscillating contributions due to electron scattering on both
impurities and acoustic phonons. The contributions due to electron-phonon
scattering are described as a result of the interference of phonon-induced and
microwave-induced resistance oscillations. In addition, microwave heating of
electrons leads to a special kind of phonon-induced oscillations. The relative
strength of different contributions and their dependence on parameters are
discussed. The interplay of numerous oscillating contributions suggests a
peculiar magnetoresistance picture in high-mobility layers at the temperatures
when electron-phonon scattering becomes important. | cond-mat_mes-hall |
Transport properties and electrical device characteristics with the
TiMeS computational platform: application in silicon nanowires: Nanoelectronics requires the development of a priori technology evaluation
for materials and device design that takes into account quantum physical
effects and the explicit chemical nature at the atomic scale. Here, we present
a cross-platform quantum transport computation tool. Using first-principles
electronic structure, it allows for flexible and efficient calculations of
materials transport properties and realistic device simulations to extract
current-voltage and transfer characteristics. We apply this computational
method to the calculation of the mean free path in silicon nanowires with
dopant and surface oxygen impurities. The dependence of transport on basis set
is established, with the optimized double zeta polarized basis giving a
reasonable compromise between converged results and efficiency. The
current-voltage characteristics of ultrascaled (3 nm length) nanowire-based
transistors with p-i-p and p-n-p doping profiles are also investigated. It is
found that charge self-consistency affects the device characteristics more
significantly than the choice of the basis set. These devices yield
source-drain tunneling currents in the range of 0.5 nA (p-n-p junction) to 2 nA
(p-i-p junction), implying that junctioned transistor designs at these length
scales would likely fail to keep carriers out of the channel in the off-state. | cond-mat_mes-hall |
Symmetry dictated universal helicity redistribution of Dirac fermions in
transport: Helicity is a fundamental property of Dirac fermions. Yet, the general rule
of how it changes in transport is still lacking. We uncover, theoretically, the
universal spinor state transformation and consequently helicity redistribution
rule in two cases of transport through potentials of electrostatic and mass
types, respectively. The former is dictated by Lorentz boost and its complex
counterpart in Klein tunneling regime, which establishes miraculously a unified
yet latent connection between helicity, Klein tunneling, and Lorentz boost. The
latter is governed by an abstract rotation group we construct, which reduces to
SO(2) when acting on the plane of effective mass and momentum. They generate
invariant submanifolds, i.e., leaves, that foliate the Hilbert space of Dirac
spinors. Our results provide a basis for unified understanding of helicity
transport, and may open a new window for exotic helicity-based physics and
applications in mesoscopic systems. | cond-mat_mes-hall |
Composite Fermions with a Warped Fermi Contour: Via measurements of commensurability features near Landau filling factor
$\nu=1/2$, we probe the shape of the Fermi contour for hole-flux composite
fermions confined to a wide GaAs quantum well. The data reveal that the
composite fermions are strongly influenced by the characteristics of the Landau
level in which they are formed. In particular, their Fermi contour is
$\textit{warped}$ when their Landau level originates from a hole band with
significant warping. | cond-mat_mes-hall |
Signatures of folded branches in the scanning gate microscopy of
ballistic electronic cavities: We demonstrate the emergence of classical features in electronic quantum
transport for the scanning gate microscopy response in a cavity defined by a
quantum point contact and a micron-sized circular reflector. The branches in
electronic flow characteristic of a quantum point contact opening on a
two-dimensional electron gas with weak disorder are folded by the reflector,
yielding a complex spatial pattern. Considering the deflection of classical
trajectories by the scanning gate tip allows to establish simple relationships
of the scanning pattern, which are in agreement with recent experimental
findings. | cond-mat_mes-hall |
Spectroscopy of double quantum dot two-spin states by tuning the
inter-dot barrier: Transport spectroscopy of two-spin states in a double quantum dot can be
performed by an AC electric field which tunes the energy detuning. However, a
problem arises when the transition rate between the states is small and,
consequently, the AC-induced current is suppressed. Here, we show that if the
AC field tunes the inter-dot tunnel barrier then for large detuning the
transition rate increases drastically resulting in high current. Multi-photon
resonances are enhanced by orders of magnitude. Our study demonstrates an
efficient way for fast two-spin transitions. | cond-mat_mes-hall |
Proximity DC squids in the long junction limit: We report the design and measurement of
Superconducting/normal/superconducting (SNS) proximity DC squids in the long
junction limit, i.e. superconducting loops interrupted by two normal metal
wires roughly a micrometer long. Thanks to the clean interface between the
metals, at low temperature a large supercurrent flows through the device. The
dc squid-like geometry leads to an almost complete periodic modulation of the
critical current through the device by a magnetic flux, with a flux periodicity
of a flux quantum h/2e through the SNS loop. In addition, we examine the entire
field dependence, notably the low and high field dependence of the maximum
switching current. In contrast with the well-known Fraunhoffer-type
oscillations typical of short wide junctions, we find a monotonous gaussian
extinction of the critical current at high field. As shown in [15], this
monotonous dependence is typical of long and narrow diffusive junctions. We
also find in some cases a puzzling reentrance at low field. In contrast, the
temperature dependence of the critical current is well described by the
proximity effect theory, as found by Dubos {\it et al.} [16] on SNS wires in
the long junction limit. The switching current distributions and hysteretic IV
curves also suggest interesting dynamics of long SNS junctions with an
important role played by the diffusion time across the junction. | cond-mat_mes-hall |
Interaction effects on thermal transport in quantum wires: We develop a theory of thermal transport of weakly interacting electrons in
quantum wires. Unlike higher-dimensional systems, a one-dimensional electron
gas requires three-particle collisions for energy relaxation. The fastest
relaxation is provided by the intrabranch scattering of comoving electrons
which establishes a partially equilibrated form of the distribution function.
The thermal conductance is governed by the slower interbranch processes which
enable energy exchange between counterpropagating particles. We derive an
analytic expression for the thermal conductance of interacting electrons valid
for arbitrary relation between the wire length and electron thermalization
length. We find that in sufficiently long wires the interaction-induced
correction to the thermal conductance saturates to an interaction-independent
value. | cond-mat_mes-hall |
Spin Coulomb drag by non-equilibrium magnetic textures: Interaction between local magnetization and conduction electrons is
responsible for a variety of phenomena in magnetic materials. We have shown
that the spin-dependent motive force induced by magnetization dynamics in a
conducting ferromagnet lead to the spin Coulomb drag effect. The spin Coulomb
drag an intrinsic friction mechanism which operates whenever the average
velocities of up-spin and down-spin electrons differ. | cond-mat_mes-hall |
Single ion implantation for single donor devices using Geiger mode
detectors: Electronic devices that are designed to use the properties of single atoms
such as donors or defects have become a reality with recent demonstrations of
donor spectroscopy, single photon emission sources, and magnetic imaging using
defect centers in diamond. Improving single ion detector sensitivity is linked
to improving control over the straggle of the ion as well as providing more
flexibility in lay-out integration with the active region of the single donor
device construction zone by allowing ion sensing at potentially greater
distances. Using a remotely located passively gated single ion Geiger mode
avalanche diode (SIGMA) detector we have demonstrated 100% detection efficiency
at a distance of >75 um from the center of the collecting junction. This
detection efficiency is achieved with sensitivity to ~600 or fewer
electron-hole pairs produced by the implanted ion. Ion detectors with this
sensitivity and integrated with a thin dielectric, for example 5 nm gate oxide,
using low energy Sb implantation would have an end of range straggle of <2.5
nm. Significant reduction in false count probability is achieved by modifying
the ion beam set-up to allow for cryogenic operation of the SIGMA detector.
Using a detection window of 230 ns at 1 Hz, the probability of a false count
was measured as 1E-1 and 1E-4 for operation temperatures of 300K and 77K,
respectively. Low temperature operation and reduced false, dark, counts are
critical to achieving high confidence in single ion arrival. For the device
performance in this work, the confidence is calculated as a probability of >98%
for counting one and only one ion for a false count probability of 1E-4 at an
average ion number per gated window of 0.015. | cond-mat_mes-hall |
Dynamical Lamb Effect in a Tunable Superconducting Qubit-Cavity System: A natural atom placed into a cavity with time-dependent parameters can be
parametrically excited due to the interaction with the quantized photon mode.
One of the channels of such a process is the dynamical Lamb effect, induced by
a nonadiabatic modulation of atomic level Lamb shift. However, in experiments
with natural atoms it is quite difficult to isolate this effect from other
mechanisms of atom excitation. We point out that a transmission line cavity
coupled with a superconducting qubit (artificial macroscopic atom) provides a
unique platform for the observation of the dynamical Lamb effect. A key idea is
to exploit a dynamically tunable qubit-resonator coupling, which was
implemented quite recently. By varying nonadiabatically the coupling, it is
possible to parametrically excite a qubit through a nonadiabatic modulation of
the Lamb shift, even if the cavity was initially empty. A dynamics of such a
coupled system is studied within the Rabi model with time-dependent coupling
constant and beyond the rotating wave approximation. An efficient method to
increase the effect through the periodic and nonadiabatic switching of a
qubit-resonator coupling energy is proposed. | cond-mat_mes-hall |
All-optical hyperpolarization of electron and nuclear spins in diamond: Low thermal polarization of nuclear spins is a primary sensitivity limitation
for nuclear magnetic resonance. Here we demonstrate optically pumped
(microwave-free) nuclear spin polarization of $^{13}\mathrm{C}$ and
$^{15}\mathrm{N}$ in $^{15}\mathrm{N}$-doped diamond. $^{15}\mathrm{N}$
polarization enhancements up to $-2000$ above thermal equilibrium are observed
in the paramagnetic system $\mathrm{N_s}^{0}$. Nuclear spin polarization is
shown to diffuse to bulk $^{13}\mathrm{C}$ with NMR enhancements of $-200$ at
room temperature and $-500$ at $\mathrm{240~K}$, enabling a route to
microwave-free high-sensitivity NMR study of biological samples in ambient
conditions. | cond-mat_mes-hall |
Many-body approach to non-Hermitian physics in fermionic systems: In previous studies, the topological invariants of 1D non-Hermitian systems
have been defined in open boundary condition (OBC) to satisfy the bulk-boundary
correspondence. The extreme sensitivity of bulk energy spectra to boundary
conditions has been attributed to the breakdown of the conventional
bulk-boundary correspondence based on the topological invariants defined under
periodic boundary condition (PBC). Here we propose non-Hermitian many-body
polarization as a topological invariant for 1D non-Hermitian systems defined in
PBC, which satisfies the bulk-boundary correspondence. Employing many-body
methodology in the non-Hermitian Su-Schrieffer-Heeger model for fermions, we
show the absence of non-Hermitian skin effect due to the Pauli exclusion
principle and demonstrate the bulk-boundary correspondence using the invariant
defined under PBC. Moreover, we show that the bulk topological invariant is
quantized in the presence of chiral or generalized inversion symmetry. Our
study suggests the existence of generalized crystalline symmetries in
non-Hermitian systems, which give quantized topological invariants that capture
the symmetry-protected topology of non-Hermitian systems. | cond-mat_mes-hall |
Equilibrium current vortices in rare-earth-doped simple metals: Dilute alloys of rare earths have played a vital role in understanding
magnetic phenomena. Here, we model the ground state of dilute 4f rare-earth
impurities in light metals. When the 4f subshells are open (but not
half-filled), the spin-orbit coupling imprints a rotational charge current of
conduction electrons around rare-earth atoms. The sign and amplitude of the
current oscillate similar to the RKKY spin polarization. We compute the
observable effect, namely the Oersted field generated by the current vortices
and the Knight shift. | cond-mat_mes-hall |
Current-controlled light scattering and asymmetric plasmon propagation
in graphene: We demonstrate that plasmons in graphene can be manipulated using a DC
current. A source-drain current lifts the forward/backward degeneracy of the
plasmons, creating two modes with different propagation properties parallel and
antiparallel to the current. We show that the propagation length of the plasmon
propagating parallel to the drift current is enhanced, while the propagation
length for the antiparallel plasmon is suppressed. We also investigate the
scattering of light off graphene due to the plasmons in a periodic dielectric
environment and we find that the plasmon resonance separates in two peaks
corresponding to the forward and backward plasmon modes. The narrower linewidth
of the forward propagating plasmon may be of interest for refractive index
sensing and the DC current control could be used for the modulation of
mid-infrared electromagnetic radiation. | cond-mat_mes-hall |
Particle-Flux Separation and Quasiexcitations in Quantum Hall Systems: The quasiexcitations of quantum Hall systems at the filling factor $\nu =
p/(2pq \pm 1)$ are studied in terms of chargeon and fluxon introduced
previously as constituents of an electron at $\nu = 1/2$. At temperatures $T <
T_{\rm PFS}(\nu)$, the phenomenon so-called particle-flux separation takes
place, and chargeons and fluxons are deconfined to behave as quasiparticles.
Bose condensation of fluxons justify the (partial) cancellation of external
magnetic field. Fluxons describe correlation holes, while chargeons describe
composite fermions. They contribute to the resistivity $\rho_{xy} = h/(\nu
e^2)$ additively. | cond-mat_mes-hall |
Percolation via combined electrostatic and chemical doping in complex
oxide films: Stimulated by experimental advances in electrolyte gating methods, we
investigate theoretically percolation in thin films of inhomogenous complex
oxides, such as La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO), induced by a combination of
bulk chemical and surface electrostatic doping. Using numerical and analytical
methods, we identify two mechanisms that describe how bulk dopants reduce the
amount of electrostatic surface charge required to reach percolation: (i)
bulk-assisted surface percolation, and (ii) surface-assisted bulk percolation.
We show that the critical surface charge strongly depends on the film thickness
when the film is close to the chemical percolation threshold. In particular,
thin films can be driven across the percolation transition by modest surface
charge densities \emph{via} surface-assisted bulk percolation. If percolation
is associated with the onset of ferromagnetism, as in LSCO, we further
demonstrate that the presence of critical magnetic clusters extending from the
film surface into the bulk results in considerable volume enhancement of the
saturation magnetization, with pronounced experimental consequences. These
results should significantly guide experimental work seeking to verify
gate-induced percolation transitions in such materials. | cond-mat_mes-hall |
Quantum criticality in a double quantum-dot system: We discuss the realization of the quantum-critical non-Fermi liquid state,
originally discovered within the two-impurity Kondo model, in double
quantum-dot systems. Contrary to the common belief, the corresponding fixed
point is robust against particle-hole and various other asymmetries, and is
only unstable to charge transfer between the two dots. We propose an
experimental set-up where such charge transfer processes are suppressed,
allowing a controlled approach to the quantum critical state. We also discuss
transport and scaling properties in the vicinity of the critical point. | cond-mat_mes-hall |
Scaling analysis of Kondo screening cloud in a mesoscopic ring with an
embedded quantum dot: The Kondo effect is theoretically studied in a quantum dot embedded in a
mesoscopic ring. The ring is connected to two external leads, which enables the
transport measurement. Using the "poor man's" scaling method, we obtain
analytical expressions of the Kondo temperature T_K as a function of the
Aharonov-Bohm phase \phi by the magnetic flux penetrating the ring. In this
Kondo problem, there are two characteristic lengths. One is the screening
length of the charge fluctuation, L_c=\hbar v_F/ |\epsilon_0|, where v_F is the
Fermi velocity and \epsilon_0 is the energy level in the quantum dot. The other
is the screening length of spin fluctuation, i.e., size of Kondo screening
cloud, L_K=\hbar v_F/ T_K. We obtain different expressions of T_K(\phi) for (i)
L_c \ll L_K \ll L, (ii) L_c \ll L \ll L_K, and (iii) L \ll L_c \ll L_K, where L
is the size of the ring. T_K is markedly modulated by \phi in cases (ii) and
(iii), whereas it hardly depends on \phi in case (i). We also derive
logarithmic corrections to the conductance at temperature T\gg T_K and an
analytical expression of the conductance at T\ll T_K, on the basis of the
scaling analysis. | cond-mat_mes-hall |
Collective modes in interacting two-dimensional tomographic Fermi
liquids: We develop an analytically solvable model for interacting two-dimensional
Fermi liquids with separate collisional relaxation rates for parity-odd and
parity-even Fermi surface deformations. Such a disparity of collisional
lifetimes exists whenever scattering is restricted to inversion-symmetric Fermi
surfaces, and should thus be a generic feature of two-dimensional Fermi
liquids. It implies an additional unanticipated "tomographic" transport regime
(in between the standard collisionless and hydrodynamic regimes) in which
even-parity modes are overdamped while odd-parity modes are collisionless. We
derive expressions for both the longitudinal and the transverse conductivity
and discuss the collective mode spectrum along the
collisionless-tomographic-hydrodynamic crossover. Longitudinal modes cross over
from zero sound in the collisionless regime to hydrodynamic first sound in the
tomographic and hydrodynamic regime, where odd-parity damping appears as a
subleading correction to the lifetime. In charged Fermi liquids with long-range
Coulomb coupling, these modes reduce to plasmons with a strongly suppressed
odd-parity correction to the damping. The transverse response, by contrast, has
a specific tomographic transport regime with two imaginary odd-parity modes,
one of which requires a finite repulsive interaction, distinct from both the
shear sound in the collisionless regime and an overdamped diffusive current
mode in the hydrodynamic limit. Our work demonstrates that there are deep
many-body aspects of interacting Fermi liquids, which are often thought to be
well understood theoretically, remaining unexplored. | cond-mat_mes-hall |
Spin-orbit induced longitudinal spin-polarized currents in non-magnetic
solids: For certain non-magnetic solids with low symmetry the occurrence of
spin-polarized longitudinal currents is predicted. These arise due to an
interplay of spin-orbit interaction and the particular crystal symmetry. This
result is derived using a group-theoretical scheme that allows investigating
the symmetry properties of any linear response tensor relevant to the field of
spintronics. For the spin conductivity tensor it is shown that only the
magnetic Laue group has to be considered in this context. Within the introduced
general scheme also the spin Hall- and additional related transverse effects
emerge without making reference to the two-current model. Numerical studies
confirm these findings and demonstrate for (Au$_{1-x}$Pt$_{\rm x}$)$_4$Sc that
the longitudinal spin conductivity may be in the same order of magnitude as the
conventional transverse one. The presented formalism only relies on the
magnetic space group and therefore is universally applicable to any type of
magnetic order. | cond-mat_mes-hall |
Friedel oscillations induced by magnetic skyrmions: from scattering
properties to all-electrical detection: Magnetic skyrmions are spin swirling solitonic defects that can play a major
role in information technology. Their future in applications and devices hinges
on their efficient manipulation and detection. Here, we explore from ab-initio
their nature as magnetic inhomongeities in an otherwise unperturbed magnetic
material, Fe layer covered by a thin Pd film and deposited on top of Ir(111)
surface. The presence of skyrmions triggers scattering processes, from which
Friedel oscillations emerge. The latter mediate interactions among skyrmions or
between skyrmions and other potential surrounding defects. In contrast to their
wavelengths, the amplitude of the oscillations depends strongly on the size of
the skyrmion. The analogy with the scattering-off atomic defects enables the
assignment of an effective scattering potential and a phase shift to the
skyrmionic particles, which can be useful to predict their behavior on the
basis of simple scattering frameworks. The induced charge ripples can be
utilized for a noninvasive all-electrical detection of skyrmions located on a
surface or even if buried a few nanometers away from the detecting electrode. | cond-mat_mes-hall |
Cotunneling renormalization in carbon nanotube quantum dots: We determine the level-shifts induced by cotunneling in a Coulomb blockaded
carbon nanotube quantum dot using leading order quasi-degenerate perturbation
theory within a single nanotube quartet. It is demonstrated that otherwise
degenerate and equally tunnel-coupled $K$ and $K'$ states are mixed by
cotunneling and therefore split up in energy except at the
particle/hole-symmetric midpoints of the Coulomb diamonds. In the presence of
an external magnetic field, we show that cotunneling induces a gate-dependent
$g$-factor renormalization, and we outline different scenarios which might be
observed experimentally, depending on the values of both intrinsic $KK'$
splitting and spin-orbit coupling. | cond-mat_mes-hall |
Electrically controlling single spin qubits in a continuous microwave
field: Large-scale quantum computers must be built upon quantum bits that are both
highly coherent and locally controllable. We demonstrate the quantum control of
the electron and the nuclear spin of a single 31P atom in silicon, using a
continuous microwave magnetic field together with nanoscale electrostatic
gates. The qubits are tuned into resonance with the microwave field by a local
change in electric field, which induces a Stark shift of the qubit energies.
This method, known as A-gate control, preserves the excellent coherence times
and gate fidelities of isolated spins, and can be extended to arbitrarily many
qubits without requiring multiple microwave sources. | cond-mat_mes-hall |
Strong influence of spin-orbit coupling on magnetotransport in
two-dimensional hole systems: With a view to electrical spin manipulation and quantum computing
applications, recent significant attention has been devoted to semiconductor
hole systems, which have very strong spin-orbit interactions. However,
experimentally measuring, identifying, and quantifying spin-orbit coupling
effects in transport, such as electrically-induced spin polarizations and
spin-Hall currents, are challenging. Here we show that the magnetotransport
properties of two dimensional (2D) hole systems display strong signatures of
the spin-orbit interaction. Specifically, the low-magnetic field Hall
coefficient and longitudinal conductivity contain a contribution that is second
order in the spin-orbit interaction coefficient and is non-linear in the
carrier number density. We propose an appropriate experimental setup to probe
these spin-orbit dependent magnetotransport properties, which will permit one
to extract the spin-orbit coefficient directly from the magnetotransport. | cond-mat_mes-hall |
Detection of finite frequency photo-assisted shot noise with a resonant
circuit: Photo-assisted transport through a mesoscopic conductor occurs when an
oscillatory (AC) voltage is superposed to the constant (DC) bias which is
imposed on this conductor. Of particular interest is the photo assisted shot
noise, which has been investigated theoretically and experimentally for several
types of samples. For DC biased conductors, a detection scheme for finite
frequency noise using a dissipative resonant circuit, which is inductively
coupled to the mesoscopic device, was developped recently. We argue that the
detection of the finite frequency photo-assisted shot noise can be achieved
with the same setup, despite the fact that time translational invariance is
absent here. We show that a measure of the photo-assisted shot noise can be
obtained through the charge correlator associated with the resonant circuit,
where the latter is averaged over the AC drive frequency. We test our
predictions for a point contact placed in the fractional quantum Hall effect
regime, for the case of weak backscattering. The Keldysh elements of the
photo-assisted noise correlator are computed. For simple Laughlin fractions,
the measured photo-assisted shot noise displays peaks at the frequency
corresponding to the DC bias voltage, as well as satellite peaks separated by
the AC drive frequency. | cond-mat_mes-hall |
Composite Fermions in Negative Effective Magnetic Field: A Monte-Carlo
Study: The method of Jain and Kamilla [PRB {\bf 55}, R4895 (1997)] allows numerical
generation of composite fermion trial wavefunctions for large numbers of
electrons in high magnetic fields at filling fractions of the form nu=p/(2mp+1)
with m and p positive integers. In the current paper we generalize this method
to the case where the composite fermions are in an effective (mean) field with
opposite sign from the actual physical field, i.e. when p is negative. We
examine both the ground state energies and the low energy neutral excitation
spectra of these states. Using particle-hole symmetry we can confirm the
correctness of our method by comparing results for the series m=1 with p>0
(previously calculated by others) to our results for the conjugate series m=1
with p <0. Finally, we present similar results for ground state energies and
low energy neutral excitations for the states with m=2 and p <0 which were not
previously addressable, comparing our results to the m=1 case and the p > 0,
m=2 cases. | cond-mat_mes-hall |
Charge trapping in the system of interacting quantum dots: We analyzed the localized charge dynamics in the system of $N$ interacting
single-level quantum dots (QDs) coupled to the continuous spectrum states in
the presence of Coulomb interaction between electrons within the dots.
Different dots geometry and initial charge configurations were considered. The
analysis was performed by means of Heisenberg equations for localized electrons
pair correlators.
We revealed that charge trapping takes place for a wide range of system
parameters and we suggested the QDs geometry for experimental observations of
this phenomenon. We demonstrated significant suppression of Coulomb
correlations with the increasing of QDs number. We found the appearance of
several time scales with the strongly different relaxation rates for a wide
range of the Coulomb interaction values. | cond-mat_mes-hall |
Low Temperature Nanoscale Electronic Transport on the MoS_2 surface: Two-probe electronic transport measurements on a Molybdenum Disulphide
(MoS_2) surface were performed at low temperature (30K) under ultra-high vacuum
conditions. Two scanning tunneling microscope tips were precisely positioned in
tunneling contact to measure the surface current-voltage characteristics. The
separation between the tips is controllably varied and measured using a high
resolution scanning electron microscope. The MoS_2 surface shows a surface
electronic gap (E_S) of 1.4eV measured at a probe separation of 50nm.
Furthermore, the two- probe resistance measured outside the electronic gap
shows 2D-like behavior with the two-probe separation. | cond-mat_mes-hall |
Crystalline Polymers with Exceptionally Low Thermal Conductivity Studied
using Molecular Dynamics: Semi-crystalline polymers have been shown to have greatly increased thermal
conductivity compared to amorphous bulk polymers due to effective heat
conduction along the covalent bonds of the backbone. However, the mechanisms
governing the intrinsic thermal conductivity of polymers remain largely
unexplored as thermal transport has been studied in relatively few polymers.
Here, we use molecular dynamics simulations to study heat transport in
polynorbornene, a polymer that can be synthesized in semi-crystalline form
using solution processing. We find that even perfectly crystalline
polynorbornene has an exceptionally low thermal conductivity near the amorphous
limit due to extremely strong anharmonic scattering. Our calculations show that
this scattering is sufficiently strong to prevent the formation of propagating
phonons, with heat being instead carried by non-propagating, delocalized
vibrational modes known as diffusons. Our results demonstrate a mechanism for
achieving intrinsically low thermal conductivity even in crystalline polymers
that may be useful for organic thermoelectrics. | cond-mat_mes-hall |
Structural and Electrical Characterization of Bi2Se3 Nanostructures
Grown by Metalorganic Chemical Vapor Deposition: We characterize nanostructures of Bi2Se3 that are grown via metalorganic
chemical vapor deposition using the precursors diethyl selenium and trimethyl
bismuth. By adjusting growth parameters, we obtain either single-crystalline
ribbons up to 10 microns long or thin micron-sized platelets. Four-terminal
resistance measurements yield a sample resistivity of 4 mOhm-cm. We observe
weak anti-localization and extract a phase coherence length l_phi = 178 nm and
spin-orbit length l_so = 93 nm at T = 0.29 K. Our results are consistent with
previous measurements on exfoliated samples and samples grown via physical
vapor deposition. | cond-mat_mes-hall |
Transport Mean Free Path for Magneto-Transverse Light Diffusion: We derive an expression for the transport mean free path $\ell^*_\perp$
associated with magneto-transverse light diffusion for a random collection of
Faraday-active
Mie scatterers. This expression relates the magneto-transverse diffusion in
multiple scattering directly to the magneto-transverse scattering of a single
scatterer. | cond-mat_mes-hall |
Coherent Phonons in Carbon Nanotubes and Graphene: We review recent studies of coherent phonons (CPs) corresponding to the
radial breathing mode (RBM) and G-mode in single-wall carbon nanotubes (SWCNTs)
and graphene. Because of the bandgap-diameter relationship, RBM-CPs cause
bandgap oscillations in SWCNTs, modulating interband transitions at terahertz
frequencies. Interband resonances enhance CP signals, allowing for chirality
determination. Using pulse shaping, one can selectively excite
speci!c-chirality SWCNTs within an ensemble. G-mode CPs exhibit
temperature-dependent dephasing via interaction with RBM phonons. Our
microscopic theory derives a driven oscillator equation with a
density-dependent driving term, which correctly predicts CP trends within and
between (2n+m) families. We also find that the diameter can initially increase
or decrease. Finally, we theoretically study the radial breathing like mode in
graphene nanoribbons. For excitation near the absorption edge, the driving term
is much larger for zigzag nanoribbons. We also explain how the armchair
nanoribbon width changes in response to laser excitation. | cond-mat_mes-hall |
Stark effect and generalized Bloch-Siegert shift in a strongly driven
two-level system: A superconducting qubit was driven in an ultrastrong fashion by an
oscillatory microwave field, which was created by coupling via the nonlinear
Josephson energy. The observed Stark shifts of the `atomic' levels are so
pronounced that corrections even beyond the lowest-order Bloch-Siegert shift
are needed to properly explain the measurements. The quasienergies of the
dressed two-level system were probed by resonant absorption via a cavity, and
the results are in agreement with a calculation based on the Floquet approach. | cond-mat_mes-hall |
Near-field thermal transport between twisted bilayer graphene: Active control of heat flow is of both fundamental and applied interest in
thermal management and energy conversion. Here, we present a fluctuational
electrodynamic study of thermal radiation between twisted bilayer graphene
(TBLG), motivated by its unusual and highly tunable plasmonic properties. We
show that near-field heat flow can vary by more than 10-fold over only a few
degrees of twist, and identify special angles leading to heat flow extrema.
These special angles are dictated by the Drude weight in the intraband optical
conductivity of TBLG, and are roughly linear with the chemical potential.
Further, we observe multiband thermal transport due to the increasing role of
interband transitions as the twist angle decreases, in analogy to monolayer
graphene in a magnetic field. Our findings are understood via the surface
plasmons in TBLG, and highlight its potential for manipulating radiative heat
flow. | cond-mat_mes-hall |
Orbital Stark effect and quantum confinement transition of donors in
silicon: Adiabatic shuttling of single impurity bound electrons to gate induced
surface states in semiconductors has attracted much attention in recent times,
mostly in the context of solid-state quantum computer architecture. A recent
transport spectroscopy experiment for the first time was able to probe the
Stark shifted spectrum of a single donor in silicon buried close to a gate.
Here we present the full theoretical model involving large-scale quantum
mechanical simulations that was used to compute the Stark shifted donor states
in order to interpret the experimental data. Use of atomistic tight-binding
technique on a domain of over a million atoms helped not only to incorporate
the full band structure of the host, but also to treat realistic device
geometries and donor models, and to use a large enough basis set to capture any
number of donor states. The method yields a quantitative description of the
symmetry transition that the donor electron undergoes from a 3D Coulomb
confined state to a 2D surface state as the electric field is ramped up
adiabatically. In the intermediate field regime, the electron resides in a
superposition between the states of the atomic donor potential and that of the
quantum dot like states at the surface. In addition to determining the effect
of field and donor depth on the electronic structure, the model also provides a
basis to distinguish between a phosphorus and an arsenic donor based on their
Stark signature. The method also captures valley-orbit splitting in both the
donor well and the interface well, a quantity critical to silicon qubits. The
work concludes with a detailed analysis of the effects of screening on the
donor spectrum. | cond-mat_mes-hall |
Phonons and Thermal Conducting Properties of Borocarbonitride (BCN)
Nanosheets: Hexagonal borocarbonitrides (BCN) are a class of 2D materials, which display
excellent catalytic activity for water splitting. Here, we report analysis of
thermal stability, phonons and thermal conductivity of BCN monolayers over a
wide range of temperatures using classical molecular dynamics simulations. Our
results show that in contrast to the case of graphene and boron nitride
monolayers, the out-of-plane phonons in BCN monolayers induce an asymmetry in
the phonon density of states at all temperatures. Despite possessing lower
thermal conducting properties compared to graphene and BN monolayers, the BCN
nanosheets do not lose thermal conductivity as much as graphene and BN in the
studied temperature range of 200-1000 K, and thus, the BCN nanosheets are
suitable for thermal interface device applications over a wide range of
temperatures. Besides their promising role in water splitting, the above
results highlight the possibility of expanding the use of BCN 2D materials in
thermal management applications and thermoelectrics. | cond-mat_mes-hall |
Sensitivity of the Power Spectra of Magnetization Fluctuations in Low
Barrier Nanomagnets to Barrier Height Modulation and Defects: Nanomagnets with small shape anisotropy energy barriers on the order of the
thermal energy have unstable magnetization that fluctuates randomly in time.
They have recently emerged as promising hardware platforms for stochastic
computing and machine learning because the random magnetization states can be
harnessed for probabilistic bits. Here, we have studied how the statistics of
the magnetization fluctuations (e.g. the power spectral density) is affected by
(i) moderate variations in the barrier height of the nanomagnet and (ii) the
presence of structural defects, in order to assess how robust the computing
platform is. We found that the power spectral density is relatively insensitive
to moderate barrier height change and also relatively insensitive to the
presence of small localized defects. However, extended (delocalized) defects,
such as thickness variations over a significant fraction of the nanomagnet,
affect the power spectral density very noticeably. As a result, small
variations in the shape (causing small variations in the barrier height), or
small localized defects, are relatively innocuous and tolerable, but
significant variation of the nanomagnet thickness is not. Consequently, tight
control over the nanomagnet thickness must be maintained for stochastic
computing applications. | cond-mat_mes-hall |
Optical Kerr Effect in Graphene: Theoretical Analysis of the Optical
Heterodyne Detection Technique: Graphene is an atomically thin two-dimensional material demonstrating strong
optical nonlinearities including harmonics generation, four wave mixing, Kerr
and other nonlinear effects. In this paper we theoretically analyze the optical
heterodyne detection (OHD) technique of measuring the optical Kerr effect (OKE)
in two-dimensional crystals and show how to relate the quantities measured in
such experiments with components of the third-order conductivity tensor
$\sigma^{(3)}_{\alpha\beta\gamma\delta}(\omega_1,\omega_2,\omega_3)$ of the
two-dimensional crystal. Using results of a recently developed quantum theory
of the third-order nonlinear electrodynamic response of graphene we analyze the
frequency, charge carrier density, temperature and other dependencies of the
OHD-OKE response of this material. We compare our results with a recent OHD-OKE
experiment in graphene and find good agreement between the theory and
experiment. | cond-mat_mes-hall |
Decay and Frequency Shift of Inter and Intravalley Phonons in Graphene
-Dirac Cone Migration-: By considering analytical expressions for the self-energies of intervalley
and intravalley phonons in graphene, we describe the behavior of D, 2D, and
D$'$ Raman bands with changes in doping ($\mu$) and light excitation energy
($E_L$). Comparing the self-energy with the observed $\mu$ dependence of the 2D
bandwidth, we estimate the wavevector $q$ of the constituent intervalley phonon
at $\hbar vq\simeq E_L/1.6$ ($v$ is electron's Fermi velocity) and conclude
that the self-energy makes a major contribution (60%) to the dispersive
behavior of the D and 2D bands. The estimation of $q$ is based on an image of
shifted Dirac cones in which the resonance decay of a phonon satisfying $q >
\omega/v$ ($\omega$ is the phonon frequency) into an electron-hole pair is
suppressed when $\mu < (vq-\omega)/2$. We highlight the fact that the decay of
an intervalley (and intravalley longitudinal optical) phonon with $q=\omega/v$
is strongly suppressed by electron-phonon coupling at an arbitrary $\mu$. This
feature is in contrast to the divergent behavior of an intravalley transverse
optical phonon, which bears a close similarity to the polarization function
relevant to plasmons. | cond-mat_mes-hall |
Structure and energetics of carbon, hexagonal boron nitride and
carbon/hexagonal boron nitride single-layer and bilayer nanoscrolls: Single-layer and bilayer carbon and hexagonal boron nitride nanoscrolls as
well as nanoscrolls made of bilayer graphene/hexagonal boron nitride
heterostructure are considered. Structures of stable states of the
corresponding nanoscrolls prepared by rolling single-layer and bilayer
rectangular nanoribbons are obtained based on the analytical model and
numerical calculations. The lengths of nanoribbons for which stable and
energetically favorable nanoscrolls are possible are determined. Barriers to
rolling of single-layer and bilayer nanoribbons into nanoscrolls and barriers
to nanoscroll unrolling are calculated. Based on the calculated barriers
nanoscroll lifetimes in the stable state are estimated. Elastic constants for
bending of graphene and hexagonal boron nitride layers used in the model are
found by density functional theory calculations. | cond-mat_mes-hall |
Bismuth antiphase domain wall: A three-dimensional manifestation of the
Su-Schrieffer-Heeger model: The Su, Schrieffer and Heeger (SSH) model, describing the soliton excitations
in polyacetylene due to the formation of antiphase domain walls (DW) from the
alternating bond pattern, has served as a paradigmatic example of
one-dimensional (1D) chiral topological insulators. While the SSH model has
been realized in photonic and plasmonic systems, there have been limited
analogues in three-dimensional (3D) electronic systems, especially regarding
the formation of antiphase DWs. Here, we propose that pristine bulk Bi, in
which the dimerization of $(111)$ atomic layers renders alternating covalent
and van der Waals bonding within and between successive $(111)$ bilayers,
respectively, serves as a 3D analogue of the SSH model. First, we confirm that
the two dimerized Bi structures belong to different Zak phases of 0 and $\pi$
by considering the parity eigenvalues and Wannier charge centers, while the
previously reported bulk topological phases of Bi remain invariant under the
dimerization reversal. Next, we demonstrate the existence of topologically
non-trivial $(111)$ and trivial $(11\bar{2})$ DWs in which the number of in-gap
DW states (ignoring spin) is odd and even respectively, and show how this
controls the interlinking of the Zak phases of the two adjacent domains.
Finally, we derive general criteria specifying when a DW of arbitrary
orientation exhibits a $\pi$ Zak phase based on the flip of parity eigenvalues.
An experimental realization of dimerization in Bi and the formation of DWs may
be achieved via intense femtosecond laser excitations that can alter the
interatomic forces and bond lengths. | cond-mat_mes-hall |
Field electron emission theory (October 2016),v2: This document provides an updated account of material originally presented in
two field electron emission (FE) tutorial lectures given at the 2016 Young
Researchers' School in Vacuum Micro/Nano Electronics, held in Saint-Petersburg
in October 2016. The aim of the tutorial lectures was to set out modern
versions of some of the basics of mainstream FE theory. This paper indicates,
in some depth, the scope, structure and content of the tutorials, and also
where some of the related published material can be found. | cond-mat_mes-hall |
Phonon-bottleneck enhanced magnetic hysteresis in a molecular paddle
wheel complex of Ru$_2^{5+}$: The ruthenium based molecular magnet
[Ru$_2$(D(3,5-Cl$_2$Ph)F)$_4$Cl(0.5H$_2$O)$\cdotp$C$_6$H$_{14}$] (hereafter
Ru$_2$) behaves as a two-level system at sufficiently low temperatures. The
authors performed spin detection by means of single-crystal measurements and
obtained magnetic hysteresis loops around zero bias as a function of field
sweeping rate. Compared to other molecular systems, Ru$_2$ presents an enhanced
irreversibility as shown by ``valleys'' of negative differential susceptibility
in the hysteresis curves. Simulations based on phonon bottleneck model are in
good qualitative agreement and suggest an abrupt spin reversal combined with
insufficient thermal coupling between sample and cryostat phonon bath. | cond-mat_mes-hall |
Spin Transfer Torque and Electric Current in Helical Edge States in
Quantum Spin Hall Devices: We study the dynamics of a quantum spin Hall edge coupled to a magnet with
its own dynamics. Using spin transfer torque principles, we analyze the
interplay between spin currents in the edge state and dynamics of the axis of
the magnet, and draw parallels with circuit analogies. As a highlighting
feature, we show that while coupling to a magnet typically renders the edge
state insulating by opening a gap, in the presence of a small potential bias,
spin-transfer torque can restore perfect conductance by transferring angular
momentum to the magnet. In the presence of interactions within the edge state,
we employ a Luttinger liquid treatment to show that the edge, when subject to a
small voltage bias, tends to form a unique dynamic rotating spin wave state
that naturally couples into the dynamics of the magnet. We briefly discuss
realistic physical parameters and constraints for observing this interplay
between quantum spin Hall and spin-transfer torque physics. | cond-mat_mes-hall |
Spin Properties of Low Density One-Dimensional Wires: We report conductance measurements of a ballistic one-dimensional (1D) wire
defined in the lower two-dimensional electron gas of a GaAs/AlGaAs double
quantum well. At low temperatures there is an additional structure at
$0.7(2e^2/h)$ in the conductance, which tends to $e^2/h$ as the electron
density is decreased. We find evidence for complete spin polarization in a
weakly disorderd 1D wire at zero magnetic field through the observation of a
conductance plateau at $e^2/h$, which strengthens in an in-plane magnetic field
and disappears with increasing electron density. In all cases studied, with
increasing temperature structure occurs at $0.6(2e^2/h)$. We suggest that the
0.7 structure is a many-body spin state excited out of, either the
spin-polarized electron gas at low densities, or the spin-degenerate electron
gas at high densities. | cond-mat_mes-hall |
Spin current generation due to differential rotation: We study nonequilibrium spin dynamics in differentially rotating systems,
deriving an effective Hamiltonian for conduction electrons in the comoving
frame. In contrast to conventional spin current generation mechanisms that
require vorticity, our theory describes spins and spin currents arising from
differentially rotating systems regardless of vorticity. We demonstrate the
generation of spin currents in differentially rotating systems, such as liquid
metals with Taylor-Couette flow. Our alternative mechanism will be important in
the development of nanomechanical spin devices. | cond-mat_mes-hall |
Wide-Band Tuneability, Nonlinear Transmission, and Dynamic
Multistability in SQUID Metamaterials: Superconducting metamaterials comprising rf SQUIDs (Superconducting QUantum
Interference Devices) have been recently realized and investigated with respect
to their tuneability, permeability and dynamic multistability properties. These
properties are a consequence of intrinsic nonlinearities due to the sensitivity
of the superconducting state to external stimuli. SQUIDs, made of a
superconducting ring interrupted by a Josephson junction, possess yet another
source of nonlinearity, which makes them widely tuneable with an applied dc
dlux. A model SQUID metamaterial, based on electric equivalent circuits, is
used in the weak coupling approximation to demonstrate the dc flux tuneability,
dynamic multistability, and nonlinear transmission in SQUID metamaterials
comprising non-hysteretic SQUIDs. The model equations reproduce the
experimentally observed tuneability patterns, and predict tuneability with the
power of an applied ac magnetic magnetic field. Moreover, the results indicate
the opening of nonlinear frequency bands for energy transmission through SQUID
metamaterials, for sufficiently strong ac fields. | cond-mat_mes-hall |
Kitaev spin models from topological nanowire networks: We show that networks of topological nanowires can realize the physics of
exactly solvable Kitaev spin models with two-body interactions. This connection
arises from the description of the low-energy theory of both systems in terms
of a tight-binding model of Majorana modes. In Kitaev spin models the Majorana
description provides a convenient representation to solve the model, whereas in
an array of topological nanowires it arises, because the physical Majorana
modes localized at wire ends permit tunnelling between wire ends and across
different Josephson junctions. We explicitly show that an array of junctions of
three wires -- a setup relevant to topological quantum computing with nanowires
-- can realize the Yao-Kivelson model, a variant of Kitaev spin models on a
decorated honeycomb lattice. Translating the results from the latter, we show
that the network can be constructed to give rise to collective states
characterized by Chern numbers \nu = 0, +/-1 and +/-2, and that defects in an
array can be associated with vortex-like quasi-particle excitations. Finally,
we analyze the stability of the collective states as well as that of the
network as a quantum information processor. We show that decoherence inducing
instabilities, be them due to disorder or phase fluctuations, can be understood
in terms of proliferation of the vortex-like quasi-particles. | cond-mat_mes-hall |
Dissipation in graphene and nanotube resonators: Different damping mechanisms in graphene nanoresonators are studied: charges
in the substrate, ohmic losses in the substrate and the graphene sheet,
breaking and healing of surface bonds (Velcro effect), two level systems,
attachment losses, and thermoelastic losses. We find that, for realistic
structures and contrary to semiconductor resonators, dissipation is dominated
by ohmic losses in the graphene layer and metallic gate. An extension of this
study to carbon nanotube-based resonators is presented. | cond-mat_mes-hall |
Disorder induced Coulomb gaps in graphene constrictions with different
aspect ratios: We present electron transport measurements on lithographically defined and
etched graphene nanoconstrictions with different aspect ratios including
different lengths (L) and widths (W). A roughly length-independent disorder
induced effective energy gap can be observed around the charge neutrality
point. This energy gap scales inversely with the width even in regimes where
the length of the constriction is smaller than its width (L<W). In very short
constrictions, we observe both resonances due to localized states or charged
islands and an elevated overall conductance level (0.1-1e2/h), which is
strongly length-dependent in the gap region. This makes very short graphene
constrictions interesting for highly transparent graphene tunneling barriers. | cond-mat_mes-hall |
Generation of hyper-entangled photon pairs in coupled microcavities: We propose and theoretically analyze a new scheme for generating
hyper-entangled photon pairs in a system of polaritons in coupled planar
microcavities. Starting from a microscopic model, we evaluate the relevant
parametric scattering processes and numerically simulate the phonon-induced
noise background under continuous-wave excitation. Our results show that,
compared to other polariton entanglement proposals, our scheme enables the
generation of photon pairs that are entangled in both path and polarization
degrees of freedom, and simultaneously leads to a strong reduction of the
photoluminesence noise background. This can significantly improve the fidelity
of the entangled photon pairs under realistic experimental conditions. | cond-mat_mes-hall |
Observation of drift and diffusion processes in Ti/TiOx/Ti memristive
devices prepared by local anodic oxidation: We demonstrate that memristive devices can be fabricated by tip-induced
oxidation of thin metallic films using atomic force microscope. Electrical
measurements of such prepared Ti/TiOx/Ti test structures confirmed their
memristive behavior and inferred diffusion of oxygen vacancies in the TiOx
barrier. Consequent Kelvin probe force microscopy studies provided evidence for
the diffusion, as well as for expected oxygen vacancy drift. Time evolution of
the space distribution of the vacancies due to the diffusion process revealed
minute-scale (at least) retention times of the devices. The work presents
technology alternative for fabrication of memristive nanodevices in geometry
favouring advantageous scanning probe microscopy studies of their in-barrier
processes, as well as widely utilizable approach to search for novel oxide
materials for perspective memristive applications. | cond-mat_mes-hall |
Localized Many-Particle Majorana Modes with Vanishing Time-Reversal
Symmetry Breaking in Double Quantum Dots: We introduce the concept of spinful many-particle Majorana modes with local
odd operator products, thereby preserving their local statistics. We consider a
superconductor-double-quantum-dot system where these modes can arise with
negligible Zeeman splitting when Coulomb interactions are present. We find a
reverse Mott-insulator transition, where the even- and odd-parity bands become
degenerate. Above this transition, Majorana operators move the system between
the odd-parity ground state, associated with elastic cotunneling, and the
even-parity ground state, associated with crossed Andreev reflection. These
Majorana modes are described in terms of one, three, and five operator
products. Parity conservation results in a 4% periodic supercurrent in the even
state and no supercurrent in the odd state. | cond-mat_mes-hall |
Anisotropic RKKY interaction in spin polarized graphene: We study the Ruderman-Kittle-Kasuya-Yosida (RKKY) interaction in the presence
of spin polarized two dimensional Dirac fermions. We show that a spin
polarization along the z-axis mediates an anisotropic interaction which
corresponds to a XXZ model interaction between two magnetic moments. For
undoped graphene, while the $x$ part of interaction keeps its constant
ferromagnetic sign, its $z$ part oscillates with the distance of magnetic
impurities, $R$. A finite doping causes that both parts of the interaction
oscillate with $R$. We explore a beating pattern of oscillations of the RKKY
interaction along armchair and zigzag lattice directions, which occurs for some
certain values of the chemical potential. The two characteristic periods of the
beating are determined by inverse of the difference and the sum of the chemical
potential and the spin polarization. | cond-mat_mes-hall |
Superconducting insulators and localization of Cooper pairs: Rapid miniaturization of electronic devices and circuits demands profound
understanding of fluctuation phenomena at the nanoscale. Superconducting
nanowires -- serving as important building blocks for such devices -- may
seriously suffer from fluctuations which tend to destroy long-range order and
suppress superconductivity. In particular, quantum phase slips (QPS)
proliferating at low temperatures may turn a quasi-one-dimensional
superconductor into a resistor or an insulator. Here, we introduce a physical
concept of QPS-controlled localization of Cooper pairs that may occur even in
uniform nanowires without any dielectric barriers being a fundamental
manifestation of the flux-charge duality in superconductors. We demonstrate --
both experimentally and theoretically -- that deep in the "insulating" state
such nanowires actually exhibit non-trivial superposition of superconductivity
and weak Coulomb blockade of Cooper pairs generated by quantum tunneling of
magnetic fluxons across the wire. | cond-mat_mes-hall |
Strain induced mobility modulation in single-layer MoS$_{2}$: In this paper the effect of biaxial and uniaxial strain on the mobility of
single-layer MoS$_{2}$ for temperatures T $>$ 100 K is investigated. Scattering
from intrinsic phonon modes, remote phonon and charged impurities are
considered along with static screening. Ab-initio simulations are utilized to
investigate the strain induced effects on the electronic bandstructure and the
linearized Boltzmann transport equation is used to evaluate the low-field
mobility under various strain conditions. The results indicate that the
mobility increases with tensile biaxial and tensile uniaxial strain along the
armchair direction. Under compressive strain, however, the mobility exhibits a
non-monotonic behavior when the strain magnitude is varied. In particular, with
a relatively small compressive strain of 1% the mobility is reduced by about a
factor of two compared to the unstrained condition, but with a larger
compressive strain the mobility partly recovers such a degradation. | cond-mat_mes-hall |
Effect of strain, thickness, and local surface environment on electron
transport properties of oxygen-terminated copper thin films: Electron transport is studied in surface oxidized single-crystal copper thin
films with a thickness of up to 5.6 nm by applying density functional theory
and density functional tight binding methods to determine electron transport
properties within the ballistic regime. The variation of the electron
transmission as a function of film thickness as well as the different
contributions to the overall electron transmission as a function of depth into
the the films is examined. Transmission at the oxidized copper film surfaces is
found to be universally low. Films with thickness greater than 2.7 nm exhibit a
similar behavior in local transmission per unit area with depth from the film
surface; transmission per unit area initially increases rapidly and then
plateaus at a depth of approximately 0.35-0.5 nm away from the surface,
dependent on surface facet. Unstrained films tend to exhibit a higher
transmission per unit area than corresponding films under tensile strain. | cond-mat_mes-hall |
Interaction effects on a Majorana zero mode leaking into a quantum dot: We have recently shown [Phys. Rev. B {\bf 89}, 165314 (2014)] that a
non--interacting quantum dot coupled to a one--dimensional topological
superconductor and to normal leads can sustain a Majorana mode even when the
dot is expected to be empty, \emph{i.e.}, when the dot energy level is far
above the Fermi level of he leads. This is due to the Majorana bound state of
the wire leaking into the quantum dot. Here we extend this previous work by
investigating the low--temperature quantum transport through an {\it
interacting} quantum dot connected to source and drain leads and side--coupled
to a topological wire. We explore the signatures of a Majorana zero--mode
leaking into the quantum dot for a wide range of dot parameters, using a
recursive Green's function approach. We then study the Kondo regime using
numerical renormalization group calculations. We observe the interplay between
the Majorana mode and the Kondo effect for different dot-wire coupling
strengths, gate voltages and Zeeman fields. Our results show that a "0.5"
conductance signature appears in the dot despite the interplay between the
leaked Majorana mode and the Kondo effect. This robust feature persists for a
wide range of dot parameters, even when the Kondo correlations are suppressed
by Zeeman fields and/or gate voltages. The Kondo effect, on the other hand, is
suppressed by both Zeeman fields and gate voltages. We show that the zero--bias
conductance as a function of the magnetic field follows a well--known
universality curve. This can be measured experimentally, and we propose that
the universal conductance drop followed by a persistent conductance of
$0.5\,e^2/h$ is evidence of the presence of Majorana--Kondo physics. These
results confirm that this "0.5" Majorana signature in the dot remains even in
the presence of the Kondo effect. | cond-mat_mes-hall |
Supercurrent carried by non-equlibrium quasiparticles in a multiterminal
Josephson junction: We theoretically study coherent multiple Andreev reflections in a biased
three-terminal Josephson junction. We demonstrate that the direct current
flowing through the junction consists of supercurrent components when the bias
voltages are commensurate. This dissipationless current depends on the phase in
the superconducting leads and stems from the Cooper pair transfer processes
induced by non-local Andreev reflections of the quasiparticles originating from
the superconducting leads. We identify supercurrent-enhanced lines in the
current and conductance maps of the recent measurement [Y. Cohen, et al., PNAS
115, 6991 (2018)] on a nanowire Josephson junction and show that the magnitude
of the phase-dependent current components is proportional to the junction
transparency with the power corresponding to the component order. | cond-mat_mes-hall |
Gate-tuned quantum oscillations of topological surface states in
beta-Ag2Te: We report the strong experimental evidence of the existence of topological
surface states with large electric field tunability and mobility in beta-Ag2Te.
Pronounced 2D SdH oscillations have been observed in beta-Ag2Te nanoplates. A
Berry phase is determined to be near pi using the Landau level fan diagram for
a relatively wide nanoplate while the largest electric field ambipolar effect
in topological insulator so far (~ 2500%) in a narrow nanoplate. The pi Berry
phase and the evolution of quantum oscillations with gate voltage (Vg) in the
nanoplates strongly indicate the presence of topological surface states in
beta-Ag2Te. Moreover, the mobility of the narrow Ag2Te nanoplate is ~ 3x10^4
cm^2s^-1V^-1 when the Fermi level is near the Dirac point. The realization of
topological surface states with large electrical tunability and high mobility
indicates that beta-Ag2Te is a promising topological insulator for fundamental
studies. | cond-mat_mes-hall |
Impurity screening and stability of Fermi arcs against Coulomband
magnetic scattering in a Weyl monopnictide: We present a quasiparticle interference study of clean and Mn surface-doped
TaAs, a prototypical Weyl semimetal, to test the screening properties as well
as the stability of Fermi arcs against Coulomb and magnetic scattering.
Contrary to topological insulators, the impurities are effectively screened in
Weyl semimetals. The adatoms significantly enhance the strength of the signal
such that theoretical predictions on the potential impact of Fermi arcs can be
unambiguously scrutinized. Our analysis reveals the existence of three
extremely short, previously unknown scattering vectors. Comparison with theory
traces them back to scattering events between large parallel segments of
spin-split trivial states, strongly limiting their coherence. In sharp contrast
to previous work [R. Batabyal et al., Sci. Adv. 2, e1600709 (2016)], where
similar but weaker subtle modulations were interpreted as evidence of
quasiparticle interference originating from Femi arcs, we can safely exclude
this being the case. Overall, our results indicate that intra- as well as
inter-Fermi arc scattering are strongly suppressed and may explain why-in spite
of their complex multiband structure-transport measurements show signatures of
topological states in Weyl monopnictides. | cond-mat_mes-hall |
AC Josephson transport through interacting quantum dots: We investigate the AC Josephson current through a quantum dot with strong
Coulomb interaction attached to two superconducting and one normal lead. To
this end, we perform a perturbation expansion in the tunneling couplings within
a diagrammatic real-time technique. The AC Josephson current is connected to
the reduced density matrix elements that describe superconducting correlations
induced on the quantum dot via proximity effect. We analyze the dependence of
the AC signal on the level position of the quantum dot, the charging energy,
and the applied bias voltages. | cond-mat_mes-hall |
Zero-bias anomaly and Kondo-assisted quasi-ballistic 2D transport: Nonequilibrium transport measurements in mesoscopic quasi-ballistic 2D
electron systems show an enhancement in the differential conductance around the
Fermi energy. At very low temperatures, such a zero-bias anomaly splits,
leading to a suppression of linear transport at low energies. We also observed
a scaling of the nonequilibrium characteristics at low energies which resembles
electron scattering by two-state systems, addressed in the framework of
two-channel Kondo model. Detailed sample-to-sample reproducibility indicates an
intrinsic phenomenon in unconfined 2D systems in the low electron-density
regime. | cond-mat_mes-hall |
Quiet SDS Josephson Junctions for Quantum Computing: Unconventional superconductors exhibit an order parameter symmetry lower than
the symmetry of the underlying crystal lattice. Recent phase sensitive
experiments on YBCO single crystals have established the d-wave nature of the
cuprate materials, thus identifying unambiguously the first unconventional
superconductor. The sign change in the order parameter can be exploited to
construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting
a degenerate ground state and a double-periodic current-phase characteristic.
Here we discuss how to make use of these special junction characteristics in
the construction of a quantum computer. Combining such junctions together with
a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit
--- a solid state implementation of a quantum bit which remains optimally
isolated from its environment. | cond-mat_mes-hall |
Magnetic states and ferromagnetic resonance in geometrically frustrated
arrays of multilayer ferromagnetic nanoparticles ordered on triangular
lattices: We present a theoretical investigation of magnetostatic interaction effects
in geometrically frustrated arrays of anisotropic multilayer ferromagnetic
nanoparticles arranged in different spatially configured systems with
triangular symmetry. We show that the interlayer magnetostatic interaction
significantly expands the opportunities to create magnetically frustrated
systems. The effects of the magnetostatic interaction in magnetization reversal
processes and the possibility to control the ferromagnetic resonance spectrum
in such systems are discussed. | cond-mat_mes-hall |
Rashba scattering in the low-energy limit: We study potential scattering in a two-dimensional electron gas with Rashba
spin-orbit coupling in the limit that the energy of the scattering electron
approaches the bottom of the lower spin-split band. Focusing on two
spin-independent circularly symmetric potentials, an infinite barrier and a
delta-function shell, we show that scattering in this limit is qualitatively
different from both scattering in the higher spin-split band and scattering of
electrons without spin-orbit coupling. The scattering matrix is purely
off-diagonal with both off-diagonal elements equal to one, and all angular
momentum channels contribute equally; the differential cross section becomes
increasingly peaked in the forward and backward scattering directions; the
total cross section exhibits quantized plateaus. These features are independent
of the details of the scattering potentials, and we conjecture them to be
universal. Our results suggest that Rashba scattering in the low-energy limit
becomes effectively one-dimensional. | cond-mat_mes-hall |
Coulomb drag in quantum circuits: We study drag effect in a system of two electrically isolated quantum point
contacts (QPC), coupled by Coulomb interactions. Drag current exhibits maxima
as a function of QPC gate voltages when the latter are tuned to the transitions
between quantized conductance plateaus. In the linear regime this behavior is
due to enhanced electron-hole asymmetry near an opening of a new conductance
channel. In the non-linear regime the drag current is proportional to the shot
noise of the driving circuit, suggesting that the Coulomb drag experiments may
be a convenient way to measure the quantum shot noise. Remarkably, the
transition to the non-linear regime may occur at driving voltages substantially
smaller than the temperature. | cond-mat_mes-hall |
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