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Spin dynamics at the singlet-triplet crossings in double quantum dot: We simulate the control of the spin states in a two-electron double quantum
dot when an external detuning potential is used for passing the system through
an anticrossing. The hyperfine coupling of the electron spins with the
surrounding nuclei causes the anticrossing of the spin states but also the
decoherence of the spin states. We calculate numerically the singlet-triplet
decoherence for different detuning values and find a good agreement with
experimental measurement results of the same setup. We predict an interference
effect due to the coupling of T0 and T+ states. | cond-mat |
Influence of the Halide Ion on the A Site Dynamics in FAPbX3 (X = Br and
Cl): The optoelectronic properties and ultimately photovoltaic performance of
hybrid lead halide perovskites, is inherently related to the dynamics of the
organic cations. Here we report on the dynamics of the formamidinium (FA)
cation in FAPbX3 perovskites for chloride and bromide varieties, as studied by
neutron spectroscopy. Elastic fixed window scan measurements showed the onset
of reorientational motion of FA cations in FAPbCl3 to occur at a considerably
higher temperature compared to that in FAPbBr3. In addition, we observed two
distinct dynamical transitions only in the chloride system, suggesting a
significant variation in the reorientational motions of the FA cation with
temperature. Quasielastic neutron scattering data analysis of FAPbCl3 showed
that in the low temperature orthorhombic phase, FA cations undergo 2-fold jump
reorientations about the C-H axis which evolve into an isotropic rotation in
the intermediate tetragonal and high temperature cubic phases. Comparing the
results with those from FAPbBr3, reveal that the time scale, barrier to
reorientation and the geometry of reorientational motion of the FA cation are
significantly different for the two halides. We note that this dependence of
the dynamic properties of the A-site cation on the halide, is unique to the FA
series; the geometry of methylammonium (MA) cation dynamics in MAPbX3 is known
to be insensitive to different halides. | cond-mat |
Ab initio calculation of carrier mobility in semiconductors including
ionized-impurity scattering: The past decade has seen the emergence of ab initio computational methods for
calculating phonon-limited carrier mobilities in semiconductors with predictive
accuracy. More realistic calculations ought to take into account additional
scattering mechanisms such as, for example, impurity and grain-boundary
scattering. In this work, we investigate the effect of ionized-impurity
scattering on the carrier mobility. We model the impurity potential by a
collection of randomly distributed Coulomb scattering centers, and we include
this relaxation channel into the ab initio Boltzmann transport equation, as
implemented in the EPW code. We demonstrate this methodology by considering
silicon, silicon carbide, and gallium phosphide, for which detailed
experimental data are available. Our calculations agree reasonably well with
experiments over a broad range of temperatures and impurity concentrations. For
each compound investigated here, we compare the relative importance of
electron-phonon scattering and ionized-impurity scattering, and we critically
assess the reliability of Matthiessen's rule. We also show that an accurate
description of dielectric screening and carrier effective masses cam improve
quantitative agreement with experiments. | cond-mat |
Electrically tunable charge and spin transitions in Landau levels of
interacting Dirac fermions in trilayer graphene: Trilayer graphene in the fractional Quantum Hall Effect regime displays a set
of unique interaction-induced transitions that can be tuned entirely by the
applied bias voltage. These transitions occur near the anti-crossing points of
two Landau levels. In a large magnetic field ($> 8$ T) the electron-electron
interactions close the anti-crossing gap, resulting in some unusual transitions
between different Landau levels. For the filling factor $\nu=\frac23$, these
transitions are accompanied by a change of spin polarization of the ground
state. For a small Zeeman energy, this provides an unique opportunity to
control the spin polarization of the ground state by fine tuning the bias
voltage. | cond-mat |
Spin relaxation in Mn12-acetate: We present a comprehensive derivation of the magnetization relaxation in a
Mn12-acetate crystal based on thermally assisted spin tunneling induced by
quartic anisotropy and weak transverse magnetic fields. The overall relaxation
rate as function of the magnetic field is calculated and shown to agree well
with data including all resonance peaks. The Lorentzian shape of the resonances
is also in good agreement with recent data. A generalized master equation
including resonances is derived and solved exactly. It is shown that many
transition paths with comparable weight exist that contribute to the relaxation
process. Previously unknown spin-phonon coupling constants are calculated
explicitly. | cond-mat |
Evolution of Landau Levels into Edge States at an Atomically Sharp Edge
in Graphene: The quantum-Hall-effect (QHE) occurs in topologically-ordered states of
two-dimensional (2d) electron-systems in which an insulating bulk-state
coexists with protected 1d conducting edge-states. Owing to a unique
topologically imposed edge-bulk correspondence these edge-states are endowed
with universal properties such as fractionally-charged quasiparticles and
interference-patterns, which make them indispensable components for QH-based
quantum-computation and other applications. The precise edge-bulk
correspondence, conjectured theoretically in the limit of sharp edges, is
difficult to realize in conventional semiconductor-based electron systems where
soft boundaries lead to edge-state reconstruction. Using scanning-tunneling
microscopy and spectroscopy to follow the spatial evolution of bulk
Landau-levels towards a zigzag edge of graphene supported above a graphite
substrate we demonstrate that in this system it is possible to realize
atomically sharp edges with no edge-state reconstruction. Our results single
out graphene as a system where the edge-state structure can be controlled and
the universal properties directly probed. | cond-mat |
New Superconducting RbFe2As2: A First-principles Investigation: RbFe2As2 has recently been reported to be a bulk superconductor with Tc = 2.6
K in the undoped state, in contrast to undoped BaFe2As2 with a magnetic ground
state. We present here the results of the first-principles calculations of the
structural, elastic and electronic properties for this newest superconductor
and discuss its behaviour in relation to other related systems.
Keywords: RbFe2As2, Electronic structure; Elastic constant;
Superconductivity.
PACS: 74.70.Dd, 74.10.+v, 74.20.Pq, 75.25.Ld | cond-mat |
Superconducting properties of lithium-decorated bilayer graphene: Present study provides a comprehensive theoretical analysis of the
superconducting phase in selected lithium-decorated bilayer graphene
nanostructures. The numerical calculations, conducted within the Eliashberg
formalism, give quantitative estimations of the most important thermodynamic
properties such as the critical temperature, specific heat, critical field and
others. It is shown that discussed lithium-graphene systems present enhancement
of their thermodynamic properties comparing to the monolayer case e.g. the
critical temperature can be raised to $\sim 15$ K. Furthermore, estimated
characteristic thermodynamic ratios exceed predictions of the
Bardeen-Cooper-Schrieffer theory suggesting that considered lithium-graphene
systems can be properly analyzed only within the strong-coupling regime. | cond-mat |
Dynamical Signatures of Rank-2 $U(1)$ Spin Liquids: Emergent $U(1)$ gauge theories and artificial photons in frustrated magnets
are outstanding examples of many-body collective phenomena. The classical and
quantum regimes of these systems provide platforms for classical and quantum
spin liquids, and are the subject of current active theoretical and
experimental investigations. Recently, realizations of rank-2 $U(1)$ (R2-U1)
gauge theories in three-dimensional frustrated magnets have been proposed. Such
systems in the quantum regime may lead to the so-called fracton ordered phases
-- a new class of topological order that has been associated with quantum
stabilizer codes and holography. However, there exist few distinguishing
characteristics of these states for their detection in real materials. Here we
focus on the classical limit, and present the dynamical spin structure factor
for a R2-U1 spin liquid state on a breathing pyrochlore lattice. Remarkably, we
find unique signatures of the R2-U1 state, and we contrast them with the
results obtained from a more conventional $U(1)$ spin liquid. These results
provide a new path of investigation for future inelastic neutron scattering
experiments on candidate materials. | cond-mat |
Muon-Spin Rotation Spectra in the Mixed Phase of High-T_c
Superconductors : Thermal Fluctuations and Disorder Effects: We study muon-spin rotation (muSR) spectra in the mixed phase of highly
anisotropic layered superconductors, specifically Bi_2+xSr_2-xCaCu_2O_8+delta
(BSCCO), by modeling the fluid and solid phases of pancake vortices using
liquid-state and density functional methods. The role of thermal fluctuations
in causing motional narrowing of muSR lineshapes is quantified in terms of a
first-principles theory of the flux-lattice melting transition. The effects of
random point pinning are investigated using a replica treatment of liquid state
correlations and a replicated density functional theory. Our results indicate
that motional narrowing in the pure system, although substantial, cannot
account for the remarkably small linewidths obtained experimentally at
relatively high fields and low temperatures. We find that satisfactory
agreement with the muSR data for BSCCO in this regime can be obtained through
the ansatz that this ``phase'' is characterized by frozen short-range
positional correlations reflecting the structure of the liquid just above the
melting transition. This proposal is consistent with recent suggestions of a
``pinned liquid'' or ``glassy'' state of pancake vortices in the presence of
pinning disorder. Our results for the high-temperature liquid phase indicate
that measurable linewidths may be obtained in this phase as a consequence of
density inhomogeneities induced by the pinning disorder. The results presented
here comprise a unified, first-principles theoretical treatment of muSR spectra
in highly anisotropic layered superconductors in terms of a controlled set of
approximations. | cond-mat |
Theory of Spin-Resolved Auger-Electron Spectroscopy from Ferromagnetic
3d-Transition Metals: CVV Auger electron spectra are calculated for a multi-band Hubbard model
including correlations among the valence electrons as well as correlations
between core and valence electrons. The interest is focused on the
ferromagnetic 3d-transition metals. The Auger line shape is calculated from a
three-particle Green function. A realistic one-particle input is taken from
tight-binding band-structure calculations. Within a diagrammatic approach we
can distinguish between the \textit{direct} correlations among those electrons
participating in the Auger process and the \textit{indirect} correlations in
the rest system. The indirect correlations are treated within second-order
perturbation theory for the self-energy. The direct correlations are treated
using the valence-valence ladder approximation and the first-order perturbation
theory with respect to valence-valence and core-valence interactions. The
theory is evaluated numerically for ferromagnetic Ni. We discuss the
spin-resolved quasi-particle band structure and the Auger spectra and
investigate the influence of the core hole. | cond-mat |
In situ TEM investigation of oxygen migration as a key mechanism for
resistive switching in Pr0.7Ca0.3MnO3: Low temperature growth Pr0.7Ca0.3MnO3 (PCMO) thin film showed high
performance in electric field induced resistance switching (RS). To understand
the micro-mechanism of RS in Metal/PCMO/Metal devices, structure evolution of
PCMO under external electric field monitored inside transmission electron
microscope (TEM) were performed. Evolution of the modulation stripe in as-grown
PCMO sample was investigated when applying electric field. The new-generated
modulation stripe gradually disappeared. These results indicate that oxygen ion
migration plays a key role in RS. | cond-mat |
Contact-Density Analysis of Lattice Polymer Adsorption Transitions: By means of contact-density chain-growth simulations, we investigate a simple
lattice model of a flexible polymer interacting with an attractive substrate.
The contact density is a function of the numbers of monomer-substrate and
monomer-monomer contacts. These contact numbers represent natural order
parameters and allow for a comprising statistical study of the conformational
space accessible to the polymer in dependence of external parameters such as
the attraction strength of the substrate and the temperature. Since the contact
density is independent of the energy scales associated to the interactions, its
logarithm is an unbiased measure for the entropy of the conformational space.
By setting explicit energy scales, the thus defined, highly general
microcontact entropy can easily be related to the microcanonical entropy of the
corresponding hybrid polymer-substrate system. | cond-mat |
High energy pseudogap and its evolution with doping in Fe-based
superconductors as revealed by optical spectroscopy: We report optical spectroscopic measurements on electron- and hole-doped
BaFe2As2. We show that the compounds in the normal state are not simple metals.
The optical conductivity spectra contain, in addition to the free carrier
response at low frequency, a temperature-dependent gap-like suppression at
rather high energy scale near 0.6 eV. This suppression evolves with the
As-Fe-As bond angle induced by electron- or hole-doping. Furthermore, the
feature becomes much weaker in the Fe-chalcogenide compounds. We elaborate that
the feature is caused by the strong Hund's rule coupling effect between the
itinerant electrons and localized electron moment arising from the multiple Fe
3d orbitals. Our experiments demonstrate the coexistence of itinerant and
localized electrons in iron-based compounds, which would then lead to a more
comprehensive picture about the metallic magnetism in the materials. | cond-mat |
Multigap superconductivity in the new BiCh$_{2}$-based layered
superconductor La$_\mathrm{0.7}$Ce$_\mathrm{0.3}$OBiSSe: The layered bismuth oxy-sulfide materials, which are structurally related to
the Fe-pnictides/chalcogenides and cuprates superconductors, have brought
substantial attention for understanding the physics of reduced dimensional
superconductors. We have examined the pairing symmetry of recently discovered
BiCh$_2$-based superconductor, La$_\mathrm{1-x}$Ce$_\mathrm{x}$OBiSSe with $x$
= 0.3, through transverse field (TF) muon spin rotation measurement, in
addition we present the results of magnetization, resistivity and zero field
(ZF) muon spin relaxation measurements. Bulk superconductivity has been
observed below 2.7 K for $x$ = 0.3, verified by resistivity and magnetization
data. The temperature dependence of the magnetic penetration depth has been
determined from TF-$\mu$SR data can be described by an isotropic two-gap $s+s$
wave model compared to a single gap $s$- or anisotropic $s$-wave models, the
resemblance with Fe-pnictides/chalcogenides and MgB$_2$. Furthermore, from the
TF-$\mu$SR data, we have determined the London's penetration depth
$\lambda_\mathrm{L}(0)$ = 452(3) nm, superconducting carrier's density
$n_\mathrm{s}$ = 2.18(1) $\times$10$^{26}$ carriers/m$^{3}$ and effective mass
enhancement $m^{*}$ = 1.66(1) $m_\mathrm{e}$, respectively. No signature of
spontaneous internal field is found down to 100 mK in ZF-$\mu$SR measurement
suggest that time-reversal symmetry is preserved in this system. | cond-mat |
Performance Analysis of 60nm gate length III-V InGaAs HEMTs: Simulations
vs. experiments: An analysis of recent experimental data for high-performance In0.7Ga0.3As
high electron mobility transistors (HEMTs) is presented. Using a fully quantum
mechanical, ballistic model, we simulate In0.7Ga0.3As HEMTs with gate lengths
of LG = 60nm, 85, and 135 nm and compare the result to the measured I-V
characteristics including draininduced barrier lowering, sub-threshold swing,
and threshold voltage variation with gate insulator thickness, as well as
on-current performance. To first order, devices with three different oxide
thicknesses and channel lengths can all be described by our ballistic model
with appropriate values of parasitic series resistance. For high gate voltages,
however, the ballistic simulations consistently overestimate the measured
on-current, and they do not show the experimentally observed decrease in
on-current with increasing gate length. With no parasitic series resistance at
all, the simulated on-current of the LG = 60 nm device is about twice the
measured current. According to the simulation, the estimated ballistic carrier
injection velocity for this device is about 2.7 x 10^7 cm/s. Because of the
importance of the semiconductor capacitance, the simulated gate capacitance is
about 2.5 times less than the insulator capacitance. Possible causes of the
transconductance degradation observed under high gate voltages in these devices
are also explored. In addition to a possible gate-voltage dependent scattering
mechanism, the limited ability of the source to supply carriers to the channel,
and the effect of nonparabolicity are likely to play a role. The drop in
on-current with increasing gate length is an indication that the devices
operate below the ballistic limit. | cond-mat |
The onset of magnetic order in fcc-Fe films on Cu(100): On the basis of a first-principles electronic structure theory of finite
temperature metallic magnetism in layered materials, we investigate the onset
of magnetic order in thin (2-8 layers) fcc-Fe films on Cu(100) substrates. The
nature of this ordering is altered when the systems are capped with copper.
Indeed we find an oscillatory dependence of the Curie temperatures as a
function of Cu-cap thickness, in excellent agreement with experimental data.
The thermally induced spin-fluctuations are treated within a mean-field
disordered local moment (DLM) picture and give rise to layer-dependent `local
exchange splittings' in the electronic structure even in the paramagnetic
phase. These features determine the magnetic intra- and interlayer interactions
which are strongly influenced by the presence and extent of the Cu cap. | cond-mat |
Topological phase transition in the quench dynamics of a one-dimensional
Fermi gas: We study the quench dynamics of a one-dimensional ultracold Fermi gas in an
optical lattice potential with synthetic spin-orbit coupling. At equilibrium,
the ground state of the system can undergo a topological phase transition and
become a topological superfluid with Majorana edge states. As the interaction
is quenched near the topological phase boundary, we identify an interesting
dynamical phase transition of the quenched state in the long-time limit,
characterized by an abrupt change of the pairing gap at a critical quenched
interaction strength. We further demonstrate the topological nature of this
dynamical phase transition from edge-state analysis of the quenched states. Our
findings provide interesting clues for the understanding of topological phase
transitions in dynamical processes, and can be useful for the dynamical
detection of Majorana edge states in corresponding systems. | cond-mat |
Novel method for photovoltaic energy conversion using surface acoustic
waves in piezoelectric semiconductors: This paper presents a novel principle for photovoltaic (PV) energy conversion
using surface acoustic waves (SAWs) in piezoelectric semiconductors. A SAW
produces a periodically modulated electric potential, which spatially
segregates photoexcited electrons and holes to the maxima and minima of the SAW
potential. The moving SAW collectively transports the carriers with the speed
of sound to the electrodes made of different materials, which extract electrons
and holes separately and generate dc output. The proposed active design is
expected to have higher efficiency than passive designs of the existing PV
devices and to produce enough energy to sustain the SAW. | cond-mat |
Twin-domain formation in epitaxial triangular lattice delafossites: Twin domains are often found as structural defects in symmetry mismatched
epitaxial thin films. The delafossite ABO2, which has a rhombohedral structure,
is a good example that often forms twin domains. Although bulk metallic
delafossites are known to be the most conducting oxides, the high conductivity
is yet to be realized in thin film forms. Suppressed conductivity found in thin
films is mainly caused by the formation of twin domains, and their boundaries
can be a source of scattering centers for charge carriers. To overcome this
challenge, the underlying mechanism for their formation must be understood, so
that such defects can be controlled and eliminated. Here, we report the origin
of structural twins formed in a CuCrO2 delafossite thin film on a substrate
with hexagonal or triangular symmetries. A robust heteroepitaxial relationship
is found for the delafossite film with the substrate, and the surface
termination turns out to be critical to determine and control the domain
structure of epitaxial delafossites. Based on such discoveries, we also
demonstrate a twin-free epitaxial thin films grown on high-miscut substrates.
This finding provides an important synthesis strategy for growing single domain
delafossite thin films and can be applied to other delafossites for epitaxial
synthesis of high-quality thin films. | cond-mat |
Influence of the coordination defects on the dynamics and the potential
energy landscape of two-dimensional silica: The main cause of the fragile-to-strong crossover of 3D silica was previously
attributed to the presence of a low energy cutoff in the potential energy
landscape. The important question emerges about the microscopic origin of this
crossover and the generalizibility to other glass-formers. In this work, the
fragile-to-strong crossover of a model 2D glassy system is analyzed via
molecular dynamics simulation, which represents 2D-silica. By separating the
sampled defect and defect-free inherent structures, we are able to identify
their respective density of state distributions with respect to energy. A low
energy cutoff is found in both distributions. It is shown that the
fragile-to-strong crossover can be quantitatively related to the parameters of
the energy landscape, involving in particular the low-energy cutoff of the
energy distribution. It is also shown that the low-energy cutoff of the
defect-states is determined by the formation energy of a specific defect
configuration, involving two silicon and no oxygen defect. The low-temperature
behavior of 2D silica is quantitatively compared with that of 3D silica,
showing surprisingly similar behavior. | cond-mat |
Possibility of Electron Pairing in Small Metallic Nanoparticles: We investigate the possibility of electron pairing in small metallic
nanoparticles at zero temperature. In these particles both electrons and
phonons are mesoscopic, i.e. modified by the nanoparticle's finite size. The
electrons, the phonons, and their interaction are described within the
framework of a simplified model. The effective electron-electron interaction is
derived from the underlying electron-phonon interaction. The effect of both
effective interaction and Coulomb interaction on the electronic spectrum is
evaluated. Results are presented for aluminum, zinc and potassium nanoparticles
containing a few hundred atoms. We find that a large portion of the aluminum
and zinc particles exhibit modifications in their electronic spectrum due to
pairing correlations, while pairing correlations are not present in the
potassium particles. | cond-mat |
Molecular Doping of Graphene: Graphene, a one-atom thick zero gap semiconductor [1, 2], has been attracting
an increasing interest due to its remarkable physical properties ranging from
an electron spectrum resembling relativistic dynamics [3-12] to ballistic
transport under ambient conditions [1-4]. The latter makes graphene a promising
material for future electronics and the recently demonstrated possibility of
chemical doping without significant change in mobility has improved graphene's
prospects further [13]. However, to find optimal dopants and, more generally,
to progress towards graphene-based electronics requires understanding the
physical mechanism behind the chemical doping, which has been lacking so far.
Here, we present the first joint experimental and theoretical investigation of
adsorbates on graphene. We elucidate a general relation between the doping
strength and whether or not adsorbates have a magnetic moment: The paramagnetic
single NO2 molecule is found to be a strong acceptor, whereas its diamagnetic
dimer N2O4 causes only weak doping. This effect is related to the peculiar
density of states of graphene, which provides an ideal situation for model
studies of doping effects in semiconductors. Furthermore, we explain recent
results on its "chemical sensor" properties, in particular, the possibility to
detect a single NO2 molecule [13]. | cond-mat |
Temperature dependence of polarization relaxation in semiconductor
quantum dots: The decay time of the linear polarization degree of the luminescence in
strongly confined semiconductor quantum dots with asymmetrical shape is
calculated in the frame of second-order quasielastic interaction between
quantum dot charge carriers and LO phonons. The phonon bottleneck does not
prevent significantly the relaxation processes and the calculated decay times
can be of the order of a few tens picoseconds at temperature $T \simeq 100$K,
consistent with recent experiments by Paillard et al. [Phys. Rev. Lett.
{\bf86}, 1634 (2001)]. | cond-mat |
Stability and decay of Bloch oscillations in presence of time-dependent
nonlinearity: We consider Bloch oscillations of Bose-Einstein condensates in presence of a
time-modulated s-wave scattering length. Generically, interaction leads to
dephasing and decay of the wave packet. Based on a cyclic-time argument, we
find---additionally to the linear Bloch oscillation and a rigid soliton
solution---an infinite family of modulations that lead to a periodic time
evolution of the wave packet. In order to quantitatively describe the dynamics
of Bloch oscillations in presence of time-modulated interactions, we employ two
complementary methods: collective-coordinates and the linear stability analysis
of an extended wave packet. We provide instructive examples and address the
question of robustness against external perturbations. | cond-mat |
Thermal-error regime in high-accuracy gigahertz single-electron pumping: Single-electron pumps based on semiconductor quantum dots are promising
candidates for the emerging quantum standard of electrical current. They can
transfer discrete charges with part-per-million (ppm) precision in nanosecond
time scales. Here, we employ a metal-oxide-semiconductor silicon quantum dot to
experimentally demonstrate high-accuracy gigahertz single-electron pumping in
the regime where the number of electrons trapped in the dot is determined by
the thermal distribution in the reservoir leads. In a measurement with
traceability to primary voltage and resistance standards, the averaged pump
current over the quantized plateau, driven by a \mbox{$1$-GHz} sinusoidal wave
in the absence of magnetic field, is equal to the ideal value of $ef$ within a
measurement uncertainty as low as $0.27$~ppm. | cond-mat |
Local modes, phonons, and mass transport in solid $^4$He: We propose a model to treat the local motion of atoms in solid $^{4}$He as a
local mode. In this model, the solid is assumed to be described by the Self
Consistent Harmonic approximation, combined with an array of local modes. We
show that in the bcc phase the atomic local motion is highly directional and
correlated, while in the hcp phase there is no such correlation. The correlated
motion in the bcc phase leads to a strong hybridization of the local modes with
the T$_{1}(110)$ phonon branch, which becomes much softer than that obtained
through a Self Consistent Harmonic calculation, in agreement with experiment.
In addition we predict a high energy excitation branch which is important for
self-diffusion. Both the hybridization and the presence of a high energy branch
are a consequence of the correlation, and appear only in the bcc phase. We
suggest that the local modes can play the role in mass transport usually
attributed to point defects (vacancies). Our approach offers a more overall
consistent picture than obtained using vacancies as the predominant point
defect. In particular, we show that our approach resolves the long standing
controversy regarding the contribution of point defects to the specific heat of
solid $^{4}$He. | cond-mat |
Theory of Underdoped Cuprates: We develop a slave-boson theory for the t-J model at finite doping which
respects an SU(2) symmetry -- a symmetry previously known to be important at
half filling. The mean field phase diagram is found to be consistent with the
phases observed in the cuprate superconductors, which contains d-wave
superconductor, spin gap, strange metal, and Fermi liquid phases. The spin gap
phase is best understood as the staggered flux phase, which is nevertheless
translationally invariant for physical quantities. The electron spectral
function shows small Fermi pockets at low doping which continuously evolve into
the large Fermi surface at high doping concentrations. | cond-mat |
Branch-entangled polariton pairs in planar microcavities and photonic
wires: A scheme is proposed for the generation of branch-entangled pairs of
microcavity polaritons through spontaneous inter-branch parametric scattering.
Branch-entanglement is achievable when there are two twin processes, where the
role of signal and idler can be exchanged between two different polariton
branches. Branch-entanglement of polariton pairs can lead to the emission of
frequency-entangled photon pairs out of the microcavity. In planar
microcavities, the necessary phase-matching conditions are fulfilled for
pumping of the upper polariton branch at an arbitrary in-plane wave-vector. The
important role of nonlinear losses due to pair scattering into high-momentum
exciton states is evaluated. The results show that the lack of protection of
the pump polaritons in the upper branch is critical. In photonic wires,
branch-entanglement of one-dimensional polaritons is achievable when the pump
excites a lower polariton sub-branch at normal incidence, providing protection
from the exciton reservoir. | cond-mat |
Quadratic heat capacity and high-field magnetic phases of V5S8: We report the observation of an unexpected quadratic temperature dependence
of the heat capacity in the vanadium sulphide metal V5S8 at low temperatures
which is independent of applied magnetic field. We find that the behaviour of
the heat capacity is consistent with an unconventional phonon spectrum which is
linear in wavevector in the c direction but quadratic in the a-b plane,
indicating a form of geometrical elastic criticality. In the case of V5S8 we
also observe an unusual intermediate transition at high magnetic fields between
the expected spin-flop and spin-flip transitions. We demonstrate that the
intermediate field-induced transition is in agreement with a model of two
sublattices with frustrated inter- and intra-sublattice spin couplings. | cond-mat |
Nonclassical rotational inertia for a supersolid under rotation: As proposed by Leggett [4], the supersolidity of a crystal is characterized
by the Non Classical Rotational Inertia (NCRI) property. Using a model of
quantum crystal introduced by Josserand, Pomeau and Rica [5], we prove that
NCRI occurs. This is done by analyzing the ground state of the aforementioned
model, which is related to a sphere packing problem, and then deriving a
theoretical formula for the inertia momentum. We infer a lower estimate for the
NCRI fraction, which is a landmark of supersolidity. | cond-mat |
Spin-wave nonreciprocity based on interband magnonic transitions: We theoretically demonstrate linear spin-wave nonreciprocity in a Ni80Fe20
nanostripe waveguide, based on interband magnonic transitions induced by a
time-reversal and spatialinversion symmetry breaking magnetic field. An
analytical coupled-mode theory of spin waves, developed to describe the
transitions which are accompanied by simultaneous frequency and wavevector
shifts of the coupled spin waves, is well corroborated by numerical
simulations. Our findings could pave the way for the realization of spin-wave
isolation and the dynamic control of spin-wave propagation in nanoscale
magnonic integrated circuits via an applied magnetic field. | cond-mat |
An electronic origin of charge order in infinite-layer nickelates: A charge order (CO) with a wavevector
$\mathbf{q}\simeq\left(\frac{1}{3},0,0\right)$ is observed in infinite-layer
nickelates. Here we use first-principles calculations to demonstrate a
charge-transfer-driven CO mechanism in infinite-layer nickelates, which leads
to a characteristic Ni$^{1+}$-Ni$^{2+}$-Ni$^{1+}$ stripe state. For every three
Ni atoms, due to the presence of near-Fermi-level conduction bands, Hubbard
interaction on Ni-$d$ orbitals transfers electrons on one Ni atom to conduction
bands and leaves electrons on the other two Ni atoms to become more localized.
We further derive a low-energy effective model to elucidate that the CO state
arises from a delicate competition between Hubbard interaction on Ni-$d$
orbitals and charge transfer energy between Ni-$d$ orbitals and conduction
bands. With physically reasonable parameters,
$\mathbf{q}=\left(\frac{1}{3},0,0\right)$ CO state is more stable than uniform
paramagnetic state and usual checkerboard antiferromagnetic state. Our work
highlights the multi-band nature of infinite-layer nickelates, which leads to
some distinctive correlated properties that are not found in cuprates. | cond-mat |
Field-angle-dependent specific heat measurements and gap determination
of a heavy fermion superconductor URu2Si2: To identify the superconducting gap structure in URu2Si2 we perform
field-angle-dependent specific heat measurements for the two principal
orientations in addition to field rotations, and theoretical analysis based on
microscopic calculations. The Sommerfeld coefficient \gamma(H)'s in the mixed
state exhibit distinctively different field-dependence. This comes from point
nodes and substantial Pauli paramagnetic effect of URu2Si2. These two features
combined give rise to a consistent picture of superconducting properties,
including a possible first order transition of Hc2 at low temperatures. | cond-mat |
Spin-splitting in GaAs 2D holes: We present quantitative measurements and calculations of the spin-orbit
induced zero-magnetic-field spin-splitting in two-dimensional (2D) hole systems
in modulation-doped GaAs (311)A quantum wells. The results show that the
splitting is large and tunable. In particular, via a combination of back- and
front-gate biases, we can tune the splitting while keeping the 2D hole density
constant. The data also reveal a surprising result regarding the
magnetoresistance (Shubnikov-de Haas) oscillations in a 2D system with
spin-split energy bands: the frequencies of the oscillations are {\it not}
simply related to the population of the spin-subbands. Next we concentrate on
the metallic-like behavior observed in these 2D holes and its relation to
spin-splitting. The data indicate that the metallic behavior is more pronounced
when two spin-subbands with unequal populations are occupied. Our measurements
of the magnetoresistance of these 2D hole systems with an in-plane magnetic
field corroborate this conclusion: while the system is metallic at zero
magnetic field, it turns insulating when one of the spin-subbands is
depopulated at high magnetic field. | cond-mat |
Generation and detection of mode-locked spin coherence in (In,Ga)As/GaAs
quantum dots by laser pulses of long duration: Using optical pulses of variable duration up to 80 ps, we report on spin
coherence initialization and its subsequent detection in n-type singly-charged
quantum dots, subject to a transverse magnetic field, by pump-probe techniques.
We demonstrate experimentally and theoretically that the spin coherence
generation and readout efficiencies are determined by the ratio of laser pulse
duration to spin precession period: An increasing magnetic field suppresses the
spin coherence signals for a fixed duration of pump and/or probe pulses, and
this suppression occurs for smaller fields the longer the pulse duration is.
The reason for suppression is the varying spin orientation due to precession
during pulse action. | cond-mat |
Stretched Non-negative Matrix Factorization: An algorithm is described and tested that carries out a non negative matrix
factorization (NMF) ignoring any stretching of the signal along the axis of the
independent variable. This extended NMF model is called StretchedNMF.
Variability in a set of signals due to this stretching is then ignored in the
decomposition. This can be used, for example, to study sets of powder
diffraction data collected at different temperatures where the materials are
undergoing thermal expansion. It gives a more meaningful decomposition in this
case where the component signals resemble signals from chemical components in
the sample. The StretchedNMF model introduces a new variable, the stretching
factor, to describe any expansion of the signal. To solve StretchedNMF, we
discretize it and employ Block Coordinate Descent framework algorithms. The
initial experimental results indicate that StretchedNMF model outperforms the
conventional NMF for sets of data with such an expansion. A further enhancement
to StretchedNMF for the case of powder diffraction data from crystalline
materials called Sparse-StretchedNMF, which makes use of the sparsity of the
powder diffraction signals, allows correct extractions even for very small
stretches where StretchedNMF struggles. As well as demonstrating the model
performance on simulated PXRD patterns and atomic pair distribution functions
(PDFs), it also proved successful when applied to real data taken from an in
situ chemical reaction experiment. | cond-mat |
Optical absorption and carrier multiplication at graphene edges in a
magnetic field: We study optical absorption at graphene edges in a transversal magnetic
field. The magnetic field bends the trajectories of particle- and hole
excitations into antipodal direction which generates a directed current. We
find a rather strong amplification of the edge current by impact ionization
processes. More concretely, the primary absorption and the subsequent carrier
multiplication is analyzed for a graphene fold and a zigzag edge. We identify
exact and approximate selection rules and discuss the dependence of the decay
rates on the initial state. | cond-mat |
Magneto-Electric Effect for Multiferroic Thin Film by Monte Carlo
Simulation: Magneto-electric effect in a multiferroic heterostructure film, i.e. a
coupled ferromagnetic-ferroelectric thin film, has been investigated through
the use of the Metropolis algorithm in Monte Carlo simulations. A classical
Heisenberg model describes the energy stored in the ferromagnetic film, and we
use a pseudo-spin model with a transverse Ising Hamiltonian to characterise the
energy of electric dipoles in the ferroelectric film. The purpose of this
article is to demonstrate the dynamic response of polarisation is driven by an
external magnetic field, when there is a linear magneto-electric coupling at
the interface between the ferromagnetic and ferroelectric components. | cond-mat |
Giant non-linear susceptibility of hydrogenic donors in silicon and
germanium: Implicit summation is a technique for the conversion of sums over
intermediate states in multiphoton absorption and the high-order susceptibility
in hydrogen into simple integrals. Here, we derive the equivalent technique for
hydrogenic impurities in multi-valley semiconductors. While the absorption has
useful applications, it is primarily a loss process; conversely, the non-linear
susceptibility is a crucial parameter for active photonic devices. For Si:P, we
predict the hyperpolarizability ranges from $\chi^{(3)}/n_{\text{3D}}=2.9 $ to
$580 \times 10^{-38}$ $\text{m}^5/\text{V}^2$ depending on the frequency, even
while avoiding resonance. Using samples of a reasonable density,
$n_{\text{3D}}$, and thickness, $L$, to produce third-harmonic generation at 9
THz, a frequency that is difficult to produce with existing solid-state
sources, we predict that $\chi^{(3)}$ should exceed that of bulk InSb and
$\chi^{(3)}L$ should exceed that of graphene and resonantly enhanced quantum
wells. | cond-mat |
Thermoelectric Effect at Quantum Limit in Two-Dimensional Organic Dirac
Fermion System with Zeeman Splitting: The thermoelectric effect in a two-dimensional (2D) massless Dirac fermion
(DF) system at the quantum limit is discussed to verify the prediction of
high-performance thermopower in an organic conductor \alpha-(BEDT-TTF)2I3.
Because of relatively large Zeeman splitting in \alpha-(BEDT-TTF)2I3, the
boundless increase of thermopower at high magnetic fields, predicted without
the Zeeman effect, is hardly expected, whereas there appears to be a broad
local maximum. This is characteristic of 2D DF systems with Zeeman splitting
and is recognized in the previous experiment. In contrast to 3D Dirac/Weyl
semimetals with robust gapless features, it might be difficult to realize
high-performance thermopower in real 2D DF systems under high magnetic fields. | cond-mat |
Thermodynamic signatures of short-range magnetic correlations in UTe$_2$: The normal-state out of which unconventional superconductivity in UTe$_2$
emerges is studied in detail using a variety of thermodynamic and transport
probes. Clear evidence for a broad Schottky-like anomaly with roughly R ln 2
entropy around $T^{*} \approx 12$K is observed in all measured quantities.
Comparison with high magnetic field transport data allows the construction of
an $H\text{-}T$ phase diagram resembling that of the ferromagnetic
superconductor URhGe. The low field electronic Gr\"uneisen parameter of $T^{*}$
and that of the metamagnetic transition at $H_m \approx 35$T are comparable
pointing to a common origin of both phenomena. Enhanced Wilson and Korringa
ratios suggests that the existence of short range ferromagnetic fluctuations
cannot be ruled out. | cond-mat |
Dynamical AC study of the critical behavior in Heisenberg spin glasses: We present some numerical results for the Heisenberg spin-glass model with
Gaussian interactions, in a three dimensional cubic lattice. We measure the AC
susceptibility as a function of temperature and determine an apparent finite
temperature transition which is compatible with the chiral-glass temperature
transition for this model. The relaxation time diverges like a power law
$\tau\sim (T-T_c)^{-z\nu}$ with $T_c=0.19(4)$ and $z\nu=5.0(5)$. Although our
data indicates that the spin-glass transition occurs at the same temperature as
the chiral glass transition, we cannot exclude the possibility of a chiral-spin
coupling scenario for the lowest frequencies investigated. | cond-mat |
Fully spin-polarized nodal loop semimetals in alkaline-metal
monochalcogenide monolayers: Topological semimetals in ferromagnetic materials have attracted enormous
attention due to the potential applications in spintronics. Using the
first-principles density functional theory together with an effective lattice
model, here we present a new family of topological semimetals with a fully
spin-polarized nodal loop in alkaline-metal monochalcogenide $MX$ ($M$ = Li,
Na, K, Rb, Cs; $X$ = S, Se, Te) monolayers. The half-metallic ferromagnetism
can be established in $MX$ monolayers, in which one nodal loop formed by two
crossing bands with the same spin components is found at the Fermi energy. This
nodal loop half-metal survives even when considering the spin-orbit coupling
owing to the symmetry protection provided by the $\mathcal{M}_{z}$ mirror
plane. The quantum anomalous Hall state and Weyl-like semimetal in this system
can be also achieved by rotating the spin from the out-of-plane to the in-plane
direction. The $MX$ monolayers hosting rich topological phases thus offer an
excellent materials platform for realizing the advanced spintronics concepts. | cond-mat |
Heat diode and engine based on quantum Hall edge states: We investigate charge and energy transport in a three-terminal quantum Hall
conductor. The peculiar properties of chiral propagation along the edges of the
sample have important consequences on the response to thermal biases. Based on
the separation of charge and heat flows, thermoelectric conversion and heat
rectification can be manipulated by tuning the scattering at gate-modulated
constrictions. Chiral motion in a magnetic field allows for a different
behaviour of left- and right-moving carriers giving rise to thermal
rectification by redirecting the heat flows. We propose our system both as an
efficient heat-to-work converter and as a heat diode. | cond-mat |
The length scale measurements of the Fractional quantum Hall state on
cylinder: Once the fractional quantum Hall (FQH) state for a finite size system is put
on the surface of a cylinder, the distance between the two ends with open
boundary conditions can be tuned as varying the aspect ratio $\gamma$. It
scales linearly as increasing the system size and therefore has a larger
adjustable range than that on disk. The previous study of the quasi-hole
tunneling amplitude on disk in Ref.~\cite{Zk2011} indicates that the tunneling
amplitudes have a scaling behavior as a function of the tunneling distance and
the scaling exponents are related to the scaling dimension and the charge of
the transported quasiparticles. However, the scaling behaviors poorly due to
the narrow range of the tunneling distance on disk. Here we systematically
study the quasiparticle tunneling amplitudes of the Laughlin state in the
cylinder geometry which shows a much better scaling behavior. Especially, there
are some corssover behaviors at two length scales when the two open edges are
close to each other. These lengths are also reflected in the bipartite
entanglement and the electron Green's function as either a singularity or a
crossover. These two critical length scales of the edge-edge distance,
$L_x^{c_1}$ and $L_x^{c_2}$, are found to be related to the dimension reduction
and back scattering point respectively. | cond-mat |
Antiferromagnetic iron based magnetoelectric compounds: The Landau free-energy of a compound that benefits from a linear coupling of
an electric field and a magnetic field includes a product of the two fields,
one polar and time-even and one axial and time-odd. In ME compounds,
expectation values of some atomic magnetic tensors are invariant with respect
to anti-inversion. An invariance shared by the Dirac monopole (an element of
charge allowed in Maxwell's equations that has not been observed) and a
Zeldovich anapole, also known as a Dirac dipole. From the science of materials
perspective, it has been established that Dirac multipoles contribute to the
diffraction of x-rays and neutrons. We identify Dirac monopoles in bulk
magnetic properties of iron tellurate (Fe2TeO6) and a spin ladder (SrFe2S2O).
Both cited compounds present a simple antiferromagnetic configuration of axial
dipoles, and their different magnetic crystal classes allow a linear ME effect.
However, the Kerr effect is symmetry allowed in the spin ladder and forbidden
in iron tellurate. Anapoles are forbidden in iron tellurate and allowed in the
spin ladder compound, a difference evident in diffraction patterns fully
informed by symmetry. More generally, we identify a raft of Dirac multipoles,
and axial multipoles beyond dipoles, visible in future experiments using
standard techniques with beams of neutrons or x-rays tuned in energy to an iron
atomic resonance. ME invariance imposes a phase relationship between nuclear
(charge) and magnetic contributions to neutron (x-ray) diffraction amplitudes.
In consequence, intensities of Bragg spots in an x-ray pattern do not change
when helicity in the primary beam is reversed. A like effect occurs in the
magnetic diffraction of polarized neutrons. | cond-mat |
Aperiodic crystals and beyond: Crystals are paradigms of ordered structures. While order was once seen as
synonymous with lattice periodic arrangements, the discoveries of
incommensurate crystals and quasicrystals led to a more general perception of
crystalline order, encompassing both periodic and aperiodic crystals. The
current definition of crystals rests on their essentially point-like
diffraction. Considering a number of recently investigated toy systems, with
particular emphasis on non-crystalline ordered structures, the limits of the
current definition are explored. | cond-mat |
Ultrapure Multilayer Graphene in Bromine Intercalated Graphite: We investigate the optical properties of bromine intercalated highly
orientated pyrolytic graphite (Br-HOPG) and provide a novel interpretation of
the data. We observe new absorption features below 620 meV which are absent in
the absorption spectrum of graphite. Comparing our results with those of
theoretical studies on graphite, single and bilayer graphene as well as recent
optical studies of multilayer graphene, we conclude that Br-HOPG contains the
signatures of ultrapure bilayer, single layer graphene, and graphite. The
observed supermetallic conductivity of Br-HOPG is identified with the presence
of very high mobility (~ 121,000 cm2V-1s-1 at room temperature and at very high
carrier density) multilayer graphene components in our sample. This could
provide a new avenue for single and multilayer graphene research. | cond-mat |
Probing the Lattice Anharmonicity of Superconducting
YBa$_2$Cu$_3$O$_{7-δ}$ Via Phonon Harmonics: We examine coherent phonons in a strongly driven sample of optimally-doped
high temperature superconductor YBa$_2$Cu$_3$O$_{7-\delta}$. We observe a
non-linear lattice response of the 4.5\,THz copper-oxygen vibrational mode at
high excitation densities, evidenced by the observation of the phonon third
harmonic and indicating the mode is strongly anharmonic. In addition, we
observe how high-amplitude phonon vibrations modify the position of the
electronic charge transfer resonance. Both of these results have important
implications for possible phonon-driven non-equilibrium superconductivity. | cond-mat |
Absorption suppression in photonic crystals: We study electromagnetic properties of periodic composite structures, such as
photonic crystals, involving lossy components. We show that in many cases a
properly designed periodic structure can dramatically suppress the losses
associated with the absorptive component, while preserving or even enhancing
its useful functionality. As an example, we consider magnetic photonic
crystals, in which the lossy magnetic component provides nonreciprocal Faraday
rotation. We show that the electromagnetic losses in the composite structure
can be reduced by up to two orders of magnitude, compared to those of the
uniform magnetic sample made of the same lossy magnetic material. Importantly,
the dramatic absorption reduction is not a resonance effect and occurs over a
broad frequency range covering a significant portion of photonic frequency
band. | cond-mat |
Electrochemical lithium intercalation in nanosized manganese oxides: X-ray amorphous manganese oxides were prepared by reduction of sodium
permanganate by lithium iodide in aqueous medium (MnOx-I) and by decomposition
of manganese carbonate at moderate temperature (MnOx-C). TEM showed that these
materials are not amorphous, but nanostructured, with a prominent spinel
substructure in MnOx-C. These materials intercalate lithium with capacities up
to 200 mAh/g at first cycle (potential window 1.8-4.3 V) and 175 mAh/g at 100th
cycle. Best performances for MnOx-C are obtained with cobalt doping. Potential
electrochemical spectroscopy shows that the initial discharge induces a 2-phase
transformation in MnOx-C phases, but not in MnOx-I ones. EXAFS and XANES
confirm the participation of manganese in the redox process, with variations in
local structure much smaller than in known long-range crystallized manganese
oxides. X-ray absorption spectroscopy also shows that cobalt in MnOx-C is
divalent and does not participate in the electrochemical reaction. | cond-mat |
Franck-Condon-Broadened Angle-Resolved Photoemission Spectra Predicted
in LaMnO3: The sudden photohole of least energy created in the photoemission process is
a vibrationally excited state of a small polaron. Therefore the photoemission
spectrum in LaMnO3 is predicted to have multiple Franck-Condon vibrational
sidebands. This generates an intrinsic line broadening approximately 0.5 eV.
The photoemission spectral function has two peaks whose central energies
disperse with band width approximately 1.2 eV. Signatures of these phenomena
are predicted to appear in angle-resolved photoemission spectra. | cond-mat |
Uncovering the Dominant Scatterer in Graphene Sheets on SiO2: We have measured the impact of atomic hydrogen adsorption on the electronic
transport properties of graphene sheets as a function of hydrogen coverage and
initial, pre-hydrogenation field-effect mobility. Our results are compatible
with hydrogen adsorbates inducing intervalley mixing by exerting a short-range
scattering potential. The saturation coverages for different devices are found
to be proportional to their initial mobility, indicating that the number of
native scatterers is proportional to the saturation coverage of hydrogen. By
extrapolating this proportionality, we show that the field-effect mobility can
reach $1.5 \times 10^4$ cm$^2$/V sec in the absence of the hydrogen-adsorbing
sites. This affinity to hydrogen is the signature of the most dominant type of
native scatterers in graphene-based field-effect transistors on SiO$_2$. | cond-mat |
Controlling dynamical entanglement in a Josephson tunneling junction: We analyze the evolution of an entangled many-body state in a Josephson
tunneling junction. A N00N state, which is a superposition of two complementary
Fock states, appears in the evolution with sufficient probability only for a
moderate many-body interaction on an intermediate time scale. This time scale
is inversely proportional to the tunneling rate. Interaction between particles
supports entanglement: The probability for creating an entangled state decays
exponentially with the number of non-interacting particles, whereas it decays
only like the inverse square root of the number of interacting particles. | cond-mat |
Exact thermodynamic limit of short-range correlation functions of the
antiferromagnetic $XXZ$-chain at finite temperatures: We evaluate numerically certain multiple integrals representing nearest and
next-nearest neighbor correlation functions of the spin-1/2 $XXZ$ Heisenberg
infinite chain at finite temperatures. | cond-mat |
Spectroscopy Study on NV Sensors in Diamond-based High-pressure Devices: Recently, the negatively charged nitrogen-vacancy (NV) center has emerged as
a robust and versatile quantum sensor in pressurized environments. There are
two popular ways to implement NV sensing in a diamond anvil cell (DAC), which
is a conventional workhorse in the high-pressure community: create implanted NV
centers (INVs) at the diamond anvil tip or immerse NV-enriched nano-diamonds
(NDs) in the pressure medium. Nonetheless, there are limited studies on
comparing the local stress environments experienced by these sensor types as
well as their performances as pressure gauges. In this work, by probing the NV
energy levels with the optically detected magnetic resonance (ODMR) method, we
experimentally reveal a dramatic difference in the partially reconstructed
stress tensors of INVs and NDs incorporated in the same DAC. Our measurement
results agree with computational simulations, concluding that INVs perceive a
more non-hydrostatic environment dominated by a uniaxial stress along the DAC
axis. This provides insights on the suitable choice of NV sensors for specific
purposes and the stress distribution in a DAC. We further propose some possible
methods, such as using NDs and nanopillars, to extend the maximum working
pressure of quantum sensing based on ODMR spectroscopy, since the maximum
working pressure could be restricted by non-hydrostaticity of the pressure
environment. Moreover, we explore more sensing applications of the NV center by
studying how pressure modifies different aspects of the NV system. We perform a
photoluminescence study using both INVs and NDs to determine the pressure
dependence of the zero-phonon line, which helps developing an all-optical
pressure sensing protocol with the NV center. We also characterize the
spin-lattice relaxation ($T_1$) time of INVs under pressure to lay a foundation
for robust pulsed measurements with NV centers in pressurized environments. | cond-mat |
Nonequilibrium hyperuniform states in active turbulence: We demonstrate that the complex spatiotemporal structure in active fluids can
feature characteristics of hyperuniformity. Using a hydrodynamic model, we show
that the transition from hyperuniformity to non-hyperuniformity and
anti-hyperuniformity depends on the strength of active forcing and can be
related to features of active turbulence without and with scaling
characteristics of inertial turbulence. Combined with identified signatures of
Levy walks and non-universal diffusion in these systems, this allows for a
biological interpretation and the speculation of non-equilibrium hyperuniform
states in active fluids as optimal states with respect to robustness and
strategies of evasion and foraging. | cond-mat |
Possible observation of phase separation near a quantum phase transition
in doubly connected ultrathin superconducting cylinders of aluminum: The kinetic energy of superconducting electrons in an ultrathin, doubly
connected superconducting cylinder, determined by the applied flux, increases
as the cylinder diameter decreases, leading to a destructive regime around
half-flux quanta and a superconductor to normal metal quantum phase transition
(QPT). Regular step-like features in resistance vs. temperature curves taken at
fixed flux values were observed near the QPT in ultrathin Al cylinders. It is
proposed that these features are most likely resulted from a phase separation
near the QPT in which normal regions nucleate in a homogeneous superconducting
cylinder. | cond-mat |
Absence of Landau damping in driven three-component Bose-Einstein
condensate in optical lattices: We explore the quantum many-body physics of a three-component Bose-Einstein
condensate (BEC) in an optical lattices driven by laser fields in $V$ and
$\Lambda$ configurations. We obtain exact analytical expressions for the energy
spectrum and amplitudes of elementary excitations, and discover symmetries
among them. We demonstrate that the applied laser fields induce a gap in the
otherwise gapless Bogoliubov spectrum. We find that Landau damping of the
collective modes above the energy of the gap is carried by laser-induced roton
modes and is considerably suppressed compared to the phonon-mediated damping
endemic to undriven scalar BECs. | cond-mat |
Interdisciplinary Discovery of Nanomaterials Based on Convolutional
Neural Networks: The material science literature contains up-to-date and comprehensive
scientific knowledge of materials. However, their content is unstructured and
diverse, resulting in a significant gap in providing sufficient information for
material design and synthesis. To this end, we used natural language processing
(NLP) and computer vision (CV) techniques based on convolutional neural
networks (CNN) to discover valuable experimental-based information about
nanomaterials and synthesis methods in energy-material-related publications.
Our first system, TextMaster, extracts opinions from texts and classifies them
into challenges and opportunities, achieving 94% and 92% accuracy,
respectively. Our second system, GraphMaster, realizes data extraction of
tables and figures from publications with 98.3\% classification accuracy and
4.3% data extraction mean square error. Our results show that these systems
could assess the suitability of materials for a certain application by
evaluation of synthesis insights and case analysis with detailed references.
This work offers a fresh perspective on mining knowledge from scientific
literature, providing a wide swatch to accelerate nanomaterial research through
CNN. | cond-mat |
Irreversible effects of memory: The steady state of a Langevin equation with short ranged memory and coloured
noise is analyzed. When the fluctuation-dissipation theorem of second kind is
not satisfied, the dynamics is irreversible, i.e. detailed balance is violated.
We show that the entropy production rate for this system should include the
power injected by ``memory forces''. With this additional contribution, the
Fluctuation Relation is fairly verified in simulations. Both dynamics with
inertia and overdamped dynamics yield the same expression for this additional
power. The role of ``memory forces'' within the fluctuation-dissipation
relation of first kind is also discussed. | cond-mat |
Kinetics of fragmentation and dissociation of two-strand protein
filaments: Coarse-grained simulations and experiments: While a significant body of investigations have been focused on the process
of protein self-assembly, much less is understood about the reverse process of
a filament breaking due to thermal motion into smaller fragments, or
depolymerization of subunits from the filament ends. Indirect evidence for
actin and amyloid filament fragmentation has been reported, although the
phenomenon has never been directly observed either experimentally or in
simulations. Here we report the direct observation of filament depolymerization
and breakup in a minimal, calibrated model of coarse-grained molecular
simulation. We quantify the orders of magnitude by which the depolymerization
rate from the filament ends $k_\mathrm{off}$ is larger than fragmentation rate
$k_{-}$ and establish the law $k_\mathrm{off}/k_- = \exp [( \varepsilon_\| -
\varepsilon_\bot) / k_\mathrm{B}T ] = \exp [0.5 \varepsilon / k_\mathrm{B}T ]$,
which accounts for the topology and energy of bonds holding the filament
together. This mechanism and the order-of-magnitude predictions are well
supported by direct experimental measurements of depolymerization of insulin
amyloid filaments. | cond-mat |
Efficient Generation of Grids and Traversal Graphs in Compositional
Spaces towards Exploration and Path Planning Exemplified in Materials: Many disciplines of science and engineering deal with problems related to
compositions, ranging from chemical compositions in materials science to
portfolio compositions in economics. They exist in non-Euclidean simplex
spaces, causing many standard tools to be incorrect or inefficient, which is
significant in combinatorically or structurally challenging spaces exemplified
by Compositionally Complex Materials (CCMs) and Functionally Graded Materials
(FGMs). Here, we explore them conceptually in terms of problem spaces and
quantitatively in terms of computational feasibility.
This work implements several essential methods specific to the compositional
(simplex) spaces through a high-performance open-source library nimplex. Most
significantly, we derive and implement an algorithm for constructing a novel
n-dimensional simplex graph data structure, which contains all discretized
compositions and all possible neighbor-to-neighbor transitions as pointer
arrays. Critically, no distance or neighborhood calculations are performed,
instead leveraging pure combinatorics and the ordering in procedurally
generated simplex grids, keeping the algorithm $\mathcal{O}(N)$, so that graphs
with billions of transitions take seconds to construct on a laptop.
Furthermore, we demonstrate how such graph representations can be combined to
express path-planning problem spaces and to incorporate prior knowledge while
keeping the problem space homogeneous. This allows for efficient deployment of
existing high-performance gradient descent, graph traversal search, and other
path optimization algorithms. | cond-mat |
Charge-stripe order in the electronic ferroelectric LuFe2O4: The structural features of the charge ordering states in LuFe2O4 are
characterized by in-situ cooling TEM observations from 300K down to 20K. Two
distinctive structural modulations, a major q1= (1/3, 1/3, 2) and a weak
q2=q1/10 + (0, 0, 3/2), have been well determined at the temperature of 20K.
Systematic analysis demonstrates that the charges at low temperatures are well
crystallized in a charge stripe phase, in which the charge density wave
behaviors in a non-sinusoidal fashion resulting in elemental electric dipoles
for ferroelectricity. It is also noted that the charge ordering and
ferroelectric domains often change markedly with lowering temperatures and
yields a rich variety of structural phenomena. | cond-mat |
Doping-dependent energy scale of the low-energy band renormalization in
(Bi,Pb)2(Sr,La)2CuO6+d: The nodal band-dispersion in (Bi,Pb)2(Sr,La)2CuO6+d (Bi2201) is investigated
over a wide range of doping by using 7-eV laser-based angle-resolved
photoemission spectroscopy. We find that the low-energy band renormalization
("kink"), recently discovered in Bi2Sr2CaCu2O8+d (Bi2212), also occurs in
Bi2201, but at a binding energy around half that in Bi2212, implying its
scaling to Tc. Surprisingly the coupling-energy dramatically increases with a
decrease of carrier concentration, showing a sharp enhancement across the
optimal doping. This strongly contrasts to other mode-couplings at higher
binding-energies (~20, ~40, and ~70 meV) with almost no doping variation in
energy scale. These nontrivial properties of the low-energy kink (material- and
doping-dependence of the coupling-energy) demonstrate the significant
correlation among the mode-coupling, the Tc, and the strong electron
correlation. | cond-mat |
Graph-based analysis of nonreciprocity in coupled-mode systems: In this work we derive the general conditions for obtaining nonreciprocity in
multi-mode parametrically-coupled systems. The results can be applied to a
broad variety of optical, microwave, and hybrid systems including recent
electro- and opto-mechanical devices. In deriving these results, we use a
graph-based methodology to derive the scattering matrix. This approach
naturally expresses the terms in the scattering coefficients as separate graphs
corresponding to distinct coupling paths between modes such that it is evident
that nonreciprocity arises as a consequence of multi-path interference and
dissipation in key ancillary modes. These concepts facilitate the construction
of new devices in which several other characteristics might also be
simultaneously optimized. As an example, we synthesize a novel three-mode
unilateral amplifier design by use of graphs. Finally, we analyze the isolation
generated in a common parametric multi-mode system, the DC-SQUID. | cond-mat |
Inherently high valley polarizations of momentum-forbidden dark excitons
in transition-metal dichalcogenide monolayers: High degree of valley polarization of optically active excitons in
transition-metal dichalcogenide monolayers (TMD-MLs) is vital in valley-based
photonic applications but known to be likely spoiled by the intrinsic
electron-hole exchange interactions. In this study, we present a theoretical
investigation of the valley and optical properties of finite-momentum dark
excitons in WSe$_2$-MLs by solving the density-functional-theory(DFT)-based
Bethe-Salpeter equation (BSE) under the guidance of symmetry analysis. %We
reveal that, in general, finite-momentum excitons are actually well immune from
the exchange-induced valley depolarization, except for those with specific
exciton momenta coincident with the $3\sigma_v$ and $3C_2'$ symmetries in the
$D_{3h}$ point group of TMD-MLs. We reveal that, unlike the bright exciton
inevitably subjected to electron-hole exchange interaction, inter-valley
finite-momentum dark excitons in WSe$_2$-MLs are well immune from the
exchange-induced valley depolarization and inherently highly valley-polarized
under the enforcement of the crystal symmetries. More importantly, the superior
valley polarizations of the inter-valley dark excitons in WSe$_2$-MLs are shown
almost fully transferable to the optical polarization in the phonon-assisted
photo-luminescences because of the native suppression of exchange-induced
depolarization in the second-order optical processes. The analysis of
phonon-assisted photo-luminescences accounts for the recently observed
brightness, high degree of optical polarization and long lifetime of the
inter-valley dark exciton states in tungsten-based TMD-MLs. | cond-mat |
Growth diagram and magnetic properties of hexagonal LuFe$_2$O$_4$ thin
films: A growth diagram of Lu-Fe-O compounds on MgO (111) substrates using pulsed
laser deposition is constructed based on extensive growth experiments.
The LuFe$_2$O$_4$ phase can only be grown in a small range of temperature and
O$_2$ pressure conditions.
An understanding of the growth mechanism of Lu-Fe-O compound films is offered
in terms of the thermochemistry at the surface.
Superparamagnetism is observed in LuFe$_2$O$_4$ film and is explained in
terms of the effect of the impurity h-LuFeO$_3$ phase and structural defects . | cond-mat |
Hyper-Raman scattering analysis of the vibrations in vitreous boron
oxide: Hyper-Raman scattering has been measured on vitreous boron oxide,
$v-$B$_2$O$_3$. This spectroscopy, complemented with Raman scattering and
infrared absorption, reveals the full set of vibrations that can be observed
with light. A mode analysis is performed based on the local D$_{3h}$ symmetry
of BO$_3$ triangles and B$_3$O$_3$ boroxol rings. The results show that in
$v-$B$_2$O$_3$ the main spectral components can be succesfully assigned using
this relatively simple model. In particular, it can be shown that the
hyper-Raman boson peak arises from external modes that correspond mainly to
librational motions of rigid boroxol rings. | cond-mat |
Phonon Dispersion Relationship and Oxygen Isotope Effect in
Superconductor LaFeAsO: In this paper we calculate ab initially the phonon dispersion relationship of
the superconductor LaFeAsO and investigate a main property in the
superconductor, the oxygen isotope effect. Based on this phonon dispersion
relationship, we find the fact that an important reason of the oxygen isotope
effect is connected with the phonon. This result agrees well with the
experimental data where the power index of the oxygen isotope effect in the
superconductor LaFeAsO is small. | cond-mat |
Real space mapping of topological invariants using artificial neural
networks: Topological invariants allow to characterize Hamiltonians, predicting the
existence of topologically protected in-gap modes. Those invariants can be
computed by tracing the evolution of the occupied wavefunctions under twisted
boundary conditions. However, those procedures do not allow to calculate a
topological invariant by evaluating the system locally, and thus require
information about the wavefunctions in the whole system. Here we show that
artificial neural networks can be trained to identify the topological order by
evaluating a local projection of the density matrix. We demonstrate this for
two different models, a 1-D topological superconductor and a 2-D quantum
anomalous Hall state, both with spatially modulated parameters. Our neural
network correctly identifies the different topological domains in real space,
predicting the location of in-gap states. By combining a neural network with a
calculation of the electronic states that uses the Kernel Polynomial Method, we
show that the local evaluation of the invariant can be carried out by
evaluating a local quantity, in particular for systems without translational
symmetry consisting of tens of thousands of atoms. Our results show that
supervised learning is an efficient methodology to characterize the local
topology of a system. | cond-mat |
Detection of the BCS transition of a trapped Fermi Gas: We investigate theoretically the properties of a trapped gas of fermionic
atoms in both the normal and the superfluid phases. Our analysis, which
accounts for the shell structure of the normal phase spectrum, identifies two
observables which are sensitive to the presence of the superfluid: the response
of the gas to a modulation of the trapping frequency, and the heat capacity.
Our results are discussed in the context of experiments on trapped Fermi gases. | cond-mat |
Quantum interferences in quasicrystals: Contributions of quantum interference effects occuring in quasicrystals are
emphasized. First conversely to metallic systems, quasiperiodic ones are shown
to enclose original alterations of their conductive properties while
downgrading long range order. Besides, origins of localization mechanisms are
outlined within the context of the original metal-insulator transition (MIT)
found in these materials. | cond-mat |
Non-Hermitian quasicrystal in dimerized lattices: Non-Hermitian quasicrystals possess PT and metal-insulator transitions
induced by gain and loss or nonreciprocal effects. In this work, we uncover the
nature of localization transitions in a generalized Aubry-Andre-Harper model
with dimerized hopping amplitudes and complex onsite potential. By
investigating the spectrum, adjacent gap ratios and inverse participation
ratios, we find an extended phase, a localized phase and a mobility edge phase,
which are originated from the interplay between hopping dimerizations and
non-Hermitian onsite potential. The lower and upper bounds of the mobility edge
are further characterized by a pair of topological winding numbers, which
undergo quantized jumps at the boundaries between different phases. Our
discoveries thus unveil the richness of topological and transport phenomena in
dimerized non-Hermitian quasicrystals. | cond-mat |
Optical and transport properties of low-dimensional semiconductor
nanostructures: The interpretation of the electronic kinetic processes in the quantum zero
dimensional nanostructures is considered. The main mechanism of the processes
is supposed to be the interaction of electrons with the optical phonons. An
emphasis is put on the recently measured effect of the long-time
photoluminescence of quantum dot samples, which is observed to occur after an
illumination of the sample by a laser pulse. In addition to this, an attention
is devoted to the possible origin of the optical effect of the blinking
(intermittence) of the optical emission of certain quantum dot samples under a
permanent optical excitation, and to another similar effect. | cond-mat |
Quantum Degenerate Fermi Gas with Spin-orbit Coupling and Crossed Zeeman
Fields: We study quantum degenerate ultra-cold Fermi gases in the presence of
artificial spin-orbit coupling and crossed Zeeman fields. We emphasize the case
where parity is violated in the excitation spectrum and compare it with the
simpler situation where parity is preserved. We investigate in detail
spectroscopic properties such as the excitation spectrum, the spectral
function, momentum distribution and density of states for the cases where
parity is preserved or violated. Similarly, we show that thermodynamic
properties such as pressure, chemical potential, entropy, specific heat,
isothermal compressibility and induced spin polarization become anisotropic as
a function of Zeeman field components, when parity is violated. Lastly, we
discuss the effects of interactions and present results for the pairing
temperature as the precursor for the transition to a superfluid state. In
particular, we find that the pairing temperature is dramatically reduced in the
weak interaction regime as parity violation gets stronger, and that the
momentum dependence of the order parameter for superfluidity violates parity
when crossed Zeeman fields are present for finite spin-orbit coupling. | cond-mat |
Orbital order in degenerate Hubbard models : A variational study: We use the Gutzwiller variational many-body theory to investigate the
stability of orbitally ordered states in a two-band Hubbard-model without spin
degrees of freedom. Our results differ significantly from earlier Hartree-Fock
calculations for this model. The Hartree-Fock phase diagram displays a large
variety of orbital orders. In contrast, in the Gutzwiller approach orbital
order only appears for densities in a narrow region around half filling. | cond-mat |
The Bose gas beyond mean field: We study a homogeneous Bose gas with purely repulsive forces. Using the Kac
scaling of the binary potential we derive analytically the form of the
thermodynamic functions of the gas for small but finite values of the scaling
parameter in the low density regime. In this way we determine dominant
corrections to the mean-field theory. It turns out that repulsive forces
increase the pressure at fixed density and decrease the density at given
chemical potential (the temperature is kept constant). They also flatten the
Bose momentum distribution. However, the present analysis cannot be extended to
the region where the mean-field theory predicts the appearence of condensate. | cond-mat |
Multiferroic Decorated Fe2O3 Monolayer Predicted from First Principles: Two-dimensional (2D) multiferroics exhibit cross-control capacity between
magnetic and electric responses in reduced spatial domain, making them well
suited for next-generation nanoscale devices; however, progress has been slow
in developing materials with required characteristic properties. Here we
identify by first-principles calculations robust 2D multiferroic behaviors in
decorated Fe2O3 monolayer, showcasing N@Fe2O3 as a prototypical case, where
ferroelectricity and ferromagnetism stem from the same origin, namely Fe
d-orbit splitting induced by the Jahn-Teller distortion and associated crystal
field changes. The resulting ferromagnetic and ferroelectric polarization can
be effectively reversed and regulated by applied electric field or strain,
offering efficient functionality. These findings establish strong materials
phenomena and elucidate underlying physics mechanism in a family of truly 2D
multiferroics that are highly promising for advanced device applications. | cond-mat |
SiQAD: A Design and Simulation Tool for Atomic Silicon Quantum Dot
Circuits: This paper introduces SiQAD, a computer-aided design tool enabling the rapid
design and simulation of atomic silicon dangling bond quantum dot patterns
capable of computational logic. Several simulation tools are included, each
able to inform the designer on various aspects of their designs: a ground-state
electron configuration finder, a non-equilibrium electron dynamics simulator,
and an electric potential landscape solver with clocking electrode support.
Simulations have been compared against past experimental results to inform the
electron population estimation and dynamic behavior. New logic gates suitable
for this platform have been designed and simulated, and a clocked wire has been
demonstrated. This work paves the way for the exploration of the vast and
fertile design space of atomic silicon dangling bond quantum dot circuits. | cond-mat |
Electronic Structure of Chromium Trihalides beyond Density Functional
Theory: We explore the electronic band structure of free standing monolayers of
chromium trihalides, CrX\textsubscript{3}{, X= Cl, Br, I}, within an advanced
\emph{ab-initio} theoretical approach based in the use of Green's function
functionals. We compare the local density approximation with the quasi-particle
self-consistent \emph{GW} approximation (QS\emph{GW}) and its self-consistent
extension (QS$G\widehat{W}$) by solving the particle-hole ladder Bethe-Salpeter
equations to improve the effective interaction \emph{W}. We show that at all
levels of theory, the valence band consistently changes shape in the sequence
Cl{\textrightarrow}Br{\textrightarrow}I, and the valence band maximum shifts
from the M point to the $\Gamma$ point. However, the details of the transition,
the one-particle bandgap, and the eigenfunctions change considerably going up
the ladder to higher levels of theory. The eigenfunctions become more
directional, and at the M point there is a strong anisotropy in the effective
mass. Also the dynamic and momentum dependent self energy shows that
QS$G\widehat{W}$ adds to the localization of the systems in comparison to the
QS\emph{GW} thereby leading to a narrower band and reduced amount of halogens
in the valence band manifold. | cond-mat |
Bayesian Optimization in Materials Science: A Survey: Bayesian optimization is used in many areas of AI for the optimization of
black-box processes and has achieved impressive improvements of the state of
the art for a lot of applications. It intelligently explores large and complex
design spaces while minimizing the number of evaluations of the expensive
underlying process to be optimized. Materials science considers the problem of
optimizing materials' properties given a large design space that defines how to
synthesize or process them, with evaluations requiring expensive experiments or
simulations -- a very similar setting. While Bayesian optimization is also a
popular approach to tackle such problems, there is almost no overlap between
the two communities that are investigating the same concepts. We present a
survey of Bayesian optimization approaches in materials science to increase
cross-fertilization and avoid duplication of work. We highlight common
challenges and opportunities for joint research efforts. | cond-mat |
Mixtures of Bose Gases Confined in a Ring Potential: The rotational properties of a mixture of two distinguishable Bose gases that
are confined in a ring potential provide novel physical effects that we
demonstrate in this study. Persistent currents are shown to be stable for a
range of the population imbalance between the two components at low angular
momentum. At higher values of the angular momentum, even small admixtures of a
second species of atoms make the persistent currents highly fragile. | cond-mat |
Phase ordering and roughening on growing films: We study the interplay between surface roughening and phase separation during
the growth of binary films. Already in 1+1 dimension, we find a variety of
different scaling behaviors depending on how the two phenomena are coupled. In
the most interesting case, related to the advection of a passive scalar in a
velocity field, nontrivial scaling exponents are obtained in simulations. | cond-mat |
A ferroelectric problem beyond the conventional scaling law: Ferroelectric (FE) size effects against the scaling law were reported
recently in ultrathin group-IV monochalcogenides, and extrinsic effects (e.g.
defects and lattice strains) were often resorted to. Via first-principles based
finite-temperature ($T$) simulations, we reveal that these abnormalities are
intrinsic to their unusual symmetry breaking from bulk to thin film. Changes of
the electronic structures result in different order parameters characterizing
the FE phase transition in bulk and in thin films, and invalidation of the
scaling law. Beyond the scaling law $T_{\text{c}}$ limit, this mechanism can
help predicting materials promising for room-$T$ ultrathin FE devices of broad
interest. | cond-mat |
Spin-Imbalance and Magnetoresistance in
Ferromagnet/Superconductor/Ferromagnet Double Tunnel Junctions: We theoretically study the spin-dependent transport in a ferromagnet/super-
conductor/ferromagnet double tunnel junction. The tunneling current in the
antiferromagnetic alignment of the magnetizations gives rise to a spin
imbalance in the superconductor. The resulting nonequilibrium spin density
strongly suppresses the superconductivity with increase of bias voltage and
destroys it at a critical voltage Vc. The results provide a new method not only
for measuring the spin polarization of ferromagnets but also for controlling
superconductivity and tunnel magnetoresistance (TMR) by applying the bias
voltage. | cond-mat |
Entropy barriers and accelerated relaxation under resetting: The zero-temperature limit of the backgammon model under resetting is
studied. The model is a balls-in-boxes model whose relaxation dynamics is
governed by the density of boxes containing just one particle. As these boxes
become rare at large times, the model presents an entropy barrier. As a
preliminary step, a related model with faster relaxation, known to be mapped to
a symmetric random walk, is studied by mapping recent results on diffusion with
resetting onto the balls-in-boxes problem. Diffusion with an absorbing target
at the origin (and diffusion constant equal to one), stochastically reset to
the unit position, is a continuum approximation to the dynamics of the
balls-in-boxes model, with resetting to a configuration maximising the number
of boxes containing just one ball. In the limit of a large system, the
relaxation time of the balls-in-boxes model under resetting is finite. The
backgammon model subject to a constant resetting rate is then studied using an
adiabatic approximation. | cond-mat |
Ettingshausen Effect around Landau Level Filling Factor nu=3 Studied by
Dynamic Nuclear Polarization: Spin current perpendicular to the electric current is investigated around
Landau level filling factor $\nu=3$ in a GaAs/AlGaAs two-dimensional electron
system. Measurements of dynamic nuclear polarization in the vicinity of the
edge of a specially designed Hall bar sample indicate that the direction of the
spin current with respect to the Hall electric field reverses its polarity at
$\nu=3$, where the dissipative current carried by holes in the spin up Landau
level is replaced with that by electrons in the spin down Landau level. | cond-mat |
Mechanics of freely-suspended ultrathin layered materials: The study of atomically thin two-dimensional materials is a young and rapidly
growing field. In the past years, a great advance in the study of the
remarkable electrical and optical properties of 2D materials fabricated by
exfoliation of bulk layered materials has been achieved. Due to the
extraordinary mechanical properties of these atomically thin materials, they
also hold a great promise for future applications such as flexible electronics.
For example, this family of materials can sustain very large deformations
without breaking. Due to the combination of small dimensions, high Young's
modulus and high crystallinity of 2D materials, they have attracted the
attention of the field of nanomechanical systems as high frequency and high
quality factor resonators. In this article, we review experiments on static and
dynamic response of 2D materials. We provide an overview and comparison of the
mechanics of different materials, and highlight the unique properties of these
thin crystalline layers. We conclude with an outlook of the mechanics of 2D
materials and future research directions such as the coupling of the mechanical
deformation to their electronic structure. | cond-mat |
Photovoltage Dynamics of the Hydroxylated Si(111) Surface Investigated
by Ultrafast Electron Diffraction: We present a novel method to measure transient photovoltage at nanointerfaces
using ultrafast electron diffraction. In particular, we report our results on
the photoinduced electronic excitations and their ensuing relaxations in a
hydroxyl-terminated silicon surface, a standard substrate for fabricating
molecular electronics interfaces. The transient surface voltage is determined
by observing Coulomb refraction changes induced by the modified space-charge
barrier within a selectively probed volume by femtosecond electron pulses. The
results are in agreement with ultrafast photoemission studies of surface state
charging, suggesting a charge relaxation mechanism closely coupled to the
carrier dynamics near the surface that can be described by a drift-diffusion
model. This study demonstrates a newly implemented ultrafast diffraction method
for investigating interfacial processes, with both charge and structure
resolution. | cond-mat |
Non-equilibrium dynamic critical scaling of the quantum Ising chain: We solve for the time-dependent finite-size scaling functions of the 1D
transverse-field Ising chain during a linear-in-time ramp of the field through
the quantum critical point. We then simulate Mott-insulating bosons in a tilted
potential, an experimentally-studied system in the same equilibrium
universality class, and demonstrate that universality holds for the dynamics as
well. We find qualitatively athermal features of the scaling functions, such as
negative spin correlations, and show that they should be robustly observable
within present cold atom experiments. | cond-mat |
Pair correlations of a spin-imbalanced Fermi gas on two-leg ladders: We study the pair correlations of a spin-imbalanced two-leg ladder with
attractive interactions, using the density matrix renormalization group method
(DMRG). We identify regions in the phase diagram spanned by the chemical
potential and the magnetic field that can harbor
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like physics. Results for the pair
structure factor, exhibiting multiple pairing wave-vectors, substantiate the
presence of FFLO-like correlations. We further discuss phase separation
scenarios induced by a harmonic trap, which differ from the case of isolated
chains. | cond-mat |
Spontaneous phase coordination and fluid pumping in model ciliary
carpets: Ciliated tissues such as in the mammalian lungs, brains, and reproductive
tracts, are specialized to pump fluid. They generate flows by the collective
activity of hundreds of thousands of individual cilia that beat in a striking
metachronal wave pattern. Despite progress in analyzing cilia coordination, a
general theory that links coordination and fluid pumping in the limit of large
arrays of cilia remains lacking. Here, we conduct in-silico experiments with
thousands of hydrodynamically-interacting cilia, and we develop a continuum
theory in the limit of infinitely-many independently beating cilia by combining
tools from active matter and classical Stokes flow. We find, in both
simulations and theory, that isotropic and synchronized ciliary states are
unstable. Traveling waves emerge regardless of initial conditions, but the
characteristics of the wave and net flows depend on cilia and tissue
properties. That is, metachronal phase coordination is a stable global
attractor in large ciliary carpets, even under finite perturbations to cilia
and tissue properties. These results support the notion that functional
specificity of ciliated tissues is interlaced with the tissue architecture and
cilia beat kinematics and open up the prospect of establishing
structure-to-function maps from cilium-level beat to tissue-level coordination
and fluid pumping. | cond-mat |
Large-scale Atomistic Simulation of Quantum Effects in SrTiO$_3$ from
First Principles: Quantum effects of lattice vibration play a major role in many physical
properties of condensed matter systems, including thermal properties such as
specific heat, structural phase transition, as well as phenomena such as
quantum crystal and quantum paraelectricity that are closely related to
zero-point fluctuations. However, realizing atomistic simulations for realistic
materials with a fully quantum-mechanical description remains a great
challenge. Here, we propose a first-principle strategy for large scale
Molecular Dynamics simulation, where high accuracy force field obtained by
Deep-Potential (DP) is combined with Quantum Thermal Bath (QTB) method to
account for quantum effects. We demonstrate the power of this DP+QTB method
using the archetypal example SrTiO$_3$, which exhibits several phenomena
induced by quantum fluctuations, such as the suppressed structure phase
transition temperature, the quantum paraelectric ground state at low
temperature and the quantum critical behavior $1/T^2$ law of dielectric
constant. Our DP+QTB strategy is efficient in simulating large scale system,
and is first principle. More importantly, quantum effects of other systems
could also be investigated as long as corresponding DP model is trained. This
strategy would greatly enrich our vision and means to study quantum behavior of
condensed matter physics. | cond-mat |
Symbiotic Optimization of the Nanolithography and RF-Plasma Etching for
Fabricating High-Quality Light-Sensitive Superconductors on the 50 nm Scale: We present results of a fabrication-process development for the lithographic
pattern transfer into the sub-100nm range by combining electron-beam
lithography and reactive dry etching to obtain high quality niobium-based
light-sensitive superconducting devices. To achieve this spatial resolution, we
systematically investigated the stability of the positive organic etching masks
ZEP 520A and PMMA 950k in different properly operated fluoride based plasma
discharges. The chemically more robust ZEP 520A was used for defining the
nanoscaled superconductors during the dry plasma etching. Our etching recipe is
appropriate for a precisely controlled removal of a number of transition
metals, their nitrides and a number of lithographic resists. Our process
yielded lightsensitive superconducting devices made from NbN with smallest
planar lateral dimensions of about 50nm with a critical temperature Tc(0) of
about 13K , which is close to the transition temperature of the unstructured
thin film. Our ultra-narrow current paths are able to permanently carry
bias-currents up to 60% of the theoretical de-pairing current-limit. | cond-mat |
Photon correlation spectroscopy on a single quantum dot embedded in a
nanowire: We have observed strong photoluminescence from a single CdSe quantum dot
embedded in a ZnSe nanowire. Exciton, biexciton and charged exciton lines have
been identified unambiguously using photon correlation spectroscopy. This
technique has provided a detailed picture of the dynamics of this new system.
This type of semi conducting quantum dot turns out to be a very efficient
single photon source in the visible. Its particular growth technique opens new
possibilities as compared to the usual self-asssembled quantum dots. | cond-mat |
Engineering Nanowire n-MOSFETs at Lg < 8 nm: As metal-oxide-semiconductor field-effect transistors (MOSFET) channel
lengths (Lg) are scaled to lengths shorter than Lg<8 nm source-drain tunneling
starts to become a major performance limiting factor. In this scenario a
heavier transport mass can be used to limit source-drain (S-D) tunneling.
Taking InAs and Si as examples, it is shown that different heavier transport
masses can be engineered using strain and crystal orientation engineering.
Full-band extended device atomistic quantum transport simulations are performed
for nanowire MOSFETs at Lg<8 nm in both ballistic and incoherent scattering
regimes. In conclusion, a heavier transport mass can indeed be advantageous in
improving ON state currents in ultra scaled nanowire MOSFETs. | cond-mat |
Machine Learning Unifies the Modelling of Materials and Molecules: Determining the stability of molecules and condensed phases is the
cornerstone of atomistic modelling, underpinning our understanding of chemical
and materials properties and transformations. Here we show that a machine
learning model, based on a local description of chemical environments and
Bayesian statistical learning, provides a unified framework to predict
atomic-scale properties. It captures the quantum mechanical effects governing
the complex surface reconstructions of silicon, predicts the stability of
different classes of molecules with chemical accuracy, and distinguishes active
and inactive protein ligands with more than 99% reliability. The universality
and the systematic nature of our framework provides new insight into the
potential energy surface of materials and molecules. | cond-mat |
Phase of phonon-induced resistance oscillations in a high-mobility
two-dimensional electron gas: We report on experimental studies of magnetoresistance oscillations that
originate from the resonant interaction of two-dimensional electrons with
thermal transverse-acoustic phonons in very high-mobility GaAs/AlGaAs quantum
wells. We find that the oscillation maxima consistently occur when a frequency
of a phonon with twice the Fermi momentum exceeds an integer multiple of the
cyclotron frequency. This observation is in contrast to to all previous
experiments associating resistance maxima with magnetophonon resonance and its
harmonics. Our experimentally obtained resonant condition is in excellent
quantitative agreement with recent theoretical proposals. | cond-mat |
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