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Chirality dependence of the radial breathing phonon mode density in
single wall carbon nanotubes: A mass and spring model is used to calculate the phonon mode dispersion for
single wall carbon nanotubes (SWNTs) of arbitrary chirality. The calculated
dispersions are used to determine the chirality dependence of the radial
breathing phonon mode (RBM) density. Van Hove singularities, usually discussed
in the context of the single particle electronic excitation spectrum, are found
in the RBM density of states with distinct qualitative differences for zig zag,
armchair and chiral SWNTs. The influence the phonon mode density has on the two
phonon resonant Raman scattering cross-section is discussed. | cond-mat_other |
Polarized emission of GaN/AlN quantum dots : single dot spectroscopy and
symmetry-based theory: We report micro-photoluminescence studies of single GaN/AlN quantum dots
grown along the (0001) crystal axis by molecular beam epitaxy on Si(111)
substrates. The emission lines exhibit a linear polarization along the growth
plane, but with varying magnitudes of the polarization degree and with
principal polarization axes that do not necessarily correspond to
crystallographic directions. Moreover, we could not observe any splitting of
polarized emission lines, at least within the spectral resolution of our setup
(1 meV). We propose a model based on the joint effects of electron-hole
exchange interaction and in-plane anisotropy of strain and/or quantum dot
shape, in order to explain the quantitative differences between our
observations and those previously reported on, e.g. CdTe- or InAs-based quantum
dots. | cond-mat_other |
Self-consistent theory for molecular instabilities in a normal
degenerate Fermi gas in the BEC-BCS crossover: We investigate within a self-consistent theory the molecular instabilities
arising in the normal state of a homogeneous degenerate Fermi gas, covering the
whole BEC-BCS crossover. These are the standard instability for molecular
formation, the BCS instability which corresponds to the formation of Cooper
pairs and the related Bose-Einstein instability. These instabilities manifest
themselves in the properties of the particle-particle vertex, which we
calculate in a ladder approximation. To find the critical temperatures
corresponding to these various instabilities, we handle the properties of the
interacting Fermi gas on the same footing as the instabilities by making use of
the same vertex. This approximate treatment is shown to be quite satisfactory
in a number of limiting situations where it agrees with known exact results.
The results for the BCS critical temperature and for the BE condensation are
found to be in fair agreement with earlier results. The threshold for formation
of molecules at rest undergoes a sizeable shift toward the BEC side, due to
quantum effects arising from the presence of the degenerate Fermi gas. This
should make its experimental observation fairly easy. This shift remains
important at least up to temperatures comparable to the Fermi energy of the
gas. | cond-mat_other |
Dynamic Many-Body Theory. II. Dynamics of Strongly Correlated Fermi
Fluids: We develop a systematic theory of multi-particle excitations in strongly
interacting Fermi systems. Our work is the generalization of the time-honored
work by Jackson, Feenberg, and Campbell for bosons, that provides, in its most
advanced implementation, quantitative predictions for the dynamic structure
function in the whole experimentally accessible energy/momentum regime. Our
view is that the same physical effects -- namely fluctuations of the wave
function at an atomic length scale -- are responsible for the correct
energetics of the excitations in both Bose and Fermi fluids. Besides a
comprehensive derivation of the fermion version of the theory and discussion of
the approximations made, we present results for homogeneous He-3 and electrons
in three dimensions. We find indeed a significant lowering of the zero sound
mode in He-3 and a broadening of the collective mode due to the coupling to
particle-hole excitations in good agreement with experiments. The most visible
effect in electronic systems is the appearance of a ``double-plasmon''
excitation. | cond-mat_other |
Work functions of self-assembled monolayers on metal surfaces: Using first-principles calculations we show that the work function of noble
metals can be decreased or increased by up to 2 eV upon the adsorption of
self-assembled monolayers of organic molecules. We identify the contributions
to these changes for several (fluorinated) thiolate molecules adsorbed on
Ag(111), Au(111) and Pt(111) surfaces. The work function of the clean metal
surfaces increases in this order, but adsorption of the monolayers reverses the
order completely. Bonds between the thiolate molecules and the metal surfaces
generate an interface dipole, whose size is a function of the metal, but it is
relatively independent of the molecules. The molecular and bond dipoles can
then be added to determine the overall work function. | cond-mat_other |
Counter-flow Induced Decoupling in Super-Fluid Turbulence: In mechanically driven superfluid turbulence the mean velocities of the
normal- and superfluid components are known to coincide: $\mathbf U_{\text{n}}
=\mathbf U_{\text{s}}$. Numerous laboratory, numerical and analytical studies
showed that under these conditions the mutual friction between the normal- and
superfluid velocity components couples also their fluctuations: $\mathbf
u'_{\text{n}}(\mathbf r,t) \approx \mathbf u'_{\text{s}}(\mathbf r,t)$ almost
at all scales. In this paper we show that this is not the case in thermally
driven superfluid turbulence; here the counterflow velocity $\mathbf
U_{\text{ns}}\equiv \mathbf U_{\text{n}} -\mathbf U_{\text{s}}\ne 0$. We
suggest a simple analytic model for the cross correlation function
$\left\langle \mathbf u'_{\text{n}}(\mathbf r,t) \cdot \mathbf
u'_{\text{s}}(\mathbf r',t)\right \rangle$ and its dependence on
$U_{\text{ns}}$. We demonstrate that $\mathbf u'_{\text{n}}(\mathbf r,t)$ and $
\mathbf u'_{\text{s}}(\mathbf r,t)$ are decoupled almost in the entire range of
separations $|\mathbf r-\mathbf r'|$ between the energy containing scale and
intervortex distance. | cond-mat_other |
On the probable wave nature of Bose crystals: At the present time, it is considered that Bose crystals are formed at the
cooling of a fluid, because the state of crystal is more favorable by energy.
It is also believed [1,2] that no ordering factor forming a crystal is present,
except for the interatomic interaction.
However, the available solutions [1,2,3] for the wave functions (WFs) of the
ground and excited states of a crystal are approximate and are obtained for
cyclic boundary conditions, which are not realized in the Nature. Here, we
present the exact solutions for the WFs of a Bose crystal with rectangular
lattice under natural zero boundary conditions.
The structure of WFs implies that 1) a crystal is formed by a standing wave
in the probability field; 2) a crystal in the ground state contains a
condensate of atoms with the wave vector \textbf{k}_l=(\pi/\bar{R}_x,
\pi/\bar{R}_y, \pi/\bar{R}_z) (\bar{R}_x, \bar{R}_y, \bar{R}_z are the periods
of the lattice) that is equal to a half of the vector of the reciprocal
lattice. These solutions indicate that the ordering factor forming a crystal is
an intense standing wave similar to a sound one. Thus, the periodicity of a
lattice is caused by that of a sound wave, but not only by the energy minimum
principle. Apparently, the crystals of other types and with different lattices
have the wave nature as well. The condensate opens a possibility to explain the
nonclassical inertia moment discovered by Kim and Chan [4,5] in solid He-4,
which testifies, probably, to the presence of a superfluid subsystem in the
crystal. | cond-mat_other |
Phase reciprocity of spin-wave excitation by a microstrip antenna: Using space-, time- and phase-resolved Brillouin light scattering
spectroscopy we investigate the difference in phase of the two
counterpropagating spin waves excited by the same microwave microstrip
transducer. These studies are performed both for backward volume magnetostatic
waves and magnetostatic surface waves in an in-plane magnetized yttrium iron
garnet film. The experiments show that for the backward volume magnetostatic
spin waves (which are reciprocal and excited symmetrically in amplitude) there
is a phase difference of $\pi$ associated with the excitation process and thus
the phase symmetry is distorted. On the contrary, for the magnetostatic surface
spin waves (which are non-reciprocal and unsymmetrical in amplitude) the phase
symmetry is preserved (there is no phase difference between the two waves
associated with the excitation). Theoretical analysis confirms this effect. | cond-mat_other |
Quantum multimode model of elastic scattering from Bose Einstein
condensates: Mean field approximation treats only coherent aspects of the evolution of a
Bose Einstein condensate. However, in many experiments some atoms scatter out
of the condensate. We study an analytic model of two counter-propagating atomic
Gaussian wavepackets incorporating dynamics of incoherent scattering processes.
Within the model we can treat processes of elastic collision of atoms into the
initially empty modes, and observe how, with growing occupation, the bosonic
enhancement is slowly kicking in. A condition for bosonic enhancement effect is
found in terms of relevant parameters. Scattered atoms form a squeezed state
that can be viewed as a multi-component condensate. Not only are we able to
calculate the dynamics of mode occupation, but also the full statistics of
scattered atoms. | cond-mat_other |
Negative refraction in nonlinear wave systems: People have been familiar with the phenomenon of wave refraction for several
centuries. Recently, a novel type of refraction, i.e., negative refraction,
where both incident and refractory lines locate on the same side of the normal
line, has been predicted and realized in the context of linear optics in the
presence of both right- and left-handed materials. In this work, we reveal, by
theoretical prediction and numerical verification, negative refraction in
nonlinear oscillatory systems. We demonstrate that unlike what happens in
linear optics, negative refraction of nonlinear waves does not depend on the
presence of the special left-handed material, but depends on suitable physical
condition. Namely, this phenomenon can be observed in wide range of oscillatory
media under the Hopf bifurcation condition. The complex Ginzburg-Landau
equation and a chemical reaction-diffusion model are used to demonstrate the
feasibility of this nonlinear negative refraction behavior in practice. | cond-mat_other |
Peculiarities in the properties of some rare-earth compounds with
orthorhombic structures: Rare-earth manganites are fascinating, because they display a wide variety of
fundamental properties from magnetism to ferroelectricity, from colossal
magnetoresistance to semi-metallicity, and because they can be used in a number
of important technological applications such as controlling a magnetic memory
by an electric field or vice versa, new types of attenuators or transducers
etc. In this paper, we present our investigation on monocrystal samples with an
orthorhombic structure, grown in two different space groups: D2h(16) for
La0.78Pb0.22MnO3 and Pr0.7Sr0.3MnO3 and D2h(9) for HoMn2O5 and TbMn2O5. The
doped perovskite manganites Ln1-x Ax MnO3 (where Ln is a rare-earth ion and A
is a divalent ion) from the group D2h(16), which crystallized in different
modifications of the perovskite structure, characterized by the parameter
deformation of the type c/√2<b<a. Many properties of these compounds
(especially the giant magnetoresistance GMR, being interesting for practical
applications) depend strongly on the carrier density, on the specific zone
structure, on the type and the quantity of dopants, on the defects of the
crystal and their magnetic structure, or on the applied magnetic fields. | cond-mat_other |
Dynamical stability of a doubly quantized vortex in a three-dimensional
condensate: The Bogoliubov equations are solved for a three-dimensional Bose-Einstein
condensate containing a doubly quantized vortex, trapped in a harmonic
potential. Complex frequencies, signifying dynamical instability, are found for
certain ranges of parameter values. The existence of alternating windows of
stability and instability, respectively, is explained qualitatively and
quantitatively using variational calculus and direct numerical solution. It is
seen that the windows of stability are much smaller for a cigar shaped
condensate than for a pancake shaped one, which is consistent with the findings
of recent experiments. | cond-mat_other |
Elastic metamaterials for independent realization of negativity in
density and stiffness: In this paper, we present the realization of an elastic metamaterial allowing
independent tuning of negative density and stiffness for elastic waves
propagating along a designated direction. In electromagnetic (or acoustic)
metamaterials, it is now possible to tune permittivity (bulk modulus) and
permeability (density) independently. Apparently, the tuning methods seem to be
directly applicable for elastic case, but no realization has yet been made due
to the unique tensorial physics of elasticity that makes wave motions coupled
in a peculiar way. To realize independent tunability, we developed a
single-phased elastic metamaterial supported by theoretical analysis and
numerical/experimental validations. | cond-mat_other |
Vortex signatures in annular Bose-Einstein condensates: We consider a Bose-Einstein condensate confined in a ``Mexican hat''
potential, with a quartic minus quadratic radial dependence. We find conditions
under which the ground state is annular in shape, with a hole in the center of
the condensate. Rotation leads to the appearance of stable multiply-quantized
vortices, giving rise to a superfluid flow around the ring. The collective
modes of the system are explored both numerically and analytically using the
Gross-Pitaevskii and hydrodynamic equations. Potential experimental schemes to
detect vorticity are proposed and evaluated, which include measuring the
splitting of collective mode frequencies, observing expansion following release
from the trap, and probing the momentum distribution of the condensate. | cond-mat_other |
A basis-set based Fortran program to solve the Gross-Pitaevskii Equation
for dilute Bose gases in harmonic and anharmonic traps: Inhomogeneous boson systems, such as the dilute gases of integral spin atoms
in low-temperature magnetic traps, are believed to be well described by the
Gross-Pitaevskii equation (GPE). GPE is a nonlinear Schroedinger equation which
describes the order parameter of such systems at the mean field level. In the
present work, we describe a Fortran 90 computer program developed by us, which
solves the GPE using a basis set expansion technique. In this technique, the
condensate wave function (order parameter) is expanded in terms of the
solutions of the simple-harmonic oscillator (SHO) characterizing the atomic
trap. Additionally, the same approach is also used to solve the problems in
which the trap is weakly anharmonic, and the anharmonic potential can be
expressed as a polynomial in the position operators x, y, and z. The resulting
eigenvalue problem is solved iteratively using either the self-consistent-field
(SCF) approach, or the imaginary time steepest-descent (SD) approach. Our
results for harmonic traps are also compared with those published by other
authors using different numerical approaches, and excellent agreement is
obtained. GPE is also solved for a few anharmonic potentials, and the influence
of anharmonicity on the condensate is discussed. Additionally, the notion of
Shannon entropy for the condensate wave function is defined and studied as a
function of the number of particles in the trap. It is demonstrated numerically
that the entropy increases with the particle number in a monotonic way. | cond-mat_other |
The effect of a Chern-Simons term on dynamical gap generation in
graphene: We study the effect of a Chern-Simons term on dynamical gap generation in a
low energy effective theory that describes some features of mono-layer
suspended graphene. We use a non-perturbative Schwinger-Dyson approach. We
solve a set of coupled integral equations for eight independent dressing
functions that describe fermion and photon degrees of freedom. We find a strong
suppression of the gap, and corresponding increase in the critical coupling, as
a function of increasing Chern-Simons coefficient. | cond-mat_other |
On the origin of Stark effect of rotons in He-II and the existence of p
= 0 condensate: Linear Stark effect of roton transition, experimentally observed through
microwave absorption in He-II (superfluid He) in the presence of varying
external electric field, is critically analysed. We find that: (i) The effect
cannot be explained in terms of conventional microscopic theory (CMT) of He-II
which presumes the existence of p = 0 condensate and concludes that He atoms
even at T = 0 have random motions and mutual collisions which do not support
the basic factor (viz. an ordered arrangement of atomic electric dipoles)
needed for its occurrence. (ii) The desired order is concluded, rather, by a
non-conventional microscopic theory (NCMT) as an intrinsic property of He-II.
Accordingly, all atoms in He-II define a closepacked arrangement of their wave
packets (CPA-WP) with identically equal nearest neighbour distance (d), per
particle zero-point energy ({\epsilon}0 = h2/8md2) and equivalent momentum,
h/2d. (iii) The CPA-WP prevent atoms from having relative motions and mutual
collisions capable of disturbing any order of atomic dipoles. As such the NCMT
and the observed Stark effect have strong mutual support; whereas the former
concludes CPA-WP necessary for the occurrence of the effect, the latter
strengthens the experimental support for the former, which means that
theobservation does not support the presence of p = 0 condensate in He-II. | cond-mat_other |
Plasmonic Resonances and Electromagnetic Forces Between Coupled Silver
Nanowires: We compute the electromagnetic response and corresponding forces between two
silver nanowires. The wires are illuminated by a plane wave which has the
electric field vector perpendicular to the axis of the wires, insuring that
plasmonic resonances can be excited. We consider a nontrivial square cross
section geometry that has dimensions on the order of $0.1 \lambda$, where
$\lambda$ is the wavelength of the incident electromagnetic field. We find that
due to the plasmonic resonance, there occurs great enhancement of the direct
and mutual electromagnetic forces that are exerted on the nanowires. The
Lippman-Schwinger volume integral equation is implemented to obtain solutions
to Maxwell's equations for various $\lambda$ and separation distances between
wires. The forces are computed using Maxwell's stress tensor and numerical
results are shown for both on and off resonant conditions. | cond-mat_other |
Thermodynamic Evidence for Water as a Quantum Mechanical Liquid: We consider general theoretical models of water and in particular the nature
of the motions of the hydrogen nuclei. If the motion of hydrogen nuclei is
classical, then the thermodynamic pressure equation of state for heavy water
wherein the hydrogen nuclei are deuterons is identical to the pressure equation
of state for light water wherein the hydrogen nuclei are protons. Since the
experimental thermodynamic phase diagram for light water is clearly measurably
different from the experimental thermodynamic phase diagram for heavy water,
one may deduce that the motions of hydrogen nuclei are quantum mechanical in
nature. This conclusion is in physical agreement with a recent analysis of
X-ray, neutron and deep inelastic neutron scattering data. | cond-mat_other |
NMR Study of Disordered Inclusions in the Quenched Solid Helium: Phase structure of rapidly quenched solid helium samples is studied by the
NMR technique. The pulse NMR method is used for measurements of spin-lattice
$T_1$ and spin-spin $T_2$ relaxation times and spin diffusion coefficient $D$
for all coexisting phases. It was found that quenched samples are two-phase
systems consisting of the hcp matrix and some inclusions which are
characterized by $D$ and $T_2$ values close to those in liquid phase. Such
liquid-like inclusions undergo a spontaneous transition to a new state with
anomalously short $T_2$ times. It is found that inclusions observed in both the
states disappear on careful annealing near the melting curve. It is assumed
that the liquid-like inclusions transform into a new state - a glass or a
crystal with a large number of dislocations. These disordered inclusions may be
responsible for the anomalous phenomena observed in supersolid region. | cond-mat_other |
Direct Energy Cascade in Two-Dimensional Compressible Quantum Turbulence: We numerically study two-dimensional quantum turbulence with a
Gross--Pitaevskii model. With the energy initially accumulated at large scale,
quantum turbulence with many quantized vortex points is generated. Due to the
lack of enstrophy conservation in this model, direct energy cascade with a
Kolmogorov--Obukhov energy spectrum $E(k) \propto k^{-5/3}$ is observed, which
is quite different from two-dimensional incompressible classical turbulence in
the decaying case. A positive value for the energy flux guarantees a
\emph{direct} energy cascade in the inertial range (from large to small
scales). After almost all the energy at the large scale cascades to the small
scale, the compressible kinetic energy realizes the thermodynamic equilibrium
state without quantized vortices. | cond-mat_other |
Effects of environmental and exciton screening in single-walled carbon
nanotubes: The ground-state exciton binding energy for single-walled carbon nanotubes
(SWCNTs) in vacuum calculated ignoring the screening of Coulomb interaction
appears to be much greater than the corresponding band gap. The most essential
contributions to the screening of electron-hole (e-h) interaction potential in
semiconducting SWCNTs, which return the ground-state exciton binding energy
into the energy gap, are considered. Our estimates on the screening effects and
exciton binding energies are in satisfactory agreement with the corresponding
experimental data for concrete nanotubes. | cond-mat_other |
Atom interferometric detection of the pairing order parameter in a Fermi
gas: We propose two interferometric schemes to experimentally detect the onset of
pair condensation in a two spin-component Fermi gas. Two atomic wave-packets
are coherently extracted from the gas at different positions and are mixed by a
matter-wave beam splitter: we show that the spatial long range order of the
atomic pairs in the gas then reflects in the atom counting statistics in the
output channels of the beam splitter. Alternatively, the same long range order
is also shown to create a matter-wave grating in the overlapping region of the
two extracted wave-packets, grating that can be revealed by a light scattering
experiment. | cond-mat_other |
Observing individual thermal motions of ions, and molecules in water
with light: We observe thermal motions of ions, and molecules in water through light
extinction, at the individual particle level. The motions appear as time
dependent intensity variations, characterized through their averaged spectra.
Theoretical spectrum derived from random motions of one particle describes
these measured spectra. This theory is used to extract diffusion constants of
liquid mixtures and solutions, that correspond to binary diffusion, and thermal
diffusion, which are consistent with previous macroscopic measurements. We also
estimate the sizes of the particles. | cond-mat_other |
Kelvin-wave turbulence generated by vortex reconnections: Reconnections of quantum vortex filaments create sharp bends which degenerate
into propagating Kelvin waves. These waves cascade their energy down-scale and
their waveaction up-scale via weakly nonlinear interactions, and this is the
main mechanism of turbulence at the scales less than the inter-vortex distance.
In case of an idealised forcing concentrated around a single scale k0, the
turbulence spectrum exponent has a pure direct cascade form -17/5 at scales
k>k0 and a pure inverse cascade form -3 at k<k0. However, forcing produced by
the reconnections contains a broad range of Fourier modes. What scaling should
one expect in this case? In this Letter I obtain an answer to this question
using the differential model for the Kelvin wave turbulence. The main result is
that the direct cascade scaling dominates, i.e. the reconnection forcing is
more or less equivalent to a low-frequency forcing. | cond-mat_other |
Influence of structure on the optical limiting properties of nanotubes: We investigate the role of carbon nanotubes structure on their optical
limiting properties. Samples of different and well-characterized structural
features are studied by optical limiting and pump-probe experiments. The
influence of the diameter's size on the nano-object is demonstrated. Indeed,
both nucleation and growth of gas bubbles are expected to be sensitive to
diameter. | cond-mat_other |
Repulsive Casimir forces produced in rectangular cavities: Possible
measurements and applications: We perform a theoretical analysis of a setup intended to measure the
repulsive (outward) Casimir forces predicted to exist inside of perfectly
conducting rectangular cavities. We consider the roles of the conductivity of
the real metals, of the temperature and surface roughness. The use of this
repulsive force to reduce friction and wear in micro and nanoelectromechanical
systems (MEMS and NEMS) is also considered. | cond-mat_other |
Simulation study of the two-dimensional Burridge-Knopoff model of
earthquakes: Spatiotemporal correlations of the two-dimensional spring-block
(Burridge-Knopoff) model of earthquakes are extensively studied by means of
numerical computer simulations. The model is found to exhibit either
``subcritical'' or ``supercritical'' behavior, depending on the values of the
model parameters. Transition between these regimes is either continuous or
discontinuous. Seismic events in the ``subcritical'' regime and those in the
``supercritical'' regime at larger magnitudes exhibit universal scaling
properties. In the ``supercritical'' regime, eminent spatiotemporal
correlations, {\it e.g.}, remarkable growth of seismic activity preceding the
mainshock, arise in earthquake occurrence, whereas such spatiotemporal
correlations are significantly suppressed in the ``subcritical'' regime.
Seismic activity is generically suppressed just before the mainshock in a close
vicinity of the epicenter of the upcoming event while it remains to be active
in the surroundings (the Mogi doughnut). It is also observed that, before and
after the mainshock, the apparent $B$-value of the magnitude distribution
decreases or increases in the ``supercritical'' or ``subcritical'' regimes,
respectively. Such distinct precursory phenomena may open a way to the
prediction of the upcoming large event. | cond-mat_other |
Material-independent crack arrest statistics: Application to indentation
experiments: An extensive experimental study of indentation and crack arrest statistics is
presented for four different brittle materials (alumina, silicon carbide,
silicon nitride, glass). Evidence is given that the crack length statistics can
be described by a universal (i.e. material independent) distribution. The
latter directly derives from results obtained when modeling crack propagation
as a depinning phenomenon. Crack arrest (or effective toughness) statistics
appears to be fully characterized by two parameters, namely, an asymptotic
crack length (or macroscopic toughness) value and a power law size dependent
width. The experimental knowledge of the crack arrest statistics at one given
scale thus gives access to its knowledge at all scales. | cond-mat_other |
Gravity through the prism of condensed matter physics: In the paper "Life, the Universe, and everything--42 fundamental questions",
Roland Allen and Suzy Lidstr\"om presented personal selection of the
fundamental questions. Here, based on the condensed matter experience, we
suggest the answers to some questions concerning the vacuum energy, black hole
entropy and the origin of gravity. In condensed matter we know both the
many-body phenomena emerging on the macroscopic level and the microscopic
(atomic) physics, which generates this emergence. It appears that the same
macroscopic phenomenon may be generated by essentially different microscopic
backgrounds. This points to various possible directions in study of the deep
quantum vacuum of our Universe. | cond-mat_other |
Extracting contact effects in organic field-effect transistors: Contact resistances between organic semiconductors and metal electrodes have
been shown to play a dominant role in electronic charge injection properties of
organic field-effect transistors. These effects are more prevalent in short
channel length devices and therefore should not be ignored when examining
intrinsic properties such as the mobility and its dependence on temperature or
gate voltage. Here we outline a general procedure to extract contact
current-voltage characteristics and the true channel mobility from the
transport characteristics in bottom contact poly(3-hexylthiophene) field-effect
transistors, for both Ohmic and nonlinear charge injection, over a broad range
of temperatures and gate voltages. Distinguishing between contact and channel
contributions in bottom contact OFETs is an important step toward improved
understanding and modeling of these devices. | cond-mat_other |
Dipolar spinor Bose-Einstein condensates: Under many circumstances, the only important two-body interaction between
atoms in ultracold dilute atomic vapors is the short-ranged isotropic s-wave
collision. Recent studies have shown, however, that situations may arise where
the dipolar interaction between atomic magnetic or electric dipole moments can
play a significant role. The long-range anisotropic nature of the dipolar
interaction greatly enriches the static and dynamic properties of ultracold
atoms. In the case of dipolar spinor condensates, the interplay between the
dipolar interaction and the spin exchange interaction may lead to nontrivial
spin textures. Here we pay particular attention to the spin vortex state that
is analogous to the magnetic vortex found in thin magnetic films. | cond-mat_other |
Crystal Structure Studies of Human Dental Apatite as a Function of Age: Studies of the average crystal structure properties of human dental apatite
as a function of the tooth-age in the range of 5-87 years are reported. The
crystallinity of the dental hydroxyapatite decreases with the tooth-age. The
a-lattice constant that is associated with the carbonate content in carbonate
apatite decreases with the tooth-age in a systematic way, whereas the c-lattice
constant does not change significantly. Thermogravimetric measurements
demonstrate an increase of the carbonate content with the tooth-age. FTIR
spectroscopy reveals both, B and A-type carbonate substitutions with the B-type
greater than the A-type substitution by a factor up to ~5. An increase of the
carbonate content as a function of the tooth-age can be deduced from the ratio
of the v2 CO3 to the v1 PO4 IR modes. | cond-mat_other |
MsSpec-DFM (Dielectric function module): Towards a multiple scattering
approach to plasmon description: We present here the MsSpec Dielectric Function module (MsSpec-DFM), which
generates dielectric functions in an electron gas or a liquid, either isolated
or embedded into an environment. In addition to standard models such as the
plasmon pole and the RPA, this module also provides more involved methods
incorporating local field corrections (in order to account for correlations),
Boltzmann-Vlasov hydrodynamical methods, the relaxation-damped Mermin and the
diffusion-damped Hu-O'Connell methods, as well as moment-based methods using
either a Nevanlinna function or a memory function. Ultimately, through the use
of form factors, the MsSpec-DFM module will be able to address a wide range of
materials such as metals, semiconductors, including inversion layers,
hetero-structures, superconductors, quantum wells, quantum wires, quantum dots,
Dirac materials such as graphene, and liquids. | cond-mat_other |
Coherent spin mixing dynamics in a spin-1 atomic condensate: We study the coherent off-equilibrium spin mixing inside an atomic
condensate. Using mean field theory and adopting the single spatial mode
approximation (SMA), the condensate spin dynamics is found to be well described
by that of a nonrigid pendulum, and displays a variety of periodic oscillations
in an external magnetic field. Our results illuminate several recent
experimental observations and provide critical insights into the observation of
coherent interaction-driven oscillations in a spin-1 condensate. | cond-mat_other |
Quantitative determination of the Hubbard model phase diagram from
optical lattice experiments by two-parameter scaling: We propose an experiment to obtain the phase diagram of the fermionic Hubbard
model, for any dimensionality, using cold atoms in optical lattices. It is
based on measuring the total energy for a sequence of trap profiles. It
combines finite-size scaling with an additional `finite-curvature scaling'
necessary to reach the homogeneous limit. We illustrate its viability in the 1D
case, simulating experimental data in the Bethe-Ansatz local density
approximation. Including experimental errors, the filling corresponding to the
Mott transition can be determined with better than 3% accuracy. | cond-mat_other |
Polariton amplification in a multiple-quantum-well resonant photonic
crystal: Based on a microscopic many-particle theory we study the amplification of
polaritons in a multiple-quantum-well resonant photonic crystal. For the
Bragg-spaced multiple quantum wells under investigation we predict that in a
typical pump-probe setup four-wave mixing processes can lead to an unstable
energy transfer from the pump into the probe and the background-free four-wave
mixing directions. We find that under certain excitation conditions this
phase-conjugate oscillation induced instability can lead to a large
amplification of the weak probe pulse. | cond-mat_other |
Laser tweezers for atomic solitons: We describe a controllable and precise laser tweezers for Bose-Einstein
condensates of ultracold atomic gases. In our configuration, a laser beam is
used to locally modify the sign of the scattering length in the vicinity of a
trapped BEC. The induced attractive interactions between atoms allow to extract
and transport a controllable number of atoms. We analyze, through numerical
simulations, the number of emitted atoms as a function of the width and
intensity of the outcoupling beam. We also study different configurations of
our system, as the use of moving beams. The main advantage of using the control
laser beam to modify the nonlinear interactions in comparison to the usual way
of inducing optical forces, i.e. through linear trapping potentials, is to
improve the controllability of the outcoupled solitary wave-packet, which opens
new possibilities for engineering macroscopic quantum states. | cond-mat_other |
Pair-Breaking in Rotating Fermi Gases: We study the pair-breaking effect of rotation on a cold Fermi gas in the
BCS-BEC crossover region. In the framework of BCS theory, which is supposed to
be qualitatively correct at zero temperature, we find that in a trap rotating
around a symmetry axis, three regions have to be distinguished: (A) a region
near the rotational axis where the superfluid stays at rest and where no pairs
are broken, (B) a region where the pairs are progressively broken with
increasing distance from the rotational axis, resulting in an increasing
rotational current, and (C) a normal-fluid region where all pairs are broken
and which rotates like a rigid body. Due to region B, density and current do
not exhibit any discontinuities. | cond-mat_other |
Spontaneous symmetry breaking of gap solitons in double-well traps: We introduce a two dimensional model for the Bose-Einstein condensate with
both attractive and repulsive nonlinearities. We assume a combination of a
double well potential in one direction, and an optical lattice along the
perpendicular coordinate. We look for dual core solitons in this model,
focusing on their symmetry-breaking bifurcations. The analysis employs a
variational approximation, which is verified by numerical results. The
bifurcation which transforms antisymmetric gap solitons into asymmetric ones is
of supercritical type in the case of repulsion; in the attraction model,
increase of the optical latttice strength leads to a gradual transition from
subcritical bifurcation (for symmetric solitons) to a supercritical one. | cond-mat_other |
STS Observations of Landau Levels at Graphite Surfaces: Scanning tunneling spectroscopy measurements were made on surfaces of two
different kinds of graphite samples, Kish graphite and highly oriented
pyrolytic graphite (HOPG), at very low temperatures and in high magnetic
fields. We observed a series of peaks in the tunnel spectra, which grow with
increasing field, both at positive and negative bias voltages. These are
associated with Landau quantization of the quasi two-dimensional electrons and
holes in graphite in magnetic fields perpendicular to the basal plane. Almost
field independent Landau levels fixed near the Fermi energy, which are
characteristic of the graphite crystalline structure, were directly observed
for the first time. Calculations of the local density of states at the graphite
surfaces allow us to identify Kish graphite as bulk graphite and HOPG as
graphite with finite thickness effectively. | cond-mat_other |
Stability of the decagonal quasicrystal in the Lennard-Jones-Gauss
system: Although quasicrystals have been studied for 25 years, there are many open
questions concerning their stability: What is the role of phason fluctuations?
Do quasicrystals transform into periodic crystals at low temperature? If yes,
by what mechanisms? We address these questions here for a simple
two-dimensional model system, a monatomic decagonal quasicrystal, which is
stabilized by the Lennard-Jones-Gauss potential in thermodynamic equilibrium.
It is known to transform to the approximant Xi, when cooled below a critical
temperature. We show that the decagonal phase is an entropically stabilized
random tiling. By determining the average particle energy for a series of
approximants, it is found that the approximant Xi is the one with lowest
potential energy. | cond-mat_other |
Computer simulation of quantum melting in hydrogen clusters: We introduce a new criterion--based on multipole dynamical correlations
calculated within Reptation Quantum Monte Carlo--to discriminate between a
melting vs. freezing behavior in quantum clusters. This criterion is applied to
small clusters of para-hydrogen molecules (both pristine and doped with a CO
cromophore), for cluster sizes around 12 molecules. This is a magic size at
which para-hydrogen clusters display an icosahedral structure and a large
stability. In spite of the similar geometric structure of CO@(pH2)_12 and
(pH2)_13, the first system has a rigid, crystalline, behavior, while the second
behaves more like a superfluid (or, possibly, a supersolid). | cond-mat_other |
Chaotic Dynamics of Spin-Valve Oscillators: Recent experimental and theoretical studies on the magnetization dynamics
driven by an electric current have uncovered a number of unprecedented rich
dynamic phenomena. We predict an intrinsic chaotic dynamics that has not been
previously anticipated. We explicitly show that the transition to chaotic
dynamics occurs through a series of period doubling bifurcations. In chaotic
regime, two dramatically different power spectra, one with a well-defined peak
and the other with a broadly distributed noise, are identified and explained. | cond-mat_other |
Standing Spin Waves in an Antiferromagnetic Molecular Cr6 Horseshoe: The antiferromagnetic molecular finite chain Cr6 was studied by inelastic
neutron scattering. The observed magnetic excitations at 2.6 and 4.3 meV
correspond, due to the open boundaries of a finite chain, to standing spin
waves. The determined energy spectrum revealed an essentially classical spin
structure. Hence, various spin-wave theories were investigated in order to
assess their potential for describing the elementary excitations of finite spin
systems. | cond-mat_other |
Study on Evolvement Complexity in an Artificial Stock Market: An artificial stock market is established based on multi-agent . Each agent
has a limit memory of the history of stock price, and will choose an action
according to his memory and trading strategy. The trading strategy of each
agent evolves ceaselessly as a result of self-teaching mechanism. Simulation
results exhibit that large events are frequent in the fluctuation of the stock
price generated by the present model when compared with a normal process, and
the price returns distribution is L\'{e}vy distribution in the central part
followed by an approximately exponential truncation. In addition, by defining a
variable to gauge the "evolvement complexity" of this system, we have found a
phase cross-over from simple-phase to complex-phase along with the increase of
the number of individuals, which may be a ubiquitous phenomenon in multifarious
real-life systems. | cond-mat_other |
On the effect of superfluid flows on the interaction of microwaves with
He II: The paper proposes a possible mechanism of interaction of microwaves with
superfluid helium that results in an experimentally observed narrow peak of
microwave absorption on the frequencies by the order of the roton frequency.
The obtained microwave photon absorption coefficient depends on the local
equilibrium distribution function which is established due to fast roton-roton
and roton-phonon interactions. With the availability of superfluid flows, the
local equilibrium distribution function depends on their velocity. The critical
velocity of the flows, at which the absorption of microwaves is replaced by
their radiation, is found. | cond-mat_other |
Dressed matter waves: We suggest to view ultracold atoms in a time-periodically shifted optical
lattice as a "dressed matter wave", analogous to a dressed atom in an
electromagnetic field. A possible effect lending support to this concept is a
transition of ultracold bosonic atoms from a superfluid to a Mott-insulating
state in response to appropriate "dressing" achieved through time-periodic
lattice modulation. In order to observe this effect in a laboratory experiment,
one has to identify conditions allowing for effectively adiabatic motion of a
many-body Floquet state. | cond-mat_other |
Linear dynamics of classical spin as Möbius transformation: Although the overwhelming majority of natural processes occurs far from the
equilibrium, general theoretical approaches to non-equilibrium phase
transitions remain scarce. Recent breakthroughs introducing description of open
dissipative systems in terms of non-Hermitian quantum mechanics allowed to
identify a class of non-equilibrium phase transitions associated with the loss
of combined parity (reflection) and time-reversal symmetries. Here we report
that time evolution of a single classical spin (e.g. monodomain ferromagnet)
governed by the Landau-Lifshitz-Gilbert-Slonczewski equation in absence of
higher-order anisotropy terms is described by a M\"{o}bius transformation in
complex stereographic coordinates. We identify the \textit{parity-time}
symmetry-breaking phase transition occurring in spin-transfer torque-driven
linear spin systems as a transition between hyperbolic and loxodromic classes
of M\"{o}bius transformations, with the critical point of the transition
corresponding to the parabolic transformation. This establishes the
understanding of non-equilibrium phase transitions as topological transitions
in configuration space. | cond-mat_other |
Adaptive Sparse Sampling for Quasiparticle Interference Imaging: Quasiparticle interference imaging (QPI) offers insight into the band
structure of quantum materials from the Fourier transform of local density of
states (LDOS) maps. Their acquisition with a scanning tunneling microscope is
traditionally tedious due to the large number of required measurements that may
take several days to complete. The recent demonstration of sparse sampling for
QPI imaging showed how the effective measurement time could be fundamentally
reduced by only sampling a small and random subset of the total LDOS. However,
the amount of required sub-sampling to faithfully recover the QPI image
remained a recurring question. Here we introduce an adaptive sparse sampling
(ASS) approach in which we gradually accumulate sparsely sampled LDOS
measurements until a desired quality level is achieved via compressive sensing
recovery. The iteratively measured random subset of the LDOS can be interleaved
with regular topographic images that are used for image registry and drift
correction. These reference topographies also allow to resume interrupted
measurements to further improve the QPI quality. Our ASS approach is a
convenient extension to quasiparticle interference imaging that should remove
further hesitation in the implementation of sparse sampling mapping schemes. | cond-mat_other |
String-Net Models with $Z_N$ Fusion Algebra: We study the Levin-Wen string-net model with a $Z_N$ type fusion algebra.
Solutions of the local constraints of this model correspond to $Z_N$ gauge
theory and double Chern-simons theories with quantum groups. For the first
time, we explicitly construct a spin-$(N-1)/2$ model with $Z_N$ gauge symmetry
on a triangular lattice as an exact dual model of the string-net model with a
$Z_N$ type fusion algebra on a honeycomb lattice. This exact duality exists
only when the spins are coupled to a $Z_N$ gauge field living on the links of
the triangular lattice. The ungauged $Z_N$ lattice spin models are a class of
quantum systems that bear symmetry-protected topological phases that may be
classified by the third cohomology group $H^3(Z_N,U(1))$ of $Z_N$. Our results
apply also to any case where the fusion algebra is identified with a finite
group algebra or a quantusm group algebra. | cond-mat_other |
GWΓ + Bethe-Salpeter equation approach for photoabsorption
spectra: Importance of self-consistent GWΓ calculations in small
atomic systems: The self-consistent GW{\Gamma} method satisfies the Ward-Takahashi identity
(i.e., the gauge invariance or the local charge continuity) for arbitrary
energy ($\omega$) and momentum ($\bf q$) transfers. Its self-consistent
first-principles treatment of the vertex $\Gamma=\Gamma_v$ or $\Gamma_W$ is
possible to first order in the bare ($v$) or dynamically-screened ($W$) Coulomb
interaction. It is developed within a linearized scheme and combined with the
Bethe-Salpeter equation (BSE) to accurately calculate photoabsorption spectra
(PAS) and photoemission (or inverse photoemission) spectra (PES)
simultaneously. The method greatly improves the PAS of Na, Na$_3$, B$_2$, and
C$_2$H$_2$ calculated using the standard one-shot $G_0W_0$ + BSE method that
results in significantly redshifted PAS by 0.8-3.1 eV, although the PES are
well reproduced already in $G_0W_0$. | cond-mat_other |
On the Nature of Self-Consistency in Density Functional Theory: A thesis providing a pedagogical introduction to the problem of achieving
self-consistency in density functional theory. Contained is an introduction to
the framework of Kohn-Sham density functional theory, leading then to the
considerations required to solve the equations of Kohn-Sham density functional
theory. Specifically, a focus is placed on where current self-consistent field
methodology is inefficient and/or fails to converge to a solution. As such,
this review spans sub-disciplines such as numerical analysis of linear and
non-linear systems, linear response theory, and general electronic structure
theory. Toward the end of the thesis, certain contemporary methods for
achieving self-consistency from literature are outlined, and a novel,
computationally efficient preconditioning strategy is proposed. This work is
implemented in the CASTEP software. | cond-mat_other |
A generalization of Wolynes factor in activated processes: Kramers-Grote-Hynes factor is the key nonequilibrium contribution to rate
constant of a reaction over and above the transition state theory rate in the
spatial limited regime. Wolynes in eighties introduced a quantum correction to
the overall rate coefficient. This is responsible for tunneling and quantum
enhancement of rate at low temperature. However, its validity is restricted to
activated tunneling region or above crossover temperature. Based on a quantum
formulation of the normal mode analysis, we suggest a generalization of Wolynes
factor and a consequent multidimensional transition state rate expression which
are valid in the deep tunneling region down to zero degree Kelvin. | cond-mat_other |
Optical chirality in gyrotropic media: symmetry approach: We discuss optical chirality in different types of gyrotropic media. Our
analysis is based on the formalism of nongeometric symmetries of Maxwell's
equations in vacuum generalized to material media with given constituent
relations. This approach enables us to derive directly conservation laws
related to the nongeometric symmetries. For isotropic chiral media, we
demonstrate that likewise free electromagnetic field, both duality and helicity
generators belong to the basis set of nongeometric symmetries that guarantees
the conservation of optical chirality. In gyrotropic crystals, which exhibit
natural optical activity, the situation is quite different from the case of
isotropic media. For light propagating along certain crystallographic
direction, there arise two distinct cases, i.~e., (1) the duality is broken but
the helicity is preserved, or (2) only the duality symmetry survives. We show
that the existence of one of these symmetries (duality or helicity) is enough
to define optical chirality. In addition, we present examples of low-symmetry
media, where optical chirality can not be defined. | cond-mat_other |
Picosecond switching of high voltage reverse-biased p+-n-n+-structures
into conductive state by pulsed lighting: An analytical theory of high voltage reverse-biased p+-n-n+-structures
picosecond switching into conducting state by pulsed lighting has been
developed and a numerical simulation of this process has been performed.
Combining the results of theory and simulation allowed us to obtain a simple
relation between the parameters of structure, light pulse, external circuit and
main characteristics of the process - the load current pulse amplitude and
duration of switching process. | cond-mat_other |
Landau-Zener tunneling with many-body quantum effects in crystals of
molecular magnets: We present a quantum interpretation of the heights in hysteresis of $Fe_{8}$
molecule at lower temperatures by treating the crystal as an Ising spin system
with the dipolar interaction between spins. Then we apply it to two limit cases
: rapid and adiabatic regions. Our theoretical analysis is in agreement with
the experimental observation in these regions, which indicates that the steps
in hysteresis loops of magnetization of Fe$_{8}$ at lower temperatures show a
pure quantum process. | cond-mat_other |
d-wave collapse and explosion of a dipolar Bose-Einstein condensate: We investigate the collapse dynamics of a dipolar condensate of 52Cr atoms
when the s-wave scattering length characterizing the contact interaction is
reduced below a critical value. A complex dynamics, involving an anisotropic,
d-wave symmetric explosion of the condensate, is observed. The atom number
decreases abruptly during the collapse. We find good agreement between our
experimental results and those of a numerical simulation of the
three-dimensional Gross-Pitaevskii equation, including contact and dipolar
interactions as well as three-body losses. The simulation indicates that the
collapse induces the formation of two vortex rings with opposite circulations. | cond-mat_other |
Self-sustained Levitation of Dust Aggregate Ensembles by Temperature
Gradient Induced Overpressures: In laboratory experiments we observe dust aggregates from 100 \mu m to 1 cm
in size composed of micrometer sized grains levitating over a hot surface.
Depending on the dust sample aggregates start to levitate at a temperature of
400 K. Levitation of dust aggregates is restricted to a pressure range between
1--40 mbar. The levitating is caused by a Knudsen compressor effect. Based on
thermal transpiration through the dust aggregates the pressure increases
between surface and aggregates. Dust aggregates are typically balanced 100 \mu
m over the surface. On a slightly concave surface individual aggregates are
trapped at the center. Ensembles of aggregates are confined in a 2D plane.
Aggregates are subject to systematic and random translational and rotational
motion. The levitated aggregates are well suited to study photophoretic or
thermophoretic forces on dust aggregates or the mutual interaction between dust
aggregates. | cond-mat_other |
Frustration of Decoherence in Open Quantum Systems: We study a model of frustration of decoherence in an open quantum system.
Contrary to other dissipative ohmic impurity models, such as the Kondo model or
the dissipative two-level system, the impurity model discussed here never
presents overdamped dynamics even for strong coupling to the environment. We
show that this unusual effect has its origins in the quantum mechanical nature
of the coupling between the quantum impurity and the environment. We study the
problem using analytic and numerical renormalization group methods and obtain
expressions for the frequency and temperature dependence of the impurity
susceptibility in different regimes. | cond-mat_other |
Dynamical T-matrix theory for high-density excitons in coupled quantum
wells: Excitons in coupled quantum wells open the possibility to reach high
densities close to equilibrium. In a recent experiment employing a lateral trap
potential, a blue shift and a broadening of the exciton emission line has been
seen (Snoke, SSC 134). The standard Hartree-Fock treatment can explain the blue
shift but fails to give a finite broadening. Starting from the (spin-dependent)
many-exciton Hamiltonian with direct and exchange potential, we present a
dynamical T-matrix calculation for the single-exciton Green's function which is
directly related to the frequency- and angle-resolved photoluminescence. The
calculated spectrum is blue shifted and broadened due to exciton-exciton
scattering. At high excitation, both the spectrum and the angular emission are
getting narrow. This is a direct manifestation for off-diagonal long range
order and a precursor of condensation. | cond-mat_other |
Algebraic theory of crystal vibrations: Singularities and zeros in
vibrations of 1D and 2D lattices: A novel method for the calculation of the energy dispersion relation (EDR)
and density of states (DOS) in one (1D) and two (2D) dimensions is introduced
and applied to linear lattices (1D) and square and hexagonal lattices (2D). The
(van Hove) singularities and (Dirac) zeros of the DOS are discussed. Results
for the 2D hexagonal lattice (graphene-like materials) are compared with
experimental data in microwave photonic crystals. | cond-mat_other |
Second layer crystalline phase of helium films on graphite: We investigate theoretically the existence at low temperature of a
commensurate (4/7) crystalline phase of a layer of either He isotope on top of
a He-4 layer adsorbed on graphite. We make use of a recently developed,
systematically improvable variational approach which allows us to treat both
isotopes on an equal footing. We confirm that no commensurate crystalline
second layer of He-4 forms, in agreement with all recent calculations.
Interestingly and more significantly, we find that even for He-3 there is no
evidence of such a phase, as the system freezes into an {\it incommensurate}
crystal at a coverage lower than that (4/7) at which a commensurate one has
been predicted, and for which experimental claims have been made. Implications
on the interpretation of recent experiments with helium on graphite are
discussed. | cond-mat_other |
Dynamics of Superflow by Mesoscopic Condensate: The shear viscosity $\eta $ of a quantum liquid in the vicinity of
$T_{\lambda}$ is examined. In liquid helium 4 above $T_{\lambda}$
($T_{\lambda}<T<3.7K$), under a strong effect of Bose statistics, the coherent
many-body wave function grows to an intermediate size between a macroscopic
level and a microscopic one. These wave functions are qualitatively different
from thermal fluctuation, and manifest themselves in the gradual decrease in
shear viscosity above $T_{\lambda}$. To formulate this phenomenon, we combine
the correlation function with fluid dynamics. Applying the Kramers-Kronig
relation to the generalized Poiseuille's formula for capillary flow, we perform
a perturbation calculation of the reciprocal $1/\eta $ with respect to the
particle interaction, and examine how the growth of coherent wave functions
gradually decreases shear viscosity. Comparing with the experimentally
determined $\eta (T)$, $\hat {\rho\cdot}_s(T)/\rho\cdot$ of such a mesoscopic
condensate is estimated to reach $10^{-5}$ just above $T_{\lambda}$. We examine
the effect of condensate size on the stability of such a superflow, and touch
upon the superflow in porous media. | cond-mat_other |
Adaptive Sparse Sampling for Quasiparticle Interference Imaging: Quasiparticle interference imaging (QPI) offers insight into the band
structure of quantum materials from the Fourier transform of local density of
states (LDOS) maps. Their acquisition with a scanning tunneling microscope is
traditionally tedious due to the large number of required measurements that may
take several days to complete. The recent demonstration of sparse sampling for
QPI imaging showed how the effective measurement time could be fundamentally
reduced by only sampling a small and random subset of the total LDOS. However,
the amount of required sub-sampling to faithfully recover the QPI image
remained a recurring question. Here we introduce an adaptive sparse sampling
(ASS) approach in which we gradually accumulate sparsely sampled LDOS
measurements until a desired quality level is achieved via compressive sensing
recovery. The iteratively measured random subset of the LDOS can be interleaved
with regular topographic images that are used for image registry and drift
correction. These reference topographies also allow to resume interrupted
measurements to further improve the QPI quality. Our ASS approach is a
convenient extension to quasiparticle interference imaging that should remove
further hesitation in the implementation of sparse sampling mapping schemes. | cond-mat_other |
Graphene may help to solve the Casimir conundrum in indium tin oxide
systems: We reconsider the long-explored problem that the magnitude of the measured
Casimir force between an Au sphere and an indium tin oxide (ITO) film decreases
significantly with no respective changes in the ITO dielectric permittivity
required by the Lifshitz theory. Two plausible resolutions of this conundrum
are discussed: the phase transition of an ITO film from metallic to dielectric
state and the modification of a film surface under the action of UV light. To
exclude the latter option, we propose an improvement in the experimental scheme
by adding a graphene sheet on top of an ITO film. The formalism is developed
allowing precise calculation of the Casimir force between an Au sphere and a
graphene sheet on top of ITO film deposited on a quartz substrate. In doing so
Au, ITO, and quartz are described by the frequency-dependent dielectric
permittivities and real graphene sheet with nonzero mass-gap parameter and
chemical potential by the polarization tensor at nonzero temperature. Numerical
computations performed both before and after the phase transition resulting
from the UV treatment show that the presence of graphene leads to only a minor
decrease in the drop of the Casimir force which remains quite measurable. At
the same time, in the presence of graphene the guess that an observed drop
originates from the modification of an ITO surface by the UV light breaks down.
Similar results are obtained for the configuration of two parallel plates
consisting of a graphene sheet, an ITO film and a quartz substrate. The
proposed experiments involving additional graphene sheets may help in
resolution of the problems arising in application of the Lifshitz theory to
real materials. | cond-mat_other |
Singlet-Triplet Relaxation in Two-electron Silicon Quantum Dots: We investigate the singlet-triplet relaxation process of a two electron
silicon quantum dot. In the absence of a perpendicular magnetic field, we find
that spin-orbit coupling is not the main source of singlet-triplet relaxation.
Relaxation in this regime occurs mainly via virtual states and is due to
nuclear hyperfine coupling. In the presence of an external magnetic field
perpendicular to the plane of the dot, the spin-orbit coupling is important and
virtual states are not required. We find that there can be strong anisotropy
for different field directions: parallel magnetic field can increase
substantially the relaxation time due to Zeeman splitting, but when the
magnetic field is applied perpendicular to the plane, the enhancement of the
spin-orbit effect shortens the relaxation time. We find the relaxation to be
orders of magnitude longer than for GaAs quantum dots, due to weaker hyperfine
and spin-orbit effects. | cond-mat_other |
Effective Range Corrections to Three-Body Recombination for Atoms with
Large Scattering Length: Few-body systems with large scattering length a have universal properties
that do not depend on the details of their interactions at short distances. The
rate constant for three-body recombination of bosonic atoms of mass m into a
shallow dimer scales as \hbar a^4/m times a log-periodic function of the
scattering length. We calculate the leading and subleading corrections to the
rate constant which are due to the effective range of the atoms and study the
correlation between the rate constant and the atom-dimer scattering length. Our
results are applied to 4He atoms as a test case. | cond-mat_other |
Long-range donor-acceptor electron transport mediated by alpha-helices: We study the long-range electron and energy transfer mediated by a polaron on
an $\alpha$-helix polypeptide chain coupled to donor and acceptor molecules at
opposite ends of the chain. We show that for specific parameters of the system,
an electron initially located on the donor can tunnel onto the $\alpha$-helix,
forming a polaron which then travels to the other extremity of the polypeptide
chain where it is captured by the acceptor. We consider three families of
couplings between the donor, acceptor and the chain, and show that one of them
can lead to a 90\% efficiency of the electron transport from donor to acceptor.
We also show that this process remains stable at physiological temperatures in
the presence of thermal fluctuations in the system. | cond-mat_other |
Mass flow through solid 4He induced by the fountain effect: Using an apparatus that allows superfluid liquid 4He to be in contact with
hcp solid \4he at pressures greater than the bulk melting pressure of the
solid, we have performed experiments that show evidence for 4He mass flux
through the solid and the likely presence of superfluid inside the solid. We
present results that show that a thermomechanical equilibrium in quantitative
agreement with the fountain effect exists between two liquid reservoirs
connected to each other through two superfluid-filled Vycor rods in series with
a chamber filled with solid 4He. We use the thermomechanical effect to induce
flow through the solid and measure the flow rate. On cooling, mass flux appears
near T = 600 mK and rises smoothly as the temperature is lowered. Near T = 75
mK a sharp drop in the flux is present. The flux increases as the temperature
is reduced below 75 mK. We comment on possible causes of this flux minimum. | cond-mat_other |
Network reachability of real-world contact sequences: We use real-world contact sequences, time-ordered lists of contacts from one
person to another, to study how fast information or disease can spread across
network of contacts. Specifically we measure the reachability time -- the
average shortest time for a series of contacts to spread information between a
reachable pair of vertices (a pair where a chain of contacts exists leading
from one person to the other) -- and the reachability ratio -- the fraction of
reachable vertex pairs. These measures are studied using conditional uniform
graph tests. We conclude, among other things, that the network reachability
depends much on a core where the path lengths are short and communication
frequent, that clustering of the contacts of an edge in time tend to decrease
the reachability, and that the order of the contacts really do make sense for
dynamical spreading processes. | cond-mat_other |
Investigation of the Casimir interaction between two magnetic metals in
comparison with nonmagnetic test bodies: We present the complete results for the dynamic experiment on measuring the
gradient of the Casimir force between magnetic (Ni-coated) surfaces of a plate
and a sphere. Special attention is paid to the description of some details of
the setup, its calibration, error analysis and background effects. Computations
are performed in the framework of the Lifshitz theory at nonzero temperature
with account of analytic corrections to the proximity force approximation and
of surface roughness using both the Drude and the plasma model approaches. The
theory of magnetic interaction between a sphere and a plate due to domain
structure of their surfaces is developed for both out-of-plane and in-plane
magnetizations in the absence and in the presence of spontaneous magnetization.
It is shown that in all cases the magnetic contribution to the measured force
gradients is much smaller than the total experimental error. The comparison
between experiment and theory is done using the rigorous statistical method. It
is shown that the theoretical approach taking into account dissipation of free
electrons is excluded by the data at a 95% confidence level. The approach
neglecting dissipation is confirmed by the data at more than 90% confidence
level. We prove that the results of experiments with Ni-Ni, Ni-Au and Au-Au
surfaces taken together cannot be reconciled with the approach including free
electrons dissipation by the introduction of any unaccounted background force,
either attractive or repulsive. | cond-mat_other |
Formation of stationary electronic states in finite homogeneous
molecular chains: Evolution of an arbitrary initial distribution of a quantummechanical
particle in a uniform molecular chain is simulated by a system of coupled
quantumclassical dynamical equations with dissipation. Stability of a uniform
distribution of the particle over the chain is studied. An asymptotical
expression is obtained for the time in which a localized state is formed. The
validity of the expression is checked by direct computational experiments. It
is shown that the time of soliton and multisoliton type states formation
depends strongly on the initial phase of the particle's wave function. It is
shown that in multisoliton states objects with a fractional electron charge
which can be observed experimentally are realized. The results obtained are
applied to synthetic uniform polynucleotide molecular chains. | cond-mat_other |
Gravity through the prism of condensed matter physics: In the paper "Life, the Universe, and everything--42 fundamental questions",
Roland Allen and Suzy Lidstr\"om presented personal selection of the
fundamental questions. Here, based on the condensed matter experience, we
suggest the answers to some questions concerning the vacuum energy, black hole
entropy and the origin of gravity. In condensed matter we know both the
many-body phenomena emerging on the macroscopic level and the microscopic
(atomic) physics, which generates this emergence. It appears that the same
macroscopic phenomenon may be generated by essentially different microscopic
backgrounds. This points to various possible directions in study of the deep
quantum vacuum of our Universe. | cond-mat_other |
Electronic states in a magnetic quantum-dot molecule: phase transitions
and spontaneous symmetry breaking: We show that a double quantum-dot system made of diluted magnetic
semiconductor behaves unlike usual molecules. In a semiconductor double quantum
dot or in a diatomic molecule, the ground state of a single carrier is
described by a symmetric orbital. In a magnetic material molecule, new ground
states with broken symmetry can appear due the competition between the
tunnelling and magnetic polaron energy. With decreasing temperature, the ground
state changes from the normal symmetric state to a state with spontaneously
broken symmetry. Interestingly, the symmetry of a magnetic molecule is
recovered at very low temperatures. A magnetic double quantum dot with
broken-symmetry phases can be used a voltage-controlled nanoscale memory cell. | cond-mat_other |
Attophysics of Thermal Phenomena in Carbon Nanotubes: In this paper heat transport in carbon nanotubes is investigated. When the
dimension of the structure is of the order of the de Broglie wave length
transport phenomena must be analysed by quantum mechanics. In this paper we
derived the Dirac type thermal equation .The solution of the equation for the
temperature fields for electrons can either be damped or can oscillate
depending on the dynamics of the scattering. Key words: Carbon nanotubes,
ultrashort laser pulses, Dirac thermal equation, temperature fields. | cond-mat_other |
Minimal size of a barchan dune: Barchans are dunes of high mobility which have a crescent shape and propagate
under conditions of unidirectional wind. However, sand dunes only appear above
a critical size, which scales with the saturation distance of the sand flux [P.
Hersen, S. Douady, and B. Andreotti, Phys. Rev. Lett. {\bf{89,}} 264301 (2002);
B. Andreotti, P. Claudin, and S. Douady, Eur. Phys. J. B {\bf{28,}} 321 (2002);
G. Sauermann, K. Kroy, and H. J. Herrmann, Phys. Rev. E {\bf{64,}} 31305
(2001)]. It has been suggested by P. Hersen, S. Douady, and B. Andreotti, Phys.
Rev. Lett. {\bf{89,}} 264301 (2002) that this flux fetch distance is itself
constant. Indeed, this could not explain the proto size of barchan dunes, which
often occur in coastal areas of high litoral drift, and the scale of dunes on
Mars. In the present work, we show from three dimensional calculations of sand
transport that the size and the shape of the minimal barchan dune depend on the
wind friction speed and the sand flux on the area between dunes in a field. Our
results explain the common appearance of barchans a few tens of centimeter high
which are observed along coasts. Furthermore, we find that the rate at which
grains enter saltation on Mars is one order of magnitude higher than on Earth,
and is relevant to correctly obtain the minimal dune size on Mars. | cond-mat_other |
Experimental evidence of solitary wave interaction in Hertzian chains: We study experimentally the interaction between two solitary waves that
approach one to another in a linear chain of spheres interacting via the Hertz
potential. When these counter propagating waves collide, they cross each other
and a phase shift respect to the noninteracting waves is introduced, as a
result of the nonlinear interaction potential. This observation is well
reproduced by our numerical simulations and it is shown to be independent of
viscoelastic dissipation at the beads contact. In addition, when the collision
of equal amplitude and synchronized counter propagating waves takes place, we
observe that two secondary solitary waves emerge from the interacting region.
The amplitude of secondary solitary waves is proportional to the amplitude of
incident waves. However, secondary solitary waves are stronger when the
collision occurs at the middle contact in chains with even number of beads.
Although numerical simulations correctly predict the existence of these waves,
experiments show that their respective amplitude are significantly larger than
predicted. We attribute this discrepancy to the rolling friction at the beads
contacts during solitary wave propagation. | cond-mat_other |
Transient heat transfer of superfluid $^4$He in nonhomogeneous
geometries -- Part I: Second sound, rarefaction, and thermal layer: Transient heat transfer in superfluid $^4$He (He II) is a complex process
that involves the interplay of the unique counterflow heat-transfer mode, the
emission of second-sound waves, and the creation of quantized vortices. Many
past researches focused on homogeneous heat transfer of He II in a uniform
channel driven by a planar heater. In this paper, we report our systematic
study of He II transient heat transfer in nonhomogeneous geometries that are
pertinent to emergent applications. By solving the He II two-fluid equation of
motion coupled with the Vinen's equation for vortex-density evolution, we
examine and compare the characteristics of transient heat transfer from planar,
cylindrical, and spherical heaters in He II. Our results show that as the
heater turns on, an outgoing second-sound pulse emerges, in which the vortex
density grows rapidly. These vortices attenuate the second sound and result in
a heated He II layer in front of the heater, i.e., the thermal layer. In the
planar case where the vortices are created throughout the space, the
second-sound pulse is continuously attenuated, leading to a strong thermal
layer that diffusively spreads following the heat pulse. On the contrary, in
the cylindrical and the spherical heater cases, vortices are created mainly in
a thin thermal layer near the heater surface. As the heat pulse ends, a
rarefaction tail develops following the second-sound pulse, in which the
temperature drops. This rarefaction tail can promptly suppress the thermal
layer and take away all the thermal energy deposited in it. The effects of the
heater size, heat flux, pulse duration, and temperature on the thermal-layer
dynamics are discussed. We also show how the peak heat flux for the onset of
boiling in He II can be studied in our model. | cond-mat_other |
Dynamics of the Bose-Hubbard model: transition from Mott insulator to
superfluid: We study the dynamics of phase transitions in the one dimensional
Bose-Hubbard model. To drive the system from Mott insulator to superfluid
phase, we change the tunneling frequency at a finite rate. We investigate the
build up of correlations during fast and slow transitions using variational
wave functions, dynamical Bogoliubov theory, Kibble-Zurek mechanism, and
numerical simulations. We show that time-dependent correlations satisfy
characteristic scaling relations that can be measured in optical lattices
filled with cold atoms. | cond-mat_other |
Non-local pair correlations in the 1D Bose gas at finite temperature: The behavior of the spatial two-particle correlation function is surveyed in
detail for a uniform 1D Bose gas with repulsive contact interactions at finite
temperatures. Both long-, medium-, and short-range effects are investigated.
The results span the entire range of physical regimes, from ideal gas, to
strongly interacting, and from zero temperature to high temperature. We present
perturbative analytic methods, available at strong and weak coupling, and
first-principle numerical results using imaginary time simulations with the
gauge-P representation in regimes where perturbative methods are invalid.
Nontrivial effects are observed from the interplay of thermally induced
bunching behavior versus interaction induced antibunching. | cond-mat_other |
Mean-field algebraic approach to the dynamics of fermions in a 1D
optical lattice: We consider a one-dimensional optical lattice of three-dimensional Harmonic
Oscillators which are loaded with neutral fermionic atoms trapped into two
hyperfine states. By means of a standard variational coherent-state procedure,
we derive an effective Hamiltonian for this quantum model and the hamiltonian
equations describing its evolution. To this end, we identify the algebra
$\mathcal L$ of two-fermion operators --describing the relevant microscopic
quantum processes of our model-- whereby the natural choice for the trial state
appears to be a so(2r) coherent state. The coherent-state parameters, playing
the role of dynamical variables for the effective Hamiltonian, are shown to
identify with the $\mathcal L$-operator expectation values thus providing a
clear physical interpretation of this algebraic mean-field picture. | cond-mat_other |
The unitary three-body problem in a trap: We consider either 3 spinless bosons or 3 equal mass spin-1/2 fermions,
interacting via a short range potential of infinite scattering length and
trapped in an isotropic harmonic potential. For a zero-range model, we obtain
analytically the exact spectrum and eigenfunctions: for fermions all the states
are universal; for bosons there is a coexistence of decoupled universal and
efimovian states. All the universal states, even the bosonic ones, have a tiny
3-body loss rate. For a finite range model, we numerically find for bosons a
coupling between zero angular momentum universal and efimovian states; the
coupling is so weak that, for realistic values of the interaction range, these
bosonic universal states remain long-lived and observable. | cond-mat_other |
Nonanalyticity of the optimized effective potential with finite basis
sets: We show that the finite-basis optimized effective potential (OEP) equations
exhibit previously unknown singular behavior.Imposing continuity, we derive new
well-behaved finite-basis-set OEP equations that determine OEP for any orbital
and any large enough potential basis sets and which adopt an analytic solution
via matrix-inversion. | cond-mat_other |
Numerical Modeling of Coexistence, Competition and Collapse of Rotating
Spiral Waves in Three-Level Excitable Media with Discrete Active Centers and
Absorbing Boundaries: Spatio-temporal dynamics of excitable media with discrete three-level active
centers (ACs) and absorbing boundaries is studied numerically by means of a
deterministic three-level model (see S. D. Makovetskiy and D. N. Makovetskii,
on-line preprint cond-mat/0410460 ), which is a generalization of Zykov-
Mikhailov model (see Sov. Phys. -- Doklady, 1986, Vol.31, No.1, P.51) for the
case of two-channel diffusion of excitations. In particular, we revealed some
qualitatively new features of coexistence, competition and collapse of rotating
spiral waves (RSWs) in three-level excitable media under conditions of strong
influence of the second channel of diffusion. Part of these features are caused
by unusual mechanism of RSWs evolution when RSW's cores get into the surface
layer of an active medium (i.~e. the layer of ACs resided at the absorbing
boundary). Instead of well known scenario of RSW collapse, which takes place
after collision of RSW's core with absorbing boundary, we observed complicated
transformations of the core leading to nonlinear ''reflection'' of the RSW from
the boundary or even to birth of several new RSWs in the surface layer. To our
knowledge, such nonlinear ''reflections'' of RSWs and resulting die hard
vorticity in excitable media with absorbing boundaries were unknown earlier.
ACM classes: F.1.1, I.6, J.2; PACS numbers: 05.65.+b, 07.05.Tp, 82.20.Wt | cond-mat_other |
Second layer crystalline phase of helium films on graphite: We investigate theoretically the existence at low temperature of a
commensurate (4/7) crystalline phase of a layer of either He isotope on top of
a He-4 layer adsorbed on graphite. We make use of a recently developed,
systematically improvable variational approach which allows us to treat both
isotopes on an equal footing. We confirm that no commensurate crystalline
second layer of He-4 forms, in agreement with all recent calculations.
Interestingly and more significantly, we find that even for He-3 there is no
evidence of such a phase, as the system freezes into an {\it incommensurate}
crystal at a coverage lower than that (4/7) at which a commensurate one has
been predicted, and for which experimental claims have been made. Implications
on the interpretation of recent experiments with helium on graphite are
discussed. | cond-mat_other |
k^-3 superfluid spectrum of highly curved interacting quantum vortices: Presented is a prediction, based on the Frenet-Serret differential geometry
of space curves, that the wave number dependence of the average kinetic energy
per unit length of two mutually interacting highly curved quantum vortex scales
as k^-3. The interacting quantum vortices are helical in shape, supporting
circularly polarized counter-propagating waves, with arbitrary curvature and
torsion. This power-law spectrum agrees with the high-k spectrum found in
precise quantum simulations of turbulent superfluidity with tangle of highly
curved and excited quantum vortices. | cond-mat_other |
Asymmetry of Endofullerenes with Silver Atoms: A series of endofullerenes Ag@C60 with different symmetry are calculated at
ab initio level. The lowest energy structure is completely asymmetrical one
(C1), in which the endo-atom has noticeably off-centre position. The
symmetrical structures are less stable. Silver atom in the Ag@C60 (C1)
endofullerene has the low negative charge and high spin density. | cond-mat_other |
Relaxation of a Goldstino-like mode due to supersymmetry breaking in
Bose-Fermi mixtures: In the presence of nonrelativistic supersymmetry, a sharp fermionic
collective mode similar to the Goldstino mode in high-energy physics was
proposed to be realized in Bose-Fermi mixtures. The Goldstino mode is relaxed
(a.k.a. decays) if supersymmetry is explicitly broken, which can be revealed as
the broadening of the corresponding spectral function. We find that the
situation shares many similarities with the electron spin resonance in magnetic
systems and adopt the well-known Kubo-Tomita theory to perform a general
analysis of the spectral function lineshape broadening of the Goldstino mode. | cond-mat_other |
Spin dynamics in high-mobility two-dimensional electron systems: Understanding the spin dynamics in semiconductor heterostructures is highly
important for future semiconductor spintronic devices. In high-mobility
two-dimensional electron systems (2DES), the spin lifetime strongly depends on
the initial degree of spin polarization due to the electron-electron
interaction. The Hartree-Fock (HF) term of the Coulomb interaction acts like an
effective out-of-plane magnetic field and thus reduces the spin-flip rate. By
time-resolved Faraday rotation (TRFR) techniques, we demonstrate that the spin
lifetime is increased by an order of magnitude as the initial spin polarization
degree is raised from the low-polarization limit to several percent. We perform
control experiments to decouple the excitation density in the sample from the
spin polarization degree and investigate the interplay of the internal HF field
and an external perpendicular magnetic field. The lifetime of spins oriented in
the plane of a [001]-grown 2DES is strongly anisotropic if the Rashba and
Dresselhaus spin-orbit fields are of the same order of magnitude. This
anisotropy, which stems from the interference of the Rashba and the Dresselhaus
spin-orbit fields, is highly density-dependent: as the electron density is
increased, the kubic Dresselhaus term becomes dominant and reduces the
anisotropy. | cond-mat_other |
Guided Quasicontinuous Atom Laser: We report the first realization of a guided quasicontinuous atom laser by rf
outcoupling a Bose-Einstein condensate from a hybrid optomagnetic trap into a
horizontal atomic waveguide. This configuration allows us to cancel the
acceleration due to gravity and keep the de Broglie wavelength constant at 0.5
$\mu$m during 0.1 s of propagation. We also show that our configuration,
equivalent to pigtailing an optical fiber to a (photon) semiconductor laser,
ensures an intrinsically good transverse mode matching. | cond-mat_other |
Topology of chiral superfluid: skyrmions, Weyl fermions and chiral
anomaly: Chiral anomaly observed in the chiral superfluid $^3$He-A is the result of
the combined effect of the real space and momentum space topologies. This
effect incorporates several topological charges in the extended $({\bf k},{\bf
r})$-space, which is beyond the conventional chiral anomaly in the relativistic
systems. | cond-mat_other |
Supersolid phases of light in extended Jaynes-Cummings-Hubbard systems: Jaynes-Cummings-Hubbard lattices provide unique properties for the study of
correlated phases as they exhibit convenient state preparation and measurement,
as well as "in situ" tuning of parameters. We show how to realize charge
density and supersolid phases in Jaynes-Cummings-Hubbard lattices in the
presence of long-range interactions. The long-range interactions are realized
by the consideration of Rydberg states in coupled atom-cavity systems and the
introduction of additional capacitive couplings in quantum-electrodynamics
circuits. We demonstrate the emergence of supersolid and checkerboard solid
phases, for calculations which take into account nearest neighbour couplings,
through a mean-field decoupling. | cond-mat_other |
Dynamic structure factor of liquid 4He across the normal-superfluid
transition: We have carried out a microscopic study of the dynamic structure factor of
liquid $^4$He across the normal-superfluid transition temperature using the
path integral Monte Carlo method. The ill-posed problem of the inverse Laplace
transform, from the imaginary-time intermediate scattering function to the
dynamic response, is tackled by stochastic optimization. Our results show a
quasi-particle peak and a small and broad multiphonon contribution. In spite of
the lack of strength in the collective peaks, we clearly identify the rapid
dropping of the roton peak amplitude when crossing the transition temperature
$T_\lambda$. Other properties such as the static structure factor, static
response, and one-phonon contribution to the response are also calculated at
different temperatures. The changes of the phonon-roton spectrum with the
temperature are also studied. An overall agreement with available experimental
data is achieved. | cond-mat_other |
Coherent tunneling by adiabatic passage in an optical waveguide system: We report on the first experimental demonstration of light transfer in an
engineered triple-well optical waveguide structure which provides a classic
analogue of Coherent Tunnelling by Adiabatic Passage (CTAP) recently proposed
for coherent transport in space of neutral atoms or electrons among
tunneling-coupled optical traps or quantum wells [A.D. Greentree et al., Phys.
Rev. B 70, 235317 (2004); K. Eckert et al., Phys. Rev. A 70, 023606 (2004)].
The direct visualization of CTAP wavepacket dynamics enabled by our simple
optical system clearly shows that in the counterintuitive passage scheme light
waves tunnel between the two outer wells without appreciable excitation of the
middle well. | cond-mat_other |
Zitterbewegung of moiré excitons in twisted MoS$_2$/WSe$_2$
hetero-bilayers: The moir\'e pattern observed in stacked non-commensurate crystal lattices,
such as hetero-bilayers of transition metal dichalcogenides, produces a
periodic modulation of their bandgap. Excitons subjected to this potential
landscape exhibit a band structure that gives rise to a quasi-particle dubbed
moir\'e exciton. In the case of MoS$_2$/WSe$_2$ hetero-bilayers, the moir\'e
trapping potential has honeycomb symmetry and, consequently, the moir\'e
exciton band structure is the same as that of a Dirac-Weyl fermion, whose mass
can be further tuned down to zero with a perpendicularly applied field. Here we
show that, analogously to other Dirac-like particles, moir\'e exciton exhibits
a trembling motion, also known as zitterbewegung, whose long timescales are
compatible with current experimental techniques for exciton dynamics. This
promotes the study of the dynamics of moir\'e excitons in van der Waals
heterostructures as an advantageous solid-state platform to probe
zitterbewegung, broadly tunable by gating and inter-layer twist angle. | cond-mat_other |
Photon correlations in a two-site non-linear cavity system under
coherent drive and dissipation: We calculate the normalized second-order correlation function for a system of
two tunnel-coupled photonic resonators, each one exhibiting a single-photon
nonlinearity of the Kerr type. We employ a full quantum formulation: the master
equation for the model, which takes into account both a coherent continuous
drive and radiative as well as non-radiative dissipation channels, is solved
analytically in steady state through a perturbative approach, and the results
are compared to exact numerical simulations. The degree of second-order
coherence displays values between 0 and 1, and divides the diagram identified
by the two energy scales of the system - the tunneling and the nonlinear Kerr
interaction - into two distinct regions separated by a crossover. When the
tunneling term dominates over the nonlinear one, the system state is
delocalized over both cavities and the emitted light is coherent. In the
opposite limit, photon blockade sets in and the system shows an insulator-like
state with photons locked on each cavity, identified by antibunching of emitted
light. | cond-mat_other |
Spin dynamics triggered by sub-terahertz magnetic field pulses: Current pulses of up to 20 A and as short as 3 ps are generated by a low
temperature grown GaAs (lt-GaAs) photoconductive switch and guided through a
coplanar waveguide, resulting in a 0.6 Tesla terahertz (THz) magnetic field
pulse. The pulse length is directly calibrated using photocurrent
autocorrelation. Magnetic excitations in Fe microstructures are studied by
time-resolved Kerr spectroscopy and compared with micromagnetic simulations. A
response within less than 10 ps to the THz electromagnetic field pulse is
found. | cond-mat_other |
Experimental observation of inter-orbital coupling: Inter-orbital coupling refers to the possibility of exciting orbital states
by otherwise orthogonal non-interacting modes, a forbidden process in photonic
lattices due to an intrinsic propagation constant detuning. In this work, using
a femtosecond laser writing technique, we experimentally demonstrate that
fundamental and excited orbital states can couple each other when located at
different spatial positions. We perform a full characterization of an
asymmetric double-well like potential and implement a scan method to
effectively map the dynamics along the propagation coordinate. Our fundamental
observation constitutes also a direct solution for a spatial mode converter
device, which could be located in any position inside a photonic glass chip. By
taking advantage of the phase structure of higher-order photonic modes and the
effective negative coupling generated, we propose a trimer configuration as a
phase beam splitter ($\pi$-BS), which could be of great relevance for
multiplexing and interference-based photonic concatenated operations. | cond-mat_other |
Cascade of vortex loops initiated by a single reconnection of quantum
vortices: We demonstrate that a single reconnection of two quantum vortices can lead to
creation of a cascade of vortex rings. Our analysis, motivated by the
analytical solution in LIA, involves high-resolution Biot-Savart and
Gross-Pitaevskii simulations. The latter showed that the rings cascade starts
on the atomic scale, with rings diameters orders of magnitude smaller than the
characteristic line spacing in the tangle. So created vortex rings may
penetrate the tangle and annihilate on the boundaries. This provides an
efficient mechanism of the vortex tangle decay in very low temperatures. | cond-mat_other |
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