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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 |
Study of superfluid $^3$He under nanoscale confinement. A new approach
to the investigation of superfluid $^3$He films: We review recent experiments in which superfluid $^3$He has been studied
under highly controlled confinement in nanofluidic sample chambers. We discuss
the experimental challenges and their resolution. These methods open the way to
a systematic investigation of the superfluidity of $^3$He films, and the
surface and edge excitations of topological superfluids. | cond-mat_other |
Creating stable molecular condensate using a generalized Raman adiabatic
passage scheme: We study the Feshbach resonance assisted stimulated adiabatic passage of an
effective coupling field for creating stable molecules from atomic Bose
condensate. By exploring the properties of the coherent population trapping
state, we show that, contrary to the previous belief, mean-field shifts need
not to limit the conversion efficiency as long as one chooses an adiabatic
passage route that compensates the collision mean-field phase shifts and avoids
the dynamical unstable regime. | cond-mat_other |
Finite Temperature Dynamics of Spin Solitons with Applications in
Thermocouples and Refrigerators: The exploitation of spin Berry phases to generate emergent fields for
producing miniaturized and high-quality inductors has enjoyed considerable
popularity among proponents of quantum technologies [Nature 586, 202 (2020)}].
Inspired by this breakthrough, we extend its mechanism to spin thermoelectrics
by probing responses of ferrimagnetic domain walls (DWs) to thermal gradients.
Similarly, voltages here stem from DW-spin collective motion, in contrast to
normal electron transport phenomena. We further develop finite-temperature
dynamics to investigate thermoelectric figures of merit and attribute
corresponding quantum superiority to ultrafast spin evolution of ferrimagnetism
with tunable non-Abelian phases. We propose a more likely cause of DW motion
towards hot or cold regions (contrary to conclusions of previous reports) and
verify existence of efficient magnon-momentum transfers. These findings deepen
our understanding of heat-driven DW kinetics and suggest profitable new
directions in an emerging realm of spincaloritronics. | cond-mat_other |
Superfluid shells for trapped fermions with mass and population
imbalance: We map out the phase diagram of strongly interacting fermions in a potential
trap with mass and population imbalance between the two spin components. As a
unique feature distinctively different from the equal-mass case, we show that
the superfluid here forms a shell structure which is not simply connected in
space. Different types of normal states occupy the trap regions inside and
outside this superfluid shell. We calculate the atomic density profiles, which
provide an experimental signature for the superfluid shell structure. | cond-mat_other |
Symbiotic Solitons in Heteronuclear Multicomponent Bose-Einstein
condensates: We show that bright solitons exist in quasi-one dimensional heteronuclear
multicomponent Bose-Einstein condensates with repulsive self-interaction and
attractive inter-species interaction. They are remarkably robust to
perturbations of initial data and collisions and can be generated by the
mechanism of modulational instability. Some possibilities for control and the
behavior of the system in three dimensions are also discussed. | cond-mat_other |
Virial theorems for trapped cold atoms: We present a general virial theorem for quantum particles with arbitrary
zero-range or finite-range interactions in an arbitrary external potential. We
deduce virial theorems for several situations relevant to trapped cold atoms:
zero-range interactions with and without Efimov effect, hard spheres, narrow
Feshbach resonances, and finite-range interactions. If the scattering length
$a$ is varied adiabatically in the BEC-BCS crossover, we find that the trapping
potential energy as a function of $1/a$ has an inflexion point at unitarity. | cond-mat_other |
Effect of fluctuations on the superfluid-supersolid phase transition on
the lattice: We derive a controlled expansion into mean field plus fluctuations for the
extended Bose-Hubbard model, involving interactions with many neighbors on an
arbitrary periodic lattice, and study the superfluid-supersolid phase
transition. Near the critical point, the impact of (thermal and quantum)
fluctuations on top of the mean field grows, which entails striking effects,
such as negative superfluid densities and thermodynamical instability of the
superfluid phase -- earlier as expected from mean-field dynamics. We also
predict the existence of long-lived "supercooled" states with anomalously large
quantum fluctuations. | cond-mat_other |
Probing the intrinsic state of a one-dimensional quantum well with a
photon-assisted tunneling: The photon-assisted tunneling (PAT) through a single wall carbon nanotube
quantum well (QW) under influence an external electromagnetic field for probing
of the Tomonaga Luttinger liquid (TLL) state is suggested. The elementary TLL
excitations inside the quantum well are density ($\rho_{\pm}$) and spin
($\sigma_{\pm} $) bosons. The bosons populate the quantized energy levels
$\epsilon^{\rho +}_n =\Delta n/ g$ and $\epsilon^{\rho -(\sigma \pm)}_n =
\Delta n$ where $\Delta = h v_F /L $ is the interlevel spacing, $n$ is an
integer number, $L$ is the tube length, $g$ is the TLL parameter. Since the
electromagnetic field acts on the $\rho_{+}$ bosons only while the neutral
$\rho_{-}$ and $\sigma_{\pm} $ bosons remain unaffected, the PAT spectroscopy
is able of identifying the $\rho_{+}$ levels in the QW setup. The spin
$\epsilon_n^{\sigma+} $ boson levels in the same QW are recognized from Zeeman
splitting when applying a d.c. magnetic field $H \neq 0$ field. Basic TLL
parameters are readily extracted from the differential conductivity curves. | cond-mat_other |
Ultracold atoms confined in an optical lattice plus parabolic potential:
a closed-form approach: We discuss interacting and non-interacting one dimensional atomic systems
trapped in an optical lattice plus a parabolic potential. We show that, in the
tight-binding approximation, the non-interacting problem is exactly solvable in
terms of Mathieu functions. We use the analytic solutions to study the
collective oscillations of ideal bosonic and fermionic ensembles induced by
small displacements of the parabolic potential. We treat the interacting boson
problem by numerical diagonalization of the Bose-Hubbard Hamiltonian. From
analysis of the dependence upon lattice depth of the low-energy excitation
spectrum of the interacting system, we consider the problems of
"fermionization" of a Bose gas, and the superfluid-Mott insulator transition.
The spectrum of the noninteracting system turns out to provide a useful guide
to understanding the collective oscillations of the interacting system,
throughout a large and experimentally relevant parameter regime. | cond-mat_other |
Full counting statistics of heteronuclear molecules from
Feshbach-assisted photo association: We study the effects of quantum statistics on the counting statistics of
ultracold heteronuclear molecules formed by Feshbach-assisted photoassociation
[Phys. Rev. Lett. {\bf 93}, 140405 (2004)]. Exploiting the formal similarities
with sum frequency generation and using quantum optics methods we consider the
cases where the molecules are formed from atoms out of two Bose-Einstein
condensates, out of a Bose-Einstein condensate and a gas of degenerate
fermions, and out of two degenerate Fermi gases with and without superfluidity.
Bosons are treated in a single mode approximation and fermions in a degenerate
model. In these approximations we can numerically solve the master equations
describing the system's dynamics and thus we find the full counting statistics
of the molecular modes. The full quantum dynamics calculations are complemented
by mean field calculations and short time perturbative expansions. While the
molecule production rates are very similar in all three cases at this level of
approximation, differences show up in the counting statistics of the molecular
fields. The intermediate field of closed-channel molecules is for short times
second-order coherent if the molecules are formed from two Bose-Einstein
condensates or a Bose-Fermi mixture. They show counting statistics similar to a
thermal field if formed from two normal Fermi gases. The coherence properties
of molecule formation in two superfluid Fermi gases are intermediate between
the two previous cases. In all cases the final field of deeply-bound molecules
is found to be twice as noisy as that of the intermediate state. This is a
consequence of its coupling to the lossy optical cavity in our model, which
acts as an input port for quantum noise, much like the situation in an optical
beam splitter. | cond-mat_other |
Electric power transfer in spin pumping experiments: Spin pumping is becoming an established method to generate voltages from
magnetic dynamics. The standard detection method of spin pumping is based on
open circuit voltage measurement across ferromagnetic (FM) and non-magnetic
(NM) bi-layers, where the inverse spin-Hall effect (ISHE) can convert spin
currents into electrical charge accumulation. In this paper, we present that it
is also possible to measure the associated electric charge current generated in
FM/NM bi-layers, by using a macroscopic closed circuitry detection method.
Using variable load resistors connected in series to the sample, we quantified
charge currents and associated electric power dissipation as a function of the
load resistance. By using basic circuit analysis, we are able to describe spin
pumping cells as a non-ideal voltage source or equivalent current source with
an internal resistor. | cond-mat_other |
The absence of fragmentation in Bose-Einstein condensates: A Bose-Einstein condensate produced by a Hamiltonian which is rotationally or
translationally symmetric is fragmented as a direct result of these symmetries.
A corresponding mean-field unfragmented state, with an identical energy to
leading order in the number of particles, can generally be constructed. As a
consequence, vanishingly weak symmetry-breaking perturbations destabilize the
fragmented state, which would thus be extremely difficult to realize
experimentally, and lead to an unfragmented condensate. | cond-mat_other |
Possible experiment for determination of the role of microscopic vortex
rings in the λ-transition in He-II: It is suggested that microscopic vortex rings (MVR) play an important role in
the \lambda-transition in helium-II and substantially determine the value of
T_{\lambda}. For very thin films of He-II, with thickness d less than the size
of the smallest MVR, the rings do not fit in and, therefore, do not exist in
such films. Consequently, for superfluid films of He-II, a peculiarity in the
form of a smoothed-out jump should be observed in the curve T_{m}(d) at the
values of thickness approximately equal to the size of the smallest MVR, d= 3 -
9 A (T_{m} is the temperature of the maximum of the broad peak on the curve of
the dependence of the specific heat on temperature). The absence of a similar
peculiarity will be an evidence that MVR do not influence the values of
T_{\lambda} and T_{m}, and do not play any key role in the \lambda-transition.
The currently available experimental data are insufficient for revealing the
predicted peculiarity. | cond-mat_other |
Phase diagram for a Bose-Einstein condensate moving in an optical
lattice: The stability of superfluid currents in a system of ultracold bosons was
studied using a moving optical lattice. Superfluid currents in a very weak
lattice become unstable when their momentum exceeds 0.5 recoil momentum.
Superfluidity vanishes already for zero momentum as the lattice deep reaches
the Mott insulator(MI) phase transition. We study the phase diagram for the
disappearance of superfluidity as a function of momentum and lattice depth
between these two limits. Our phase boundary extrapolates to the critical
lattice depth for the superfluid-to-MI transition with 2% precision. When a
one-dimensional gas was loaded into a moving optical lattice a sudden
broadening of the transition between stable and unstable phases was observed. | cond-mat_other |
Endurance Write Speed Tradeoffs in Nonvolatile Memories: We derive phenomenological model for endurance-write time switching tradeoff
for nonvolatile memories with thermally activated switching mechanisms. The
model predicts linear to cubic dependence of endurance on write time for metal
oxide memristors and flash memories, which is partially supported by
experimental data for the breakdown of metal-oxide thin films. | cond-mat_other |
A Tonks Giradeau Gas in the Presence of a Local Potential: The physics of a Tonks-Giradeau Gas in the presence of a local potential is
studied. In order to evaluate the single particle density matrix (SPDM) of the
many-body ground state, the Wiger-Jordan transformation is used. The
eigenvector with the largest eigenvalue of the SPDM corresponds to the
"Bose-Einstein Condensate"(BEC) State. We find that the "BEC" state density at
the positon of the local potential decreases, as expected, in the case of a
repulsive potential. For an attractive potential, it decreases or increases
depending on the strength of the potential. The superfluidity of this system is
investigated both numerically and perturbatively. An experimental method for
detecting the effect of an impurity in a Tonks-Giradueau gas is discussed. | cond-mat_other |
Topologically trapped vortex molecules in Bose-Einstein condensates: In a numerical experiment based on Gross-Pitaevskii formalism, we demonstrate
unique topological quantum coherence in optically trapped Bose-Einstein
condensates (BECs). Exploring the fact that vortices in rotating BEC can be
pinned by a geometric arrangement of laser beams, we show the parameter range
in which vortex-antivortex molecules or multiquantum vortices are formed as a
consequence of the optically imposed symmetry. Being low-energy states, we
discuss the conditions for spontaneous nucleation of these unique molecules and
their direct experimental observation, and provoke the potential use of the
phase print of an antivortex or a multiquantum vortex when realized in
unconventional circumstances. | cond-mat_other |
Equation of state of cubic boron nitride at high pressures and
temperatures: We report accurate measurements of the equation of state (EOS) of cubic boron
nitride by x-ray diffraction up to 160 GPa at 295 K and 80 GPa in the range
500-900 K. Experiments were performed on single-crystals embedded in a
quasi-hydrostatic pressure medium (helium or neon). Comparison between the
present EOS data at 295 K and literature allows us to critically review the
recent calibrations of the ruby standard. The full P-V-T data set can be
represented by a Mie-Gr\"{u}neisen model, which enables us to extract all
relevant thermodynamic parameters: bulk modulus and its first
pressure-derivative, thermal expansion coefficient, thermal Gr\"{u}neisen
parameter and its volume dependence. This equation of state is used to
determine the isothermal Gr\"{u}neisen mode parameter of the Raman TO band. A
new formulation of the pressure scale based on this Raman mode, using
physically-constrained parameters, is deduced. | cond-mat_other |
The Plastic Flow of Solid 4He through a Porous Membrane: The flow velocity of solid 4He through a porous membrane frozen into a
crystal has been measured in the temperature interval 0.1 - 1.8 K. A flat
capacitor consisting of a metalized plastic porous membrane and a bulk
electrode is applied and the gap in the capacitor was filled with examined
helium. The flow of helium through the membrane pores is caused by a d.c.
voltage applied to the capacitor plates. Above T~1K the velocity of solid 4He
flow decreases with lowering temperature following the Arrhenius law with the
activation energy of the process closed to that of vacancies. At low
temperatures the velocity is practically independent of temperature, which
suggests a transition in 4He from the classical thermally activated
vacancy-related flow to the quantum plastic flow. | cond-mat_other |
Dynamics of matter-wave and optical fields in superradiant scattering
from Bose-Einstein condensates: We study superradiant scattering off Bose-Einstein condensates by solving the
semiclassical Maxwell-Schroedinger equations describing the coupled dynamics of
matter-wave and optical fields. Taking the spatial dependence of these fields
along the condensate axis into account, we are able to reproduce and explain
many of the characteristic features observed in the experiments of Inouye et
al. [Science 285, 571 (1999)] and Schneble et al. [Science 300, 475 (2003)],
such as the shape of the atomic side-mode distributions for forward and
backward scattering, the spatial asymmetry between forward and backward side
modes, and the depletion of the condensate center observed for forward
scattering. | cond-mat_other |
Long-range correlation energies and off-diagonal interactions for the
$π$ electronic systems: The long-range correlation energies and the off-diagonal interactions are
studied and a general formula for correlation energy $E_c$ of the $\pi$
electron systems is given, which is beyond the nearest-neighbor
electron-electron interactions and includes the off-diagonal interactions. It
is found that the effects of the off-diagonal interactions $W$ and $X$ on the
correlation energies are opposite, but the influence of $X$ on the correlation
energies is much stronger than that of $W$ on the correlation energies, and the
correlation energies decrease with increasing the screening effect. | cond-mat_other |
Critical temperature of a trapped Bose gas: comparison of theory and
experiment: We apply the Projected Gross-Pitaevskii equation (PGPE) formalism to the
experimental problem of the shift in critical temperature $T_c$ of a
harmonically confined Bose gas as reported in Gerbier \emph{et al.} [Phys. Rev.
Lett. \textbf{92}, 030405 (2004)]. The PGPE method includes critical
fluctuations and we find the results differ from various mean-field theories,
and are in best agreement with experimental data. To unequivocally observe
beyond mean-field effects, however, the experimental precision must either
improve by an order of magnitude, or consider more strongly interacting
systems. This is the first application of a classical field method to make
quantitative comparison with experiment. | cond-mat_other |
Low-energy effective theory of the toric code model in a parallel field: We determine analytically the phase diagram of the toric code model in a
parallel magnetic field which displays three distinct regions. Our study relies
on two high-order perturbative expansions in the strong- and weak-field limit,
as well as a large-spin analysis. Calculations in the topological phase
establish a quasiparticle picture for the anyonic excitations. We obtain two
second-order transition lines that merge with a first-order line giving rise to
a multicritical point as recently suggested by numerical simulations. We
compute the values of the corresponding critical fields and exponents that
drive the closure of the gap. We also give the one-particle dispersions of the
anyonic quasiparticles inside the topological phase. | cond-mat_other |
Estimation of the spatial decoherence time in circular quantum dots: We propose a simple phenomenological model to estimate the spatial
decoherence time in quantum dots. The dissipative phase space dynamics is
described in terms of the density matrix and the corresponding Wigner function,
which are derived from a master equation with Lindblad operators linear in the
canonical variables. The formalism was initially developed to describe
diffusion and dissipation in deep inelastic heavy ion collisions, but also an
application to quantum dots is possible. It allows us to study the dependence
of the decoherence rate on the dissipation strength, the temperature and an
external magnetic field, which is demonstrated in illustrative calculations on
a circular GaAs one-electron quantum dot. | cond-mat_other |
The Linear and Non-linear Magnetic Response of a Tri-Uranium Single
Molecule Magnet: We report here low temperature magnetization isotherms for the single
molecule magnet, $(UO_2-L)_3$. By analyzing the low temperature magnetization
in terms of $M= X_1*B + X_3*B^3$ we extract the linear susceptibility $X_1$ and
the leading order nonlinear susceptibility $X_3$. We find that $X_1$ exhibits a
peak at a temperature of $T_1=10.4 K$ with $Chi_3$ also exhibiting a peak but
at a reduced temperature $T3 = 5 K$. At the lowest temperatures the isotherms
exhibit a critical field $B_c = 11.5 T$ marked by a clear point of inflection.
A minimal Hamiltonian employing S=1 (pseudo) spins with only a single energy
scale (successfully used to model the behavior of bulk f-electron metamagnets)
is shown to provide a good description of the observed linear scaling between
$T_1, T_3$ and $B_c$. We further show that a Heisenberg Hamiltonian previously
employed by Carretta et al. (2013 J. Phys.Cond. Matt. 25 486001) to model this
single molecule magnet gives formulas for the angle averaged susceptibilities
(in the Ising limit) very similar to those of the minimal model. | cond-mat_other |
Adiabatic Transport of Bose-Einstein Condensate in Double- and
Triple-Well Traps: By using a close similarity between multi-photon and tunneling population
transfer schemes, we propose robust adiabatic methods for the transport of
Bose-Einstein condensate (BEC) in double- and triple-well traps. The
calculations within the mean-field approximation (Gross-Pitaevskii equation)
show that irreversible and complete transport takes place even in the presence
of the non-linear effects caused by interaction between BEC atoms. The transfer
is driven by adiabatic time-dependent monitoring the barriers and well depths.
The proposed methods are universal and can be applied to a variety of systems
and scenarios. | cond-mat_other |
Applying matrix product operators to model systems with long-range
interactions: An algorithm is presented which computes a translationally invariant matrix
product state approximation of the ground state of an infinite 1D system; it
does this by embedding sites into an approximation of the infinite
``environment'' of the chain, allowing the sites to relax, and then merging
them with the environment in order to refine the approximation. By making use
of matrix product operators, our approach is able to directly model any
long-range interaction that can be systematically approximated by a series of
decaying exponentials. We apply our techniques to compute the ground state of
the Haldane-Shastry model and present results. | cond-mat_other |
S-mixing and quantum tunneling of the magnetization in molecular
nanomagnets: The role of $S$-mixing in the quantum tunneling of the magnetization in
nanomagnets has been investigated. We show that the effect on the tunneling
frequency is huge and that the discrepancy (more than 3 orders of magnitude in
the tunneling frequency) between spectroscopic and relaxation measurements in
Fe$_8$ can be resolved if $S$-mixing is taken into account. | cond-mat_other |
Electron sound in metals: This paper is devoted to the investigation of electron sound -- oscillations
of the electron distribution function coupled with elastic deformation and
propagating with the Fermi velocity. The amplitude-phase relations
characterizing the behavior of the electron sound in Ga single crystals are
determined experimentally. A model problem of excitation of electron sound in a
compensated metal with equivalent bands is solved for a finite sample with
diffusive scattering of electrons at the interfaces. It was found that the
displacement amplitude of the receiving interface is two orders of magnitude
larger than the elastic amplitude of the wave due to electron pressure. It was
established that the changes occurring in the amplitude and phase of the
electron sound waves at a superconducting transition do not depend on the path
traversed by the wave, i.e. they refer only to the behavior of the
transformation coefficient. | cond-mat_other |
Full transmission through perfect-conductor subwavelength hole arrays: Light transmission through 2D subwavelength hole arrays in perfect-conductor
films is shown to be complete (100%) at some resonant wavelengths even for
arbitrarily narrow holes. Conversely, the reflection on a 2D planar array of
non-absorbing scatterers is shown to be complete at some wavelengths regardless
how weak the scatterers are. These results are proven analytically and
corroborated by rigorous numerical solution of Maxwell's equations. This work
supports the central role played by dynamical diffraction during light
transmission through subwavelength hole arrays and it provides a systematics to
analyze more complex geometries and many of the features observed in connection
with transmission through hole arrays. | cond-mat_other |
Entanglement and errors in the control of spins by optical coupling: We analyze the optical quantum control of impurity spins in proximity to a
quantum dot. A laser pulse creates an exciton in the dot and controls the spins
by indirect coupling. We show how to determine the control parameters using as
an illustration the production of maximal spin entanglement. We consider errors
in the quantum control due to the exciton radiative recombination. The control
errors in the adiabatic and nonadiabatic case are compared to the threshold
needed for scalable quantum computing. | cond-mat_other |
Photoinduced vibronic coupling in two-level dissipative systems: Interaction of an electron system with a strong electromagnetic wave leads to
rearrangement both the electron and vibrational energy spectra of a dissipative
system. For instance, the optically coupled electron levels become split in the
conditions of the ac Stark effect that gives rise to appearance of the
nonadiabatic coupling between the electron and vibrational motions. The
nonadiabatic coupling exerts a substantial impact on the electron and phonon
dynamics and must be taken into account to determine the system wave functions.
In this paper, the vibronic coupling induced by the ac Stark effect is
considered. It is shown that the interaction between the electron states
dressed by an electromagnetic field and the forced vibrations of reservoir
oscillators under the action of rapid changing of the electron density with the
Rabi frequency is responsible for establishment of the photoinduced vibronic
coupling. However, if the resonance conditions for the optical phonon frequency
and the transition frequency of electrons in the dressed state basis are
satisfied, the vibronic coupling is due to the electron-phonon interaction.
Additionally, photoinduced vibronic coupling results in appearance of the
doubly dressed states which are formed by both the electron-photon and
electron-vibrational interactions. | cond-mat_other |
Continuum Mechanics for Quantum Many-Body Systems: The Linear Response
Regime: We derive a closed equation of motion for the current density of an
inhomogeneous quantum many-body system under the assumption that the
time-dependent wave function can be described as a geometric deformation of the
ground-state wave function. By describing the many-body system in terms of a
single collective field we provide an alternative to traditional approaches,
which emphasize one-particle orbitals. We refer to our approach as continuum
mechanics for quantum many-body systems. In the linear response regime, the
equation of motion for the displacement field becomes a linear fourth-order
integro-differential equation, whose only inputs are the one-particle density
matrix and the pair correlation function of the ground-state. The complexity of
this equation remains essentially unchanged as the number of particles
increases. We show that our equation of motion is a hermitian eigenvalue
problem, which admits a complete set of orthonormal eigenfunctions under a
scalar product that involves the ground-state density. Further, we show that
the excitation energies derived from this approach satisfy a sum rule which
guarantees the exactness of the integrated spectral strength. Our formulation
becomes exact for systems consisting of a single particle, and for any
many-body system in the high-frequency limit. The theory is illustrated by
explicit calculations for simple one- and two-particle systems. | cond-mat_other |
Evolution of Electronic and Vibrational Polarity of NaF Nanocrystals
from Diatomic to Bulk: A Density Functional Study: Density functional theory (DFT) is used to study vibrations, electrical
dipole moments, and polarizabilities of NaF clusters. Because of prior
experimental and theoretical studies, this is a good model system for tracking
the evolution of the properties from diatomic molecule to bulk crystal. The
ratio of vibrational to electronic contributions to the polarizability
increases dramatically with size N in the closed shell clusters (NaF)_N. The
open shell system Na_14F_13 has a greatly enhanced electronic polarizability.
Contrary to previous studies on this system which treated only the outer
electron by quantum mechanics, we find the O_h cubic structure to be stable
relative to the polar distorted structures such as C_3v. | cond-mat_other |
On non-markovian nature of stock trading: Using a relationship between the moments of the probability distribution of
times between the two consecutive trades (intertrade time distribution) and the
moments of the distribution of a daily number of trades we show, that the
underlying point process generating times of the trades is an essentially
non-markovian long-range memory one. Further evidence for the long-range memory
nature of this point process is provided by the powerlike correlation between
the intertrade time intervals. The data set includes all trades in EESR stock
on the Moscow International Currency Exchange in January 2003 - September 2003
and in Siemens, Commerzbank and Karstadt stocks traded on the Xetra electronic
stock exchange of Deutsche Boerse in October 2002. | cond-mat_other |
Manifolds of quasi-constant SOAP and ACSF fingerprints and the resulting
failure to machine learn four-body interactions: Atomic fingerprints are commonly used for the characterization of local
environments of atoms in machine learning and other contexts. In this work, we
study the behavior of two widely used fingerprints, namely the smooth overlap
of atomic positions (SOAP) and the atom-centered symmetry functions (ACSF),
under finite changes of atomic positions and demonstrate the existence of
manifolds of quasi-constant fingerprints. These manifolds are found numerically
by following eigenvectors of the sensitivity matrix with quasi-zero
eigenvalues. The existence of such manifolds in ACSF and SOAP causes a failure
to machine learn four-body interactions such as torsional energies that are
part of standard force fields. No such manifolds can be found for the Overlap
Matrix (OM) fingerprint due to its intrinsic many-body character. | cond-mat_other |
Possible display of phason mode of electromagnons in TbMnO3: The interaction of light in terahertz frequency region with electromagnons in
a sinusoidal incommensurate magnetic state in TbMnO3 is studied. A significant
change in the frequency dependence of the dielectric constant e_zz near the
temperature of the phase transition from sinusoidal magnetic ordering to spiral
spin structure with a spontaneous electric poiarization is predicted. Phason
mode of this phase transition is the mode of electromagnon with ME coupling
proportional to the wave number of modulation structure. | cond-mat_other |
Dynamical Instability of a Doubly Quantized Vortex in a Bose-Einstein
condensate: Doubly quantized vortices were topologically imprinted in $|F=1>$ $^{23}$Na
condensates, and their time evolution was observed using a tomographic imaging
technique. The decay into two singly quantized vortices was characterized and
attributed to dynamical instability. The time scale of the splitting process
was found to be longer at higher atom density. | cond-mat_other |
Stability and excitations of solitons in 2D Bose-Einstein condensates: The small oscillations of solitons in 2D Bose-Einstein condensates are
investigated by solving the Kadomtsev-Petviashvili equation which is valid when
the velocity of the soliton approaches the speed of sound. We show that the
soliton is stable and that the lowest excited states obey the same dispersion
law as the one of the stable branch of excitations of a 1D gray soliton in a 2D
condensate. The role of these states in thermodynamics is discussed. | cond-mat_other |
Sandpiles and superconductors: dual variational formulations for
critical-state problems: Similar evolutionary variational inequalities appear as convenient
formulations for continuous models for sandpile growth, magnetization of
type-II superconductors, and evolution of some other dissipative systems
characterized by the multiplicity of metastable states, long-range
interactions, avalanches, and hysteresis. The origin of this similarity is that
these are quasistationary models of equilibrium in which the multiplicity of
metastable states is a consequence of a unilateral condition of equilibrium
(critical-state constraint). Existing variational formulations for
critical-state models of sandpiles and superconductors are convenient for
modelling only the "primary" variables (evolving pile shape and magnetic field,
respectively). The conjugate variables (the surface sand flux and the electric
field) are also of interest in various applications. Here we derive dual
variational formulations, similar to mixed variational inequalities in
plasticity, for the sandpile and superconductor models. These formulations are
used in numerical simulations and allow us to approximate simultaneously both
the primary and dual variables. | cond-mat_other |
Discontinuities in fourth sound waves in superfluid helium: Formation of fourth-sound shock waves in narrow channels filled with
superfluid helium is studied. Physical and mathematical conditions at the
surface of discontinuity are established. These conditions differ somewhat from
those in case of first- and second-sound waves. The velocity of discontinuity
coincides with that of fourth sound. The jumps of temperature and the
superfluid velocity are shown to be of the first order as to the pressure
jumps. | cond-mat_other |
Inhibition of Transport of a Bose-Einstein Condensate in a Random
Potential: We observe the suppression of the 1D transport of an interacting elongated
Bose-Einstein condensate in a random potential with a standard deviation small
compared to the typical energy per atom, dominated by the interaction energy.
Numerical solutions of the Gross-Pitaevskii equation reproduce well our
observations. We propose a scenario for disorder-induced trapping of the
condensate in agreement with our observations. | cond-mat_other |
Aging in Citation Networks: In many growing networks, the age of the nodes plays an important role in
deciding the attachment probability of the incoming nodes. For example, in a
citation network, very old papers are seldom cited while recent papers are
usually cited with high frequency. We study actual citation networks to find
out the distribution $T(t)$ of $t$, the time interval between the published and
the cited paper. For different sets of data we find a universal behaviour:
$T(t) \sim t^{-0.9}$ for $t \leq t_c$ and $T(t) \sim t^{-2}$ for $t>t_c$ where
$t_c \sim O(10)$. | cond-mat_other |
Tunneling in a uniform one-dimensional superfluid: emergence of a
complex instanton: In a uniform ring-shaped one-dimensional superfluid, quantum fluctuations
that unwind the order parameter need to transfer momentum to quasiparticles
(phonons). We present a detailed calculation of the leading exponential factor
governing the rate of such phonon-assisted tunneling in a weakly-coupled Bose
gas at a low temperature $T$. We also estimate the preexponent. We find that
for small superfluid velocities the $T$-dependence of the rate is given mainly
by $\exp(-c_s P/ 2T)$, where $P$ is the momentum transfer, and $c_s$ is the
phonon speed. At low $T$, this represents a strong suppression of the rate,
compared to the non-uniform case. As a part of our calculation, we identify a
complex instanton, whose analytical continuation to suitable real-time segments
is real and describes formation and decay of coherent quasiparticle states with
nonzero total momenta. | cond-mat_other |
Electron guiding through insulating nanocapillaries: We simulate the electron transmission through insulating Mylar (PET)
capillaries. We show that the mechanisms underlying the recently discovered
electron guiding are fundamentally different from those for ion guiding.
Quantum reflection and multiple near-forward scattering rather than the
self-organized charge-up are key to the transmission along the capillary axis
irrespective of the angle of incidence. We find surprisingly good agreement
with recent data. Our simulation suggests that electron guiding should also be
observable for metallic capillaries. | cond-mat_other |
Special relativity description of the heat propagation in Minkowski
spacetime: In this paper we investigate the heat transport induced by continuous laser
beams up to an intensity of about 1029 Watt/cm2. We maintain that up to this
intensity nonlinear effects are negligible and that the application of the
linear hyperbolic heat transport equation is fully justifiable. We show that
the Fourier diffusion equation gives the speed of diffusion, v > c and breaks
the causality of the thermal processes in Minkowski space-time. For hyperbolic
heat transport v<c and causality is valid Key words: high energy continuous
laser beams, causality, Minkowski space-time. | cond-mat_other |
Rare-earth solid-state qubits: Quantum bits (qubits) are the basic building blocks of any quantum computer.
Superconducting qubits have been created with a 'top-down' approach that
integrates superconducting devices into macroscopic electrical circuits [1-3],
whereas electron-spin qubits have been demonstrated in quantum dots [4-6]. The
phase coherence time (Tau2) and the single qubit figure of merit (QM) of
superconducting and electron-spin qubits are similar -- Tau2 ~ microseconds and
QM ~10-1000 below 100mK -- and it should be possible to scale-up these systems,
which is essential for the development of any useful quantum computer.
Bottom-up approaches based on dilute ensembles of spins have achieved much
larger values of tau2 (up to tens of ms) [7, 8], but these systems cannot be
scaled up, although some proposals for qubits based on 2D nanostructures should
be scalable [9-11]. Here we report that a new family of spin qubits based on
rare-earth ions demonstrates values of Tau2 (~ 50microseconds) and QM (~1400)
at 2.5 K, which suggests that rare-earth qubits may, in principle, be suitable
for scalable quantum information processing at 4He temperatures. | 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 |
Resonantly enhanced tunneling of Bose-Einstein condensates in periodic
potentials: We report on measurements of resonantly enhanced tunneling of Bose-Einstein
condensates loaded into an optical lattice. By controlling the initial
conditions of our system we were able to observe resonant tunneling in the
ground and the first two excited states of the lattice wells. We also
investigated the effect of the intrinsic nonlinearity of the condensate on the
tunneling resonances. | 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 |
Relaxation and decoherence in a resonantly driven qubit: Relaxation and decoherence of a qubit coupled to environment and driven by a
resonant ac field are investigated by analytically solving Bloch equation of
the qubit. It is found that the decoherence of a driven qubit can be decomposed
into intrinsic and field-dependent ones. The intrinsic decoherence time equals
to the decoherence time of the qubit in free decay while the field-dependent
decoherence time is identical with the relaxation time of the qubit in driven
oscillation. Analytical expressions of the relaxation and decoherence times are
derived and applied to study a microwave-driven SQUID flux qubit. The results
are in excellent agreement with those obtained by numerically solving the
master equation. The relations between the relaxation and decoherence times of
a qubit in free decay and driven oscillation can be used to extract the
decoherence and thus dephasing times of the qubit by measuring its population
evolution in free decay and resonantly driven oscillation. | cond-mat_other |
A Semiclassical Model for Molecular Localization in Ammonia: The pedagogic two stste system of the ammonia molecule is used to illustrate
the phenomenon of environment induced molecular localization in pyramidal
molecules. A semiclassical model is used to describe a gas of pyramidal
molecules interacting via hard ball collisions. This modifies the tunnelling
dynamics between the classical equilibrium configurations of an isolated
molecule. For sufficiently high pressures, the model explains molecular
localization in these classical configurations. The decrease in the inversion
line frequency of ammonia, noted upon increase in pressure, is also studied. | cond-mat_other |
Resonant control of spin dynamics in ultracold quantum gases by
microwave dressing: We study experimentally interaction-driven spin oscillations in optical
lattices in the presence of an off-resonant microwave field. We show that the
energy shift induced by this microwave field can be used to control the spin
oscillations by tuning the system either into resonance to achieve near-unity
contrast or far away from resonance to suppress the oscillations. Finally, we
propose a scheme based on this technique to create a flat sample with either
singly- or doubly-occupied sites, starting from an inhomogeneous Mott
insulator, where singly- and doubly-occupied sites coexist. | cond-mat_other |
The role of Helium-3 impurities in the stress induced roughening of
superclimbing dislocations in solid Helium-4: We analyze the stress induced and thermally assisted roughening of a forest
of superclimbing dislocations in a Peierls potential in the presence of
Helium-3 impurities and randomly frozen in static stresses. It is shown that
the temperature of the dip $T_d$ in the flow rate observed by Ray and Hallock
(Phys.Rev. Lett. {\bf 105}, 145301 (2010)) is determined by the energy of the
impurity activation from dislocation core. However, it is suppressed by,
essentially, the logarithm of the impurity fraction. The width of the dip is
determined by inhomogeneous fluctuations of the stresses and is shown to be
much smaller than $T_d$. | cond-mat_other |
Formulas and equations for finding scattering data from the
Dirichlet-to-Neumann map with nonzero background potential: For the Schrodinger equation at fixed energy with a potential supported in a
bounded domain we give formulas and equations for finding scattering data from
the Dirichlet-to-Neumann map with nonzero background potential. For the case of
zero background potential these results were obtained in [R.G.Novikov,
Multidimensional inverse spectral problem for the equation
-\Delta\psi+(v(x)-Eu(x))\psi=0, Funkt. Anal. i Ego Prilozhen 22(4), pp.11-22,
(1988)]. | cond-mat_other |
Breakdown of Potential Flow to Turbulence around a Sphere Oscillating in
Superfluid He-4 above the Critical Velocity: The onset of turbulent flow around an oscillating sphere in superfluid $^4$He
is known to occur at a critical velocity $v_c \sim \sqrt{\kappa\omega}$ where
$\kappa$ is the circulation quantum and $\omega$ is the oscillation frequency.
But it is also well known that initially in a first up-sweep of the oscillation
amplitude, $v_c$ can be considerably exceeded before the transition occurs,
thus leading to a strong hysteresis in the velocity sweeps. The velocity
amplitude $v_c^*
> v_c$ where the transition finally occurs is related to the density $L_0$ of
the remanent vortices in the superfluid. Moreover, at temperatures below ca.
0.5 K and in a small interval of velocity amplitudes between $v_c$ and a
velocity that is about 2% larger, the flow pattern is found to be unstable,
switching intermittently between potential flow and turbulence. From time
series recorded at constant temperature and driving force the distribution of
the excess velocities $\Delta v = v_c^* - v_c$ is obtained and from that the
failure rate. Below 0.1 K we also can determine the distribution of the
lifetimes of the phases of potential flow. Finally, the frequency dependence of
these results is discussed. | cond-mat_other |
Quantum liquid of repulsively bound pairs of particles in a lattice: Repulsively interacting particles in a periodic potential can form bound
composite objects, whose dissociation is suppressed by a band gap. Nearly pure
samples of such repulsively bound pairs of cold atoms -- "dimers" -- have
recently been prepared by Winkler et al. [Nature 441, 853 (2006)]. We here
derive an effective Hamiltonian for a lattice loaded with dimers only and
discuss its implications to the many-body dynamics of the system. We find that
the dimer-dimer interaction includes strong on-site repulsion and
nearest-neighbor attraction which always dominates over the dimer kinetic
energy at low temperatures. The dimers then form incompressible,
minimal-surface "droplets" of a quantum lattice liquid. For low lattice
filling, the effective Hamiltonian can be mapped onto the spin-1/2 XXZ model
with fixed total magnetization which exhibits a first-order phase transition
from the "droplet" to a "gas" phase. This opens the door to studying first
order phase transitions using highly controllable ultracold atoms. | cond-mat_other |
Coherent adiabatic theory of two-electron quantum dot molecules in
external spin baths: We derive an accurate molecular orbital based expression for the coherent
time evolution of a two-electron wave function in a quantum dot molecule where
the electrons interact with each other, with external time dependent
electromagnetic fields and with a surrounding nuclear spin reservoir. The
theory allows for direct numerical modeling of the decoherence in quantum dots
due to hyperfine interactions. Calculations result in good agreement with
recent singlet-triplet dephasing experiments by Laird et. al. [Phys. Rev. Lett.
97, 056801 (2006)], as well as analytical model calculations. Furthermore, it
is shown that using a much faster electric switch than applied in these
experiments will transfer the initial state to excited states where the
hyperfine singlet-triplet mixing is negligible. | cond-mat_other |
The density maximum of He4 at the lambda point modeled by the stochastic
quantum hydrodynamic analogy: The lambda point in liquid He4 is a well established phenomenon acknowledged
as an example of Bose-Einstain condensation. This is generally accepted, but
there are serious discrepancies between the theory and experimental results,
namely the lower value of the transition temperature Tl and the negative value
of dTl /dP. These discrepancies can be explained in term of the quantum
stochastic hydrodynamic analogy (QSHA). The QSHA shows that at the
He4I\textregisteredHe4II superfluid transition the quantum coherence length lc
becomes of order of the distance up to which the wave function of a couple of
He4 atoms extends itself. In this case, the He42 state is quantum and the
quantum pseudo-potential brings a repulsive interaction that leads to the
negative dTl /dP behavior. This fact overcomes the difficulty to explain the
phenomenon by introducing a Hamiltonian inter-atomic repulsive potential that
would obstacle the gas-liquid transition. | cond-mat_other |
Pseudospin excitations in coaxial nanotubes: In a 2DEG confined to two coaxial tubes the `tube degree of freedom' can be
described in terms of pseudospin-1/2 dynamics. The presence of tunneling
between the two tubes leads to a collective oscillation known as pseudospin
resonance. We employ perturbation theory to examine the dependence of the
frequency of this mode with respect to a coaxial magnetic field for the case of
small intertube distances. Coulomb interaction leads to a shift of the
resonance frequency and to a finite lifetime of the pseudospin excitations. The
presence of the coaxial magnetic field gives rise to pronounced peaks in the
shift of the resonance frequency. For large magnetic fields this shift vanishes
due to the effects of Zeeman splitting. Finally, an expression for the
linewidth of the resonance is derived. Numerical analysis of this expression
suggests that the linewidth strongly depends on the coaxial magnetic field,
which leads to several peaks of the linewidth as well as regions where damping
is almost completely suppressed. | cond-mat_other |
Superfluid hydrodynamics in fractal dimension space: The complex behavior of liquid ${}^4$He and liquid ${}^3$He in nanoporous
media is determined by influence of randomly distributed geometrical
confinement as well as by significant contribution from the atoms near walls.
In the present paper fractional Schrodinger equation has been used for deriving
two-fluid hydrodynamical equations for describing the motion of superfluid
helium in the fractal dimension space. Nonlinear equations for oscillations of
pressure and temperature are obtained and coupling of pressure and temperature
oscillations is observed. Moreover coupling should disappear at very low
temperatures which provide an experimental test for this theory. | cond-mat_other |
The scaling of the density of states in systems with resonance states: Resonance states of a two-electron quantum dot are studied using a
variational expansion with both real basis-set functions and complex scaling
methods. We present numerical evidence about the critical behavior of the
density of states in the region where there are resonances. The critical
behavior is signaled by a strong dependence of some features of the density of
states with the basis-set size used to calculate it. The resonance energy and
lifetime are obtained using the scaling properties of the density of states | cond-mat_other |
Luttinger parameter of quasi-one-dimensional para-H2: We have studied the ground-state properties of para-hydrogen in one dimension
and in quasi-one-dimensional configurations using the path integral ground
state Monte Carlo method. This method produces zero-temperature exact results
for a given interaction and geometry. The quasi-one-dimensional setup has been
implemented in two forms: the inner channel inside a carbon nanotube coated
with H$_2$ and a harmonic confinement of variable strength. Our main result is
the dependence of the Luttinger parameter on the density within the stable
regime. Going from one dimension to quasi-one dimension, keeping the linear
density constant, produces a systematic increase of the Luttinger parameter.
This increase is however not enough to reach the superfluid regime and the
system always remain in the quasi-crystal regime, according to Luttinger liquid
theory. | cond-mat_other |
On the Laser Stimulation of Low-Energy Nuclear Reactions in Deuterated
Palladium: Models to account for the observed experimental results for low-energy
nuclear reactions in palladium-deuteride systems are presented along with
calculated results. The crucial idea is a mechanism of improved probability for
the needed penetration of the Coulomb barrier for a D-D reaction. This
facilitation occurs, in general, with the formation of D^- ions at special
frequency modes (e.g. via phonons) and, specifically for the laser-stimulated
case, with utilization of enhanced optical potential at a selected interface.
Both mechanisms may work individually, or together, to increase the probability
of barrier penetration. | cond-mat_other |
Coherent optical control of spin-spin interaction in doped
semiconductors: We provide a theory of laser-induced interaction between spins localized by
impurity centers in a semiconductor host. By solving exactly the problem of two
localized spins interacting with one itinerant exciton, an analytical
expression for the induced spin-spin interaction is given as a function of the
spin separation, laser energy, and intensity. We apply the theory to shallow
neutral donors (Si) and deep rare-earth magnetic impurities (Yb) in III-V
semiconductors. When the photon energy approaches a resonance related to
excitons bound to the impurities, the coupling between the localized spins
increases, and may change from ferromagnetic to anti-ferromagnetic. This
light-controlled spin interaction provides a mechanism for the quantum control
of spins in semiconductors for quantum information processing; it suggests the
realization of spin systems whose magnetic properties can be controlled by
changing the strength and the sign of the spin-spin interaction. | cond-mat_other |
Detection of vortex coherent structures in superfluid turbulence: Filamentary regions of high vorticity irregularly form and disappear in the
turbulent flows of classical fluids. We report an experimental comparative
study of these so-called " coherent structures " in a classical versus quantum
fluid, using liquid helium with a superfluid fraction varied from 0% up to 83%.
The low pressure core of the vorticity filaments is detected by pressure probes
located on the sidewall of a 78-cm-diameter Von K\'arm\'an cell driven up to
record turbulent intensity (R $\lambda$ $\sim$ $\sqrt$ Re 10000). The
statistics of occurrence, magnitude and relative distribution of the filaments
in a classical fluid are found indistinguishable from their superfluid
counterpart, namely the bundles of quantized vortex lines. This suggest that
the internal structure of vortex filaments, as well as their dissipative
properties have a negligible impact on their macroscopic dynamics, such as
lifetime and intermittent properties. | cond-mat_other |
Correlation functions of cold bosons in an optical lattice: We investigate the experiment of collapses and revivals of matter wave field
in more detail. To this end we calculate the lowest-order correlation functions
of the Bose field. We compare predictions of the total Fock state with the
commonly used coherent state approximation. We also show how to observe an
interference pattern for the celebrated Mott state. | cond-mat_other |
Li Doping Effect on Properties and Phase Transfomations of Knbo3: Dielectric permittivity and infrared reflectivity spectra of Li doped KNbO3
single crystals have been studied for the first time for K1-xLixNbO3 (KLN) with
x = 0.015, 0.02, 0.065. It was found that like in KTaO3, Li admixture results
in appearance of dielectric relaxation with the relaxation parameters very
close to those in KTaO3 quantum paraelectric. It was attributed to 90-grad
dipole reorientation of Li+ <100> off centers substituted K+, which appear to
be presents as in paraelectric cubic phase as in ferroelectric phase down to
low temperatures. Besides, Li doping is accompanied by increasing of the
cubic-tetragonal phase transition point, decreasing of
tetragonal-orthorhombic-rhombohedral phase transition points and TO soft mode
stiffen at room temperature. | cond-mat_other |
Vortices in Bose-Einstein Condensates: Some Recent Developments: In this brief review we summarize a number of recent developments in the
study of vortices in Bose-Einstein condensates, a topic of considerable
theoretical and experimental interest in the past few years. We examine the
generation of vortices by means of phase imprinting, as well as via dynamical
instabilities. Their stability is subsequently examined in the presence of
purely magnetic trapping, and in the combined presence of magnetic and optical
trapping. We then study pairs of vortices and their interactions, illustrating
a reduced description in terms of ordinary differential equations for the
vortex centers. In the realm of two vortices we also consider the existence of
stable dipole clusters for two-component condensates. Last but not least, we
discuss mesoscopic patterns formed by vortices, the so-called vortex lattices
and analyze some of their intriguing dynamical features. A number of
interesting future directions are highlighted. | cond-mat_other |
Photon recycling in Fabry-Perot micro-cavities based on Si$_3$N$_4$
waveguides: We present a numerical analysis and preliminary experimental results on
one-dimensional Fabry-Perot micro-cavities in Si$_3$N$_4$ waveguides. The
Fabry-Perot micro-cavities are formed by two distributed Bragg reflectors
separated by a straight portion of waveguide. The Bragg reflectors are composed
by a few air slits produced within the Si$_3$N$_4$ waveguides. In order to
increase the quality factor of the micro-cavities, we have minimized, with a
multiparametric optimization tool, the insertion loss of the reflectors by
varying the length of their first periods (those facing the cavity). To explain
the simulation results the coupling of the fundamental waveguide mode with
radiative modes in the Fabry-Perot micro-cavities is needed. This effect is
described as a recycling of radiative modes in the waveguide. To support the
modelling, preliminary experimental results of micro-cavities in Si$_3$N$_4$
waveguides realized with Focused Ion Beam technique are reported. | cond-mat_other |
Theoretical Studies on the Scanning Tunneling Microscope: The thesis explores calculating tunneling current densities between planar
conducting electrodes in an STM. It considers factors like bias voltages and
the separation between electrodes, using Fermi energy and work functions. Pauli
blocking effects on forward and reverse current densities are introduced,
comparing Airy function solutions with WKB results. A 'Russell Potential' is
defined for field lines, considering non-linearities and image force effects. A
multi-slice method using the transfer matrix approach calculates tunneling
currents for the Russell Potential and trapezoid + image force potentials. The
Simmons image potential's unreasonable enhancement of tunneling currents
prompts the construction of models with distributed charge, which show
negligible image effects compared to trapezoidal potentials. Tunneling currents
increase with bias voltage, decrease exponentially with tip-sample distance,
and rise with increasing tip curvature radius. The resolving power of the STM
degrades with blunter tips, higher bias voltages, and increased tip-sample
distances. In essence, the thesis covers calculating tunneling current
densities in STM, exploring different potential models and their effects. It
discusses factors influencing tunneling behavior, like bias voltage, tip-sample
distance, and tip curvature, revealing limitations in imaging capabilities
under certain conditions. | cond-mat_other |
Large scale numerical simulations of "ultrametric" long-range depinning: The depinning of an elastic line interacting with a quenched disorder is
studied for long range interactions, applicable to crack propagation or
wetting. An ultrametric distance is introduced instead of the Euclidean
distance, allowing for a drastic reduction of the numerical complexity of the
problem. Based on large scale simulations, two to three orders of magnitude
larger than previously considered, we obtain a very precise determination of
critical exponents which are shown to be indistinguishable from their Euclidean
metric counterparts. Moreover the scaling functions are shown to be unchanged.
The choice of an ultrametric distance thus does not affect the universality
class of the depinning transition and opens the way to an analytic real space
renormalization group approach. | cond-mat_other |
Semiclassical and quantum polarons in crystaline acetanilide: Crystalline acetanilide is a an organic solid with peptide bond structure
similar to that of proteins. Two states appear in the amide I spectral region
having drastically different properties: one is strongly temperature dependent
and disappears at high temperatures while the other is stable at all
temperatures. Experimental and theoretical work over the past twenty five years
has assigned the former to a selftrapped state while the latter to an extended
free exciton state. In this article we review the experimental and theoretical
developments on acetanilide paying particular attention to issues that are
still pending. Although the interpretation of the states is experimentally
sound, we find that specific theoretical comprehension is still lacking. Among
the issues that that appear not well understood is the effective dimensionality
of the selftrapped polaron and free exciton states. | cond-mat_other |
Bose-Einstein condensates of polar molecules: anisotropic interactions =
anisotropic mass: So far the theory of Bose-Einstein condensates (BEC) of polar molecules was
based on an ad hoc generalization of equations for spherical atoms. Here I
adopt a rigorous pseudo-potential approach to low-energy dipolar interactions
and derive a non-linear mean-field Schrodinger equation for a
harmonically-trapped condensate. I show that, effectively, the dipolar
interactions alter molecular mass. The resulting effective mass is anisotropic:
to the leading order the mass is altered only for the motion along the
polarizing field. For a typical BEC of spin-polarized magnetically-interacting
alkali-metal atoms the effective atomic mass is reduced by 10% from its bare
value. For a BEC of polar molecules the mass may be reduced by a factor of a
1,000. | cond-mat_other |
Bright solitary waves of atomic Bose-Einstein condensates under rotation: We analyse the rotation of bright solitary waves formed of atomic
Bose-Einstein condensates with attractive atomic interactions. By employing a
variational technique and assuming an irrotational quadrupolar flow field, we
map out the variational solutions in the rotating frame. In particular, we show
that rotation has a considerable stabilising effect on the system,
significantly raising the critical threshold for collapse of the bright
solitary waves. | cond-mat_other |
Reaction Enhanced Diffusion in Spherical Membranes: The reversible reactions like A+B <-> C in the many-component diffusive
system affect the diffusive properties of the constituents. The effective
conjugation of irreversible processes of different dimensionality takes place
due to the stationarity in the system and can lead to essential increase of the
resulting diffusive fluxes. The exact equations for the spatial concentration
profiles of the components are difficult to treat analytically. We solve
approximately the equations for the concentration profiles of the
reaction-diffusion components in the spherical geometry in the application to
the problem of the enhanced oxygen transfer through a biological membrane and
to the mathematically similar problem of surface diffusion in a solid body. In
the latter case the spherical geometry can be an adequate tool for describing
the surface of a real solid body which can be modeled as a fractal object
formed of sequences of spherical surfaces with different radii. | cond-mat_other |
Levels of self-consistency in the GW approximation: We perform $GW$ calculations on atoms and diatomic molecules at different
levels of self-consistency and investigate the effects of self-consistency on
total energies, ionization potentials and on particle number conservation. We
further propose a partially self-consistent $GW$ scheme in which we keep the
correlation part of the self-energy fixed within the self-consistency cycle.
This approximation is compared to the fully self-consistent $GW$ results and to
the $G W_0$ and the $G_0W_0$ approximations. Total energies, ionization
potentials and two-electron removal energies obtained with our partially
self-consistent $GW$ approximation are in excellent agreement with fully
self-consistent $GW$ results while requiring only a fraction of the
computational effort. We also find that self-consistent and partially
self-consistent schemes provide ionization energies of similar quality as the
$G_0W_0$ values but yield better total energies and energy differences. | cond-mat_other |
Magnetization dynamics in dysprosium orthoferrites via inverse Faraday
effect: The ultrafast non-thermal control of magnetization has recently become
feasible in canted antiferromagnets through photomagnetic instantaneous pulses
[A.V. Kimel {\it et al.}, Nature {\bf 435}, 655 (2005)]. In this experiment
circularly polarized femtosecond laser pulses set up a strong magnetic field
along the wave vector of the radiation through the inverse Faraday effect,
thereby exciting non-thermally the spin dynamics of dysprosium orthoferrites. A
theoretical study is performed by using a model for orthoferrites based on a
general form of free energy whose parameters are extracted from experimental
measurements. The magnetization dynamics is described by solving coupled
sublattice Landau-Lifshitz-Gilbert equations whose damping term is associated
with the scattering rate due to magnon-magnon interaction. Due to the inverse
Faraday effect and the non-thermal excitation, the effect of the laser is
simulated by magnetic field Gaussian pulses with temporal width of the order of
hundred femtoseconds. When the field is along the z-axis, a single resonance
mode of the magnetization is excited. The amplitude of the magnetization and
out-of-phase behavior of the oscillations for fields in z and -z directions are
in good agreement with the cited experiment. The analysis of the effect of the
temperature shows that magnon-magnon scattering mechanism affects the decay of
the oscillations on the picosecond scale. Finally, when the field pulse is
along the x-axis, another mode is excited, as observed in experiments. In this
case the comparison between theoretical and experimental results shows some
discrepancies whose origin is related to the role played by anisotropies in
orthoferrites. | cond-mat_other |
Financial heat machine: We consider dynamics of financial markets as dynamics of expectations and
discuss such a dynamics from the point of view of phenomenological
thermodynamics. We describe a financial Carnot cycle and the financial analogue
of a heat machine. We see, that while in physics a perpetuum mobile is
absolutely impossible, in economics such mobile may exist under some
conditions. Our thermodynamical model for the financial market induces a rather
unusual interpretation of the role of financial crises. In contrast to the
common point of view, in our model financial crises play a crucial role in
functioning of the modern financial market. This is an important (concluding)
stage of any financial cycle that is analogous to the stage of cooling in the
ordinary Carnot cycle. A financial cycle could not be completed without such a
stage as well as the ordinary Carnot cycle. Thus, in spite its destructive (at
the first sight) consequences the stage or financial crises is as well
important as the stage of "boiling of the financial market" ("heating of
expectations") | cond-mat_other |
Ultrasensitive nanoelectromechanical mass detection: We describe the application of nanoelectromechanical systems (NEMS) to
ultrasensitive mass detection. In these experiments, a modulated flux of atoms
was adsorbed upon the surface of a 32.8 MHz NEMS resonator within an ultrahigh
vacuum environment. The mass-induced resonance frequency shifts by these
adsorbates were then measured to ascertain a mass sensitivity of 2.53x10^-18 g.
In these initial measurements, this sensitivity is limited by the noise in the
NEMS displacement transducer; the ultimate, limits of the technique are set by
fundamental phase noise processes. Our results and analysis indicate that mass
sensing of individual molecules will be realizable with optimized NEMS devices. | cond-mat_other |
Topological Constraints on the Charge Distributions for the Thomson
Problem: The method of Morse theory is used to analyze the distributions of unit
charges interacting through a repulsive force and constrained to move on the
surface of a sphere -- the Thomson problem. We find that, due to topological
reasons, the system may organize itself in the form of pentagonal structures.
This gives a qualitative account for the interesting ``pentagonal buttons''
discovered in recent numerical work. | cond-mat_other |
Comment on the paper "Quasi-particle approach for lattice Hamiltonians
with large coordination numbers" by P. Navez, F. Queisser and R. Schützhold
- J. Phys. A: Math. Theor. 47 225004 (2014): This comment regards a central aspect of the referred-to paper, the issue of
convergence of the large coordination-number expansion. Perturbation expansions
of expressions containing a large number of parameters are generally invalid
due to the non-analyticity of the expanded expressions. I refer to recent work
where these issues are analyzed and discussed in detail in relation to a
benchmark example of a cluster model. As discussed therein, methods which are
uncontrollable and for which their convergence is not foreseeable are not only
useless but can mislead, particularly if models derived from them are used to
interpret experiments. | cond-mat_other |
A Guided Walk Down Wall Street: an Introduction to Econophysics: This article contains the lecture notes for the short course ``Introduction
to Econophysics,'' delivered at the II Brazilian School on Statistical
Mechanics, held in Sao Carlos, Brazil, in February 2004. The main goal of the
present notes is twofold: i) to provide a brief introduction to the problem of
pricing financial derivatives in continuous time; and ii) to review some of the
related problems to which physicists have made relevant contributions in recent
years. | cond-mat_other |
Observation of Majorana Quasiparticles Surface States in Superfluid
${^3}$He-B by Heat Capacity Measurements: We report about direct measurements of heat capacity of Majorana
quasiparticles in superfluid ${^3}$He-B which appear near the surface of the
experimental bolometer on the coherence length ${\xi}$. Two bolometers with
different surface-to-volume ratios were used which allows us to have different
calibrated contributions from Majorana quasiparticles to the ${^3}$He heat
capacity. Estimations of possible impact of ${^3}$He layers adsorbed on the
walls of the bolometer have been done. | cond-mat_other |
Stability of low-dimensional multicomponent Bose gases: I show that in low dimensions the interactions in dilute Bose mixtures are
strongly renormalized, which leads to a considerable change of stability
conditions compared to the mean-field results valid in the high-density regime.
Estimates are given for the two-component Bose-Hubbard model and for the
Rb(87)-K(41) mixture. | cond-mat_other |
Non-local density correlations as signal of Hawking radiation in BEC
acoustic black holes: We have used the analogy between gravitational systems and non-homogeneous
fluid flows to calculate the density-density correlation function of an atomic
Bose-Einstein condensate in the presence of an acoustic black hole. The
emission of correlated pairs of phonons by Hawking-like process results into a
peculiar long-range density correlation. Quantitative estimations of the effect
are provided for realistic experimental configurations. | cond-mat_other |
Dynamic localization in an effective tight binding Hamiltonian model
with a rapidly oscillating homogeneous electric field on a lattice: By the Magnus-Floquet approach we calculate the effective Hamiltonian for a
charged particle on the lattice subject to a homogeneous high frequency
oscillating electric field. The obtained result indicate the absence of dynamic
localization of the particle for any value of the lattice constant and electric
field applied, which completes the limit results obtained by Dunlap and Kenkre. | cond-mat_other |
Critical Velocities for Roton and Super-Flow Quantum Turbulence in
Liquid $^4$He: Two different types of transitions of the superfluid $^4$He to quantum
turbulence regimes are studied for $1{\rm D}$ geometry in the case when the
influence of the normal fluid on superfluid flow is suppressed. It is shown
that the roton mechanism of transition to quantum turbulence leads to a
critical velocity satisfying the relation $v_c\propto d^{-1/4}$. In the
super-flow mechanism, the transition to quantum turbulence arises when the
"quantum Reynolds number" is about $10^3$ and the critical velocity depends on
channel size $d$ as $v_c\propto d^{-1}$ in agreement with the equations of
motion for a superfluid component of the liquid $^4$He being disturbed by small
fluctuations of the normal fluid. | cond-mat_other |
Hard-core bosons on optical superlattices: Dynamics and relaxation in
the superfluid and insulating regimes: We study the ground-state properties and nonequilibrium dynamics of hard-core
bosons confined in one-dimensional lattices in the presence of an additional
periodic potential (superlattice) and a harmonic trap. The dynamics is analyzed
after a sudden switch-on or switch-off of the superlattice potential, which can
bring the system into insulating or superfluid phases, respectively. A collapse
and revival of the zero-momentum peak can be seen in the first case. We study
in detail the relaxation of these integrable systems towards equilibrium. We
show how after relaxation time averages of physical observables, like the
momentum distribution function, can be predicted by means of a generalization
of the Gibbs distribution. | cond-mat_other |
Modification to the pre-factor of the semiclassical propagator: We modify the pre-factor of the semiclassical propagator to improve its
efficiency in practical implementations. The new pre-factor represents the
smooth portion of an orbit's contribution, and leads to fast convergence in
numerical calculations. As an illustration of the accuracy and efficiency of
the resultant propagator, we numerically calculate overlaps between quantum and
semiclassical wave functions, as well as low-lying spectrum density in a
10-dimensional system contains unstable classical orbits. This sheds light on
applying semiclassical propagator to high dimensional systems. | cond-mat_other |
Giant magnetic anisotropy of the bulk antiferromagnets IrMn and IrMn3: Theoretical predictions of the magnetic anisotropy of antiferromagnetic
materials are demanding due to a lack of experimental techniques which are
capable of a direct measurement of this quantity. At the same time it is highly
significant due to the use of antiferromagnetic components in magneto-resistive
sensor devices where the stability of the antiferromagnet is of upmost
relevance. We perform an ab-initio study of the ordered phases of IrMn and
IrMn3, the most widely used industrial antiferromagnets. Calculating the form
and the strength of the magnetic anisotropy allows the construction of an
effective spin model, which is tested against experimental measurements
regarding the magnetic ground state and the Neel temperature. Our most
important result is the extremely strong second order anisotropy for IrMn3
appearing in its frustrated triangular magnetic ground state, a surprising fact
since the ordered L12 phase has a cubic symmetry. We explain this large
anisotropy by the fact that cubic symmetry is locally broken for each of the
three Mn sub-lattices. | cond-mat_other |
Electronic structure via potential functional approximations: The universal functional of Hohenberg-Kohn is given as a coupling-constant
integral over the density as a functional of the potential. Conditions are
derived under which potential-functional approximations are variational.
Construction via this method and imposition of these conditions are shown to
greatly improve the accuracy of the non-interacting kinetic energy needed for
orbital-free Kohn-Sham calculations. | cond-mat_other |
Experimental realization of BCS-BEC crossover physics with a Fermi gas
of atoms: This thesis presents experiments probing physics in the crossover between
Bose-Einstein condensation (BEC) and BCS superconductivity using an ultracold
gas of atomic fermions. Scattering resonances in these ultracold gases (known
as Feshbach resonances) provide the unique ability to tune the fermion-fermion
interactions. The work presented here pioneered the use of fermionic Feshbach
resonances as a highly controllable and tunable system ideal for studying the
cusp of the BCS-BEC crossover problem. Here pairs of fermionic atoms have some
properties of diatomic molecules and some properties of Cooper pairs. I present
studies of a normal Fermi gas at a Feshbach resonance and the work required to
cool the gas to temperatures where superfluidity in the crossover is predicted.
These studies culminated in our observation of a phase transition at the cusp
of the BCS-BEC crossover through condensation of fermionic atom pairs. I also
discuss subsequent work that confirmed the crossover nature of the pairs in
these condensates. | cond-mat_other |
Commensurability and hysteretic evolution of vortex configurations in
rotating optical lattices: We present a theoretical study of vortices within a harmonically trapped
Bose-Einstein condensate in a rotating optical lattice. Due to the competition
between vortex-vortex interactions and pinning to the optical lattice we find a
very complicated energy landscape, which leads to hysteretic evolution. The
qualitative structure of the vortex configurations depends on the
commensurability between the vortex density and the site density -- with
regular lattices when these are commensurate, and concentric rings when they
are not. We model the imaging of these structures by calculating time-of-flight
column densities. As in the absence of the optical lattice, the vortices are
much more easily observed in a time-of-flight image than \emph{in-situ}. | cond-mat_other |
Diffusion and localization for the Chirikov typical map: We consider the classical and quantum properties of the "Chirikov typical
map", proposed by Boris Chirikov in 1969. This map is obtained from the well
known Chirikov standard map by introducing a finite number $T$ of random phase
shift angles. These angles induce a random behavior for small time scales
($t<T$) and a $T$-periodic iterated map which is relevant for larger time
scales ($t>T$). We identify the classical chaos border $k_c\sim T^{-3/2} \ll 1$
for the kick parameter $k$ and two regimes with diffusive behavior on short and
long time scales. The quantum dynamics is characterized by the effect of
Chirikov localization (or dynamical localization). We find that the
localization length depends in a subtle way on the two classical diffusion
constants in the two time-scale regime. | cond-mat_other |
Trapped Fermions across a Feshbach resonance with population imbalance: We investigate the phase separation of resonantly interacting fermions in a
trap with imbalanced spin populations, both at zero and at finite temperatures.
We directly minimize the thermodynamical potential under the local density
approximation instead of using the gap equation, as the latter may give
unstable solutions. On the BEC side of the resonance, one may cross three
different phases from the trap center to the edge; while on the BCS side or at
resonance, typically only two phases show up. We compare our results with the
recent experiment, and the agreement is remarkable. | cond-mat_other |
On the free rotation of a molecule embedded in helium-4 clusters: The fact, that $^4$He atoms on different concentric circular paths around the
axis of a quantum vortex move with identically equal angular momentum, which
represents an important aspect of superfluidity of He-II, has been used to
discover a model which can explain the {\it typical nature} of experimentally
observed $N$ (number of $^4He$ atoms) dependence of the rotational constant
($B$) of the rotor part of a cluster M:He$_N$. It reveals how exactly
superfluidity is related to the said dependence of $B$ on $N$. We believe that
this model, when used with simulation techniques, would render results that
would agree closely with experiments. | cond-mat_other |
Laser Pulse Amplification with Bose-Einstein Condensates: This paper has been withdrawn by the author. | cond-mat_other |
Microstructure of He II in the presence of boundaries: We have studied the microstructure of a system of interacting Bose particles
under zero boundary conditions and have found two possible orderings. One
ordering is traditional and is characterized by the Bogolyubov dispersion law
E^2 = (h^2 k^2/2m)^{2} + qn\nu(k)[h^2 k^2/m] (with q=1) at a weak interaction.
The second one is new and is characterized by the same dispersion law, but with
q=2^{-f}, where $f$ is the number of noncyclic coordinates. At a weak
interaction, the ground-state energy is less for the new solution. The
boundaries affect the bulk microstructure due to the difference of the
topologies of closed and open systems. | cond-mat_other |
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