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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 |
Electron effective mobility in strained Si/Si1-xGex MOS devices using
Monte Carlo simulation: Based on Monte Carlo simulation, we report the study of the inversion layer
mobility in n-channel strained Si/ Si1-xGex MOS structures. The influence of
the strain in the Si layer and of the doping level is studied. Universal
mobility curves mueff as a function of the effective vertical field Eeff are
obtained for various state of strain, as well as a fall-off of the mobility in
weak inversion regime, which reproduces correctly the experimental trends. We
also observe a mobility enhancement up to 120 % for strained Si/ Si0.70Ge0.30,
in accordance with best experimental data. The effect of the strained Si
channel thickness is also investigated: when decreasing the thickness, a
mobility degradation is observed under low effective field only. The role of
the different scattering mechanisms involved in the strained Si/ Si1-xGex MOS
structures is explained. In addition, comparison with experimental results is
discussed in terms of SiO2/ Si interface roughness, as well as surface
roughness of the SiGe substrate on which strained Si is grown. | cond-mat_other |
Adiabatic and Non-Adiabatic Contributions to the Free Energy from the
Electron-Phonon Interaction for Na, K, Al, and Pb: We calculate the adiabatic contributions to the free energy due to the
electron--phonon interaction at intermediate temperatures, $0 \leqslant k_{B} T
< \epsilon_{F}$ for the elemental metals Na, K, Al, and Pb. Using our
previously published results for the nonadiabatic contributions we show that
the adiabatic contribution, which is proportional to $T^{2}$ at low
temperatures and goes as $T^{3}$ at high temperatures, dominates the
nonadiabatic contribution for temperatures above a cross--over temperature,
$T_{c}$, which is between 0.5 and 0.8 $T_{m}$, where $T_{m}$ is the melting
temperature of the metal. The nonadiabatic contribution falls as $T^{-1}$ for
temperatures roughly above the average phonon frequency. | cond-mat_other |
Superradiant scattering from a hydrodynamic vortex: We show that sound waves scattered from a hydrodynamic vortex may be
amplified. Such superradiant scattering follows from the physical analogy
between spinning black holes and hydrodynamic vortices. However a sonic horizon
analogous to the black hole event horizon does not exist unless the vortex
possesses a central drain, which is challenging to produce experimentally. In
the astrophysical domain, superradiance can occur even in the absence of an
event horizon: we show that in the hydrodynamic analogue, a drain is not
required and a vortex scatters sound superradiantly. Possible experimental
realization in dilute gas Bose-Einstein condensates is discussed. | cond-mat_other |
The angular momentum of a magnetically trapped atomic condensate: For an atomic condensate in an axially symmetric magnetic trap, the sum of
the axial components of the orbital angular momentum and the hyperfine spin is
conserved. Inside an Ioffe-Pritchard trap (IPT) whose magnetic field (B-field)
is not axially symmetric, the difference of the two becomes surprisingly
conserved. In this paper we investigate the relationship between the values of
the sum/difference angular momentums for an atomic condensate inside a magnetic
trap and the associated gauge potential induced by the adiabatic approximation.
Our result provides significant new insight into the vorticity of magnetically
trapped atomic quantum gases. | 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 |
Remarks on the lattice Green's Function for the anisotropic Face
Centered Cubic Lattice: An expression for the Green's function (GF) of anisotropic face centered
cubic lattice is evaluated analytically and numerically for a single impurity
problem. The density of states (DOS), phase shift and scattering cross section
are expressed in terms of complete elliptic integrals of the first kink. | cond-mat_other |
Effects of an oscillating field on pattern formation in a ferromagnetic
thin film: Analysis of patterns traveling at a low velocity: Magnetic domain patterns under an oscillating field is studied theoretically
by using a simple Ising-like model. We propose two ways to investigate the
effects of the oscillating field. The first one leads to a model in which
rapidly oscillating terms are averaged out, and the model can explain the
existence of the maximum amplitude of the field for the appearance of patterns.
The second one leads to a model that includes the delay of the response to the
field, and the model suggests the existence of a traveling pattern which moves
very slowly compared with the time scale of the driving field. | cond-mat_other |
Fidelity and Quantum phase transition for the Heisenberg chain with the
next-nearest-neighbor interaction: In this paper, we investigate the fidelity for the Heisenberg chain with the
next-nearest-neighbor interaction (or the $J_1-J_2$ model) and analyze its
connections with quantum phase transition. We compute the fidelity between the
ground states and find that the phase transition point of the $J_1-J_2$ model
can not be well characterized by the ground state fidelity for finite-size
systems. Instead, we introduce and calculate the fidelity between the first
excited states. Our results show that the quantum transition can be well
characterized by the fidelity of the first excited state even for a small-size
system. | cond-mat_other |
Quantum levitation of nanoparticles seen with ultracold neutrons: Analyzing new experiments with ultracold neutrons (UCNs) we show that
physical adsorption of nanoparticles/nano-droplets, levitating in high-excited
states in a deep and broad potential well formed by van der
Waals/Casimir-Polder (vdW/CP) forces results in new effects on a cross-road of
fundamental interactions, neutron, surface and nanoparticle physics. Accounting
for the interaction of UCNs with nanoparticles explains a recently discovered
intriguing small heating of UCNs in traps. It might be relevant to the striking
conflict of the neutron lifetime experiments with smallest reported
uncertainties by adding false effects there. | cond-mat_other |
Fast partial decoherence of a superconducting flux qubit in a spin bath: The superconducting flux qubit has two quantum states with opposite magnetic
flux. Environment of nuclear spins can find out the direction of the magnetic
flux after a decoherence time $\tau_0$ inversely proportional to the magnitude
of the flux and the square root of the number of spins. When the Hamiltonian of
the qubit drives fast coherent Rabi oscillations between the states with
opposite flux, then flux direction is flipped at a constant rate $\omega$ and
the decoherence time $\tau=\omega\tau_0^2$ is much longer than $\tau_0$.
However, on closer inspection decoherence actually takes place on two
timescales. The long time $\tau$ is a time of full decoherence but a part of
quantum coherence is lost already after the short time $\tau_0$. This fast
partial decoherence biases coherent flux oscillations towards the initial flux
direction and it can affect performance of the superconducting devices as
qubits. | cond-mat_other |
Adiabatic self-trapped states in carbon nanotubes: We study here polaron (soliton) states of electrons or holes in a model
describing carbon-type nanotubes. In the Hamiltonian of the system we take into
account the electron-phonon interaction that arises from the deformation
dependencies of both the on-site and the hopping interaction energies. Using an
adiabatic approximation, we derive the equations for self-trapped electron
states in zigzag nanotubes. We find the ground states of an electron in such a
system and show that the polaron states can have different symmetries depending
on the strength of the electron-phonon coupling. Namely, at relatively weak
coupling the polarons possess quasi-one-dimensional (quasi-1D) properties and
have an azimuthal symmetry. When the coupling constant exceeds some critical
value, the azimuthal symmetry breaks down and the polaron spreads out in more
than one dimension.
We also study polarons that are formed by the electrons in the conducting
band (or by holes in the valence band) in semiconducting carbon nanotubes. We
show that their properties are more complex than those of quasi-1D ground state
polarons. In particular, polarons in semiconducting carbon nanotubes possess an
inner structure: being self-trapped along the nanotube axis they exhibit some
modulations around the nanotube. | cond-mat_other |
Electrical conductivity in granular media and Branly's coherer: A simple
experiment: We show how a simple laboratory experiment can illustrate certain electrical
transport properties of metallic granular media. At a low critical imposed
voltage, a transition from an insulating to a conductive state is observed.
This transition comes from an electro-thermal coupling in the vicinity of the
microcontacts between grains where microwelding occurs. Our apparatus allows us
to obtain an implicit determination of the microcontact temperature, which is
analogous to the use of a resistive thermometer. The experiment also
illustrates an old problem, the explanation of Branly's coherer effect - a
radio wave detector used for the first wireless radio transmission, and based
on the sensitivity of the metal fillings conductivity to an electromagnetic
wave. | cond-mat_other |
Thermally induced rotons in two-dimensional dilute Bose gases: We show that roton-like excitations are thermally induced in a
two-dimensional dilute Bose gas as a consequence of the strong phase
fluctuations in two dimensions. At low momentum, the roton-like excitations
lead for small enough temperatures to an anomalous phonon spectrum with a
temperature dependent exponent reminiscent of the Kosterlitz-Thouless
transition. Despite the anomalous form of the energy spectrum, it is shown that
the corresponding effective theory of vortices describes the usual
Kosterlitz-Thouless transition. The possible existence of an anomalous normal
state in a small temperature interval is also discussed. | cond-mat_other |
Self-similar space-filling sphere packings in three and four dimensions: Inversive geometry can be used to generate exactly self-similar space-filling
sphere packings. We present a construction method in two dimensions and
generalize it to search for packings in higher dimensions. We newly discover 29
three-dimensional and 13 four-dimensional topologies of which 10 and 5,
respectively, are bearings. To distinguish and characterize the packing
topologies, we numerically estimate their fractal dimensions and we analyze
their contact networks. | cond-mat_other |
Influence of Exchange Scattering on Superfluid He-3 states in Nematic
Aerogel: The superfluid state in bulk liquid $^3$He is realized in the form of A or B
phases. Uniaxially anisotropic aerogel (nafen) stabilizes transition from the
normal to the polar superfluid state which on further cooling transitions to
the axipolar orbital glass state (Phys. Rev. Lett. {\bf 115}, 165304 (2015)).
This is the case in nafen aerogel preplated by several atomic layers of $^4$He.
When pure liquid $^3$He fills the same nafen aerogel a solid-like layer of
$^3$He atoms coats the aerogel structure. The polar state is not formed anymore
and a phase transition occurs directly to the axipolar phase (Phys. Rev. Lett.
{\bf 120}, 075301 (2018). The substitution of $^4$He by $^3$He atoms at the
aerogel surface changes the potential and adds the exchange scattering of
quasiparticles on the aerogel strands. A calculation shows that both of these
effects can decrease the degree of anisotropy of scattering and suppress the
polar phase formation. The derived anisotropy of the spin diffusion coefficient
in globally anisotropic aerogel is determined by the same parameter which
controls the polar state emergence which allows one to check the effect of
anisotropy change for different types of covering. | cond-mat_other |
Partially incoherent gap solitons in Bose-Einstein condensates: We construct families of incoherent matter-wave solitons in a repulsive
degenerate Bose gas trapped in an optical lattice (OL), i.e., gap solitons, and
investigate their stability at zero and finite temperature, using the
Hartree-Fock-Bogoliubov equations. The gap solitons are composed of a coherent
condensate, and normal and anomalous densities of incoherent vapor co-trapped
with the condensate. Both intragap and intergap solitons are constructed, with
chemical potentials of the components falling in one or different bandgaps in
the OL-induced spectrum. Solitons change gradually with temperature. Families
of intragap solitons are completely stable (both in direct simulations, and in
terms of eigenvalues of perturbation modes), while the intergap family may have
a very small unstable eigenvalue (nevertheless, they feature no instability in
direct simulations). Stable higher-order (multi-humped) solitons, and bound
complexes of fundamental solitons are found too. | cond-mat_other |
Mechanisms limiting the coherence time of spontaneous magnetic
oscillations driven by DC spin-polarized currents: The spin-transfer torque from a DC spin-polarized current can generate
highly-coherent magnetic precession in nanoscale magnetic-multilayer devices.
By measuring linewidths of spectra from the resulting resistance oscillations,
we argue that the coherence time can be limited at low temperature by thermal
deflections about the equilibrium magnetic trajectory, and at high temperature
by thermally-activated transitions between dynamical modes. Surprisingly, the
coherence time can be longer than predicted by simple macrospin simulations. | cond-mat_other |
Electronic states in 1/1 Cd6Yb and 1/1 Cd6Ca: Relativistic, correlation,
and structural effects: The electronic structure of the rational approximants 1/1 Cd6Yb and 1/1 Cd6Ca
to the stable icosahedral CdYb and CdCa quasicrystals is studied by the
full-potential linear augmented plane wave method. A comparison is made between
several structural models. We show that the (relativistic) spin-orbit (SO)
interaction and electronic correlations that are not described by the usual
local density approximation, are essential for an accurate description of the
electronic structure. In particular, we show that the SO interaction is
responsible for a splitting of the Cd-4d and Yb-4f peaks, and that the
experimental peak positions can be reproduced by including a Hubbard U term in
the Hamiltonian [U(Cd) = 5.6 eV, U(Yb) = 3.1 eV]. Our results show very good
agreement with a photo-emission (PE) spectrum of 1/1 Cd6Yb [R. Tamura, Y.
Murao, S. Takeuchi, T. Kiss, T. Yokoya, and S. Shin, Phys. Rev. B 65, 224207
(2002)] and a 350 eV PE spectrum of 1/1 Cd6Ca, which we present in this paper.
Without the relativistic and correlation effects even a qualitative agreement
with the PE spectra cannot be achieved. | cond-mat_other |
Magneto-optic far-infrared study of Sr$_{14}$Cu$_{24}$O$_{41}$: triplet
excitations in chains: Using far-infrared spectroscopy we have studied the magnetic field and
temperature dependence of the spin gap modes in the chains of
Sr$_{14}$Cu$_{24}$O$_{41}$. Two triplet modes T$_1$ and T$_2$ were found in the
center of the Brillouin zone at $\Delta_1=9.65$ meV and $\Delta_2=10.86$ meV in
zero magnetic field. The T$_1$ mode was excited when the electric field vector
${\bf E}$ of the light was polarized along the b axis (perpendicular to the
planes of chains and ladders) and T$_2$ was excited for ${\bf E}\parallel {\bf
a}$ (perpendicular to the chains and along the rungs). Up to the maximum
magnetic field of 18 T, applied along the chains, the electron $g$ factors of
these two modes were similar, $g_{1c}=2.049$ and $g_{2c}=2.044$. Full linewidth
at half maximum for both modes was 1 cm$^{-1}$ (0.12 meV) at 4K and increased
with $T$. The temperature dependence of mode energies and line intensities was
in agreement with the inelastic neutron scattering results from two groups
[Matsuda {\it et al.}, Phys. Rev. B {\bf 59}, 1060 (1999) and Regnault {\it et
al.}, Phys. Rev. B {\bf 59}, 1055 (1999)]. The T$_1$ mode has not been observed
by inelastic neutron scattering in the points of the $k$-space equivalent to
the center of the Brillouin zone. Our study indicates that the zone structure
model of magnetic excitations of Sr$_{14}$Cu$_{24}$O$_{41}$ must be modified to
include a triplet mode at 9.65 meV in the center of the magnetic Brillouin
zone. | cond-mat_other |
Low-frequency noise and tunnelling magnetoresistance in
Fe(110)/MgO(111)/Fe(110) epitaxial magnetic tunnel junctions: We report on tunnelling magnetoresistance (TMR), current-voltage (IV)
characteristics and low frequency noise in epitaxially grown
Fe(110)/MgO(111)/Fe(110) magnetic tunnel junctions (MTJs) with dimensions from
2x2 to 20x20 um2. The evaluated MgO energy barrier (0.50+/-0.08 eV), the
barrier width (13.1+/-0.5 angstrom) as well as the resistance times area
product (7+/-1 Mohmsum2) show relatively small variation, confirming a high
quality epitaxy and uniformity of all MTJs studied. The noise power, though
exhibiting large variation, was observed to be roughly anticorrelated with the
TMR. Surprisingly, for the largest junctions we observed a strong enhancement
of the normalized low-frequency noise in the antiparallel magnetic
configuration. This behaviour could be related to an interplay between the
magnetic state and the local barrier defects structure of the epitaxial MTJs | cond-mat_other |
Physical and mathematical justification of the numerical Brillouin zone
integration of the Boltzmann rate equation by Gaussian smearing: Scatterings of electrons at quasiparticles or photons are very important for
many topics in solid state physics, e.g., spintronics, magnonics or photonics,
and therefore a correct numerical treatment of these scatterings is very
important. For a quantum-mechanical description of these scatterings Fermi's
golden rule is used in order to calculate the transition rate from an initial
state to a final state in a first-order time-dependent perturbation theory. One
can calculate the total transition rate from all initial states to all final
states with Boltzmann rate equations involving Brillouin zone integrations. The
numerical treatment of these integrations on a finite grid is often done via a
replacement of the Dirac delta distribution by a Gaussian. The Dirac delta
distribution appears in Fermi's golden rule where it describes the energy
conservation among the interacting particles. Since the Dirac delta
distribution is a not a function it is not clear from a mathematical point of
view that this procedure is justified. We show with physical and mathematical
arguments that this numerical procedure is in general correct, and we comment
on critical points. | cond-mat_other |
On the evolution of higher order fluxes in non-equilibrium
thermodynamics: The connection between the balance structure of the evolution equations of
higher order fluxes and different forms of the entropy current is investigated
on the example of rigid heat conductors. Compatibility conditions of the
theories are given. Thermodynamic closure relations are derived. | cond-mat_other |
Quantum fluctuations of a vortex in a dilute Bose-Einstein condensate: A vortex in a quasi two-dimensional Bose-Einstein condensate is subject to
the Magnus force and can be effectively described as a planar particle in a
uniform magnetic field. Quantization of this effective particle leads to the
lowest Landau level where the most localized wave function is a gaussian. In
this gaussian state vortex position seems to fluctuate with an average
magnitude set by the magnetic width of the gaussian. We readdress this problem
using the number-conserving version of the Bogoliubov theory. We find that the
Bogoliubov mode that might be interpreted as a fluctuation of vortex position
actually does not contribute to position fluctuation at all. The only non-zero
contribution comes from phonons but it is an order of magnitude less than the
simple estimate, based on the magnetic width of the effective gaussian wave
packet. | cond-mat_other |
Ground State of Quantum Jahn-Teller Model: Selftrapping vs Correlated
Phonon-assisted Tunneling: Ground state of the quantum Jahn-Teller model with broken rotational symmetry
was investigated by the variational approach in two cases: a lattice and a
local ones. Both cases differ by the way of accounting for the nonlinearity
hidden in the reflection-symmetric Hamiltonian. In spite of that the ground
state energy in both cases shows the same features: there appear two regions of
model parameters governing the ground state: the region of dominating
selftrapping modified by the quantum effects and the region of dominating
phonon-assisted tunneling (antiselftrapping). In the local case (i) the effect
of quantum fluctuations and anharmonicity due to the two-mode correlations is
up to two orders larger than contributions due to the reflection effects of
two-center wave function; (ii) the variational results for the ground state
energy were compared with exact numerical results. The coincidence is the
better the more far away from the transition region at the E$\otimes$e symmetry
where the variational approach fails. | 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 |
Momentum distribution dynamics of a Tonks-Girardeau gas: Bragg
reflections of a quantum many-body wavepacket: The dynamics of the momentum distribution and the reduced single-particle
density matrix (RSPDM) of a Tonks-Girardeau (TG) gas is studied in the context
of Bragg-reflections of a many-body wavepacket. We find strong suppression of a
Bragg-reflection peak for a dense TG wavepacket; our observation illustrates
dependence of the momentum distribution on the interactions/wavefunction
symmetry. The momentum distribution is calculated with a fast algorithm based
on a formula expressing the RSPDM via a dynamically evolving single-particle
basis. | cond-mat_other |
Velocity correlations in dense granular flows: Velocity fluctuations of grains flowing down a rough inclined plane are
experimentally studied. The grains at the free surface exhibit fluctuating
motions, which are correlated over few grains diameters. The characteristic
correlation length is shown to depend on the inclination of the plane and not
on the thickness of the flowing layer. This result strongly supports the idea
that dense granular flows are controlled by a characteristic length larger than
the particle diameter. | cond-mat_other |
First-principles calculations of the phonon dispersion curves of H on
Pt(111): We have calculated the surface phonon dispersion curves for H on Pt(111),
using first-principles, total energy calculations based on a mixed-basis set
and norm-conserving pseudopotentials. Linear response theory and the harmonic
approximation are invoked. For one monolayer of H in the preferred adsorption
site (fcc hollow) vibrational modes polarized parallel and perpendicular to the
surface are found, respectively, at 73.5 meV and 142.6 meV, at the Γ point
of the surface Brillouin zone. The degeneracy of the parallel mode is lifted at
the zone boundaries, yielding energies of 69.6 meV and 86.3 meV at the M point
and 79.4 meV and 80.8 meV at the K point. The dispersion curves for H
adsorption at the hcp hollow site differ only slightly from the above. In
either case, H adsorption has considerable impact on the substrate modes; in
particular the surface mode in the gap in the bulk phonon spectrum (around M
point) is pushed into the bulk band. For on-top H adsorption, modes polarized
parallel and perpendicular to the surface have respective energies of 47.4 meV
and 277.2 meV, at the Γ point. The former disperses to 49.1 meV and 59.5
meV at the M point and to 56 meV and 56.7 meV at the K point. The H vibrational
mode polarized perpendicular to the surface shows little dispersion, in all
three cases considered. Insights are obtained from the hybridization of the H
and Pt electronic states. | cond-mat_other |
Self-focusing magnetostatic beams in thin magnetic films: The possibility of generation of stable self-focusing beams in in-plane
magnetized thin magnetic films is considered and theoretical conditions for the
existence of such localized solutions are discussed. It is shown that for the
definite direction between static magnetizing field and preferential direction
of radiation from microwave antenna the problem reduces to the one-dimensional
nonlinear Schroedinger equation. For such angles it is possible to generate
stable self-focusing beams. Particular values of beam width and propagation
angles versus magnitude of magnetizing field are calculated in order to suggest
the realistic experimental setup for the observation of discovered effect. | cond-mat_other |
Heteronuclear molecules in an optical dipole trap: We report on the creation and characterization of heteronuclear KRb Feshbach
molecules in an optical dipole trap. Starting from an ultracold gas mixture of
K-40 and Rb-87 atoms, we create as many as 25,000 molecules at 300 nK by rf
association. Optimizing the association process, we achieve a conversion
efficiency of 25%. We measure the temperature dependence of the rf association
process and find good agreement with a phenomenological model that has
previously been applied to Feshbach molecule creation by slow magnetic-field
sweeps. We also present a measurement of the binding energy of the
heteronuclear molecules in the vicinity of the Feshbach resonance and provide
evidence for Feshbach molecules as deeply bound as 26 MHz. | cond-mat_other |
Coherence-enhanced imaging of a degenerate Bose gas: We present coherence-enhanced imaging, an in situ technique that uses Raman
superradiance to probe the spatial coherence properties of an ultracold gas.
Applying this method, we obtain a spatially resolved measurement of the
condensate number and more generally, of the first-order spatial correlation
function in a gas of $^{87}$Rb atoms. We observe the enhanced decay of
propagating spin gratings in high density regions of a Bose condensate, a decay
we ascribe to collective, non-linear atom-atom scattering. Further, we directly
observe spatial inhomogeneities that arise generally in the course of extended
sample superradiance. | cond-mat_other |
Spectrum and Screening cloud in the central spin model: We consider an electronic spin in a quantum dot, coupled to the surrounding
nuclear spins via inhomogeneous antiferromagnetic hyperfine interactions and
subject to a uniform field, which is described by Gaudin's central spin model.
We study spectral properties, the two-point correlation functions, and the
magnetization profile in the ground state and in low-lying excited states,
which characterizes the structure of the cloud of nuclear spins screening the
electron spin. A close connection to the pair occupation probability in the
BCS-model is established. Using the exact Bethe Ansatz solution of that model
and arguments of integrability, we can distinguish between contributions from
purely classical physics and from quantum fluctuations. | cond-mat_other |
The role of the spin in quasiparticle interference: Quasiparticle interference patterns measured by scanning tunneling microscopy
(STM) can be used to study the local electronic structure of metal surfaces and
high temperature superconductors. Here, we show that even in non-magnetic
systems the spin of the quasiparticles can have a profound effect on the
interference patterns. On Bi(110), where the surface state bands are not
spin-degenerate, the patterns are not related to the dispersion of the
electronic states in a simple way. In fact, the features which are expected for
the spin-independent situation are absent and the observed interference
patterns can only be interpreted by taking spin-conserving scattering events
into account. | cond-mat_other |
Multiple time scales and the exponential Ornstein-Uhlenbeck stochastic
volatility model: We study the exponential Ornstein-Uhlenbeck stochastic volatility model and
observe that the model shows a multiscale behavior in the volatility
autocorrelation. It also exhibits a leverage correlation and a probability
profile for the stationary volatility which are consistent with market
observations. All these features make the model quite appealing since it
appears to be more complete than other stochastic volatility models also based
on a two-dimensional diffusion. We finally present an approximate solution for
the return probability density designed to capture the kurtosis and skewness
effects. | cond-mat_other |
Coherent control theory and experiment of optical phonons in diamond: The coherent control of optical phonons has been experimentally demonstrated
in various physical systems. While the transient dynamics for optical phonons
can be explained by phenomenological models, the coherent control experiment
cannot be explained due to the quantum interference. Here, we theoretically
propose the generation and detection processes of the optical phonons and
experimentally confirm our theoretical model using the diamond optical phonon
by the double-pump-probe type experiment. | cond-mat_other |
Propagation of self-localised Q-ball solitons in the $^3$He universe: In relativistic quantum field theories, compact objects of interacting bosons
can become stable owing to conservation of an additive quantum number $Q$.
Discovering such $Q$-balls propagating in the Universe would confirm
supersymmetric extensions of the standard model and may shed light on the
mysteries of dark matter, but no unambiguous experimental evidence exists. We
report observation of a propagating long-lived $Q$-ball in superfluid $^3$He,
where the role of $Q$-ball is played by a Bose-Einstein condensate of magnon
quasiparticles. We achieve accurate representation of the $Q$-ball Hamiltonian
using the influence of the number of magnons, corresponding to the charge $Q$,
on the orbital structure of the superfluid $^3$He order parameter. This
realisation supports multiple coexisting $Q$-balls which in future allows
studies of $Q$-ball dynamics, interactions, and collisions. | cond-mat_other |
Superfluid Edge Dislocation: Transverse Quantum Fluid: Recently, it has been argued by Kuklov et al., that unusual features
associated with the superflow-through-solid effect observed in solid He4 can be
explained by unique properties of dilute distribution of superfluid edge
dislocations. We demonstrate that stability of supercurrents controlled by
quantum phase slips (instantons), and other exotic infrared properties of the
superfluid dislocations readily follow from a one-dimensional quantum liquid
distinguished by an effectively infinite compressibility (in the absence of
Peierls potential) associated with the edge dislocation's ability to climb.
This establishes a new class of quasi-one-dimensional superfluid states that
remain stable and long-range ordered despite their low dimensionality. We
propose an experiment to test our mass-current--pressure characteristic
prediction. | cond-mat_other |
Generalisation of Gilbert damping and magnetic inertia parameter as a
series of higher-order relativistic terms: The phenomenological Landau-Lifshitz-Gilbert (LLG) equation of motion remains
as the cornerstone of contemporary magnetisation dynamics studies, wherein the
Gilbert damping parameter has been attributed to first-order relativistic
effects. To include magnetic inertial effects the LLG equation has previously
been extended with a supplemental inertia term and the arising inertial
dynamics has been related to second-order relativistic effects. Here we start
from the relativistic Dirac equation and, performing a Foldy-Wouthuysen
transformation, derive a generalised Pauli spin Hamiltonian that contains
relativistic correction terms to any higher order. Using the Heisenberg
equation of spin motion we derive general relativistic expressions for the
tensorial Gilbert damping and magnetic inertia parameters, and show that these
tensors can be expressed as series of higher-order relativistic correction
terms. We further show that, in the case of a harmonic external driving field,
these series can be summed and we provide closed analytical expressions for the
Gilbert and inertial parameters that are functions of the frequency of the
driving field. | cond-mat_other |
Fundamental dissipation due to bound fermions in the zero-temperature
limit: The ground state of a fermionic condensate is well protected against
perturbations in the presence of an isotropic gap. Regions of gap suppression,
surfaces and vortex cores which host Andreev-bound states, seemingly lift that
strict protection. Here we show that the role of bound states is more subtle:
when a macroscopic object moves in superfluid $^3$He at velocities exceeding
the Landau critical velocity, little to no bulk pair breaking takes place,
while the damping observed originates from the bound states covering the moving
object. We identify two separate timescales that govern the bound state
dynamics, one of them much longer than theoretically anticipated, and show that
the bound states do not interact with bulk excitations. | cond-mat_other |
Effective magnetic fields in degenerate atomic gases induced by light
beams with orbital angular momenta: We investigate the influence of two resonant laser beams on the mechanical
properties of degenerate atomic gases. The control and probe beams of light are
considered to have Orbital Angular Momenta (OAM) and act on the three-level
atoms in the Electromagnetically Induced Transparency (EIT) configuration. The
theory is based on the explicit analysis of the quantum dynamics of cold atoms
coupled with two laser beams. Using the adiabatic approximation, we obtain an
effective equation of motion for the atoms driven to the dark state. The
equation contains a vector potential type interaction as well as an effective
trapping potential. The effective magnetic field is shown to be oriented along
the propagation direction of the control and probe beams containing OAM. Its
spatial profile can be controlled by choosing proper laser beams. We
demonstrate how to generate a constant effective magnetic field, as well as a
field exhibiting a radial distance dependence. The resulting effective magnetic
field can be concentrated within a region where the effective trapping
potential holds the atoms. The estimated magnetic length can be considerably
smaller than the size of the atomic cloud. | cond-mat_other |
Exact soliton solution of Spin Chain with a external magnetic field in
linear wave background: Employing a simple, straightforward Darboux transformation we construct exact
N-soliton solution for anisotropic spin chain driven by a external magnetic
field in linear wave background. As a special case the explicit one- and
two-soliton solution dressed by the linear wave corresponding to magnon in
quantum theory is obtained analytically and its property is discussed in
detail. The dispersion law, effective soliton mass, and the energy of each
soliton are investigated as well. Our result show that the stability criterion
of soliton is related with anisotropic parameter and the amplitude of the
linear wave. | cond-mat_other |
Mass flux and solid growth in solid 4He: 60 mK - 700 mK: We use the thermo-mechanical effect to create a chemical potential difference
between two liquid reservoirs connected to each other through Vycor rods in
series with solid hcp 4He to confirm that a DC flux of atoms takes place below
600 mK, but find that the flux falls abruptly in the vicinity of 80 mK. It is
impossible to add density to a solid freshly made at 60 mK and samples freshly
made at 60 mK do not allow mass flux, even when raised in temperature to 200
mK. Solids created above 300 mK and cooled to 60 mK accept added density and
demonstrate finite mass flux. | cond-mat_other |
Range separation combined with the Overhauser model: Application to the
H$_2$ molecule along the dissociation curve: The combination of density-functional theory with other approaches to the
many-electron problem through the separation of the electron-electron
interaction into a short-range and a long-range contribution (range separation)
is a successful strategy, which is raising more and more interest in recent
years. We focus here on a range-separated method in which only the short-range
correlation energy needs to be approximated, and we model it within the
"extended Overhauser approach". We consider the paradigmatic case of the H$_2$
molecule along the dissociation curve, finding encouraging results. By means of
very accurate variational wavefunctions, we also study how the effective
electron-electron interaction appearing in the Overhauser model should be in
order to yield the exact correlation energy for standard Kohn-Sham density
functional theory. | cond-mat_other |
p-wave Feshbach molecules: We have produced and detected molecules using a p-wave Feshbach resonance
between 40K atoms. We have measured the binding energy and lifetime for these
molecules and we find that the binding energy scales approximately linearly
with magnetic field near the resonance. The lifetime of bound p-wave molecules
is measured to be 1.0 +/- 0.1 ms and 2.3 +/- 0.2 ms for the m_l = +/- 1 and m_l
= 0 angular momentum projections, respectively. At magnetic fields above the
resonance, we detect quasi-bound molecules whose lifetime is set by the
tunneling rate through the centrifugal barrier. | cond-mat_other |
Anyons from fermions with conventional two-body interactions: Emergent anyons are the key elements of the topological quantum computation
and topological quantum memory. We study a two-component fermion model with
conventional two-body interaction in an open boundary condition and show that
several subsets in the low-lying excitations obey the same fusion rules as
those of the toric code model. Those string-like non-local excitations in a
given subset obey mutual semionic statistics. We show how to peel off one of
such subset from other degenerate subsets and manipulate anyons in cold dipolar
Fermi atoms or cold dipolar fermionic heteronuclear molecules in optical
lattices by means of the established techniques. | cond-mat_other |
Spin motive forces due to magnetic vortices and domain walls: We study spin motive forces, i.e, spin-dependent forces, and voltages induced
by time-dependent magnetization textures, for moving magnetic vortices and
domain walls. First, we consider the voltage generated by a one-dimensional
field-driven domain wall. Next, we perform detailed calculations on
field-driven vortex domain walls. We find that the results for the voltage as a
function of magnetic field differ between the one-dimensional and vortex domain
wall. For the experimentally relevant case of a vortex domain wall, the
dependence of voltage on field around Walker breakdown depends qualitatively on
the ratio of the so-called $\beta$-parameter to the Gilbert damping constant,
and thus provides a way to determine this ratio experimentally. We also
consider vortices on a magnetic disk in the presence of an AC magnetic field.
In this case, the phase difference between field and voltage on the edge is
determined by the $\beta$ parameter, providing another experimental method to
determine this quantity. | cond-mat_other |
Magnetic Domain Wall Pumping by Spin Transfer Torque: We show that spin transfer torque from direct spin-polarized current applied
parallel to a magnetic domain wall (DW) induces DW motion in a direction
independent of the current polarity. This unidirectional response of the DW to
spin torque enables DW pumping: long-range DW displacement driven by
alternating current. Our numerical simulations reveal that DW pumping can be
resonantly amplified through excitation of internal degrees of freedom of the
DW by the current. | cond-mat_other |
Heteronuclear quantum gas mixtures: This PhD tutorial article is a review of our experiments on heteronuclear
quantum gas mixtures at the University of Hamburg. We introduce basic
properties of trapped Fermi-Bose mixtures and demonstrate the achievement of
large quantum degenerate mixtures of 40K and 87Rb. Using heteronuclear Feshbach
resonances, we show how the heteronuclear interaction can be tuned, allowing us
to induce phase separation and collapse for large repulsive and attractive
interactions, respectively. We realize Fermi-Bose mixtures in 3D optical
lattices as a novel quantum many-body system and study coherence properties of
the mixture. Combining our experiments on lattices and Feshbach resonances, we
present the first realization of ultracold heteronuclear Feshbach molecules.
The molecules are created at individual sites of a 3D optical lattice. We
discuss lifetime, binding energy and rf association efficiency in terms of a
universal model and give an outlook for possible future developments. | cond-mat_other |
Dynamics of a Bright Soliton in Bose-Einstein condensates with
Time-Dependent Atomic Scattering Length in an Expulsive Parabolic Potential: We present a family of exact solutions of one-dimensional nonlinear
Schr\"odinger equation, which describe the dynamics of a bright soliton in
Bose-Einstein condensates with the time-dependent interatomic interaction in an
expulsive parabolic potential. Our results show that, under the safe range of
parameters, the bright soliton can be compressed into very high local matter
densities by increasing the absolute value of atomic scattering length, which
can provide an experimental tool for investigating the range of validity of the
one-dimensional Gross-Pitaevskii equation. We also find that the number of
atoms in the bright soliton keeps dynamic stability: a time-periodic atomic
exchange is formed between the bright soliton and the background. | cond-mat_other |
Cross-sections of Andreev scattering by quantized vortex rings in 3He-B: We studied numerically the Andreev scattering cross-sections of
three-dimensional isolated quantized vortex rings in superfluid 3He-B at
ultra-low temperatures. We calculated the dependence of the cross-section on
the ring's size and on the angle between the beam of incident thermal
quasiparticle excitations and the direction of the ring's motion. We also
introduced, and investigated numerically, the cross-section averaged over all
possible orientations of the vortex ring; such a cross-section may be
particularly relevant for the analysis of experimental data. We also analyzed
the role of screening effects for Andreev reflection of quasiparticles by
systems of vortex rings. Using the results obtained for isolated rings we found
that the screening factor for a system of unlinked rings depends strongly on
the average radius of the vortex ring, and that the screening effects increase
with decreasing the rings' size. | cond-mat_other |
Modelling the term structure of interest rates á la
Heath-Jarrow-Morton but with non Gaussian fluctuations: We consider a generalization of the Heath Jarrow Morton model for the term
structure of interest rates where the forward rate is driven by Paretian
fluctuations. We derive a generalization of It\^{o}'s lemma for the calculation
of a differential of a Paretian stochastic variable and use it to derive a
Stochastic Differential Equation for the discounted bond price. We show that it
is not possible to choose the parameters of the model to ensure absence of
drift of the discounted bond price. Then we consider a Continuous Time Random
Walk with jumps driven by Paretian random variables and we derive the large
time scaling limit of the jump probability distribution function (pdf). We show
that under certain conditions defined in text the large time scaling limit of
the jump pdf in the Fourier domain is \tilde{omega}_t(k,t) \sim \exp{-K/(\ln(k
t))^2} and is different from the case of a random walk with Gaussian
fluctuations. We also derive the master equation for the jump pdf and discuss
the relation of the master equation to Distributed Order Fractional Diffusion
Equations. | cond-mat_other |
High domain wall velocities induced by current in ultrathin Pt/Co/AlOx
wires with perpendicular magnetic anisotropy: Current-induced domain wall (DW) displacements in an array of ultrathin
Pt/Co/AlOx wires with perpendicular magnetic anisotropy have been directly
observed by wide field Kerr microscopy. DWs in all wires in the array were
driven simultaneously and their displacement on the micrometer-scale was
controlled by the current pulse amplitude and duration. At the lower current
densities where DW displacements were observed (j less than or equal to 1.5 x
10^12 A/m^2), the DW motion obeys a creep law. At higher current density (j =
1.8 x 10^12 A/m^2), zero-field average DW velocities up to 130 +/- 10 m/s were
recorded. | cond-mat_other |
Strongly Interacting Atoms and Molecules in a 3D Optical Lattice: We report on the realization of a strongly interacting quantum degenerate gas
of fermionic atoms in a three-dimensional optical lattice. We prepare a
band-insulating state for a two-component Fermi gas with one atom per spin
state per lattice site. Using a Feshbach resonance, we induce strong
interactions between the atoms. When sweeping the magnetic field from the
repulsive side towards the attractive side of the Feshbach resonance we induce
a coupling between Bloch bands leading to a transfer of atoms from the lowest
band into higher bands. Sweeping the magnetic field across the Feshbach
resonance from the attractive towards the repulsive side leads to two-particle
bound states and ultimately to the formation of molecules. From the fraction of
formed molecules we determine the temperature of the atoms in the lattice. | cond-mat_other |
Density modulations in an elongated Bose-Einstein condensate released
from a disordered potential: We observe large density modulations in time-of-flight images of elongated
Bose-Einstein condensates, initially confined in a harmonic trap and in the
presence of weak disorder. The development of these modulations during the
time-of-flight and their dependence with the disorder are investigated. We
render an account of this effect using numerical and analytical calculations.
We conclude that the observed large density modulations originate from the weak
initial density modulations induced by the disorder, and not from initial phase
fluctuations (thermal or quantum). | cond-mat_other |
On surface plasmon polariton wavepacket dynamics in metal-dielectric
heterostructures: The WKB equations for dynamics of the surface plasmon polariton (SPP)
wavepacket are studied. The dispersion law for the SPP in the metal-dielectric
heterostructure with varying thickness of a perforated dielectric layer is
rigorously calculated and investigated using the scattering matrix method. Two
channels of the SPP wavepacket optical losses related to the absorption in a
metal and to the SPP leakage are analyzed. It is shown that change of the
dielectric layer thickness acts on the SPP as an external force leading to
evolution of its quasimomentum and to the wavepacket reversal or even to the
optical Bloch oscillations (BO). Properties of these phenomena are investigated
and discussed. Typical values of the BO amplitude are about tens of microns and
the period is around tens or hundreds of femtoseconds. | cond-mat_other |
Nanoscale spin-polarization in dilute magnetic semiconductor (In,Mn)Sb: Results of point contact Andreev reflection (PCAR) experiments on (In,Mn)Sb
are presented and analyzed in terms of current models of charge conversion at a
superconductor-ferromagnet interface. We investigate the influence of surface
transparency, and study the crossover from ballistic to diffusive transport
regime as contact size is varied. Application of a Nb tip to a (In,Mn)Sb sample
with Curie temperature Tc of 5.4 K allowed the determination of
spin-polarization when the ferromagnetic phase transition temperature is
crossed. We find a striking difference between the temperature dependence of
the local spin polarization and of the macroscopic magnetization, and
demonstrate that nanoscale clusters with magnetization close to the saturated
value are present even well above the magnetic phase transition temperature. | cond-mat_other |
Phase diagram of a Bose gas near a wide Feshbach resonance: In this paper, we study the phase diagram of a homogeneous Bose gas with a
repulsive interaction near a wide Feshbach resonance at zero temperature. The
Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the
scattering length $a$ exceeds a critical value depending on the atom density
$n$, $na^3>0.035$, the molecular excitation energy is imaginary and the atomic
BEC state is dynamically unstable against molecule formation. The BEC state of
diatomic molecules has lower energy, where the atomic excitation is gapped and
the molecular excitation is gapless. However when the scattering length is
above another critical value, $na^3>0.0164$, the molecular BEC state becomes a
unstable coherent mixture of atoms and molecules. In both BEC states, the
binding energy of diatomic molecules is reduced due to the many-body effect. | cond-mat_other |
Simple derivation of the frequency dependent complex heat capacity: This paper gives a simple derivation of the well-known expression of the
frequency dependent complex heat capacity in modulated temperature experiments.
It aims at clarified again that the generalized calorimetric susceptibility is
only due to the non-equilibrium behaviour occurring in the vicinity of
thermodynamic equilibrium of slow internal degrees of freedom of a sample when
the temperature oscillates at a well determined frequency. | cond-mat_other |
PT-Symmetric Electronics: We show both theoretically and experimentally that a pair of inductively
coupled active LRC circuits (dimer), one with amplification and another with an
equivalent amount of attenuation, display all the features which characterize a
wide class of non-Hermitian systems which commute with the joint parity-time PT
operator: typical normal modes, temporal evolution, and scattering processes.
Utilizing a Liouvilian formulation, we can define an underlying PT-symmetric
Hamiltonian, which provides important insight for understanding the behavior of
the system. When the PT-dimer is coupled to transmission lines, the resulting
scattering signal reveals novel features which reflect the PT-symmetry of the
scattering target. Specifically we show that the device can show two different
behaviors simultaneously, an amplifier or an absorber, depending on the
direction and phase relation of the interrogating waves. Having an exact
theory, and due to its relative experimental simplicity, PT-symmetric
electronics offers new insights into the properties of PT-symmetric systems
which are at the forefront of the research in mathematical physics and related
fields. | cond-mat_other |
Number statistics of molecules formed from ultra-cold atoms: We calculate the number statistics of a single-mode molecular field excited
by photoassociation or via a Feshbach resonance from an atomic Bose-Einstein
condensate (BEC), a normal atomic Fermi gas and a Fermi system with pair
correlations (BCS state). We find that the molecule formation from a BEC is a
collective process that leads for short times to a coherent molecular state in
the quantum optical sense. Atoms in a normal Fermi gas, on the other hand, are
converted into molecules independently of each other and result for short times
in a molecular state analogous to that of a classical chaotic light source. The
BCS situation is intermediate between the two and goes from producing an
incoherent to a coherent molecular field with increasing gap parameter. | cond-mat_other |
Determination of anisotropic dipole moments in self-assembled quantum
dots using Rabi oscillations: By investigating the polarization-dependent Rabi oscillations using
photoluminescence spectroscopy, we determined the respective transition dipole
moments of the two excited excitonic states |Ex> and |Ey> of a single
self-assembled quantum dot that are nondegenerate due to shape anisotropy. We
find that the ratio of the two dipole moments is close to the physical
elongation ratio of the quantum dot. | cond-mat_other |
Chern-number spin Hamiltonians for magnetic nano-clusters by DFT methods: Combining field-theoretical methods and ab-initio calculations, we construct
an effective Hamiltonian with a single giant-spin degree of freedom, capable of
the describing the low-energy spin dynamics of ferromagnetic metal nanoclusters
consisting of up to a few tens of atoms. In our procedure, the magnetic moment
direction of the Kohn-Sham SDFT wave-function is constrained by means of a
penalty functional, allowing us to explore the entire parameter space of
directions, and to extract the magnetic anisotropy energy and Berry curvature
functionals. The average of the Berry curvature over all magnetization
directions is a Chern number - a topological invariant that can only take on
values equal to multiples of one half, representing the dimension of the
Hilbert space of the effective spin system. The spin Hamiltonian is obtained by
quantizing the classical anisotropy energy functional, after performing a
change of variables to a constant Berry curvature space. The purpose of this
article is to examine the impact of the topological effect from the Berry
curvature on the low-energy total-spin-system dynamics. To this end, we study
small transition metal clusters: Co$_{n}$ ($n=2,...,5$), Rh$_{2}$, Ni$_{2}$,
Pd$_{2}$, Mn$_{x}$N$_{y}$, Co$_{3}$Fe$_{2}$. | cond-mat_other |
3He on preplated graphite: By using the diffusion Monte Carlo method, we obtained the full phase diagram
of $^3$He on top of graphite preplated with a solid layer of $^4$He. All the
$^4$He atoms of the substrate were explicitly considered and allowed to move
during the simulation. We found that the ground state is a liquid of density
0.007 $\pm$ 0.001 \AA$^{-2}$, in good agreement with available experimental
data. This is significantly different from the case of $^3$He on clean
graphite, in which both theory and experiment agree on the existence of a
gas-liquid transition at low densities. Upon an increase in $^3$He density, we
predict a first-order phase transition between a dense liquid and a registered
7/12 phase, the 4/7 phase being found metastable in our calculations. At larger
second-layer densities, a final transition is produced to an incommensurate
triangular phase. | cond-mat_other |
Dipole moments from atomic-number-dependent potentials in analytic
density-functional theory: Molecular dipole moments of analytic density-functional theory are
investigated. The effect of element-dependent exchange potentials on these
moments are examined by comparison with conventional quantum-chemical methods
and experiment for the subset of the extended G2 set of molecules that have
nonzero dipole moment. Fitting the Kohn-Sham [Phys. Rev. 140, A1133 (1965)]
potential itself makes a mean absolute error of less than 0.1 Debye. Variation
of alpha (Slater's [Phys. Rev. 81, 385 (1951)] exchange parameter) values has
far less effect on dipole moments than on energies. It is argued that in
variable alpha methods one should choose the smaller of the two rather than the
geometric mean of the two alpha values for the heteroatomic part of the
linear-combination-atomic-orbital density. Calculations on the dipole moment of
NH2(CH)24NO2 are consistent with earlier calculations and show that varying the
differences between alpha values for atoms with different atomic numbers has
only short-ranged electrostatic effects. | cond-mat_other |
Terahertz Kerr effect: We have observed optical birefringence in liquids induced by single-cycle THz
pulses with field strengths exceeding 100 kV/cm. The induced change in
polarization is proportional to the square of the THz electric field. The
time-dependent THz Kerr signal is composed of a fast electronic response that
follows the individual cycles of the electric field and a slow exponential
response associated with molecular orientation. | cond-mat_other |
Efficient unidirectional nanoslit couplers for surface plasmons: Plasmonics is based on surface plasmon polariton (SPP) modes which can be
laterally confined below the diffraction limit, thereby enabling ultracompact
optical components. In order to exploit this potential, the fundamental
bottleneck of poor light-SPP coupling must be overcome. In established SPP
sources (using prism, grating} or nanodefect coupling) incident light is a
source of noise for the SPP, unless the illumination occurs away from the
region of interest, increasing the system size and weakening the SPP intensity.
Back-side illumination of subwavelength apertures in optically thick metal
films eliminates this problem but does not ensure a unique propagation
direction for the SPP. We propose a novel back-side slit-illumination method
based on drilling a periodic array of indentations at one side of the slit. We
demonstrate that the SPP running in the array direction can be suppressed, and
the one propagating in the opposite direction enhanced, providing localized
unidirectional SPP launching. | cond-mat_other |
Numerical study of gas dynamic processes in conditions of silver
nanocluster deposition experiments: In this paper, experiments on the deposition of silver nanoclusters are
analysed numerically using improved DSMC method. These experiments were made in
the Institute of Thermophysics SB RAS recent years, in the context of the
development of bactericidal nanocomposite coatings deposition technology. In
this paper we analyze the gas-dynamic effects in experimental conditions: the
parameters of the gas stream from the silver vapor source, the flow of silver
vapors inside the source and in the outflowing jet under different conditions,
the movement of silver nanoclusters of different masses (up to 1024 atoms) in
buffer gas, expecting different places of nanocluster formation in a source.
The simulations revealed a strong nonisothermal state of the source, which was
later confirmed experimentally, the site of nanocluster nulceus formation in a
source (subcooled confuser of the nozzle) and, finally, show that the process
of nucleation of nanoclusters inside a source is of heterogeneous nature. The
width of the silver nanocluster jet (of mass 1024 atoms) reaching target
according to sumulations, is in good agreement with that obtained in the
experiment coating profile on the target substrate in a form of narrow strip
made of stainless steel, that further validates the numerical simulations. | cond-mat_other |
Fundamental dissipation due to bound fermions in the zero-temperature
limit: The ground state of a fermionic condensate is well protected against
perturbations in the presence of an isotropic gap. Regions of gap suppression,
surfaces and vortex cores which host Andreev-bound states, seemingly lift that
strict protection. Here we show that the role of bound states is more subtle:
when a macroscopic object moves in superfluid $^3$He at velocities exceeding
the Landau critical velocity, little to no bulk pair breaking takes place,
while the damping observed originates from the bound states covering the moving
object. We identify two separate timescales that govern the bound state
dynamics, one of them much longer than theoretically anticipated, and show that
the bound states do not interact with bulk excitations. | cond-mat_other |
Effect of the Zero-Mode on the Response of a Trapped Bose-Condensed Gas: The dynamical response of a trapped Bose-Einstein condensate (BEC) is
formulated consistently with quantum field theory and is numerically evaluated.
We regard the BEC as a manifestation of the breaking of the global phase
symmetry. Then, the Goldstone theorem implies the existence of a zero energy
excitation mode (the zero-mode). We calculate the effect of the zero-mode on
the response frequency and show that the contribution of the zero-mode to the
first excitation mode is not so important in the parameter set realized in the
existing experiment. This is the reason that experimental results can be
described using the Bogoliubov prescription, although it breaks the consistency
of the description in quantum field theory. | cond-mat_other |
Nucleation of helium in liquid lithium: Fusion energy stands out as a promising alternative for a future decarbonised
energy system. To be sustainable, future fusion nuclear reactors will have to
produce their own tritium. In the so-called breeding blanket of a reactor, the
neutron bombardment of lithium will produce the desired tritium, but also
helium, which can trigger nucleation mechanisms owing to the very low
solubility of helium in liquid metals. An understanding of the underlying
microscopic processes is important for improving the efficiency, sustainability
and reliability of the fusion energy conversion process. A spontaneous creation
of helium drops or bubbles in the liquid metal used as breeding material in
some designs may be a serious issue for the performance of the breeding
blankets. This phenomenon has yet to be fully studied and understood. This work
aims to provide some insight on the behavior of lithium and helium mixtures at
experimentally corresponding operating conditions (843 K and pressures between
0.1 and 7 GPa). We report a microscopic study of the thermodynamic, structural
and dynamical properties of lithium-helium mixtures, as a first step to the
simulation of the environment in a nuclear fusion power plant. We introduce a
microscopic model devised to describe the formation of helium drops in the
thermodynamic range considered. A transition from a miscible homogeneous
mixture to a phase-separated one, in which helium drops are nucleated, is
observed as the pressure is increased above 0.175 GPa. The diffusion
coefficient of lithium (2 {\AA} 2 /ps) is in excellent agreement with reference
experimental data, whereas the diffusion coefficient of helium is in the range
of 1 {\AA} 2 /ps and tends to decrease as pressure increases. The radii of
helium drops have been found to be between 1 and 2 {\AA}. | cond-mat_other |
Quantum Ratchets at High Temperatures: Using the continued-fraction method we solve the Caldeira-Leggett master
equation in the phase-space (Wigner) representation to study Quantum ratchets.
Broken spatial symmetry, irreversibility and periodic forcing allows for a net
current in these systems. We calculate this current as a function of the force
under adiabatic conditions. Starting from the classical limit we make the
system quantal. In the quantum regime tunnel events and over-barrier wave
reflection phenomena modify the classical result. Finally, using the
phase-space formalism we give some insights about the decoherence in these
systems. | cond-mat_other |
Three-dimensional character of atom-chip-based rf-dressed potentials: We experimentally investigate the properties of radio-frequency-dressed
potentials for Bose-Einstein condensates on atom chips. The three-dimensional
potential forms a connected pair of parallel waveguides. We show that
rf-dressed potentials are robust against the effect of small magnetic-field
variations on the trap potential. Long-lived dipole oscillations of condensates
induced in the rf-dressed potentials can be tuned to a remarkably low damping
rate. We study a beam-splitter for Bose-Einstein condensates and show that a
propagating condensate can be dynamically split in two vertically separated
parts and guided along two paths. The effect of gravity on the potential can be
tuned and compensated for using a rf-field gradient. | cond-mat_other |
Quench dynamics and non equilibrium phase diagram of the Bose-Hubbard
model: We investigate the time evolution of correlations in the Bose-Hubbard model
following a quench from the superfluid to the Mott insulating phase. For large
values of the final interaction strength the system approaches a distinctly
non-equilibrium steady state that bears strong memory of the initial
conditions. In contrast, when the final interaction strength is comparable to
the hopping, the correlations are rather well approximated by those at thermal
equilibrium. The existence of two distinct non-equilibrium regimes is
surprising given the non-integrability of the Bose-Hubbard model. We relate
this phenomena to the role of quasi-particle interactions in the Mott
insulating state. | cond-mat_other |
Static Electric Field in a 1D Systems without Boundaries: In this brief report, we show that in a 1D system with unit-cell doubling,
the coefficient of the $\theta$-term is not only determined the topological
index, $\int i\bra{u_k}\frac{\d}{\d k}\ket{u_k}{\rm d}k$. Specifically, the
relative position between the electronic orbitals and the ions also alters the
coefficient. This resolves a paradox when we apply our previous result to the
Su-Shreiffer-Heeger model where the two ground states related by a lattice
translation have $\theta$ differed by $\pi$. We also show that the static
dielectric screening is the same with or without boundaries, on the contrary to
what we have commented in our previous paper. | cond-mat_other |
The Kelvin-wave cascade in the vortex filament model: The energy transfer mechanism in zero temperature superfluid turbulence of
helium-4 is still a widely debated topic. Currently, the main hypothesis is
that weakly nonlinear interacting Kelvin waves transfer energy to sufficiently
small scales such that energy is dissipated as heat via phonon excitations.
Theoretically, there are at least two proposed theories for Kelvin-wave
interactions. We perform the most comprehensive numerical simulation of weakly
nonlinear interacting Kelvin-waves to date and show, using a specially designed
numerical algorithm incorporating the full Biot-Savart equation, that our
results are consistent with nonlocal six-wave Kelvin wave interactions as
proposed by L'vov and Nazarenko. | cond-mat_other |
Polarization fine-structure and enhanced single-photon emission of
self-assembled lateral InGaAs quantum dot molecules embedded in a planar
micro-cavity: Single lateral InGaAs quantum dot molecules have been embedded in a planar
micro-cavity in order to increase the luminescence extraction efficiency. Using
a combination of metal-organic vapor phase and molecular beam epitaxy samples
could be produced that exhibit a 30 times enhanced single-photon emission rate.
We also show that the single-photon emission is fully switchable between two
different molecular excitonic recombination energies by applying a lateral
electric field. Furthermore, the presence of a polarization fine-structure
splitting of the molecular neutral excitonic states is reported which leads to
two polarization-split classically correlated biexciton exciton cascades. The
fine-structure splitting is found to be on the order of 10 micro-eV. | cond-mat_other |
Correlations in Ultracold Trapped Few-Boson Systems: Transition from
Condensation to Fermionization: We study the correlation properties of the ground states of few ultracold
bosons, trapped in double wells of varying barrier height in one dimension.
Extending previous results on the signature of the transition from a
Bose-condensed state via fragmentation to the hard-core limit, we provide a
deeper understanding of that transition by relating it to the loss of coherence
in the one-body density matrix and to the emerging long-range tail in the
momentum spectrum. These are accounted for in detail by discussing the natural
orbitals and their occupations. Our discussion is complemented by an analysis
of the two-body correlation function. | cond-mat_other |
Shape dynamics during deposit of simple metal clusters on rare gas
matrices: Using a combined quantum mechanical/classical method, we study the collisions
of small Na clusters on large Ar clusters as a model for cluster deposit. We
work out basic mechanisms by systematic variation of collision energy, system
sizes, and orientations. The soft Ar material is found to serve as an extremely
efficient shock absorber. The collisional energy is quickly transfered at first
impact and the Na clusters are always captured by the Ar surface. The
distribution of the collision energy into the Ar system proceeds very fast with
velocity of sound. The relaxation of shapes goes at a slower pace using times
of several ps. It produces a substantial rearrangement of the Ar system while
the Na cluster remains rather robust. | cond-mat_other |
Adiabatic Connection in the Low-Density Limit: In density functional theory (DFT), the exchange-correlation functional can
be exactly expressed by the adiabatic connection integral. It has been noticed
that as lambda goes to infinity, the lambda^(-1) term in the expansion of
W(lambda) vanishes. We provide a simple but rigorous derivation to this exact
condition in this work. We propose a simple parametric form for the integrand,
satisfying this condition, and show that it is highly accurate for
weakly-correlated two-electron systems. | cond-mat_other |
Sodium Bose-Einstein Condensates in an Optical Lattice: The phase transition from a superfluid to a Mott insulator has been observed
in a $^{23}$Na Bose-Einstein condensate. A dye laser detuned $\approx 5$nm red
of the Na $3^2$S$ \to 3^2$P$_{1/2}$ transition was used to form the three
dimensional optical lattice. The heating effects of the small detuning as well
as the three-body decay processes constrained the timescale of the experiment.
Certain lattice detunings were found to induce a large loss of atoms. These
loss features were shown to be due to photoassociation of atoms to vibrational
levels in the Na$_2$ $(1) ^3\Sigma_g^+$ state. | cond-mat_other |
High intermodulation gain in a micromechanical Duffing resonator: In this work we use a micromechanical resonator to experimentally study small
signal amplification near the onset of Duffing bistability. The device consists
of a PdAu beam serving as a micromechanical resonator excited by an adjacent
gate electrode. A large pump signal drives the resonator near the onset of
bistability, enabling amplification of small signals in a narrow bandwidth. To
first order, the amplification is inversely proportional to the frequency
difference between the pump and signal. We estimate the gain to be about 15dB
for our device. | cond-mat_other |
Extreme times in financial markets: We apply the theory of continuous time random walks to study some aspects of
the extreme value problem applied to financial time series. We focus our
attention on extreme times, specifically the mean exit time and the mean
first-passage time. We set the general equations for these extremes and
evaluate the mean exit time for actual data. | cond-mat_other |
Dynamics of the vortex line density in superfluid counterflow turbulence: Describing superfluid turbulence at intermediate scales between the
inter-vortex distance and the macroscale requires an acceptable equation of
motion for the density of quantized vortex lines $\cal{L}$. The closure of such
an equation for superfluid inhomogeneous flows requires additional inputs
besides $\cal{L}$ and the normal and superfluid velocity fields. In this paper
we offer a minimal closure using one additional anisotropy parameter $I_{l0}$.
Using the example of counterflow superfluid turbulence we derive two coupled
closure equations for the vortex line density and the anisotropy parameter
$I_{l0}$ with an input of the normal and superfluid velocity fields. The
various closure assumptions and the predictions of the resulting theory are
tested against numerical simulations. | cond-mat_other |
Non-equilibrium dynamics of a Bose-Einstein condensate in an optical
lattice: The dynamical evolution of a Bose-Einstein condensate trapped in a
one-dimensional lattice potential is investigated theoretically in the
framework of the Bose-Hubbard model. The emphasis is set on the
far-from-equilibrium evolution in a case where the gas is strongly interacting.
This is realized by an appropriate choice of the parameters in the Hamiltonian,
and by starting with an initial state, where one lattice well contains a
Bose-Einstein condensate while all other wells are empty. Oscillations of the
condensate as well as non-condensate fractions of the gas between the different
sites of the lattice are found to be damped as a consequence of the collisional
interactions between the atoms. Functional integral techniques involving
self-consistently determined mean fields as well as two-point correlation
functions are used to derive the two-particle-irreducible (2PI) effective
action. The action is expanded in inverse powers of the number of field
components N, and the dynamic equations are derived from it to next-to-leading
order in this expansion. This approach reaches considerably beyond the
Hartree-Fock-Bogoliubov mean-field theory, and its results are compared to the
exact quantum dynamics obtained by A.M. Rey et al., Phys. Rev. A 69, 033610
(2004) for small atom numbers. | cond-mat_other |
Bound states of attractive Bose-Einstein condensates in shallow traps in
two and three dimensions: Using variational and numerical solutions of the mean-field Gross-Pitaevskii
equation for attractive interaction (with cubic or Kerr nonlinearity) we show
that a stable bound state can appear in a Bose-Einstein condensate (BEC) in a
localized exponentially-screened radially-symmetric harmonic potential well in
two and three dimensions. We also consider an axially-symmetric configuration
with zero axial trap and a exponentially-screened radial trap so that the
resulting bound state can freely move along the axial direction like a soliton.
The binding of the present states in shallow wells is mostly due to the
nonlinear interaction with the trap playing a minor role. Hence these BEC
states are more suitable to study the effect of the nonlinear force on the
dynamics. We illustrate the highly nonlinear nature of breathing oscillation of
these states. Such bound states could be created in BECs and studied in the
laboratory with present knowhow. | cond-mat_other |
Stationary waves in a supersonic flow of a two-component Bose gas: A stationary wave pattern occurring in a flow of a two-component
Bose-Einstein condensate past an obstacle is studied. We consider the general
case of unequal velocities of two superfluid components. The Landau criterium
applied to the two-component system determines a certain region in the velocity
space in which superfluidity may take place. Stationary waves arise out of this
region, but under the additional condition that the relative velocity of the
components does not exceed some critical value. Under increase of the relative
velocity the spectrum of the excitations becomes complex valued and the
stationary wave pattern is broken. In case of equal velocities two sets of
stationary waves that correspond to the lower and the upper Bogolyubov mode can
arise. If one component flows and the other is at rest only one set of waves
may emerge. Two or even three interfere sets of waves may arise if the
velocities approximately of equal value and the angle between the velocities is
close to pi/2. In two latter cases the stationary waves correspond to the lower
mode and the densities of the components oscillate out-of-phase. The ratio of
amplitudes of the components in the stationary waves is computed. This quantity
depends on the relative velocity, is different for different sets of waves, and
varies along the crests of the waves. For the cases where two or three waves
interfere the density images are obtained. | cond-mat_other |
Quantum Monte Carlo study of ring-shaped polariton parametric
luminescence in a semiconductor microcavity: We present a quantum Monte Carlo study of the quantum correlations in the
parametric luminescence from semiconductor microcavities in the strong
exciton-photon coupling regime. As already demonstrated in recent experiments,
a ring-shaped emission is obtained by applying two identical pump beams with
opposite in-plane wavevectors, providing symmetrical signal and idler beams
with opposite in-plane wavevectors on the ring. We study the squeezing of the
signal-idler difference noise across the parametric instability threshold,
accounting for the radiative and non-radiative losses, multiple scattering and
static disorder. We compare the results of the complete multimode Monte Carlo
simulations with a simplified linearized quantum Langevin analytical model. | cond-mat_other |
Bose-Einstein condensate in a quartic potential: Static and Dynamic
properties: In this paper, we present a theoretical study of a Bose-Einstein condensate
of interacting bosons in a quartic trap in one, two, and three dimensions.
Using Thomas-Fermi approximation, suitably complemented by numerical solutions
of the Gross-Pitaevskii equation, we study the ground sate condensate density
profiles, the chemical potential, the effects of cross-terms in the quartic
potential, temporal evolution of various energy components of the condensate,
and width oscillations of the condensate. Results obtained are compared with
corresponding results for a bose condensate in a harmonic confinement. | cond-mat_other |
Potential energy threshold for nano-hillock formation by impact of slow
highly charged ions on a CaF$_2$(111) surface: We investigate the formation of nano-sized hillocks on the (111) surface of
CaF$_2$ single crystals by impact of slow highly charged ions. Atomic force
microscopy reveals a surprisingly sharp and well-defined threshold of potential
energy carried into the collision of about 14 keV for hillock formation.
Estimates of the energy density deposited suggest that the threshold is linked
to a solid-liquid phase transition (``melting'') on the nanoscale. With
increasing potential energy, both the basal diameter and the height of the
hillocks increase. The present results reveal a remarkable similarity between
the present predominantly potential-energy driven process and track formation
by the thermal spike of swift ($\sim$ GeV) heavy ions. | cond-mat_other |
Nanoelectromechanical systems based on multi-walled nanotubes:
nanothermometer, nanorelay and nanoactuator: We report on three new types of nanoelectromechanical systems based on carbon
nanotubes: an electromechanical nanothermometer, a nanorelay and a nanomotor.
The nanothermometer can be used for accurate temperature measurements in
spatially localized regions with dimensions of several hundred nanometers. The
nanorelay is a prototype of a memory cell, and the nanoactuator can be used for
transformation of the forward force into the relative rotation of the walls.
Relative motion of the walls in these nanosystems is defined by the shape of
the interwall interaction energy surface. Ab initio and semi-empirical
calculations have been used to estimate the operational characteristics and
dimensions of these nanosystems. | cond-mat_other |
Thomas-Fermi Screening in Graphene: The in-plane static screening of the field originated by a charge placed in a
graphene sheet is investigated. A self-consistent field equation in the real
space domain is obtained by using a suitable Thomas-Fermi procedure. Exact and
approximated (for qualitative considerations) solutions are presented. In the
case of a charged sheet, the screened potential presents a tail dependent on
the free carrier density whose importance is connected with the local features
of the impurity system. Early conclusions about Thomas-Fermi screening in
graphene are revised. | cond-mat_other |
Exciton Spin Dynamics in Semiconductor Quantum Wells: In this paper we will review Exciton Spin Dynamics in Semiconductor Quantum
Wells. The spin properties of excitons in nanostructures are determined by
their fine structure. We will mainly focus in this review on GaAs and InGaAs
quantum wells which are model systems. | cond-mat_other |
Nonlinear Electrokinetics at large applied voltages: The classical theory of electrokinetic phenomena assumes a dilute solution of
point-like ions in chemical equilibrium with a surface whose double-layer
voltage is of order the thermal voltage, $k_BT/e = 25$ mV. In nonlinear
``induced-charge'' electrokinetic phenomena, such as AC electro-osmosis,
several Volts $\approx 100 k_BT/e$ are applied to the double layer, so the
theory breaks down and cannot explain many observed features. We argue that,
under such a large voltage, counterions ``condense'' near the surface, even for
dilute bulk solutions. Based on simple models, we predict that the double-layer
capacitance decreases and the electro-osmotic mobility saturates at large
voltages, due to steric repulsion and increased viscosity of the condensed
layer, respectively. The former suffices to explain observed high frequency
flow reversal in AC electro-osmosis; the latter leads to a salt concentration
dependence of induced-charge flows comparable to experiments, although a
complete theory is still lacking. | cond-mat_other |
Numerical observation of Hawking radiation from acoustic black holes in
atomic Bose-Einstein condensates: We report numerical evidence of Hawking emission of Bogoliubov phonons from a
sonic horizon in a flowing one-dimensional atomic Bose-Einstein condensate. The
presence of Hawking radiation is revealed from peculiar long-range patterns in
the density-density correlation function of the gas. Quantitative agreement
between our fully microscopic calculations and the prediction of analog models
is obtained in the hydrodynamic limit. New features are predicted and the
robustness of the Hawking signal against a finite temperature discussed. | cond-mat_other |
Resonant Dimer Relaxation in Cold Atoms with a Large Scattering Length: Efimov physics refers to universal phenomena associated with a discrete
scaling symmetry in the 3-body problem with a large scattering length. The
first experimental evidence for Efimov physics was the recent observation of a
resonant peak in the 3-body recombination rate for 133Cs atoms with large
negative scattering length. There can also be resonant peaks in the atom-dimer
relaxation rate for large positive scattering length. We calculate the
atom-dimer relaxation rate as a function of temperature and show how
measurements of the relaxation rate can be used to determine accurately the
parameters that govern Efimov physics. | cond-mat_other |
Ratchet, pawl and spring Brownian motor: We present a model for a thermal Brownian motor based on Feynman's famous
ratchet and pawl device. Its main feature is that the ratchet and the pawl are
in different thermal baths and connected by an harmonic spring. We simulate its
dynamics, explore its main features and also derive an approximate analytical
solution for the mean velocity as a function of the external torque applied and
the temperatures of the baths. Such theoretical predictions and the results
from numerical simulations agree within the ranges of the approximations
performed. | cond-mat_other |
Quantum phases of hardcore bosons with long-range interactions on a
square lattice: We study the ground-state phase diagrams of hardcore bosons with long-range
interactions on a square lattice using the linear spin-wave theory and a
cluster mean-field method. Specifically, we consider the two types of
long-range interaction: One consists only of the nearest- and
next-nearest-neighbor interactions, and the other is the dipole-dipole
interaction that decays with the interparticle distance $r$ as $\sim r^{-3}$.
It is known from previous analyses by quantum Monte Carlo methods that a
checkerboard supersolid (CSS) is absent in the ground-state phase diagram of
the former case while it is present in the latter. In the former, we find that
quantum fluctuations around mean-field solutions are enhanced by the direct
competition between the checkerboard and striped solid orders and that they
destabilize the CSS phase. On the other hand, the emergence of the CSS phase in
the latter case can be attributed to the absence of such a competition with
other solid orders. We also show that the cluster mean-field method allows for
the determination of phase boundaries in a precise quantitative manner when
scaling with respect to the cluster size is taken into account. It is found
that the phase transition between the superfluid and the solid (or CSS) is of
the first order in the vicinity of the particle-hole symmetric line. | cond-mat_other |
Density-functional theory of nonequilibrium tunneling: Nanoscale optoelectronics and molecular-electronics systems operate with
current injection and nonequilibrium tunneling, phenomena that challenge
consistent descriptions of the steady-state transport. The current affects the
electron-density variation and hence the inter- and intra-molecular bonding
which in turn determines the transport magnitude. The standard approach for
efficient characterization of steady-state tunneling combines ground-state
density functional theory (DFT) calculations (of an effective scattering
potential) with a Landauer-type formalism and ignores all actual many-body
scattering. The standard method also lacks a formal variational basis. This
paper formulates a Lippmann-Schwinger collision density functional theory
(LSC-DFT) for tunneling transport with full electron-electron interactions.
Quantum-kinetic (Dyson) equations are used for an exact reformulation that
expresses the variational noninteracting and interacting many-body scattering
T-matrices in terms of universal density functionals. The many-body
Lippmann-Schwinger (LS) variational principle defines an implicit equation for
the exact nonequilibrium density. | cond-mat_other |
On surface plasmon polariton wavepacket dynamics in metal-dielectric
heterostructures: The WKB equations for dynamics of the surface plasmon polariton (SPP)
wavepacket are studied. The dispersion law for the SPP in the metal-dielectric
heterostructure with varying thickness of a perforated dielectric layer is
rigorously calculated and investigated using the scattering matrix method. Two
channels of the SPP wavepacket optical losses related to the absorption in a
metal and to the SPP leakage are analyzed. It is shown that change of the
dielectric layer thickness acts on the SPP as an external force leading to
evolution of its quasimomentum and to the wavepacket reversal or even to the
optical Bloch oscillations (BO). Properties of these phenomena are investigated
and discussed. Typical values of the BO amplitude are about tens of microns and
the period is around tens or hundreds of femtoseconds. | cond-mat_other |
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