2,946 research outputs found
Signatures of superfluidity for Feshbach-resonant Fermi gases
We consider atomic Fermi gases where Feshbach resonances can be used to
continuously tune the system from weak to strong interaction regime, allowing
to scan the whole BCS-BEC crossover. We show how a probing field transferring
atoms out of the superfluid can be used to detect the onset of the superfluid
transition in the high- and BCS regimes. The number of transferred atoms,
as a function of the energy given by the probing field, peaks at the gap
energy. The shape of the peak is asymmetric due to the single particle
excitation gap. Since the excitation gap includes also a pseudogap
contribution, the asymmetry alone is not a signature of superfluidity.
Incoherent nature of the non-condensed pairs leads to broadening of the peak.
The pseudogap and therefore the broadening decay below the critical
temperature, causing a drastic increase in the asymmetry. This provides a
signature of the transition.Comment: Revised version, accepted to Phys. Rev. Letters. Figures changed,
explanations adde
Influence of pure-dephasing by phonons on exciton-photon interfaces: Quantum microscopic theory
We have developed a full quantum microscopic theory to analyze the time
evolution of transversal and longitudinal components of an exciton-single
photon system coupled to bulk acoustic phonons. These components are subjected
to two decay processes. One is radiative relaxation and the other is
pure-dephasing due to exciton-phonon interaction. The former results in a decay
with an exponent linear to time, while the latter causes a faster initial decay
than the radiative decay. We analyzed the dependence of the components on the
duration of the input one-photon pulse, temperature, and radiative relaxation
rates. Such a quantitative analysis is important for the developments of
atom-photon interfaces which enable coherent transfer of quantum information
between photons and atomic systems. We found that, for a GaAs spherical quantum
dot in which the exciton interacts with bulk phonons, the maximal probability
of the excited state can be increased up to 75 %. This probability can be
considered as the efficiency for quantum information transfer from photon to
exciton.Comment: 9pages, 5figure
Polarons and Molecules in a Two-Dimensional Fermi Gas
We study an impurity atom in a two-dimensional Fermi gas using variational
wave functions for (i) an impurity dressed by particle-hole excitations
(polaron) and (ii) a dimer consisting of the impurity and a majority atom. In
contrast to three dimensions, where similar calculations predict a sharp
transition to a dimer state with increasing interspecies attraction, we show
that the polaron ansatz always gives a lower energy. However, the exact
solution for a heavy impurity reveals that both a two-body bound state and
distortions of the Fermi sea are crucial. This reflects the importance of
particle-hole pairs in lower dimensions and makes simple variational
calculations unreliable. We show that the energy of an impurity gives important
information about its dressing cloud, for which both ans\"atze give inaccurate
results.Comment: 5 pages, 2 figures, minor change
Conductance enhancement due to the resonant tunneling into the subgap vortex core states in normal metal/superconductor ballistic junctions
We investigate the low-energy quantum transport in the ballistic normal
metal-insulator -superconductor junction in the presence of magnetic field
creating Abrikosov vortices in the superconductor. Within the Bogolubov- de
Gennes theory we show that the presence of the subgap quasiparticle states
localized within the vortex cores near the junction interface leads to the
strong resonant enhancement of the Andreev reflection probability, and the
normal-to supercurrent conversion. The corresponding increase of the charge
conductance of the junction is determined by the distance from the vortex chain
to the junction interface, which can be controlled by the applied magnetic
field. The effect that we study provides a tool for probing the vortex core
states by the measurements of charge transport across the applied magnetic
field.Comment: 8 pages, 3 figure
Transport phenomena in three-dimensional system close to the magnetic quantum critical point: The conserving approximation with the current vertex corrections
It is known that various transport coefficients strongly deviate from
conventional Fermi-liquid behaviors in many electron systems which are close to
antiferromagnetic (AF) quantum critical points (QCP). For example, Hall
coefficients and Nernst coefficients in three-dimensional heavy fermion CeCoIn5
and CeCu6-xAux increase strikingly at low temperatures, whose overall behaviors
are similar to those in high-Tc cuprates. These temperature dependences are too
strong to explain in terms of the relaxation time approximation. To elucidate
the origin of these anomalous transport phenomena in three-dimensional systems,
we study the current vertex corrections (CVC) based on the fluctuation exchange
(FLEX) approximation, and find out decisive role of the CVC. The main finding
of the present paper is that the Hall coefficient and the Nernst coefficient
strongly increase thanks to the CVC in the vicinity of the AF QCP, irrespective
of dimensionality. We also study the relaxation time of quasi-particles, and
find that "hot points" and "cold lines" are formed in general three-dimensional
systems due to strong AF fluctuations.Comment: 11 pages, 18 figures. Accepted for publication in Phys. Rev.
Pairing based cooling of Fermi gases
We propose a pairing-based method for cooling an atomic Fermi gas. A three
component (labels 1, 2, 3) mixture of Fermions is considered where the
components 1 and 2 interact and, for instance, form pairs whereas the component
3 is in the normal state. For cooling, the components 2 and 3 are coupled by an
electromagnetic field. Since the quasiparticle distributions in the paired and
in the normal states are different, the coupling leads to cooling of the normal
state even when initially (notation ).
The cooling efficiency is given by the pairing energy and by the linewidth of
the coupling field. No superfluidity is required: any type of pairing, or other
phenomenon that produces a suitable spectral density, is sufficient. In
principle, the paired state could be cooled as well but this requires
. The method has a conceptual analogy to cooling based on
superconductor -- normal metal (SN) tunneling junctions. Main differences arise
from the exact momentum conservation in the case of the field-matter coupling
vs. non-conservation of momentum in the solid state tunneling process.
Moreover, the role of processes that relax the energy conservation requirement
in the tunneling, e.g. thermal fluctuations of an external reservoir, is now
played by the linewidth of the field. The proposed method should be
experimentally feasible due to its close connection to RF-spectroscopy of
ultracold gases which is already in use.Comment: Journal version 4 pages, 4 figure
Mott Insulator to Superfluid transition in Bose-Bose mixtures in a two-dimensional lattice
We perform a numeric study (Worm algorithm Monte Carlo simulations) of
ultracold two-component bosons in two-dimensional optical lattices. We study
how the Mott insulator to superfluid transition is affected by the presence of
a second superfluid bosonic species. We find that, at fixed interspecies
interaction, the upper and lower boundaries of the Mott lobe are differently
modified. The lower boundary is strongly renormalized even for relatively low
filling factor of the second component and moderate (interspecies) interaction.
The upper boundary, instead, is affected only for large enough filling of the
second component. Whereas boundaries are renormalized we find evidence of
polaron-like excitations. Our results are of interest for current experimental
setups.Comment: 4 pages, 3 figures, accepted as PRA Rapid Communicatio
Reflection high-energy electron diffraction patterns of CrSi_2 films on (111) silicon
Highly oriented films of the semiconducting transition metal silicide, CrSi2, were grown on (111) silicon substrates, with the matching crystallographic faces being CrSi_2(001)/Si(111). Reflection high‐energy electron diffraction (RHEED) yielded symmetric patterns of sharp streaks. The expected streak spacings for different incident RHEED beam directions were calculated from the reciprocal net of the CrSi_2(001) face and shown to match the observed spacings. The predominant azimuthal orientation of the films was thus determined to be CrSi_2〈210〉∥Si〈110〉. This highly desirable heteroepitaxial relationship may be described with a common unit mesh of 51 Å^2 and a mismatch of −0.3%. RHEED also revealed the presence of limited film regions of a competing azimuthal orientation, CrSi_2〈110〉∥Si〈110〉. A new common unit mesh for this competing orientation is suggested; it possesses an area of 612 Å^2 and a mismatch of −1.2%
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