108 research outputs found
Collective mode of homogeneous superfluid Fermi gases in the BEC-BCS crossover
We perform a detailed study of the collective mode across the whole BEC-BCS
crossover in fermionic gases at zero temperature, covering the whole range of
energy beyond the linear regime. This is done on the basis of the dynamical BCS
model. We recover first the results of the linear regime in a simple form. Then
specific attention is payed to the non linear part of the dispersion relation
and its interplay with the continuum of single fermionic excitations. In
particular we consider in detail the merging of collective mode into the
continuum of single fermionic excitations. This occurs not only on the BCS side
of the crossover, but also slightly beyond unitarity on the BEC side. Another
remarkable feature is the very linear behaviour of the dispersion relation in
the vicinity of unitarity almost up to merging with the continuum. Finally,
while on the BEC side the mode is quite analogous to the Bogoliubov mode, a
difference appear at high wavevectors. On the basis of our results we determine
the Landau critical velocity in the BEC-BCS crossover which is found to be
largest close to unitarity. Our investigation has revealed interesting
qualitative features which would deserve experimental exploration as well as
further theoretical studies by more sophisticated means.Comment: 21 page
Biexciton oscillator strength
Our goal is to provide a physical understanding of the elementary coupling
between photon and biexciton and to derive the physical characteristics of the
biexciton oscillator strength, following the procedure we used for trion.
Instead of the more standard two-photon absorption, this work concentrates on
molecular biexciton created by photon absorption in an exciton gas. We first
determine the appropriate set of coordinates in real and momentum spaces to
describe one biexciton as two interacting excitons. We then turn to second
quantization and introduce the "Fourier transform in the exciton sense" of the
biexciton wave function which is the relevant quantity for oscillator strength.
We find that, like for trion, the oscillator strength for the formation of one
biexciton out of one photon plus a \emph{single} exciton is extremely small: it
is one biexciton volume divided by one sample volume smaller than the exciton
oscillator strength. However, due to their quantum nature, trion and biexciton
have absorption lines which behave quite differently. Electrons and trions are
fermionic particles impossible to pile up all at the same energy. This would
make the weak trion line spread with electron density, the peak structure only
coming from singular many-body effects. By contrast, the bosonic nature of
exciton and biexciton makes the biexciton peak mainly rise with exciton
density, this rise being simply linear if we forget many-body effects between
the photocreated exciton and the excitons present in the sample
Huge Enhancement of Impurity Scattering due to Critical Valence Fluctuations in a Ce-Based Heavy Electron System
On the basis of the Ward-Pitaevskii identity, the residual resistivity
is shown to exhibit huge enhancement around the quantum critical
point of valence transition in Ce-based heavy electron systems. This explains a
sharp peak of observed in CeCuGe under the pressure at
16GPa where the superconducting trasition temperature also exhibit the
sharp peak.Comment: 5 pages, 1 figur
Atomic lattice excitons: from condensates to crystals
We discuss atomic lattice excitons (ALEs), bound particle-hole pairs formed
by fermionic atoms in two bands of an optical lattice. Such a system provides a
clean setup to study fundamental properties of excitons, ranging from
condensation to exciton crystals (which appear for a large effective mass ratio
between particles and holes). Using both mean-field treatments and 1D numerical
computation, we discuss the properities of ALEs under varying conditions, and
discuss in particular their preparation and measurement.Comment: 19 pages, 15 figures, changed formatting for journal submission,
corrected minor errors in reference list and tex
Only Fermi-Liquids are Metals
Any singular deviation from Landau Fermi-liquid theory appears to lead, for
arbitrarily small concentration of impurities coupling to a non-conserved
quantity, to a vanishing density of states at the chemical potential and
infinite resistivity as temperature approaches zero. Applications to
copper-oxide metals including the temperature dependence of the anisotropy in
resistivity, and to other cases of non Fermi-liquids are discussed.Comment: 11 pages,revtex, 1 Postscript figur
Effective Lagrangians for BCS Superconductors at T=0
We show that the low frequency, long wavelength dynamics of the phase of the
pair field for a BCS-type s-wave superconductor at T=0 is equivalent to that of
a time-dependent non-linear Schr\"odinger Lagrangian (TDNLSL), when terms
required by Galilean invariance are included. If the modulus of the pair field
is also allowed to vary, the system is equivalent to two coupled TDNLSL's.
We also refer the interested reader to our earlier paper, `Nonlinear
Schrodinger equation for superconductors' [cond-mat/9312099], for a different
line of derivationComment: Latex, 13 page
Fermi-Liquid Interactions in d-Wave Superconductor
This article develops a quantitative quasiparticle model of the
low-temperature properties of d-wave superconductors which incorporates both
Fermi-liquid effects and band-structure effects. The Fermi-liquid interaction
effects are found to be classifiable into strong and negligible renormalizaton
effects, for symmetric and antisymmetric combinations of the energies of
and quasiparticles, respectively. A particularly
important conclusion is that the leading clean-limit temperature-dependent
correction to the superfluid density is not renormalized by Fermi-liquid
interactions, but is subject to a Fermi velocity (or mass) renormalization
effect. This leads to difficulties in accounting for the penetration depth
measurements with physically acceptable parameters, and hence reopens the
question of the quantitative validity of the quasiparticle picture.Comment: 4 page
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