206 research outputs found
Sum Rule for the Optical Absorption of an Interacting Many-Polaron Gas
A sum rule for the first frequency moment of the optical absorption of a
many-polaron system is derived, taking into account many-body effects in the
system of constituent charge carriers of the many-polaron system. In our
expression for the sum rule, the electron-phonon coupling and the many-body
effects in the electron (or hole) system formally decouple, so that the
many-body effects can be treated to the desired level of approximation by the
choice of the dynamical structure factor of the electron (hole) gas. We
calculate correction factors to take into account both low and high
experimental cutoff frequencies.Comment: 16 pages, 3 figures, revised version; new results adde
Path integral formulation of the tunneling dynamics of a superfluid Fermi gas in an optical potential
To describe the tunneling dynamics of a stack of two-dimensional fermionic
superfluids in an optical potential, we derive an effective action functional
from a path integral treatment. This effective action leads, in the saddle
point approximation, to equations of motion for the density and the phase of
the superfluid Fermi gas in each layer. In the strong coupling limit (where
bosonic molecules are formed) these equations reduce to a discrete nonlinear
Schrodinger equation, where the molecular tunneling amplitude is reduced for
large binding energies. In the weak coupling (BCS) regime, we study the
evolution of the stacked superfluids and derive an approximate analytical
expression for the Josephson oscillation frequency in an external harmonic
potential. Both in the weak and intermediate coupling regimes the detection of
the Josephson oscillations described by our path integral treatment constitutes
experimental evidence for the fermionic superfluid regime.Comment: 13 pages + 2 figure
Pseudogap and preformed pairs in the imbalanced Fermi gas in two dimensions
The physics of the pseudogap state is intimately linked with the pairing
mechanism that gives rise to superfluidity in quantum gases and to
superconductivity in high-Tc cuprates, and therefore, both in quantum gases and
superconductors, the pseudogap state and preformed pairs have been under
intensive experimental scrutiny. Here, we develop a path integral treatment
that provides a divergence-free description of the paired state in
two-dimensional Fermi gases. Within this formalism, we derive the pseudogap
temperature and the pair fluctuation spectral function, and compare these
results with the recent experimental measument of the pairing in the
two-dimensional Fermi gas. The removal of the infrared divergence in the number
equations is shown both numerically and analytically, through a study of the
long-wavelength and low-energy limit of the pair fluctuation density. Besides
the pseudogap temperature, also the pair formation temperature and the critical
temperature for superfluidity are derived. The latter corresponds to the
Berezinski-Kosterlitz-Thouless (BKT) temperature. The pseudogap temperature,
which coincides with the pair formation temperature in mean field, is found to
be suppressed with respect to the pair formation temperature by fluctuations.
This suppression is strongest for large binding energies of the pairs. Finally,
we investigate how the pair formation temperature, the pseudogap temperature
and the BKT temperature behave as a function of both binding energy and
imbalance between the pairing partners in the Fermi gas. This allows to set up
phase diagrams for the two-dimensional Fermi gas, in which the superfluid
phase, the phase-fluctuating quasicondensate, and the normal state can be
identified.Comment: 17 pages, 6 figure
- …