129 research outputs found
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
Soliton core filling in superfluid Fermi gases with spin-imbalance
In this paper the properties of dark solitons in superfluid Fermi gases with
spin-imbalance are studied by means of a recently developed effective field
theory [S. N. Klimin, J. Tempere, G. Lombardi, J. T. Devreese, Eur. Phys. J. B
88, 122 (2015)] suitable to describe the BEC-BCS crossover in ultracold gases
in an extended range of temperatures as compared to the usual Ginzburg-Landau
treatments. The spatial profiles for the total density and for the density of
the excess-spin component, and the changes of their properties across the
BEC-BCS crossover are examined in different conditions of temperature and
imbalance. The presence of population imbalance is shown to strongly affect the
structure of the soliton excitation by filling its core with unpaired atoms.
This in turn influences the dynamical properties of the soliton since the
additional particles in the core have to be dragged along thus altering the
effective mass.Comment: 9 pages, 9 figure
Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film
Electrons on liquid helium can form different phases depending on density,
and temperature. Also the electron-ripplon coupling strength influences the
phase diagram, through the formation of so-called "ripplonic polarons", that
change how electrons are localized, and that shifts the transition between the
Wigner solid and the liquid phase. We use an all-coupling, finite-temperature
variational method to study the formation of a ripplopolaron Wigner solid on a
liquid helium film for different regimes of the electron-ripplon coupling
strength. In addition to the three known phases of the ripplopolaron system
(electron Wigner solid, polaron Wigner solid, and electron fluid), we define
and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the
transitions between these four phases and calculate the corresponding phase
diagrams. This reveals a reentrant melting of the electron solid as a function
of temperature. The calculated regions of existence of the Wigner solid are in
agreement with recent experimental data.Comment: 12 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1012.4576, arXiv:0709.4140 by other author
Wigner lattice of ripplopolarons in a multielectron bubble in helium
The properties of ripplonic polarons in a multielectron bubble in liquid
helium are investigated on the basis of a path-integral variational method. We
find that the two-dimensional electron gas can form deep dimples in the helium
surface, or ripplopolarons, to solidify as a Wigner crystal. We derive the
experimental conditions of temperature, pressure and number of electrons in the
bubble for this phase to be realized. This predicted state is distinct from the
usual Wigner lattice of electrons, in that it melts by the dissociation of the
ripplopolarons, when the electrons shed their localizing dimple as the pressure
on the multielectron bubble drops below a critical value.Comment: 19 pages, 4 figure
Imbalanced d-wave superfluids in the BCS-BEC crossover regime at finite temperatures
Singlet pairing in a Fermi superfluid is frustrated when the amounts of
fermions of each pairing partner are unequal. The resulting `imbalanced
superfluid' has been realized experimentally for ultracold atomic gases with
s-wave interactions. Inspired by high-temperature superconductivity, we
investigate the case of d-wave interactions, and find marked differences from
the s-wave superfluid. Whereas s-wave imbalanced Fermi gases tend to phase
separate in real space, in a balanced condensate and an imbalanced normal halo,
we show that the d-wave gas can phase separate in reciprocal space so that
imbalance and superfluidity can coexist spatially. We show that the mechanism
explaining this property is the creation of polarized excitations in the nodes
of the gap. The Sarma mechanism, present only at nonzero temperatures for the
s-wave case, is still applicable in the temperature zero limit for the d-wave
case. As a result, the d-wave BCS superfluid is more robust with respect to
imbalance, and a region of the phase diagram can be identified where the s-wave
BCS superfluidity is suppressed whereas the d-wave superfluidity is not. When
these results are extended into the BEC limit of strongly bound molecules, the
symmetry of the order parameter matters less. The effects of fluctuations
beyond mean field is taken into account in the calculation of the structure
factor and the critical temperature. The poles of the structure factor
(corresponding to bound molecular states) are less damped in the d-wave case as
compared to s-wave. On the BCS side of the unitarity limit, the critical
temperature follows the temperature corresponding to the pair binding energy
and as such will also be more robust against imbalance. Possible routes for the
experimental observation of the d-wave superfluidity have been discussed.Comment: 22 pages, 7 figure
Finite temperature effective field theory and two-band superfluidity in Fermi gases
We develop a description of fermionic superfluids in terms of an effective
field theory for the pairing order parameter. Our effective field theory
improves on the existing Ginzburg - Landau theory for superfluid Fermi gases in
that it is not restricted to temperatures close to the critical temperature.
This is achieved by taking into account long-range fluctuations to all orders.
The results of the present effective field theory compare well with the results
obtained in the framework of the Bogoliubov - de Gennes method. The advantage
of an effective field theory over Bogoliubov - de Gennes calculations is that
much less computation time is required. In the second part of the paper, we
extend the effective field theory to the case of a two-band superfluid. The
present theory allows us to reveal the presence of two healing lengths in the
two-band superfluids, to analyze the finite-temperature vortex structure in the
BEC-BCS crossover, and to obtain the ground state parameters and spectra of
collective excitations. For the Leggett mode our treatment provides an
interpretation of the observation of this mode in two-band superconductors.Comment: 17 pages, 11 figures. In the published version [EPJB 88, 122 (2015)],
there is a misprint in expressions (20) and (21). There must be "E_k" instead
of "\xi_k" in the arguments of the functions "f_n" in those two formulae. In
the present version, this misprint is correcte
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