187 research outputs found
Extracting the condensate density from projection experiments with Fermi gases
A debated issue in the physics of the BCS-BEC crossover with trapped Fermi
atoms is to identify characteristic properties of the superfluid phase.
Recently, a condensate fraction was measured on the BCS side of the crossover
by sweeping the system in a fast (nonadiabatic) way from the BCS to the BEC
sides, thus ``projecting'' the initial many-body state onto a molecular
condensate. We analyze here the theoretical implications of these projection
experiments, by identifying the appropriate quantum-mechanical operator
associated with the measured quantities and relating them to the many-body
correlations occurring in the BCS-BEC crossover. Calculations are presented
over wide temperature and coupling ranges, by including pairing fluctuations on
top of mean field.Comment: 4 pages, 4 figure
BCS-BEC crossover at finite temperature for superfluid trapped Fermi atoms
We consider the BCS-BEC crossover for a system of trapped Fermi atoms at
finite temperature, both below and above the superfluid critical temperature,
by including fluctuations beyond mean field. We determine the superfluid
critical temperature and the pair-breaking temperature as functions of the
attractive interaction between Fermi atoms, from the weak- to the
strong-coupling limit (where bosonic molecules form as bound-fermion pairs).
Density profiles in the trap are also obtained for all temperatures and
couplings.Comment: revised version, to be published in Phys. Rev. Let
Stripe ordering and two-gap model for underdoped cuprates
The evidence of edge-gaps around the M-points in the metallic state of
underdoped cuprates has triggered a very active debate on their origin. We
first consider the possibility that this spectroscopic feature results from a
quasi-static charge ordering taking place in the underdoped regime. It comes
out that to explain the coexistence of gaps and arcs on the Fermi surface the
charge modulation should be in an eggbox form. In the lack of evidences for
that, we then investigate the local pairing induced by charge-stripe
fluctuations. A proper description of the strong anisotropy of both the
interactions and the Fermi velocities requires a two-gap model for pairing. We
find that a gap due to incoherent pairing forms near the M-points, while
coherence is established by the stiffness of the pairing near the nodal points.
The model allows for a continuos evolution from a pure BCS pairing (over- and
optimally doped regime) to a mixed boson-fermion model (heavily underdoped
regime).Comment: 4 pages, Proceedings of M2S-HTS
Pairing effects in the normal phase of a two-dimensional Fermi gas
In a recent experiment [M. Feld et al., Nature 480, 75 (2011); B. Froehlich
et al., Phys. Rev. Lett. 109,130403 (2012)], a pairing gap was detected in a
two-dimensional (2D) Fermi gas with attractive interaction at temperatures
where superfluidity does not occur. The question remains open as to whether
this gap is a pseudogap phenomenon or is due to a molecular state. In this
paper, by using a t-matrix approach, we reproduce quite well the experimental
data for a 2D Fermi gas, and set the boundary between the pseudogap and
molecular regimes. We also show that pseudogap phenomena occurring in 2D and 3D
can be related through a variable spanning the BCS-BEC crossover in a universal
way.Comment: 10 pages, 9 figures; final versio
Temperature and coupling dependence of the universal contact intensity for an ultracold Fermi gas
Physical properties of an ultracold Fermi gas in the temperature-coupling
phase diagram can be characterized by the contact intensity C, which enters the
pair-correlation function at short distances and describes how the two-body
problem merges into its surrounding. We show that the local order established
by pairing fluctuations about the critical temperature Tc of the superfluid
transition considerably enhances the contact C in a temperature range where
pseudogap phenomena are maximal. Our ab initio results for C in a trap compare
well with recently available experimental data over a wide coupling range. An
analysis is also provided for the effects of trap averaging on C.Comment: 5 pages, 5 figure
Charge and spin inhomogeneity as a key to the physics of the high Tc cuprates
We present a coherent scenario for the physics of cuprate superconductors,
which is based on a charge-driven inhomogeneity, i.e. the ``stripe phase''. We
show that spin and charge critical fluctuations near the stripe instability of
strongly correlated electron systems provide an effective interaction between
the quasiparticles, which is strongly momentum, frequency, temperature and
doping dependent. This accounts for the various phenomena occurring in the
overdoped, optimally and underdoped regimes both for the normal and the
superconductive phase.Comment: 6 pages, 1 enclosed figure, proceedings of LT2
Competition between final-state and pairing-gap effects in the radio-frequency spectra of ultracold Fermi atoms
The radio-frequency spectra of ultracold Fermi atoms are calculated by
including final-state interactions affecting the excited level of the
transition, and compared with the experimental data. A competition is revealed
between pairing-gap effects which tend to push the oscillator strength toward
high frequencies away from threshold, and final-state effects which tend
instead to pull the oscillator strength toward threshold. As a result of this
competition, the position of the peak of the spectra cannot be simply related
to the value of the pairing gap, whose extraction thus requires support from
theoretical calculations.Comment: 4 pages, 3 figures, final version published in Phys. Rev. Let
Fermi Surface and gap parameter in high-Tc superconductors: the Stripe Quantum Critical Point scenario
We study the single-particle spectral properties of electrons coupled to
quasicritical charge and spin fluctuations close to a stripe-phase, which is
governed by a Quantum Critical Point near optimum doping. We find that spectral
weight is transferred from the quasiparticle peak to incoherent dispersive
features. As a consequence the distribution of low-laying spectral weight is
modified with respect to the quasiparticle Fermi surface. The interplay of
charge and spin fluctuations reproduces features of the observed Fermi surface,
such as the asymmetric suppression of spectral weight near the M points of the
Brillouin zone.
Within the model, we also analyze the interplay between repulsive spin and
attractive charge fluctuations in determining the symmetry and the peculiar
momentum dependence of the superconducting gap parameter. When both spin and
charge fluctuations are coupled to the electrons, we find -wave
gap symmetry in a wide range of parameter. A crossover - vs -wave
symmetry of the gap may occur when the strength of charge fluctuations
increases with respect to spin fluctuations.Comment: 18 pages, 3 included figures, to be published on Physica
Dispersions, weights, and widths of the single-particle spectral function in the normal phase of a Fermi gas
The dispersions, weights, and widths of the peaks of the single-particle
spectral function in the presence of pair correlations, for a Fermi gas with
either attractive or repulsive short-range inter-particle interaction, are
determined in the normal phase over a wide range of wave vectors, with a
twofold purpose. The first one is to determine how these dispersions identify
both an energy scale known as the pseudo-gap near the Fermi wave vector, as
well as an additional energy scale related to the contact C at large wave
vectors. The second one is to differentiate the behaviors of the repulsive gas
from the attractive one in terms of crossing versus avoided crossing of the
dispersions near the Fermi wave vector. An analogy will also be drawn between
the occurrence of the pseudo-gap physics in a Fermi gas subject to pair
fluctuations and the persistence of local spin waves in the normal phase of
magnetic materials.Comment: 18 pages, 21 figure
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