199 research outputs found
Low-Lying Collective Excitations of Superconductors and Charged Superfluids
We investigate theoretically the momentum-dependent frequency and damping of
low-lying collective excitations of superconductors and charged superfluids in
the BCS-BEC crossover regime. The study is based on the Gaussian
pair-and-density fluctuation method for the propagator of Gaussian fluctuations
of the pair and density fields. Eigenfrequencies and damping rates are
determined in a mutually consistent nonperturbative way as complex poles of the
fluctuation propagator. Particular attention is paid to new features with
respect to preceding theoretical studies, which were devoted to collective
excitations of superconductors in the far BCS regime. We find that at a
sufficiently strong coupling, new branches of collective excitations appear,
which manifest different behavior as functions of the momentum and the
temperature.Comment: 10 pages, 5 figures, Proceedings of SuperFluctuations 2022 Conference
(Padova, 2022
Collective excitations of superfluid Fermi gases near the transition temperature
Studying the collective pairing phenomena in a two-component Fermi gas, we
predict the appearance near the transition temperature of a well-resolved
collective mode of quadratic dispersion. The mode is visible both above and
below in the system's response to a driving pairing field. When
approaching from below, the phononic and pair-breaking branches,
characteristic of the zero temperature behavior, reduce to a very low
energy-momentum region when the pair correlation length reaches its critical
divergent behavior ; elsewhere, they are
replaced by the quadratically-dispersed pairing resonance, which thus acts as a
precursor of the phase transition. In the strong-coupling and Bose-Einstein
Condensate regime, this mode is a weakly-damped propagating mode associated to
a Lorentzian resonance. Conversely, in the BCS limit it is a relaxation mode of
pure imaginary eigenenergy. At large momenta, the resonance disappears when it
is reabsorbed by the lower-edge of the pairing continuum. At intermediate
temperatures between 0 and , we unify the newly found collective phenomena
near with the phononic and pair-breaking branches predicted from previous
studies, and we exhaustively classify the roots of the analytically continued
dispersion equation, and show that they provided a very good summary of the
pair spectral functions.Comment: 44 pages, 17 figures, accepted in Physical Review A (2021
Phononic collective excitations in superfluid Fermi gases at nonzero temperatures
We study the phononic collective modes of the pairing field and
their corresponding signature in both the order-parameter and density response
functions for a superfluid Fermi gas at all temperatures below in the
collisionless regime. The spectra of collective modes are calculated within the
Gaussian Pair Fluctuation approximation. We deal with the coupling of these
modes to the fermionic continuum of quasiparticle-quasihole excitations by
performing a non-perturbative analytic continuation of the pairing field
propagator. At low temperature, we recover the known exponential temperature
dependence of the damping rate and velocity shift of the Anderson-Bogoliubov
branch. In the vicinity of , we find analytically a weakly-damped
collective mode whose velocity vanishes with a critical exponent of , and
whose quality factor diverges logarithmically with , thereby clarifying
an existing debate in the literature (Andrianov et al. Th. Math. Phys. 28, 829,
Ohashi et al. J. Phys. Jap. 66, 2437). A transition between these two phononic
branches is visible at intermediary temperatures, particularly in the BCS limit
where the phase-phase response function displays two maxima.Comment: 31 pages, 15 figure
Pair-Breaking Collective Branch in BCS Superconductors and Superfluid Fermi Gases
We demonstrate the existence of a collective excitation branch in the
pair-breaking continuum of superfluid Fermi gases and BCS superconductors. At
zero temperature, we analytically continue the equation on the collective mode
energy in Anderson's Random Phase Approximation or Gaussian fluctuations
through its branch cut associated with the continuum, and obtain the full
complex dispersion relation, including in the strong coupling regime. The
branch exists as long as the chemical potential is positive and the wave
number below (with m the fermion mass). In the long
wavelength limit, the branch varies quadratically with the wave number, with a
complex effective mass that we compute analytically for an arbitrary
interaction strength.Comment: 6-7 pages, 4 figures, in English et en fran\c{c}ai
Bose-Einstein condensation of Efimovian triples in the unitary Bose gas
In an atomic Bose-Einstein condensate quenched to the unitary regime, we
predict the sequential formation of a significant fraction of condensed pairs
and triples. At short-distances, we demonstrate the two-body and Efimovian
character of the condensed pairs and triples, respectively. As the system
evolves, the size of the condensed pairs and triples becomes comparable to the
interparticle distance, such that many-body effects become significant. The
structure of the condensed triples depends on the relative size of Efimov
states to density scales. Unexpectedly, we find universal condensed triples in
the limit where these scales are well-separated. Our findings provide a new
framework for understanding dynamics in the unitary regime as the Bose-Einstein
condensation of few-body composites
Cumulant theory of the unitary Bose gas: Prethermal and Efimovian dynamics
We study the quench of a degenerate ultracold Bose gas to the unitary regime,
where interactions are as strong as allowed by quantum mechanics. We lay the
foundations of a cumulant theory able to capture simultaneously the three-body
Efimov effect and ergodic evolution. After an initial period of rapid quantum
depletion, a universal prethermal stage is established characterized by a
kinetic temperature and an emergent Bogoliubov dispersion law while the
microscopic degrees of freedom remain far-from-equilibrium. Integrability is
then broken by higher-order interaction terms in the many-body Hamiltonian,
leading to a momentum-dependent departure from power law to decaying
exponential behavior of the occupation numbers at large momentum. We find also
signatures of the Efimov effect in the many-body dynamics and make a precise
identification between the observed beating phenomenon and the binding energy
of an Efimov trimer. Throughout the work, our predictions for a uniform gas are
quantitatively compared with experimental results for quenched unitary Bose
gases in uniform potentials.Comment: 34 pages, 12 figure
Higgs Oscillations in a Unitary Fermi Superfluid
Symmetry-breaking phase transitions are central to our understanding of states of matter. When a continuous symmetry is spontaneously broken, new excitations appear that are tied to fluctuations of the order parameter. In superconductors and fermionic superfluids, the phase and amplitude can fluctuate independently, giving rise to two distinct collective branches. However, amplitude fluctuations are difficult to both generate and measure, as they do not couple directly to the density of fermions and have only been observed indirectly to date. Here, we excite amplitude oscillations in an atomic Fermi gas with resonant interactions by an interaction quench. Exploiting the sensitivity of Bragg spectroscopy to the amplitude of the order parameter, we measure the time-resolved response of the atom cloud, directly revealing amplitude oscillations at twice the frequency of the gap. The magnitude of the oscillatory response shows a strong temperature dependence, and the oscillations appear to decay faster than predicted by time-dependent Bardeen-Cooper-Schrieffer theory applied to our experimental setup.</p
Material-independent crack arrest statistics: Application to indentation experiments
An extensive experimental study of indentation and crack arrest statistics is
presented for four different brittle materials (alumina, silicon carbide,
silicon nitride, glass). Evidence is given that the crack length statistics can
be described by a universal (i.e. material independent) distribution. The
latter directly derives from results obtained when modeling crack propagation
as a depinning phenomenon. Crack arrest (or effective toughness) statistics
appears to be fully characterized by two parameters, namely, an asymptotic
crack length (or macroscopic toughness) value and a power law size dependent
width. The experimental knowledge of the crack arrest statistics at one given
scale thus gives access to its knowledge at all scales
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