1,720,963 research outputs found
Inclusion of pairing fluctuations in the differential equation for the gap parameter for superfluid fermions in the presence of nontrivial spatial constraints
Full text linkMost theoretical treatments of inhomogeneous superconductivity/fermionic superfluidity have been based on the Bogoliubov-deGennes equations (or, else, on their various simplified forms), which implement a standard mean-field decoupling in the presence of spatial inhomogeneities. This approach is reliable even at finite temperature for weak interparticle attraction, when the Cooper pair size is much larger than the average interparticle distance (corresponding to the BCS limit of the BCS-BEC crossover). However, it loses accuracy for increasing attraction when the Cooper pair size becomes comparable or even smaller than the average interparticle distance (corresponding to the BEC limit of the BCS-BEC crossover), in particular when finite-temperature effects are considered. In these cases, inclusion of pairing fluctuations beyond mean field is required, a task that turns out to be especially difficult in the presence of inhomogeneities. Here, we implement the inclusion of pairing fluctuations directly on a coarse-graining version of the Bogoliubov-deGennes equations, which makes it simpler and faster to obtain a solution over the whole sector of the temperature-coupling phase diagram of the BCS-BEC crossover in the broken-symmetry phase. We apply this method in the presence of a supercurrent flow, such that problems related to the Josephson effect throughout the BCS-BEC crossover can be addressed under a variety of circumstances. This is relevant in the view of recent experimental data with ultracold Fermi atoms, to which the results of the present approach are shown to favorably compare in the companion paper
Critical current throughout the BCS-BEC crossover with the inclusion of pairing fluctuations
Full text linkThe present work aims at providing a systematic analysis of the current density versus momentum characteristics for a fermionic superfluid throughout the BCS-BEC crossover, even in the fully homogeneous case. At low
temperatures, where pairing fluctuations are not strong enough to invalidate a quasiparticle approach, a sharp
threshold for the inception of a back-flow current is found, which sets the onset of dissipation and identifies the
critical momentum according to Landau. This momentum is seen to smoothly evolve from the BCS to the BEC
regimes, whereby a single expression for the single-particle current density that includes pairing fluctuations
enables us to incorporate on equal footing two quite distinct dissipative mechanisms, namely, pair breaking
and phonon excitations in the two sides of the BCS-BEC crossover, respectively. At finite temperature, where
thermal fluctuations broaden the excitation spectrum and make the dissipative (kinetic and thermal) mechanisms
intertwined with each other, an alternative criterion due to Bardeen is instead employed to signal the loss of
superfluid behavior. In this way, detailed comparison with available experimental data in linear and annular
geometries is significantly improved with respect to previous approaches, thereby demonstrating the crucial role
played by quantum fluctuations in renormalizing the single-particle excitation spectrum
Strong Fulde-Ferrell Larkin-Ovchinnikov pairing fluctuations in polarized Fermi systems
Full text linkWe calculate the pair susceptibility of an attractive spin-polarized Fermi gas in the normal phase, as a function of the pair momentum. Close to unitarity, we find a strong enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing fluctuations over an extended region of the temperature-polarization phase diagram, which manifests itself as a pronounced peak in the pair-momentum distribution at a finite pair momentum. This peak should be amenable to experimental observation at achievable temperatures in a boxlike trapping potential, as a fingerprint of FFLO pairing. Our calculations rest on a self-consistent t-matrix approach which, for the unitary balanced Fermi gas, has been validated against experimental data for several thermodynamic quantities
Josephson current flowing through a nontrivial geometry: Role of pairing fluctuations across the BCS-BEC crossover
Full text linkA realistic description of the Josephson effect at finite temperature with ultra-cold Fermi gases embedded in nontrivial geometrical constraints (typically, a trap plus a barrier) requires appropriate consideration of pairing fluctuations that arise in inhomogeneous environments. Here, we apply the theoretical approach developed in the companion article, where the inclusion of pairing fluctuations beyond mean field across the BCS-BEC crossover at finite temperature is combined with a detailed description of the gap parameter in a nontrivial geometry. In this way, we are able to account for the
experimental results on the Josephson critical current, reported both at low temperature for various couplings across the BCS-BEC crossover and as a function of temperature at unitarity. Besides validating the theoretical approach of the companion article, our numerical results reveal generic
features of the Josephson effect which may not readily emerge from an analysis of corresponding experiments with condensed-matter samples owing to the unique intrinsic flexibility of experiments with ultra-cold gases
Evolution of an attractive polarized Fermi gas: From a Fermi liquid of polarons to a non-Fermi liquid at the Fulde-Ferrell-Larkin-Ovchinnikov quantum critical point
Full text linkThe evolution of an attractive polarized two-component Fermi gas at zero
temperature is analyzed as its polarization is progressively decreased, from
full polarization (corresponding to the polaronic limit) down to a critical
polarization when superfluidity sets in. This critical polarization and the
nature of the associated superfluid instability are determined within a fully
self-consistent -matrix approach implemented exactly at zero temperature. In
this way, the polarization-vs-coupling phase diagram at zero temperature is
constructed throughout the whole BCS-BEC crossover. Depending on the coupling
strength of the inter-particle interaction between the two components, the
superfluid instability can be either toward a Fulde-Ferrel-Larkin-Ovchinnikov
(FFLO) phase or toward a standard polarized BCS phase. The evolution with
polarization of the quasi-particle parameters in the normal Fermi gas turns out
to be notably different in the two cases. When the instability is toward a
polarized BCS superfluid, quasi-particles in the proximity of the two Fermi
surfaces remain well defined for all polarizations. When the instability is
instead toward an FFLO superfluid, precursor effects become apparent upon
approaching the FFLO quantum critical point (QCP), where the quasi-particle
residues vanish and the effective masses diverge. This behavior leads to a
complete breakdown of the quasi-particle picture characteristic of a Fermi
liquid, similarly to what occurs in heavy-fermion materials at an
antiferromagnetic QCP. At unitarity, the system is further investigated at
finite temperature, making it possible to identify a non-Fermi liquid region in
the temperature-vs-polarization phase diagram associated with the underlying
FFLO QCP.Comment: 24 pages, 10 figures; modified Fig. 5 and text discussing it, added
Appendix A7 and Fig. 10, added References, other minor correction
Pair correlations in the normal phase of an attractive Fermi gas
Full text linkIn a recent paper (2019 Phys. Rev. A 99, 053617), the total number of fermion pairs in a spin-balanced two-component Fermi gas of 6Li atoms was experimentally probed in the normal phase above the superfluid critical temperature, in order to investigate the sectors of pseudogap and preformed-pair in the temperature-coupling phase diagram. Here, we present a theoretical account of these experimental results in terms of an ab initio self-consistent t-matrix calculation, which emphasizes the role of the pair-correlation function between opposite-spin fermions at equilibrium. Good agreement is found between the available experimental data and the theoretical results obtained with no adjustable paramete
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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