1,720,962 research outputs found
Bound states in a superfluid vortex: A detailed study along the BCS-BEC crossover
No full textThe bound states that can occur in a superfluid vortex have recently called for attention owing to the capability
of detecting them experimentally. However, a detailed theoretical account for the presence of these vortex bound states is still lacking for all temperatures in the superfluid phase and couplings along the BCS-BEC crossover. Here, we fill this gap and present a systematic theoretical study based on the Bogoliubov–de Gennes equations for the bound states that occur over the two characteristic (inner and outer) spatial ranges in which the extension of a superfluid vortex can be partitioned. It is found that the total number of bound states decreases from the BCS (weak-coupling) side of the crossover toward the intermediate-coupling region where they are still present, whereas the bound states disappear upon entering the BEC (strong-coupling) side. A scaling relation is also obtained that connects the number of bound states in the inner spatial range of the vortex to the depth and width of the vortex itself. A criterion is finally provided in terms of the local density of states, to distinguish where a given fermionic superfluid is located in the coupling-temperature phase diagram of the BCS-BEC crossover
Quantitative comparison between theoretical predictions and experimental results for the BCS-BEC crossover
No full textTheoretical predictions for the Bardeen-Cooper-Schrieffer–Bose-Einstein condensation crossover of
trapped Fermi atoms are compared with recent experimental results for the density profiles of 6Li. The
calculations rest on a single theoretical approach that includes pairing fluctuations beyond mean-field.
Excellent agreement with experimental results is obtained. Theoretical predictions for the zerotemperature
chemical potential and gap at the unitarity limit are also found to compare extremely
well with Quantum Monte Carlo simulations and with recent experimental results
Optimizing the proximity effect along the BCS side of the BCS-BEC crossover
No full textThe proximity effect, which arises at the interface between two fermionic superfluids with different critical
temperatures, is examined with a nonlocal (integral) equation whose kernel contains information about the size
of Cooper pairs that leak across the interface. This integral approach avoids reference to the boundary conditions
at the interface that would be required with a differential approach. The temperature dependence of the pair
penetration depth on the normal side of the interface is determined over a wide temperature range also varying
the interparticle coupling along the BCS side of the BCS-BEC crossover independently on both sides of the
interface. In this way, the size of Cooper pairs evolves from being much larger than (BCS limit) the interparticle
distance to being comparable with (unitarity limit, halfway between the BCS and BEC limits) the interparticle
distance. Conditions are then found for which the proximity effect is optimized in terms of the extension
of the pair penetration depth
Nonlocal equation for the superconducting gap parameter
No full textThe properties are considered in detail of a nonlocal (integral) equation for the superconducting gap parameter, which is obtained by a coarse-graining procedure applied to the Bogoliubov-de Gennes (BdG) equations over the whole coupling-versus-temperature phase diagram associated with the superfluid phase. It is found that the limiting size of the coarse-graining procedure, which is dictated by the range of the kernel of this integral equation, corresponds to the size of the Cooper pairs over the whole coupling-versus-temperature phase diagram up to the critical temperature, even when Cooper pairs turn into composite bosons on the BEC side of the BCS-BEC crossover. A practical method is further implemented to solve numerically this integral equation in an efficient way, which is based on a novel algorithm for calculating the Fourier transforms. Application of this method to the case of an isolated vortex, throughout the BCS-BEC crossover and for all temperatures in the superfluid phase, helps clarifying the nature of the length scales associated with a single vortex and the kinds of details that are in practice disposed off by the coarse-graining procedure on the BdG equations
Entanglement between pairing and screening in the Gorkov-Melik-Barkhudarov correction to the critical temperature throughout the BCS-BEC crossover
Full text linkThe problem of the theoretical description of the critical temperature Tc of a Fermi superfluid dates back to
the work by Gorkov and Melik-Barkhudarov (GMB), who addressed it for a weakly coupled (dilute) superfluid
in what would today be referred to as the (extreme) BCS (weak-coupling) limit of the BCS-BEC crossover.
The point made in this context by GMB was that particle-particle (pairing) excitations, which are responsible
for superfluidity to occur below Tc, and particle-hole excitations, which give rise to screening also in a normal
system, get effectively disentangled from each other in the BCS limit, thus yielding a reduction by a factor of 2.2
of the value of Tc obtained when neglecting screening effects. Subsequent work on this topic, that was aimed at
extending the original GMB argument away from the BCS limit with diagrammatic methods, has tout court kept
this disentangling between pairing and screening throughout the BCS-BEC crossover, without realizing that the
conditions for it to be valid are soon violated away from the BCS limit. Here, we reconsider this problem from a
more general perspective and argue that pairing and screening are intrinsically entangled with each other along
the whole BCS-BEC crossover but for the BCS limit considered by GMB, with the particle-hole excitations soon
transmuting into particle-particle excitations away from this limit.We substantiate our argument by performing a
detailed numerical calculation of the GMB diagrammatic contribution suitably extended to the whole BCS-BEC
crossover, where the fullwave-vector and frequency dependence occurring in the repeated in-medium two-particle
scattering is duly taken into account. Our numerical calculations are tested against analytic results available in
both the BCS and BEC limits, and the contribution of the GMB diagrammatic term to the scattering length of
composite bosons in the BEC limit is highlighted. We calculate Tc throughout the BCS-BEC crossover and find
that it agrees quite well with quantum Monte Carlo calculations and experimental data available in the unitarity
regime
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
The BCS–BEC crossover: From ultra-cold Fermi gases to nuclear systems
Full text linkInternational audienceThis report addresses topics and questions of common interest in the fields of ultra-cold gases and nuclear physics in the context of the BCS–BEC crossover. By this crossover, the phenomena of Bardeen–Cooper–Schrieffer (BCS) superfluidity and Bose–Einstein condensation (BEC), which share the same kind of spontaneous symmetry breaking, are smoothly connected through the progressive reduction of the size of the fermion pairs involved as the fundamental entities in both phenomena. This size ranges, from large values when Cooper pairs are strongly overlapping in the BCS limit of a weak inter-particle attraction, to small values when composite bosons are non-overlapping in the BEC limit of a strong inter-particle attraction, across the intermediate unitarity limit where the size of the pairs is comparable with the average inter-particle distance.The BCS–BEC crossover has recently been realized experimentally, and essentially in all of its aspects, with ultra-cold Fermi gases. This realization, in turn, has raised the interest of the nuclear physics community in the crossover problem, since it represents an unprecedented tool to test fundamental and unanswered questions of nuclear many-body theory. Here, we focus on the several aspects of the BCS–BEC crossover, which are of broad joint interest to both ultra-cold Fermi gases and nuclear matter, and which will likely help to solve in the future some open problems in nuclear physics (concerning, for instance, neutron stars). Similarities and differences occurring in ultra-cold Fermi gases and nuclear matter will then be emphasized, not only about the relative phenomenologies but also about the theoretical approaches to be used in the two contexts. Common to both contexts is the fact that at zero temperature the BCS–BEC crossover can be described at the mean-field level with reasonable accuracy. At finite temperature, on the other hand, inclusion of pairing fluctuations beyond mean field represents an essential ingredient of the theory, especially in the normal phase where they account for precursor pairing effects.After an introduction to present the key concepts of the BCS–BEC crossover, this report discusses the mean-field treatment of the superfluid phase, both for homogeneous and inhomogeneous systems, as well as for symmetric (spin- or isospin-balanced) and asymmetric (spin- or isospin-imbalanced) matter. Pairing fluctuations in the normal phase are then considered, with their manifestations in thermodynamic and dynamic quantities. The last two Sections provide a more specialized discussion of the BCS–BEC crossover in ultra-cold Fermi gases and nuclear matter, respectively. The separate discussion in the two contexts aims at cross communicating to both communities topics and aspects which, albeit arising in one of the two fields, share a strong common interest
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