arXiv subjects

Verdiana Piselli

Publications and source records attributed to Verdiana Piselli.

5 recordsLinked to original sources

arXiv2024

Inclusion of pairing fluctuations in a semiclassical approach: The case of study of the Josephson effect

Recent refinements on a semiclassical approach are reviewed, aiming at describing the inhomogeneous local gap parameter in the presence of non-trivial spatial geometries and at taking into account at the same time pairing fluctuations beyond mean field. The method is applied to describe the Josephson effect over the wide range of physical conditions related to recent experiments on this topic performed with ultra-cold Fermi gases.

cond-mat.supr-con↗

arXiv2024

Josephson current flowing through a nontrivial geometry: The role of pairing fluctuations across the BCS-BEC crossover

A 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 [Pisani \emph{et al.}, Phys. Rev. B {\bf 108}, 214503 (2023)], 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.

cond-mat.supr-con↗

arXiv2024

Inclusion of pairing fluctuations in the differential equation for the gap parameter for superfluid fermions in the presence of nontrivial spatial constraints

Most 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 inter-particle attraction, when the Cooper pair size is much larger than the average inter-particle distance (corresponding to the BCS limit of the BCS-BEC crossover). However, it looses accuracy for increasing attraction when the Cooper pair size becomes comparable or even smaller than the average inter-particle 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 super-current 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 ultra-cold Fermi atoms, to which the results of the present approach favorably compare.

cond-mat.supr-con↗

arXiv2024

Critical current throughout the BCS-BEC crossover with the inclusion of pairing fluctuations

The 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 quasi-particle 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.

cond-mat.supr-con↗

arXiv2020

Josephson effect at finite temperature along the BCS-BEC crossover

The Josephson current-phase characteristics, that arise when a supercurrent flows across two fermionic superfluids separated by a potential barrier, can be controlled by varying either the inter-particle coupling or the temperature. While the coupling dependence has been addressed in detail both theoretically and experimentally for an attractive Fermi gas undergoing the BCS-BEC crossover, a corresponding study of the temperature dependence of the Josephson characteristics is still lacking in this context. Here, we investigate the combined coupling and temperature dependence of the Josephson characteristics in a systematic way for a wide set of barriers, within ranges of height and width that can be experimentally explored. Our study smoothly connects the two limiting cases, of non-overlapping composite bosons at low temperature described by the Gross-Piatevskii equation, and of strongly overlapping Cooper pairs near the critical temperature described by the Ginzburg-Landau equation. In this way, we are able to explore several interesting effects related to how the current-phase characteristics evolve along the BCS-BEC crossover as a function of temperature and of barrier shape. These effects include the coherence length outside the barrier and the pair penetration length inside the barrier (which is related to the proximity effect), as well as the temperature evolution of the Landau criterion in the limit of a vanishingly small barrier. A comparison is also presented between the available experimental data for the critical current and our theoretical results over a wide range of couplings along the BCS-BEC crossover.

cond-mat.supr-con↗