Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSnake instability of dark solitons across the BEC-BCS crossover: an effective field theory perspective
65
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
In the present article the snake instability mechanism for dark solitons in superfluid Fermi gases is studied in the context of a recently developed effective field theory [Eur. Phys. J. B 88, 122 (2015)]. This theoretical treatment has proven to be suitable to study stable dark solitons in quasi-1D setups across the BEC-BCS crossover. In this manuscript the nodal plane of the stable soliton solution is perturbed by adding a transverse modulation. The numerical solution of the system of coupled nonlinear differential equations describing the amplitude of the perturbation leads to the instability spectra which are calculated for a wide range of interaction regimes and compared to other theoretical predictions. The maximum transverse size that the atomic cloud can have in order to preserve the stability is estimated, and the effects of spin-imbalance on this critical length are examined, revealing a stabilization of the soliton with increasing imbalance.
rate research
Read More
Dark solitons in superfluid Bose gases decay through the snake instability mechanism, unless they are strongly confined. Recent experiments in superfluid Fermi gases have also interpreted soliton decay via this mechanism. However, we show using both an effective field numerical simulation and a perturbative analysis that there is a qualitative difference between soliton decay in the BEC- and BCS-regimes. On the BEC-side of the interaction domain, the characteristic snaking deformations are induced by fluctuations of the amplitude of the order parameter, while on the BCS-side, fluctuations of the phase destroy the soliton core through the formation of local Josephson currents. The latter mechanism is qualitatively different from the snaking instability and this difference should be experimentally detectable.
Reconnecting vortices in a superfluid allow for the energy transfer between different length scales and its subsequent dissipation. The present picture assumes that the dynamics of a reconnection is driven mostly by the phase of the order parameter, and this statement can be justified in the case of Bose-Einstein Condensates (BECs), where vortices have a simple internal structure. Therefore, it is natural to postulate that the reconnection dynamics in the vicinity of the reconnection moment is universal. This expectation has been confirmed in numerical simulations for BECs and experimentally for the superfluid ${}^4$He. Not much has been said about this relation in the context of Fermi superfluids. In this article we aim at bridging this gap, and we report our findings, which reveal that the reconnection dynamics conforms with the predicted universal behaviour across the entire BCS-BEC crossover. The universal scaling also survives for spin-imbalanced systems, where unpaired fermions induce a complex structure of the colliding vortices.
We developed a comprehensive semiclassical theory of solitons in one dimensional systems at BCS-BEC crossover to provide a semiclassical explanation of their excitation spectra. Our semiclassical results agree well with the exact solutions on both the deep BCS and deep BEC side and explain qualitatively the smooth crossover between them. Especially, we showed that the minimum energy of the $S=1/2$ excitation is achieved exactly at the Fermi momentum $k_F=pi n/2$, where $nm_F$ ($m_F$ is the mass of the fermionic atom) is the total mass density of the system. This momentum remains unchanged along the whole crossover, whether the mass is contained in the bosonic molecules as on the deep BEC side or in the fermionic atoms as on the deep BCS side. This phenomenon comes about as a result of a special feature of one dimensional systems that the conventional quasiparticle is not stable with respect to soliton formation. It is valid not only in exactly solvable models but also on the level of semiclassical theory. Besides, we also resolved the inconsistency of existing semiclassical theory with the exact solution of soliton-like $S=0$ excitations on the deep BCS side by a new proposal of soliton configuration.
We report on the observation of the Josephson effect between two strongly interacting fermionic superfluids coupled through a thin tunneling barrier. We prove that the relative population and phase are canonically conjugate dynamical variables, coherently oscillating throughout the entire crossover from molecular Bose-Einstein condensates (BEC) to Bardeen-Cooper-Schrieffer (BCS) superfluids. We measure the plasma frequency and we extract the Josephson coupling energy, both exhibiting a non-monotonic behavior with a maximum near the crossover regime. We also observe the transition from coherent to dissipative dynamics, which we directly ascribe to the propagation of vortices through the superfluid bulk. Our results highlight the robust nature of resonant superfluids, opening the door to the study of the dynamics of superfluid Fermi systems in the presence of strong correlations and fluctuations.
Strongly correlated Fermi systems with pairing interactions become superfluid below a critical temperature $T_c$. The extent to which such pairing correlations alter the behavior of the liquid at temperatures $T > T_c$ is a subtle issue that remains an area of debate, in particular regarding the appearance of the so-called pseudogap in the BCS-BEC crossover of unpolarized spin-$1/2$ nonrelativistic matter. To shed light on this, we extract several quantities of crucial importance at and around the unitary limit, namely: the odd-even staggering of the total energy, the spin susceptibility, the pairing correlation function, the condensate fraction, and the critical temperature $T_c$, using a non-perturbative, constrained-ensemble quantum Monte Carlo algorithm.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
