Equilibration of a finite-temperature binary Bose gas formed by population transfer

Abstract

We consider an equilibrium single-species homogeneous Bose gas from which a proportion of the atoms are instantaneously and coherently transferred to a second species, thereby forming a binary Bose gas in a non-equilibrium initial state. We study the ensuing evolution towards a new equilibrium, mapping the dynamics and final equilibrium state out as a function of the population transfer and the interspecies interactions by means of classical field methods. While in certain regimes, the condensate fractions are largely unaffected by the population transfer process, in others, particularly for immiscible interactions, one or both condensate fractions are vastly reduced to a new equilibrium value.

Critical velocity for vortex nucleation in a finite-temperature Bose gas

“…However, whereas the GPE conventionally describes the condensate only, ψ(r, t) now describes the entire multi-mode 'classical' gas [28,29]. The classical field method has been used to model phenomena beyondmean-field effects, including thermal equilibration dynamics [36,38,40,41], condensate fractions [39], critical temperatures [42], correlation functions [43], spontaneous production of vortex-antivortex pairs in quasi-2D gases [44], thermal dissipation of vortices [45], and related effects in binary condensates [40,46,47].…”

“…In effect, an ultraviolet cutoff is introduced, n k (t) = 0 for k > k max , where k = |k| and the maximum described wave vector amplitude is k max = √ 3π/∆. The ensuing evolution from the strongly nonequilibrium initial conditions has been outlined previously [36,40]. Initially the mode occupation numbers n k are uniformly distributed over wavenumber k, up to the cutoff.…”

Nonequilibrium kinetic theory for trapped binary condensates

“…Interestingly, the nature of multi-component systems means that novel transport processes, for example spin-changing collisions will contribute to both damping processes as well as noise for these systems. Both the stochastic (projected) GPE and the closely-related classical field [103] and truncated Wigner [104][105][106] approaches have an explicit cutoff in energy, with all higher-lying (pure thermal) atoms treated stationary. In contrast to these, the ZNG formalism explicitly treats all modes dynamically and selfconsistently, but it does not include the effect of fluctuations of the phase of the non-condensate atoms, which in turn limits its applicability to temperatures not too close to the transition.…”

“…There are several other complementary approaches to tackling the coupled dynamics of multi-component condensates, including the number-conserving approach [111], the stochastic Gross-Pitaevskii formal-ism [107][108][109]148], as well as classical field [103] and truncated Wigner [104][105][106] treatments.…”

“…In the context of multi-component condensates, which have been extensively studied with coupled Gross-Pitaevskii equations (GPEs) [31][32][33][91][92][93][94][95][96][97][98][99], or their dissipative generalisations [100][101][102], their finite temperature dynamics remains a partly open problem. Approaches considered to date include classical field [103], truncated Wigner [104][105][106], coupled stochastic projected Gross-Pitaevskii equations [107][108][109][110], or numberconserving approaches [111]. However, the detailed dynamics far from the critical region are expected to be better described by a model that fully accounts for all condensate and thermal cloud dynamics.…”