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arXiv:1008.0529 · Physical Review A

Paired phases and Bose-Einstein condensation of spin-one bosons with attractive interaction

G. Pełka, Krzysztof Byczuk, J. Tworzydło

Published Mar 15, 2011• 3 citations
arXivHTMLPDF

Abstract

We analyze paired phases of cold bosonic atoms with spin $S=1$ and with an attractive interaction. We derive mean-field self-consistent equations for the matrix order parameter describing such paired bosons on an optical lattice. The possible solutions are classified according to their symmetries. In particular, we find that the self-consistent equations for the SO(3) symmetric phase are of the same form as those for scalar bosons with attractive interactions. This singlet phase may exhibit either the BCS-type pairing instability (BCS phase) or the Bose-Einstein condensation (BEC) quasiparticle condensation together with the BCS-type pairing (BEC phase) for an arbitrary attraction ${U}_{0}$ in the singlet channel of the two-body interaction. We show that both condensate phases become stable if a repulsion ${U}_{2}$ in the quintet channel is above a critical value, which depends on ${U}_{0}$ and other thermodynamic parameters.

DOI
10.1103/physreva.83.033612

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We analyze paired phases of cold bosonic atoms with spin S=1S=1S=1 and with an attractive interaction. We derive mean-field self-consistent equations for the matrix order parameter describing such paired bosons on an optical lattice. The possible solutions are classified according to their symmetries. In particular, we find that the self-consistent equations for the SO(3) symmetric phase are of the same form as those for scalar bosons with attractive interactions. This singlet phase may exhibit either the BCS-type pairing instability (BCS phase) or the Bose-Einstein condensation (BEC) quasiparticle condensation together with the BCS-type pairing (BEC phase) for an arbitrary attraction U0{U}_{0}U0​ in the singlet channel of the two-body interaction. We show that both condensate phases become stable if a repulsion U2{U}_{2}U2​ in the quintet channel is above a critical value, which depends on U0{U}_{0}U0​ and other thermodynamic parameters.
We study a two-component Bose gas with a symmetric spin-orbit coupling, and find that two atoms can form a bound state with any intra- or inter-species scattering length. Consequently, in the dilute limit, the BCS-pairing state of bosons can be formed with weakly attractive interspecies and repulsive intraspecies interactions. The quasiparticle excitation energies are anisotropic due to spin-orbit coupling. This BCS-pairing state is energetically favored over Bose-Einstein condensation (BEC) of atoms at low densities. As the density increases, there is a first-order transition from the BCS to BEC states.
Thermodynamical properties of an interacting boson system at finite temperatures and zero chemical potential are studied within the framework of the Skyrme-like mean-field toy model. It is assumed that the mean field contains both attractive and repulsive terms. Self-consistency relations between the mean field and thermodynamic functions are derived. It is shown that, for sufficiently strong attractive interactions, this system develops a first-order phase transition via the formation of a Bose condensate. An interesting prediction of the model is that the condensed phase is characterized by a constant total density of particles. It is shown that the energy density exhibits a jump at the critical temperature.
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Abstract The stable Bardeen–Schrieffer–Cooper (BCS) pairing state of a bosonic system has long been sought theoretically and experimentally. Here we propose that a stable BCS state of bosons can be realized in a binary Bose gas with s-wave intra-species repulsion and an inter-species attraction in the mean-field-stable region. We find that above the Bose–Einstein condensation (BEC) transition temperature, there is a phase transition from the normal state to a BCS state driven by inter-species pairing. When the temperature decreases, another phase transition from the BCS state to a mixed state featuring both atomic BEC and inter-species pairing occurs. As the temperature is further lowered, the mixed state is eventually taken over by the pure BEC state. We present the phase diagram of this system and discuss its experimental implications.