Unconventional Bose-Einstein condensations from spin-orbit coupling

TL;DR

This paper explores unconventional Bose-Einstein condensates induced by spin-orbit coupling, revealing spontaneous time-reversal symmetry breaking and novel spin textures in cold atom and semiconductor systems.

Contribution

It demonstrates that spin-orbit coupling leads to unconventional BECs with broken time-reversal symmetry, beyond the traditional positive-definite wavefunction paradigm.

Findings

Quantum zero-point energy selects spin-spiral condensates.

Spontaneous generation of half-quantum vortices and skyrmion spin textures.

Time-reversal symmetry is spontaneously broken in these condensates.

Abstract

According to the "no-node" theorem, many-body ground state wavefunctions of conventional Bose-Einstein condensations (BEC) are positive-definite, thus time-reversal symmetry cannot be spontaneously broken. We find that multi-component bosons with spin-orbit coupling provide an unconventional type of BECs beyond this paradigm. We focus on the subtle case of isotropic Rashba spin-orbit coupling and the spin-independent interaction. In the limit of the weak confining potential, the condensate wavefunctions are frustrated at the Hartree-Fock level due to the degeneracy of the Rashba ring. Quantum zero-point energy selects the spin-spiral type condensate through the "order-from-disorder" mechanism. In a strong harmonic confining trap, the condensate spontaneously generates a half-quantum vortex combined with the skyrmion type of spin texture. In both cases, time-reversal symmetry is spontaneously broken. These phenomena can be realized in both cold atom systems with artificial spin-orbit couplings generated from atom-laser interactions and exciton condensates in semi-conductor systems.