Breaking of Goldstone modes in two component Bose-Einstein condensate

TL;DR

This paper investigates how Goldstone modes in two-component Bose-Einstein condensates decay and how their behavior changes near phase transitions, revealing enhanced friction effects that could be experimentally observed.

Contribution

It provides a detailed analysis of Goldstone mode decay in two-component BECs, especially near phase separation and critical points, highlighting the coupling effects and their experimental implications.

Findings

Decay rate changes from $k^5$ to $k^{5/2}$ at phase separation

Goldstone mode becomes ill-defined at critical spin fluctuations

Enhanced friction observed near phase transition points

Abstract

We study the decay rate $\Gamma(k)$ of density excitations of two-component Bose-Einstein condensates at zero temperature. Those excitations, where the two components oscillate in phase, include the Goldstone mode resulting from condensation. While within Bogoliubov approximation the density sector and the spin (out-of-phase) sector are independent, they couple at the three-phonon level. For a Bose-Bose mixture we find that the Belyaev decay is slightly modified due to the coupling with the gapless spin mode. At the phase separation point the decay rate changes instead from the standard $k^5$ to a $k^{5/2}$ behaviour due to the parabolic nature of the spin mode. In presence of coherent coupling between the two components the spin sector is gapped and, away from the ferromagnetic-like phase transition point, the decay of density mode is not affected. On the other hand at the transition point, when the spin fluctuations become critical, the Goldstone mode is not well defined anymore since $\Gamma(k)\propto k$. As a consequence, we show that the friction induced by a moving impurity is enahnced -- a feature which could be experimentally tested. Our results apply to every non-linear 2-component quantum hydrodynamic Hamiltonian which is time-reversal invariant, and possesses an $U(1)\times {\mathbf Z}_2$ symmetry.