Spontaneous solitons in the thermal equilibrium of a quasi-one-dimensional Bose gas

TL;DR

This paper demonstrates that spontaneous solitons naturally occur in the thermal equilibrium of a weakly-interacting 1D Bose gas, revealing new insights into the behavior of quasicondensates without external perturbations.

Contribution

It shows that solitons form spontaneously in 1D Bose gases at thermal equilibrium, linking their emergence to thermal occupation of Type II excitations in the Lieb-Liniger model.

Findings

Spontaneous solitons are observed in thermal equilibrium states.

Soliton formation is explained by thermal occupation of Type II excitations.

This challenges previous notions that solitons require external forcing.

Abstract

Solitons, or non-destructible local disturbances, are important features of many one-dimensional (1D) nonlinear wave phenomena, from water waves in narrow canals to light pulses in optical fibers. In ultra-cold gases, they have long been sought, and were first observed to be generated by phase-imprinting. More recently, their spontaneous formation in 1D gases was predicted as a result of the Kibble-Zurek mechanism, rapid evaporative cooling, and dynamical processes after a quantum quench. Here we show that they actually occur generically in the thermal equilibrium state of a weakly-interacting elongated Bose gas, without the need for external forcing or perturbations. This reveals a major new quality to the experimentally widespread quasicondensate state. It can be understood via thermal occupation of the famous and somewhat elusive Type II excitations in the Lieb-Liniger model of a uniform 1D gas.