Spontaneous solitons in the thermal equilibrium of a   quasi-one-dimensional Bose gas
  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. 
T. Karpiuk, P. Deuar, P. Bienias, E. Witkowska, K. Pawlowski, M.   Gajda, K. Rzazewski, M. Brewczyk
cond-mat.quant-gas