Correlated Coherent States - Quantum Analogue Of Thermal States.(On The   Problem Of Incorporating Thermodynamics Into Quantum Theory)
  In this paper, we show that correlated coherent states (CCSs) are the most adequate candidates for the role of quantum analogues of the thermal states. The main result of our study reduces to the fact that quantum thermal effects under conditions of the equilibrium of an object with the stochastic environment at any temperatures can be explained consistently only on the basis of the set of CCSs. 
A. D. Sukhanov, O. N. Golubjeva, and V. G. Bar'yakhtar
quant-ph