Nonclassicality of single quantum excitation of a thermal field in   thermal environments
  The nonclassicality of single photon-added thermal states in the thermal channel is investigated by exploring the volume of the negative part of the Wigner function. The Wigner functions become positive when the decay time exceeds a threshold value $\gamma{t}_c$, which only depends on the effective temperature or mean thermal photon number of the thermal channel, but not depends on the effective temperature of the initial thermal state. This phenomenon is similar with the case of single quantum excitation of classical coherent states in thermal channel. Furthermore, we firstly demonstrate $\gamma{t}_c$ is the same for arbitrary pure or mixed nonclassical optical fields with zero population in vacuum state.   PACS numbers: 42.50.Dv, 03.65.Yz, 05.40.Ca 
Shang-Bin Li