Title: Annihilation operators associated with unstable vacua in non-equilibrium thermo-field dynamics
Author(s):

	K. Yoshida (Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Ibaraki 305-8571, Japan) ,
	T. Arimitsu (Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Ibaraki 305-8571, Japan)

Out of thermal equilibrium state, the vacuum is unstable and evolves in time. Consequently, the annihilation operators associated with the unstable vacuum depend on time. This dissipative time-evolution of quantum systems can be systematically treated, within the canonical operator formalism referred to as non-equilibrium thermo-field dynamics. Given is an alternative route to derive the time-dependent annihilation operators within the formalism. As an example, time-dependent annihilation operators for the systems of bosonic and fermionic semi-free fields are derived.

Key words: non-equilibrium thermo-field dynamics, annihilation operator, unstable vacuum, dissipative quantum systems, time-dependent Bogoliubov transformation
PACS: 11.10.Wx, 05.30.-d

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