Theoretical Study of Quantum Coherence in Spin-Boson System : Condensed Matter and Statistical Physics

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We study a two-state system which is coupled to a bosonic environment with the Ohmic spectral density. In this paper, two methods are used. One is the quantum Langevin equation and the other the reduced density matrix. Through these investigations the time reversal symmetry is emphasized because the spectral density of the environment gives rise to a long-ranged collision effect on the spin part. By using the second method, we find, at T=0, (1) a purely oscillatory motion for α<α_c (α is a dimensionless dissipation coefficient and α_c≃0.27); (2) a divergent frequency of the oscillation for α=α_c; (3) a localization for α>α_c. On the other hand, a simple decoupling of the Langevin equation merely gives the lowest order calculation in α. It is notable that the approximation used here is reliable regardless of a range of α. Concerning this a detailed illustration is shown.
理論物理学刊行会の論文
1989-12-25