PHYSICA ELSEVIER Physica A 226 (1996) 137-151 Relaxation from a metastable state due to quantum fluctuation Hiroto Kobayashi, Naomichi Hatano, Akira Terai ~, Masuo Suzuki Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 9 November 1995 Abstract Relaxation from a metastable state is studied in a spin-boson model. We observe a large oscillation due to the interaction between a spin and bosons. The interaction dependence of the temporal behavior of the magnetization is also studied. PACS: 76.20.+q; 05.30.-d; 73.40.Gk; 75.10.Jm Keywords: Relaxation; Metastable state; Quantum fluctuation; Spin-boson model 1. Introduction The macroscopic quantum phenomenon has been one of the most interesting topics in recent years. An especially intriguing problem is macroscopic tunneling relaxation from a metastable state. Chudnovsky [ 1,2] considered a magnetic fine particle in a magnetic field which is anti-parallel to the magnetic moment. Owing to thermal and quantum fluctuations, the magnetic moment tunnels from the metastable anti-parallel state to the stable parallel state. They discussed the tunneling rate by means of a path integral. They thereby predicted that the quantum tunneling rate is large enough to be observed experimentally; the quantum tunneling dominates the thermal tunneling at low temperatures, and hence the total tunneling rate remains finite even at zero temperature. This prediction on the temperature dependence of the tunneling rate is quantitatively consistent with recent experiments [3,4]. I Present address: Department of Physics, Kitasato University, Sagamihara, Kanagawa 228, Japan. 0378-4371/96/$15.00 t~) 1996 Elsevier Science B.V. All rights reserved SSDI0378-4371 (95)00473-4