DISSIPATIVE QUANTUM TUNNELING OF A SINGLE MICROSCOPIC DEFECT IN A MESOSCOPIC METAL

The transition rates of individual two-state systems in a mesoscopic disordered metal (Bi) are studied via time-dependent conductance fluctuations. A striking increase in the transition rates is observed as the temperature T is decreased below 1 K for defects with energy splitting epsilon < k(B)T. In contrast, the transition rates decrease monotonically as T is lowered if epsilon > k(B)T. We show that these phenomena are a manifestation of dissipative quantum tunneling and estimate, for one particular defect, the defect-conduction-electron coupling parameter, alpha almost-equal-to 0.24.