Dissipative quantum tunneling of a single defect in a disordered metal

We have studied the dynamics of single bistable defects in submicron disordered Bi wires at temperatures 0.1-2 K. The wires are sufficiently small so that the motion of a single defect can be detected as a random telegraph signal in the resistance time trace. The amplitude of the resistance jumps increases as the temperature is lowered due to the physics of universal conductance fluctuations. The defect transitions obey Poisson statistics and detailed balance, indicating that the defect is a two-state system. We interpret the signal as arising from incoherent tunneling of an atom or group of atoms between the two wells of a double-well potential. The temperature dependence of the tunneling rates agrees quantitatively with the predictions of dissipative quantum tunneling theory, which describes tunneling of a defect in the presence of strong dissipation from the electron bath. Our measurements exploit the previous discovery that the energy asymmetry epsilon of the defect varies with magnetic field. By varying both the temperature and magnetic field, we can probe the behavior of the tunneling rates spanning two very different regimes, from kT much less than epsilon to kT much greater than epsilon. Data from a single defect at several values of magnetic field suggest that the defect-bath coupling constant alpha and the renormalized tunneling matrix element Delta(r) are nearly independent of magnetic field.