A study was made of the dynamics of single bistable defects in sub-micron disordered wires at temperatures of between 0.1 and 2K. The wires were so small that the motion of a single defect could be detected as a random telegraph signal in the resistance versus time trace. It was found that the amplitude of the resistance jumps increased as the temperature was decreased; due to the effect of universal conductance fluctuations. The defect transitions obeyed Poisson statistics and detailed balance; thus indicating that the defect was a 2-state system. The signal was explained in terms of the incoherent tunnelling of an atom, or group of atoms, between the 2 wells of a double-well potential. The temperature dependence of the tunnelling rate agreed quantitatively with the predictions of dissipative quantum tunnelling theory. The latter described the tunnelling of a defect in the presence of strong dissipation that arose from the electron bath. The present measurements exploited the fact that the energy asymmetry of the defect varied as a function of the magnetic field. By varying both the temperature and the field, it was possible to monitor the behavior of the tunneling over a wide range of conditions. Data for a single defect at several values of the magnetic field suggested that the defect-bath coupling constant, and the renormalized tunnelling matrix element, were almost independent of the field.

K.Chun, N.O.Birge: Physical Review B, 1996, 54[7], 4629-37