Magnetic field tuned energy of a single two-level system in a meso- scopic metal

We study the effect of an external magnetic field H on an individual tunneling system by measuring the time-dependent bistable conductance of a submicron Bi sample near 1 K. The amplitude of the dynamic conductance fluctuations is a random function of H. The transition rates are significantly modified by the field, from which we infer an H-dependent energy splitting. This splitting is symmetric near H=0, but exhibits nonmonotonic features at higher fields, possibly arising from the coupling of local electron-density fluctuations to the defect.