CONTROLLING LOW-TEMPERATURE TUNNELING DYNAMICS WITH EXTERNAL FIELDS

The tunneling of a particle between two states, as influenced by coupl ing the particle to an external field, is investigated. A spin-boson H amiltonian is used to describe the tunnel system and its interaction w ith a medium and is augmented by a term coupling the dipole moment of the particle to the external held. A projection operator method is int roduced that provides equations of motion for the particle's density m atrix and permits discussion of external held effects on tunneling eve n at very low temperatures. These equations of motion can exhibit solu tions characterized by population relaxation with a rate constant that is an average over the period of the external held. As an application of this theory, the modification by an external field of the tunnelin g rate of a defect in a metal, where the system-medium interaction is characterized by an Ohmic spectrum, is evaluated. At sufficiently low temperatures, for experimentally relevant parameter values, suitably c hosen (constant and/or sinusoidal), external fields can greatly suppre ss or enhance the tunneling rate relative to the field-independent val ue.