NONLINEAR DYNAMICS OF A DRIVEN 2-LEVEL TUNNELING DEFECT

With increasing external-field amplitude the dynamics of a driven two- level system shows a cross-over from the weak-coupling or linear-respo nse behaviour to a non-linear regime which is characterised by the app earance of Rabi oscillations, a field-dependent dynamical susceptibili ty, saturation of energy dissipation and additional frequencies in the correlation function. The system is no longer in thermal equilibrium with the heat bath; any initial distribution will relax towards a stat ionary state with a time-dependent polarisation vector. These features are derived from the propagator of the two-level system which is calc ulated using a perturbative scheme with a well-defined small parameter ; the rotating-wave approximation of the Bloch equations is avoided. T he dynamical behaviour is discussed by means of correlation and respon se functions.