THE PERTURBED STATIC PATH APPROXIMATION AT FINITE-TEMPERATURE - OBSERVABLES AND STRENGTH FUNCTIONS

We present an approximation scheme for calculating observables and str ength functions of finite fermionic systems at finite temperature such as hot nuclei. The approach is formulated within the framework of the Hubbard-Stratonovich transformation and goes beyond the static path a pproximation and the RPA by taking into account small amplitude time-d ependent fluctuations around each static value of the auxiliary fields . We show that this perturbed static path approach can be used systema tically to obtain good approximations for observable expectation value s and for low moments of the strength function. The approximation for the strength function itself, extracted by an analytic continuation fr om the imaginary-time response function, is not always reliable, and w e discuss the origin of the discrepancies and possible improvements. O ur results are tested in a solvable many-body model. (C) 1997 Elsevier Science B.V.