Improved spectral functions for quantum kinetics

We improve the calculation of the two-time-dependent spectral functions which determine the memory kernel of the retarded quantum kinetics by using an interpolation between the ultrashort time limit of the dissipative part of the self-energy and its stationary limit. By these means we get a spectral function which decays in the form of a power of a hyperbolic secans. For short time intervals the decay of the retarded Green function starts with zero slope, while the long-time behaviour is exponential. The real part of the Fourier transform of a hyperbolic secans is again a hyperbolic secans which is localised enough in frequency space, so that run-away effects of the asymptotic kinetics are avoided. Copyright (C) 1996 Published by Elsevier Science Ltd