TEMPERATURE DERIVATIVE OF THE SUPERFLUID DENSITY IN THE ATTRACTIVE HUBBARD-MODEL

Based on extensions of the grand-canonical quantum Monte Carlo algorit hm to incorporate magnetic fields, we provide numerical data confirmin g the existence of a Kosterlitz-Thouless transition in the attractive Hubbard model. Here, we calculate the temperature derivative of the su perfluid density, [partial derivative betaD(s)(beta)]/partial derivati ve beta, to pin down the transition. The latter quantity is obtained f rom the difference in internal energy of systems which differ by a pha se twist pi/2 in the boundary condition along one lattice direction. A way from half-band filling, [partial derivative betaD(s)(beta)]/partia l derivative beta shows a response which increases with lattice size a t the transition temperature. In contrast, such a signal is not observ ed at half-band filling.