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

Based on extensions of the grand-canonical quantum Monte Carlo algorithm to incorporate magnetic fields, we provide numerical data confirming the existence of a Kosterlitz-Thouless transition in the attractive Hubbard model. Here, we calculate the temperature derivative of the superfluid density, [partial derivative betaD(s)(beta)]/partial derivative beta, to pin down the transition. The latter quantity is obtained from the difference in internal energy of systems which differ by a phase twist pi/2 in the boundary condition along one lattice direction. Away from half-band filling, [partial derivative betaD(s)(beta)]/partial derivative beta shows a response which increases with lattice size at the transition temperature. In contrast, such a signal is not observed at half-band filling.