UNIVERSAL CONDUCTIVITY OF 2D BOSE SYSTEMS WITH DAMPING

We study the superconductor-insulator transition of a 2-dimensional Bo se-Hubbard model. In a coarse-grained free-energy functional descripti on we evaluate the electromagnetic response functions of the system. T he real part of the longitudinal conductivity is characterized by an e xcitation gap, whereas the imaginery part describes a capacitor. In an ideal system, under certain conditions a universal conductance is fou nd at the transition. If we add low frequency dissipation to the model a different value of the universal conductance is found, but still it is independent of the strength of the dissipation.