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Paper
Insulator, metal, or superconductor: The criteria
Abstract
The appropriate infinite-wavelength qi→0 and zero-frequency ω→0 limits of the paramagnetic current-current correlation function provide criteria for determining whether a system is insulating, metallic, or superconducting. Here we discuss these criteria for lattice models and present Monte Carlo data for the two-dimensional positive- and negative-U Hubbard models that imply that the ground state of the half-filled positive-U Hubbard model is an insulator, the doped state is a metal, and the negative-U Hubbard model is a superconductor. © 1993 The American Physical Society.