Vortex-pair unbinding in the normal state of two-dimensional short-coherence-length superconductors

We study the superconducting transition in the attractive Hubbard model in two dimensions using the self-consistent T-matrix approximation. We demonst rate that for large systems, this approximate method produces XY critical s caling in the correlation length and pair susceptibility. Our data also exh ibits XY finite-size scaling. For the parameters considered, the critical r egime is quite large, extending beyond 5 times T-KTB. We discuss the role o f vortex-pair unbinding in the normal state in the context of pseudogap beh aviour and the relevance to the physics of underdoped cuprates.