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 demonstrate that for large systems, this approximate method produces XY critical scaling in the correlation length and pair susceptibility. Our data also exhibits XY finite-size scaling. For the parameters considered, the critical regime is quite large, extending beyond 5 times TKTB. We discuss the role of vortex-pair unbinding in the normal state in the context of pseudogap behaviour and the relevance to the physics of underdoped cuprates. (orig.)