## BCS-BEC crossover in the two-dimensional attractive Hubbard model: Variational cluster approach    [PDF]

Tatsuya Kaneko, Yukinori Ohta
Variational cluster approximation is used to study the superconducting ground state in the two-dimensional attractive Hubbard model. We show that the calculated Bogoliubov quasiparticle spectra and condensation amplitude clearly exhibit the character of the Cooper pairs in momentum space and that the coherence length $\xi$ evaluated from the condensation amplitude demonstrates the smooth crossover from a weakly paired BCS state ($\xi\gg 1$) to a BEC state of tightly bound pairs ($\xi\ll 1$). The calculated kinetic and potential energies in the superconducting and normal ground states indicate that the superconducting state in the weak-coupling region is driven by the gain in potential energy, while in the strong-coupling region, it is driven by the gain in kinetic energy.
View original: http://arxiv.org/abs/1308.0664