First-Order Pairing Transition and Single-Particle Spectral Function in the Attractive Hubbard Model
Physical Review Letters
A Dynamical Mean Field Theory analysis of the attractive Hubbard model is carried out. We focus on the normal state upon restricting to solutions where superconducting order is not allowed. Nevertheless a clear first-order pairing transition as a function of the coupling takes place at all the electron densities out of half-filling. The transition occurs between a Fermi liquid, stable for U < U_c, and an insulating bound pairs phase for U > U_c. The spectral function in the Fermi liquid phase
... constituted by a low energy structure around the Fermi level (similar to the Kondo resonance of the repulsive half-filled model), which disappears discontinuously at U=U_c, and two high energy features (lower and upper Hubbard bands), which persist in the insulating phase.