Stabilization of the vortex-liquid state by strong pairing interaction

We theoretically investigate qualitative features of the field-temperature (H−T) phase diagram of superconductors with strong attractive interaction lying in the BCS-BEC crossover regime. Starting with a simple attractive Hubbard model, we estimate three kinds of characteristic fields, i.e., the pair-formation field H*, the vortex-liquid-formation field Hc2, and the vortex-lattice-formation field Hmelt. The region between Hc2 and Hmelt, as well as that between Hc2 and H*, is found to be enlarged as the interaction is stronger. In other words, a strong attractive interaction can stabilize both the vortex-liquid and preformed-pair regions. We also point out the expected particle-density dependence of the H−T phase diagram.