Pseudogap phenomena in the BCS pairing model

Satoshi Fujimoto

We investigate pseudogap phenomena realized in the BCS pairing model with a long but finite interaction range. We calculate the single-particle self-energy in all orders exactly in the temperature range where the superconducting fluctuation propagator is Gaussian-like. It is found that vertex corrections to the self-energy, which are discarded in the previous studies, are crucially important for the pseudogap of the single-particle density of states in higher order calculations.

Douglas J. Scalapino, Thomas Maier, Peter Staar +1 more

Thomas Maier, Peter Staar, Douglas J. Scalapino

Evidence that the pseudogap (PG) in a near-optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ sample destroys the BCS logarithmic pairing instability [1] raises again the question of the role of the PG in the high-temperature superconducting cuprates [2]. The elimination of the BCS instability is consistent with the view that the PG competes with superconductivity. However, as noted in [1], the onset of superconductivity with a $T_c \sim 90$ K suggests an alternative scenario in which the PG reflects the formation of short range pairing correlations. Here, we report results obtained from a dynamic cluster quantum Monte Carlo approximation (DCA) for a 2D Hubbard model and conclude that (1) the PG, like the superconductivity, arises due to short-range antiferromagnetic correlations and (2) contrary to the usual case in which the pairing instability arises from the Cooper instability, here, the strength of the spin-fluctuations increases as the temperature decreases leading to the pairing instability.

James F. Annett, J. P. Wallington

Giancarlo Calvanese Strinati

Ryota Watanabe, Shunji Tsuchiya, Yoji Ohashi

We investigate single-particle excitations and strong-coupling effects in the BCS-BEC crossover regime of a superfluid Fermi gas. Including phase and amplitude fluctuations of the superfluid order parameter within a $T$-matrix theory, we calculate the superfluid density of states (DOS), as well as single-particle spectral weight, over the entire BCS-BEC crossover region below the superfluid transition temperature ${T}_{\mathrm{c}}$. We clarify how the pseudogap in the normal state evolves into the superfluid gap, as one passes through ${T}_{\mathrm{c}}$. While the pseudogap in DOS continuously evolves into the superfluid gap in the weak-coupling BCS regime, the superfluid gap in the crossover region is shown to appear in DOS after the pseudogap disappears below ${T}_{\mathrm{c}}$. In the phase diagram with respect to the temperature and interaction strength, we determine the region where strong pairing fluctuations dominate over single-particle properties of the system. Our results would be useful for the study of strong-coupling phenomena in the BCS-BEC crossover regime of a superfluid Fermi gas.