Abstract:
The pseudogap (PG) region of high transition temperature superconductors has drawn more and more interest in recent years. This region resides above Tc and spans a broad range of hole densities in the hole-doped regime. It serves as the ”Normal State” of high-Tc superconductors, however many peculiar properties have been observed and make it one of the most exotic states. Specifically, novel tunneling spectroscopy which allows continuous temperature varying reveals that the PG region and superconducting region share many similar spectral features and the spectra evolve smoothly through Tc ; On the other hand, photoemission experiment on PG region also shows distinct traits. The gap along Fermi surface deviates from the standard d-wave type. Notably there is a gapless Fermi arc as well as a kink that separates the Fermi surface into two parts. Many theoretical models have been proposed to unveil the mystery. They generally fall into two categories, and the controversial point is whether PG is another energy scale which competing with superconducting state, or it is just a result of thermal phase fluctuation. In the phase fluctuation scenario, Cooper pairs are already formed in the PG region but thermal fluctuations break their long-range phase coherence. And Tc is considered to be a Berezinsky-Kosterlitz-Thouless (BKT) transition. In our numerical studies, we simulate this thermal phase fluctuation by a two-dimensional classical XY model, which is the paradigm for BKT transition. Then we incorporate these phase fluctuations into the mean-field Bogoliubov-de Gennes equations. Our calculation shows that major experimental features for PG can be qualitatively understood by the phase-fluctuating model. Moreover, our simulation also indicates that the momentum-dependence of gap size along Fermi surface, which was originally considered as a support to the competing order explanation, could also be a consequence of phase fluctuation. Furthermore, we study the impact of the atomic-scale order parameter inhomogeneity on phase fluctuation. Our simulation result indicates that Tc will decrease as inhomogeneity increases, which is consistent with experimental observations. Finally several inspirations for increasing Tc are discussed and given.