Crossover Between BCS Superconductor and Doped Mott Insulator in Two-dimensional Hubbard Model

When: Friday, 10 July (2015), 12:00h
Place: Departamento de Física Teórica de la Materia Condensada, Facultad Ciencias, Module 5, Seminar Room (5th Floor).
Speaker: Masao Ogata, Department of Physics, Graduate School of Science, University of Tokyo, Japan.

With high-T_c cuprates in mind, the properties of correlated d-wave superconducting (SC) and antiferromagnetic (AF) states are studied for the Hubbard model on square lattices using a variational Monte Carlo method.In order to understand the metal-insulator transition (Mott transition), we find that doubloon (doubly-occupied state)-holon (unoccupied state) bound states play essential roles. Using a simple trial wave functions including doublon-holon bound states, we find that the first-order metal-insulator transition without direct relevance to magnetic orders occurs at U=U_c, which is approximately the band width. This transition originates in the binding or unbinding of a doublon to a holon. On the other hand, at finite doping, a sharp crossover from normal state to SC state occurs at U_crossover/t∼10. As the doping decreases, U_crossover is smoothly connected to the Mott transition point at half filling. Comparing the doping dependence of SC correlation functions with experiments, we find that the effective value of U for cuprates should be larger than the bandwidth, for which the t–J model is valid.