Crossover between BCS Superconductor and Doped Mott Insulator of d-wave Pairing State in Two-Dimensional Hubbard Model
Abstract
With high-Tc cuprates in mind, properties of correlated dx2-y2-wave superconducting (SC) and antiferromagnetic (AF) states are studied for the Hubbard (t-t'-U) model on square lattices, using a variational Monte Carlo method. We employ simple trial wave functions including only crucial parameters, such as a doublon-holon binding factor indispensable to describe correlated SC and normal states as doped Mott insulators. U/t, t'/t and δ (doping rate) dependence of relevant quantities are systematically calculated. As U/t increases, a sharp crossover of SC properties occurs at Uco/t 10 from a conventional BCS type to a kinetic-energy-driven type for any t'/t. As δ decreases, Uco/t is smoothly connected to the Mott transition point at half filling. For U/t 5, steady superconductivity corresponding to the cuprates is not found, whereas the d-wave SC correlation function Pd∞ rapidly increases for U/t 6 and becomes maximum at U=Uco. Comparing the δ dependence of Pd∞ with experimentally observed dome-shaped Tc and condensation energy, we find that the effective value of U for the cuprates should be larger than the band width, for which the t-J model is valid. Analyzing the kinetic energy, we reveal that for U>Uco only doped holes (electrons) become charge carriers, which will make a small Fermi surface (hole pocket), but for U<Uco all the electrons (holes) contribute to conduction and will make an ordinary large Fermi surface, which is contradictory to the feature of cuprates. By introducing a proper negative (positive) t'/t, the SC (AF) state is stabilized. In the underdoped regime, the strength of SC for U>Uco is determined by two factors, i.e., the AF spin correlation, which creates singlet pairs (pseudogap), and the charge mobility dominated by Mott physics.