Crossover of superconducting properties and kinetic-energy gain in two-dimensional Hubbard model

TL;DR

This study investigates superconductivity in the two-dimensional Hubbard model near half filling, revealing a crossover from BCS to a new type of pairing at a critical interaction strength, with implications for cuprate superconductors.

Contribution

It introduces an advanced variational Monte Carlo approach considering second-order processes, demonstrating a crossover in superconducting behavior and energy gain mechanisms.

Findings

Superconductivity is stable with d_x^2-y^2-wave symmetry.

A crossover occurs at U≈12t from BCS to a new superconducting type.

Condensation energy varies exponentially with coupling strength.

Abstract

Superconductivity in the Hubbard model on a square lattice near half filling is studied using an optimization (or correlated) variational Monte Carlo method. Second-order processes of the strong-coupling expansion are considered in the wave functions beyond the Gutzwiller factor. Superconductivity of d_x^2-y^2-wave is widely stable, and exhibits a crossover around U=U_co\sim 12t from a BCS type to a new type. For U\gsim U_co (U\lsim U_co), the energy gain in the superconducting state is derived from the kinetic (potential) energy. Condensation energy is large and \propto exp(-t/J) [tiny] on the strong [weak] coupling side of U_co. Cuprates belong to the strong-coupling regime.