Coulomb Enhanced Superconducting Pair Correlations in the Frustrated Quarter-Filled Band

TL;DR

This study uses numerical methods to show that electron-electron interactions enhance superconducting correlations specifically at half-filling in a frustrated 2D Hubbard model, shedding light on unconventional superconductivity.

Contribution

It demonstrates that strong electron correlations enhance superconducting pair correlations only near half-filling in a frustrated 2D Hubbard model, linking this to singlet formation and spin gaps.

Findings

Enhanced pair correlations at carrier concentration ~0.5

Suppression of correlations at other fillings

Agreement between ground state and quantum Monte Carlo results

Abstract

A necessary condition for superconductivity (SC) driven by electron correlations is that electron-electron (e-e) interactions enhance superconducting pair-pair correlations, relative to the noninteracting limit. We report high-precision numerical calculations of the ground state within the frustrated two-dimensional (2D) Hubbard Hamiltonian for a wide range of carrier concentration $\rho$ (0 < $\rho$ <1) per site. We find that long range superconducting pair correlations are enhanced only for $\rho$ $\simeq$ 0.5. At all other fillings e-e interactions mostly suppress pair correlations. The enhancement of pair correlations is driven by the strong tendency to local singlet bond formation and spin gap (SG) in $\rho$ = 0.5, in lattices with quantum fluctuation. We also report determinantal quantum Monte Carlo (DQMC) calculations that are in strong agreement with our ground state results. Our work provides a key ingredient to the mechanism of SC in the 2D organic charge-transfer solids (CTS), and many other unconventional superconductors with frustrated crystal lattices and $\rho$ $\simeq$ 0.5, while explaining the absence of SC in structurally related materials with substantially different $\rho$.