Many-variable variational Monte Carlo study of superconductivity in two-band Hubbard models with an incipient band

Daichi Kato,Kazuhiko Kuroki

doi:10.1103/physrevresearch.2.023156

Abstract

We study superconductivity in two-band models where one of the bands does or does not intersect the Fermi level depending on the parameter values. Applying a many-variable variational Monte-Carlo method for a Hubbard model on two-leg ladder and bilayer square lattices, we show that superconductivity can be enhanced in a parameter regime where the edge of one of the bands is near the Fermi energy, that is, when the band is incipient. The resemblence of the present results to those obtained by a weak coupling method in a recent study suggests that, even in the large $U$ regime, the suppression of the near-zero-energy spin fluctuations and the development of finite energy spin fluctuations are the key factors for the enhancement of superconductivity by an incipient band.

Highlights

A purely electronic mechanism of superconductivity is expected to exhibit extremely high Tc due to the large energy scale of the pairing glue originating from quantum fluctuations, such as spin, charge, and orbital fluctuations
We study Hubbard models on the two-leg ladder and bilayer square lattices, using a manyvariable variational Monte Carlo method [48,49], which can be considered reliable in the strong-coupling regime [50]
The partial dwave superconducting phase exhibits a dome structure around tr/tl ∼ 1.5, which is reminiscent of a previous fluctuation exchange (FLEX) study on the two-leg ladder Hubbard model without t [52]

Summary

Introduction

A purely electronic mechanism of superconductivity is expected to exhibit extremely high Tc due to the large energy scale of the pairing glue originating from quantum fluctuations, such as spin, charge, and orbital fluctuations. In the early days of the study of iron-based superconductors, it was considered that the Fermi surface nesting between electron and hole Fermi surfaces, combined with Hubbard U , induces spin fluctuations, which in turn act as a pairing interaction around a certain wave vector Q if the gap sign changes across Q. This kind of superconducting gap is referred to as the s± pairing [1,2,3,4,5,6]. Removing the hole pocket is expected to destroy the spin-fluctuation-mediated pairing interaction and suppress Tc rapidly

Results

Discussion

Conclusion