On the theory of the spin gap in bilayer cuprates

TL;DR

This paper presents a theoretical model showing how weak interplane antiferromagnetic interactions, enhanced by in-plane correlations, can produce a high-temperature spin gap in bilayer cuprates, explaining experimental observations.

Contribution

The paper introduces a model demonstrating that in-plane correlations significantly boost interplane pairing, leading to a higher spin gap temperature than traditional BCS theory predicts.

Findings

Interplane interaction induces spin pairing at temperatures ~λ.

In-plane correlations enhance pairing, raising T* above BCS estimates.

Model explains observed spin gap in YBa2Cu3O6.6 below 150K.

Abstract

We formulate and solve a model of two planes of antiferromagnetically correlated electrons coupled together by a weak antiferromagnetic interaction of strength $\lambda$. We show that in-plane antiferromagnetic correlations dramatically enhance the pairing effect of the interplane interaction. For the case where the in-plane correlation length $\kappa^{-1} \sim T^{-1/2}$, we find that the interaction $\lambda$ leads to spin pairing at a temperature $T^{*} \sim \lambda$, much higher than the usual BCS result $exp(-J/\lambda)$. We suggest that this is a possible explanation of the spin gap effects observed below $T^{*} \sim 150K$ in $YBa_2Cu_3O_{6.6}$.