Triplanar Model for the Gap and Penetration Depth in YBCO

TL;DR

This paper develops a triplanar model for YBCO that incorporates intra- and interplane pairing, explaining the anisotropic gap symmetry and penetration depth behavior consistent with experimental observations.

Contribution

It introduces a coupled three-layer BCS framework capturing orthorhombic symmetry and mixed s- and d-wave pairing in YBCO.

Findings

Orthorhombic symmetry with mixed s- and d-wave gaps

Temperature dependence of penetration depth matches experiments

In-plane anisotropy explained by the model

Abstract

YBaCuO_7 is a trilayer material with a unit cell consisting of a CuO_2 bilayer with a CuO plane of chains in between. Starting with a model of isolated planes coupled through a transverse matrix element, we consider the possibility of intra as well as interplane pairing within a nearly antiferromagnetic Fermi liquid model. Solutions of a set of three coupled BCS equations for the gap exhibit orthorhombic symmetry with s- as well as d-wave contributions. The temperature dependence and a-b in plane anisotropy of the resulting penetration depth is discussed and compared with experiment.