Angular position of nodes in the superconducting gap of YBCO

TL;DR

This study investigates the angular dependence of thermal conductivity in YBCO, revealing a four-fold symmetry indicative of d-wave superconductivity with nodes at specific angles, and constraining the s-wave component.

Contribution

The paper provides experimental evidence for the angular position of nodes in YBCO's superconducting gap and limits the s-wave component in its order parameter.

Findings

Four-fold symmetry in thermal conductivity indicating d-wave gap with nodes at 45° intervals

Exclusion of macroscopic artifacts as the cause of observed symmetry

Upper limit of 10% on s-wave component in the superconducting order parameter

Abstract

The thermal conductivity of a YBCO single crystal has been studied as a function of the relative orientation of the crystal axes and a magnetic field rotating in the Cu-O planes. Measurements were carried out at several temperatures below T_c and at a fixed field of 30 kOe. A four-fold symmetry characteristic of a superconducting gap with nodes at odd multiples of 45 degrees in k-space was resolved. Experiments were performed to exclude a possible macroscopic origin for such a four-fold symmetry such as sample shape or anisotropic pinning. Our results impose an upper limit of 10% on the weight of the s-wave component of the essentially d-wave superconducting order parameter of YBCO.