Order Parameter Symmetry in Doped YBCO Systems

TL;DR

This paper investigates the symmetry of the superconducting order parameter in doped YBCO systems using a two-band model, revealing how phase choices can produce both s- and d-wave features and explaining experimental contradictions.

Contribution

It introduces a two-band model incorporating tunnelling effects that explains the coexistence of s- and d-wave symmetries in YBCO superconductors.

Findings

Phase diagram showing symmetry shifts with doping

Presence of both s- and d-wave features explained

Node positions on Fermi surface vary with doping

Abstract

Intense experimental and theoretical efforts have recently been brought to bear on the determination of the symmetry of superconducting order parameter in high T$_c$ Cuprates as this would shed light on the nature of microscopic interactions that are responsible for superconductivity in these unusual systems. We use a two band model that reproduces the Fermi surface seen in ARPES experiments of Shen et al., and study the symmetry of order parameter in the YBCO system. The model incorporates the effects of tunnelling in the c-direction between the planes and chains of these Cuprates. It is observed that a suitable choice of the phase of the pair wave-function in the planes and chains lead to both s- and d-wave like features. We calculate the detailed phase diagram of this system, observe the shift in the position of the node(s) of the order parameter(s) on the Fermi surface with doping and explain the seemingly contradictory experimental observations about the presence of both s- and/or d-wave symmetry in these systems.