Superconducting Order Parameter in Bi-Layer Cuprates: Occurrence of $\pi $ Phase Shifts in Corner Junctions

TL;DR

This paper investigates the symmetry of the superconducting order parameter in bi-layer cuprates, revealing that $\pi$ phase shifts can arise from repulsive interactions and are not exclusive to magnetic mechanisms, with implications for $T_c$.

Contribution

It classifies allowed symmetries in bi-layer cuprates using models based on measured spin spectra and Coulomb effects, highlighting the generality of $\pi$ phase shifts.

Findings

$\pi$ phase shifts occur due to repulsive interactions, independent of magnetic mechanisms.

$\pi$ shifts can be associated with orthorhombic s-symmetry with opposite phase sub-band gaps.

Discussion on how these symmetries influence the critical temperature $T_c$.

Abstract

We study the order parameter symmetry in bi-layer cuprates such as YBaCuO, where interesting $\pi$ phase shifts have been observed in Josephson junctions. Taking models which represent the measured spin fluctuation spectra of this cuprate, as well as more general models of Coulomb correlation effects, we classify the allowed symmetries and determine their associated physical properties. $\pi $ phase shifts are shown to be a general consequence of repulsive interactions, independent of whether a magnetic mechanism is operative. While it is known to occur in d-states, this behavior can also be associated with (orthorhombic) s-symmetry when the two sub-band gaps have opposite phase. Implications for the magnitude of $T_c$ are discussed.