A Displacive Structural Transformation in the CuO_2 Planes of YBa_2Cu_3O_x at the Underdoped-Overdoped Phase Separation Line

TL;DR

This paper reports a structural phase transformation in the CuO_2 planes of YBa_2Cu_3O_x at the underdoped-overdoped boundary, linking structural changes to superconducting properties and phase separation.

Contribution

It identifies a displacive structural transformation at the phase boundary, providing insight into the structural basis of superconductivity in YBa_2Cu_3O_x.

Findings

Large static displacements of Cu2 atoms in overdoped regime

Presence of a gap in O2,3 Raman shifts

Structural change correlates with superconducting transition split

Abstract

A structural phase transformation in the CuO_2 planes of YBa_2Cu_3O_x has been observed at the onset of the overdoped regime, x=6.95. We have measured as a function of x the dimpling in the CuO_2 planes by EXAFS, and the O2,3 in-phase (A_1g) mode by Raman scattering. The data show for x>6.95 anomalously large static displacements of the Cu2 atoms off the O2,3 layer, and a gap in the distribution of the O2,3 in-phase Raman shifts. We conclude the structure of overdoped YBa_2Cu_3O_x to be a martensitic form of the structure most favourable for the superconductivity, and possibly at the origin of the split superconducting transition in the overdoped regime.