Magnetic correlations on the full chains of Ortho-II YBa$_2$Cu$_3$O$_{6.5}$

TL;DR

This paper models how magnetic correlations and disorder in Ortho-II YBa₂Cu₃O₆.₅ influence NMR line shapes, revealing magnetization effects near oxygen vacancies and explaining temperature-dependent line broadening.

Contribution

It introduces a coupled chain-plane Hubbard model with Hartree-Fock calculations to explain NMR line broadening and bimodal shapes in high-Tc superconductor YBCO due to magnetic correlations and disorder.

Findings

Disorder-induced magnetization explains low-temperature NMR broadening.

Nonuniform Cu valence accounts for high-temperature bimodal line shape.

Proximity effects cause anisotropic magnetization on planar Cu.

Abstract

We propose that the NMR line shape on the chain Cu in the stoichiometric high-$T_c$ superconductor Ortho-II YBa$_2$Cu$_3$O$_{6.5}$ is determined by the magnetization induced on Cu near O vacancies, due to strong magnetic correlations in the chains. An unrestricted Hartree-Fock calculation of a coupled chain-plane Hubbard model with nearest-neighbor d-wave pairing interaction shows that the broadening of NMR lines is consistent with disorder-induced magnetization at low temperatures. In addition, we give a possible explanation of the anomalous bimodal line shape observed at high temperatures in terms of nonuniform Cu valence in the chains. The proximity between chains and CuO plane induces anisotropic magnetization on the planar Cu, and broadens the plane NMR lines in accordance with that of the chain lines, in agreement with experiment. We discuss implications of the model for other experiments on underdoped YBCO.