The NMR of High Temperature Superconductors without Anti-Ferromagnetic Spin Fluctuations

TL;DR

This paper presents a microscopic theory explaining NMR anomalies in high-temperature superconductors without invoking antiferromagnetic spin fluctuations, based on ab-initio band structure calculations and interband pairing.

Contribution

It introduces a new band structure model derived from ab-initio calculations that accounts for NMR observations without antiferromagnetic fluctuations, advancing understanding of cuprate superconductors.

Findings

Quantitative explanation of NMR anomalies without antiferromagnetic fluctuations

Identification of two bands at the Fermi level involving Cu and O orbitals

Proposal of an interband pairing mechanism for superconductivity

Abstract

A microscopic theory for the NMR anomalies of the planar Cu and O sites in superconducting La_1.85Sr_0.15CuO_4 is presented that quantitatively explains the observations without the need to invoke anit-ferromagnetic spin fluctuations on the planar Cu sites and its significant discrepancy with the observed incommensurate neutron spin fluctuations. The theory is derived from the recently published ab-initio band structure calculations that correct LDA computations tendency to overestimate the self-coulomb repulsion for the half-filled Cu d_x2-y2 orbital for these ionic systems. The new band structure leads to two bands at the Fermi level with holes in the Cu d_z2 and apical O p_z orbitals in addition to the standard Cu d_x2-y2 and planar O p_sigma orbitals. This band structure is part of a new theory for the cuprates that explains a broad range of experiments and is based upon the formation of Cooper pairs comprised of a k up spin electron from one band and a -k down spin electron from another band (Interband Pairing Model).