Spin excitations and mechanisms of superconductivity in cuprates

TL;DR

This paper develops a microscopic theory of spin excitations in cuprates, revealing their evolution with doping and their crucial role in high-temperature superconductivity, emphasizing spin interactions over phonons or weak Coulomb effects.

Contribution

It derives an exact dynamic spin susceptibility within the t-J model and explains the emergence of the resonance mode without relying on superconducting gap opening.

Findings

Spin excitation spectrum transitions from spin-wave to overdamped paramagnons with doping.

Resonance mode at Q emerges due to spin gap, not superconducting gap.

Spin excitations are central to d-wave pairing in cuprates.

Abstract

A microscopic theory of spin excitations in strongly-correlated electronic systems within the t-J model is discussed. An exact representation for the dynamic spin susceptibility is derived. In the normal state, the excitation spectrum reveals a crossover from spin-wave-like excitations at low doping to overdamped paramagnons above the optimal doping. At low temperatures, the resonance mode at the antiferromagnetic wave vector Q = \pi(1,1) emerges which is explained by a strong suppression of the spin excitation damping caused by a spin gap at Q rather than by opening of a superconducting gap. A major role of spin excitations in the d-wave superconducting pairing in cuprates is stressed in discussing mechanisms of high-Tc superconductivity within the Hubbard model in the limit of strong correlations, while electron-phonon interaction and a well-screened weak Coulomb interaction are not essential.