The Spin Gap in the Context of the Boson-Fermion Model for High $T_c$ Superconductivity

TL;DR

This paper explores the origin of the spin gap in high-temperature superconductors by modeling a mixture of localized electron pairs and itinerant electrons, revealing how magnetic correlations and a pseudogap emerge with decreasing temperature.

Contribution

It introduces a boson-fermion model explaining the spin gap phenomenon through local exchange interactions and the development of antiferromagnetic correlations in high T_c superconductors.

Findings

Spin gap observed in neutron scattering and NMR correlates with pseudogap formation.

Antiferromagnetic correlations develop near localized pairs as temperature decreases.

A spin wave-like spectrum emerges in the itinerant electron subsystem.

Abstract

The issue of the spin gap in the magnetic susceptibility $\chi''(q,\omega)$ in high T_c superconductors is discussed within a scenario of a mixture of localized tightly bound electron pairs in singlet states (bi-polarons) and itinerant electrons. Due to a local exchange between the two species of charge carriers, antiferromagnetic correlations are induced amongst the itinerant electrons in the vicinity of the sites containing the bound electron pairs. As the temperature is lowered these exchange processes become spatially correlated leading to a spin wave-like spectrum in the subsystem of the itinerant electrons. The onset of such coherence is accompanied by the opening of a pseudo gap in the density of states of the electron subsystem whose temperature dependence is reflected in that of $\chi'' (q,\omega)$ near $q =(\pi,\pi)$ where a ``spin gap'' is observed by inelastic neutron scattering and NMR.