Classical and quantum spin liquids

TL;DR

This paper reviews classical and quantum spin liquids, exotic disordered magnetic phases that remain disordered at zero temperature, focusing on their classification and experimental signatures.

Contribution

It provides a comprehensive overview of the classification and distinguishing features of classical and quantum spin liquids, emphasizing their unique correlation and excitation properties.

Findings

Classical and quantum spin liquids lack local order parameters.

They possess distinct correlation and excitation features.

Experimental probes can differentiate between these phases.

Abstract

When considering magnetic systems in the thermodynamic limit and at low enough temperature, one finds typically magnetically ordered phases. In contrast, in the high-temperature regime, the interactions between the spin degrees of freedom become less relevant and the system loses its order: this is a paramagnet. This phenomenon of phase transition has been well understood using statistical mechanics and simple modelling. In these short lecture notes, we will review the possibility that a many-body magnetic system may remain magnetically disordered down to zero-temperature, both for classical or quantum spins. These exotic phases of matter are known, respectively, as classical and quantum spin liquids. We will address in particular the question of classification of these classical or quantum disordered phases. Indeed, while they have no local order parameter by definition, they can still possess different qualitative features related e.g. to the nature of their correlations or elementary excitations, which could be probed experimentally.