A snapshot review on quantifying quantum spin liquids

Quantum spin liquids (QSLs) have emerged as one of the most actively investigated states of correlated quantum matter, yet classification remains ambiguous due to the absence of a universally accepted parameter definition. The frustration index ( f=|θCW |/Tc ) is proposed here as the dominant classification criterion. It directly measures how far a spin configuration deviates from the equilibrium order its exchange interactions would otherwise favor, the defining signature of frustration, and is obtainable from a routine susceptibility measurement. Thermodynamic and spectroscopic probes offer confirmatory evidence, but their reliance on sub-Kelvin cryogenics and reactor- or beamline-scale facilities limits their use as universal standards. Standardizing f on a single reference temperature resolves much of the inconsistency in the literature, where reported values vary by orders of magnitude. This framework is applied across candidate families, establishing frustration as the primary criterion and dynamic probes as a confirmatory characterization.