Emergent low-energy thermodynamics and universal $B/T$ scaling in $α$-RuCl$_3$

Abstract

Quantum spin liquids are exotic states of matter in which strongly interacting spins remain disordered down to zero temperature and host fractionalized excitations. This has motivated an extensive search for their realization in materials. Among candidate systems, $α$-RuCl$_3$ has drawn particular attention as a platform for the quantum spin liquid proposed by Kitaev, in which the low-energy excitations in applied magnetic fields are interpreted as itinerant Majorana fermions and local flux excitations. However, conflicting experimental signatures have cast doubt on whether a Kitaev quantum spin liquid-like state is really the right starting point to interpret the low-energy states in $α$-RuCl$_3$, motivating a closer look at the nature of these excitations. Here we report an extensive high field study of specific heat up to 35 T on $α$-RuCl$_3$. We find that the low-energy thermodynamics exhibit universal $B/T$ scaling with a single effective energy scale proportional to the applied field as long as the magnetic field is not close to the $c$-axis. This behavior persists continuously from the antiferromagnetic phase boundary to the highest fields, suggesting that the perturbative Kitaev limit, with its Majorana and flux excitations, may not be an appropriate starting point for describing the low-energy thermodynamics of $α$-RuCl$_3$ for fields away from the $c$-axis.