# Field-induced quantum criticality in the Kitaev system $\alpha$-RuCl$_3$

## TL;DR

This study investigates the quantum critical behavior of $ ext{RuCl}_3$ under magnetic fields, revealing a field-induced quantum critical point and a spin-excitation gap, advancing understanding of Kitaev quantum spin liquids.

## Contribution

It provides experimental evidence of a field-induced quantum critical point in $ ext{RuCl}_3$ and relates findings to a specific theoretical honeycomb model.

## Key findings

- Suppression of antiferromagnetic order with increasing magnetic field.
- Observation of a spin-excitation gap above the critical field.
- Universal scaling behavior indicating quantum criticality.

## Abstract

$\alpha$-RuCl$_3$ has attracted enormous attention since it has been proposed as a prime candidate to study fractionalized magnetic excitations akin to Kitaev's honeycomb-lattice spin liquid. We have performed a detailed specific-heat investigation at temperatures down to $0.4$ K in applied magnetic fields up to $9$ T for fields parallel to the $ab$ plane. We find a suppression of the zero-field antiferromagnetic order, together with an increase of the low-temperature specific heat, with increasing field up to $\mu_0H_c\approx 6.9$ T. Above $H_c$, the magnetic contribution to the low-temperature specific heat is strongly suppressed, implying the opening of a spin-excitation gap. Our data point toward a field-induced quantum critical point (QCP) at $H_c$; this is supported by universal scaling behavior near $H_c$. Remarkably, the data also reveal the existence of a small characteristic energy scale well below $1$~meV above which the excitation spectrum changes qualitatively. We relate the data to theoretical calculations based on a $J_1$--$K_1$--$\Gamma_1$--$J_3$ honeycomb model.