Anomalous and anisotropic nonlinear susceptibility in the proximate Kitaev magnet \u03b1-RuCl3

Ludwig Holleis,Jeroen Van Den Brink,B S Shivaram,Zhijie Fan,Gia-Wei Chern,Satoshi Nishimoto,Joseph C Prestigiacomo,Michael Osofsky

doi:10.1038/s41535-021-00364-z

Abstract

The leading order nonlinear (NL) susceptibility, χ3, in a paramagnet is negative and diverges as T → 0. This divergence is destroyed when spins correlate and the NL response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the NL magnetic response. Here, we study the NL susceptibilities in the proximate Kitaev magnet α-RuCl3, which differs from the expected antiferromagnetic behavior. For T < Tc = 7.5 K and field B in the ab-plane, we obtain contrasting NL responses in low (<2 T) and high field regions. For low fields, the NL behavior is dominated by a quadratic response (positive χ2), which shows a rapid rise below Tc. This large χ2 > 0 implies a broken sublattice symmetry of magnetic order at low temperatures. Classical Monte Carlo (CMC) simulations in the standard K − H − Γ model secure such a quadratic B dependence of M, only for T ≈ Tc with χ2 being zero as T → 0. It is also zero for all temperatures in exact diagonalization calculations. On the other hand, we find an exclusive cubic term (χ3) that describes the high field NL behavior well. χ3 is large and positive both below and above Tc crossing zero only for T > 50 K. In contrast, for B ∥ c-axis, no separate low/high field behaviors are measured and only a much smaller χ3 is apparent.

Highlights

Since the demonstration by Kitaev[1] of the existence of a quantum spin liquid state in two dimensions through exact calculations on a honeycomb lattice, there has been an intense experimental search for its realization in real material systems
Despite the presence of a very large exchange energy, these systems do not order down to comparatively low temperatures. When they do order magnetically, the features observed in both microscopic probes such as neutron scattering and macroscopic measurements such as magnetometry and thermal response are highly unusual and contrast dramatically with what is expected from conventional magnets
In the proximate Kitaev spin liquid candidate, α-RuCl3 neutron scattering experiments show a low-temperature magnetic excitation spectrum consisting of sharp spin wave peaks and a continuum associated with fractional excitations[6,8]

Summary

Introduction

Since the demonstration by Kitaev[1] of the existence of a quantum spin liquid state in two dimensions through exact calculations on a honeycomb lattice, there has been an intense experimental search for its realization in real material systems. We find a quadratic field dependence of M, giving rise to a significantly positive χ2, which exists only in the ordered state and has a large nonzero value even as T → 0.

Results

Conclusion