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

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.

Emergent glassiness in disorder-free Kitaev model

“…Another experiment has indicated multiple phase transitions in the same field region based on the anomalies in thermal (both longitudinal and Hall) conductivity [63] Evidence of magnetic excitations [64] and phonon anomlies [65] are also presented in experiments in the same field region (before polarized phase appears). More interestingly, this is roughly the same magnetic field region where a recent experiment finds a signature of glassiness [20].…”

“…Spin glasses, both classical and quantum, are often found in systems with spatial disorder. In an exciting recent experiment, glassiness is found in a nearly disorderfree α-RuCl 3 at low temperatures and intermediate magnetic fields [20], as seen in anomalous non-linear susceptibilities. Is α-RuCl 3 , a Kitaev proximity system, showing us a way to intrinsic glassiness in disorder-free and generic quantum spin liquid systems?…”

Nonlinear response of the Kitaev honeycomb lattice model in a weak magnetic field

“…Very recently the 2DCS of one-dimensional Ising model has been investigated by implementing matrix-product state numerical simulations 60,61 . Nonlinear responses of KM in the context of high-harmonic generation (HHG) has been studied theoretically 62 and anomalous behavior of the Kitaev spin liquid candidate α-RuCl 3 for static nonlinear susceptibilities has also been reported 63 . It has been shown that the anyonic statistic of quasiparticles can be revealed by nonlinear pumpprobe spectroscopy 64 .…”