The range of non-Kitaev terms and fractional particles in α-RuCl3

Abstract

Significant efforts have focused on the magnetic excitations of relativistic Mott insulators, predicted to realize the Kitaev quantum spin liquid (QSL). This exactly solvable model involves a highly entangled state resulting from bond-dependent Ising interactions that pro-1 arXiv:2003.09274v1 [cond-mat.str-el] 18 Mar 2020 duce excitations which are non-local in terms of spin flips. A key challenge in real materials is identifying the relative size of the non-Kitaev terms and their role in the emergence or suppression of fractional excitations. Here, we identify the energy and temperature boundaries of non-Kitaev interactions by direct comparison of the Raman susceptibility of α-RuCl 3 with quantum Monte Carlo (QMC) results for the Kitaev QSLs. Moreover, we further confirm the fractional nature of the magnetic excitations, which is given by creating a pair of fermionic quasiparticles. Interestingly, this fermionic response remains valid in the non-Kitaev range. Our results and focus on the use of the Raman susceptibility provide a stringent new test for future theoretical and experimental studies of QSLs. Exotic excitations with fractional quantum numbers are a key characteristic of QSLs 1-4 , which result from the long range entanglement of these non-trivial topological phases 5-7 . Originating from frustrated magnetic interactions, the fractional nature inspires an overarching goal of studying QSLs, realizing topological quantum computing immune to decoherence, with high operating temperatures from large exchange interactions 8, 9 . The last decade has seen great progress towards identifying the fractional excitations of QSLs 10-18 . Attention has focused on relativistic Mott insulators that are close to the exactly solvable Kitaev model with a QSL ground state. In materials such as A 2 IrO 3 (A = Cu, Li or Na) 4, 8, 19-23 and α-RuCl 3 24-27 , the large spin-orbit coupling and Coulomb repulsion result in j ef f = 1/2 moments on a honeycomb lattice 2, 9, 28-35 .According to the pure Kiteav model, in these materials spin flips could produce Z 2 gauge fluxes and dispersive Majorana fermions. 14, 36 .

High-temperature magnetic anomaly in the Kitaev hyperhoneycomb compound β−Li2IrO3

“…However, many recent studies have found high temperature signatures of these exotic states or proximity thereto. For example, recent spectroscopic and thermodynamic studies of α-RuCl 3 [11][12][13][14][15][16][17][18] , have reported evidence for the onset of nearest-neighbor Kitaev correlations, consistent with a proposal by Motome and co-authors of a thermal crossover from a paramagnet to a spin-"fractionalized" state [19][20][21] . Similar studies have extended these conclusions to α, β, γ−Li 2 IrO 3 and α−Na 2 IrO 3 systems 22-24 . In the case of the β− and γ-Li 2 IrO 3 systems this temperature range is also associated with strong deviations from Curie-Weiss susceptibility, and a dramatic re-ordering of the principal axes of magnetization.…”

“…On the other hand, evidence for such fractionalization in this temperature range has been reported in α-RuCl 3 and α-Li 2 IrO 3 . [11][12][13][14][15][16]18,62,63 The fact that this appears as a crossover in the α-type structures and a phase transition in β, γ-type structures may simply reflect the different dimensionality of the materials.…”

Effect of structural disorder on the Kitaev magnet Ag3LiIr2O6

“…One of the most promising theoretical models of QSLs is the Kitaev model based on interacting spin-1/2 ions on a two-dimensional honeycomb lattice with bond-dependent Ising axes [4]. The prime candidates for the Kitaev model are α-Li 2 IrO 3 , Na 2 IrO 3 , and α-RuCl 3 , but all three compounds order magnetically at low temperatures [5][6][7][8][9][10]. Recently, a new class of intercalated Kitaev magnets was synthesized via a topochemical exchange of the interlayer Li/Na atoms in α- [11][12][13][14][15].…”

Non-Loudon-Fleury Raman scattering in spin-orbit coupled Mott insulators

“…In recent years, there has been a series of Raman studies (both experimental and theoretical) on a range of spin-orbit coupled (SOC) Mott insulators, with a view to elucidate the nature of their magnetic excitations (and lattice dynamics) and their proximity to the so-called Kitaev quantum spin liquid ground states . Most saliently, the reported Raman scattering data in the Kitaev candidate materials α-RuCl 3 [13,14,[17][18][19] and the three-dimensional (3D) iridates β-Li 2 IrO 3 and γ -Li 2 IrO 3 [16], revealed signatures of both multiparticle continua, characteristics of the proximate spin liquid phase, and sharp peaks, characteristic of magnon excitations of the low-temperature ordered phases. These results call for a close reexamination of the Raman scattering theory applied to strong spin-orbit coupled Mott insulators.…”