On the proximate Kitaev quantum-spin liquid α-RuCl₃: thermodynamics, excitations and continua

Abstract

This topical review provides an overview over recent thermodynamic, infrared, and THz results on the proximate Kitaev spin-liquid. Quantum-spin liquids are exotic phases characterized by the absence of magnetic ordering even at the lowest temperatures and by the occurrence of fractionalized spin excitations. Among those, Kitaev spin liquids are most fascinating as they belong to the rare class of model systems, that can be solved analytically by decomposing localized spins S = 1/2 into Majorana fermions. The main aim of this review is to summarize experimental evidence obtained by THz spectroscopy and utilizing heat-capacity experiments, which point to the existence of fractionalized excitations in the spin-liquid state, which in α-RuCl3 exists at temperatures just above the onset of magnetic order or at in-plane magnetic fields just beyond the quantum-critical point where antiferromagnetic order becomes suppressed. Thermodynamic and spectroscopic results are compared to theoretical predictions and model calculations. In addition, we document recent progress in elucidating the sub-gap (<1 eV) electronic structure of the 4d 5 ruthenium electrons to characterize their local electronic configuration. The on-site excitation spectra of the d electrons below the optical gap can be consistently explained using a spin–orbit coupling constant of ∼170 meV and the concept of multiple spin–orbital excitations. Furthermore, we discuss the phonon spectra of the title compound including rigid-plane shear and compression modes of the single molecular layers. In recent theoretical concepts it has been shown that phonons can couple to Majorana fermions and may play a substantial role in establishing the half-integer thermal quantum Hall effect observed in this material.

Polar magnetism and chemical bond in α-RuCl3

“…Ruthenium trichloride is one of the most widely used reagents [1], while the micaceous black allotrope α-RuCl 3 is much discussed in the context of a Kitaev state [2][3][4][5][6]. Signatures of the state have been reported on the basis of neutron scattering [7][8][9], nuclear magnetic resonance (NMR) [10], terahertz spectroscopy [11], microwave absorption [12], specific heat measurements [13], NMR spectroscopy [14], and Raman spectroscopy [15].…”

Nonequilibrium dynamics of α-RuCl3 – a time-resolved magneto-optical spectroscopy study

“…8 The 4d 5 electronic configuration of Ru is in a low-spin state with S = 1/2, giving rise to J eff = 1/2 pseudospins due to the presence of the octahedral crystal-field splitting and large spin–orbit coupling. 9,10 The on-site interaction U opens a gap inside the J eff = 1/2 band, creating the upper Hubbard band (UHB) and lower Hubbard band (LHB), respectively. These properties are essential prerequisites to realize the Kitaev model in condensed matter systems.…”

Pressure-induced dimerization and collapse of antiferromagnetism in the Kitaev material α−Li2IrO3

“…In solid-state materials, this model can be realized in spin-orbit-coupled oxides and halides with the edge-sharing geometry of transition-metal octahedra [2,3]. The notable examples are Na 2 IrO 3 [4], different polymorphs of Li 2 IrO 3 [5], and α-RuCl 3 [6]. However, due to other interactions (such as Heisenberg J and off-diagonal interactions) beyond the Kitaev term K [7], the aforementioned Kitaev candidates all display long-range magnetic order at low temperatures.…”