Abstract
The van der Waals Kitaev magnet α-RuCl3 has recently garnered considerable attention due to its possible realization of topological spin liquids. Combining Raman spectroscopy with numerical calculations, we report here the thickness dependence of electronic structure and ensuing low-energy excitations for exfoliated α-RuCl3. We observe two pronounced peaks at A1 = 249 meV and A2 = 454 meV, which are assigned to single and double spin-orbit (SO) excitons, respectively. Our numerical calculations support this interpretation by reproducing their spectral energy and shape with the electronic parameters: SO coupling λ = 140 meV, Hund’s coupling JH = 350 meV, and on-site Coulomb interaction U = 2.35 eV. The multiple SO excitons persist down to a single layer, whereas their peaks shift slightly to lower energy. For frequencies below 350 cm−1, both a magnetic continuum and phonons show noticeable thickness dependence. These results demonstrate that a SO entangled jeff = 1/2 picture remains valid in a monolayer limit despite the presence of lattice distortions.
Highlights
Two-dimensional van der Waals (2D vdW) materials have offered an excellent platform to explore electronic, optical, and magnetic phenomena emergent in a dimensional crossover from bulk to the 2D limit as well as to enable interface engineering by forming heterostructures[1,2]
The 2D Kitaev candidate α-RuCl3— a graphene-like quantum magnet is a valuable addition to the family of 2D vdW magnetic materials, in which the edge-sharing RuCl6 octahedra constitute RuCl3 layers connected by weak interlayer vdW interactions[7,8,9,10]
The Kitaev honeycomb lattice is an analytically solvable spin model known to harbor quantum spin liquids and Majorana quasiparticles as a consequence of spin fractionalization[11]
Summary
Two-dimensional van der Waals (2D vdW) materials have offered an excellent platform to explore electronic, optical, and magnetic phenomena emergent in a dimensional crossover from bulk to the 2D limit as well as to enable interface engineering by forming heterostructures[1,2]. Given that a SO-entangled state in αRuCl3 is delicately conditioned by the interplay of spin-orbit coupling (SOC), Coulomb interaction, and trigonal crystal field, a layer-dependent electronic structure of atomically thin α-RuCl3 should be elucidated for better understanding the vdW Kitaev magnet in its 2D limit. We admit that prior Raman studies have addressed a thickness dependence of phonons and magnetic excitations at low energies 28–33.
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