Physical Review X (Mar 2018)

Spin-Orbital Excitations in Ca_{2}RuO_{4} Revealed by Resonant Inelastic X-Ray Scattering

  • L. Das,
  • F. Forte,
  • R. Fittipaldi,
  • C. G. Fatuzzo,
  • V. Granata,
  • O. Ivashko,
  • M. Horio,
  • F. Schindler,
  • M. Dantz,
  • Yi Tseng,
  • D. E. McNally,
  • H. M. Rønnow,
  • W. Wan,
  • N. B. Christensen,
  • J. Pelliciari,
  • P. Olalde-Velasco,
  • N. Kikugawa,
  • T. Neupert,
  • A. Vecchione,
  • T. Schmitt,
  • M. Cuoco,
  • J. Chang

DOI
https://doi.org/10.1103/PhysRevX.8.011048
Journal volume & issue
Vol. 8, no. 1
p. 011048

Abstract

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The strongly correlated insulator Ca_{2}RuO_{4} is considered as a paradigmatic realization of both spin-orbital physics and a band-Mott insulating phase, characterized by orbitally selective coexistence of a band and a Mott gap. We present a high resolution oxygen K-edge resonant inelastic x-ray scattering study of the antiferromagnetic Mott insulating state of Ca_{2}RuO_{4}. A set of low-energy (about 80 and 400 meV) and high-energy (about 1.3 and 2.2 eV) excitations are reported, which show strong incident light polarization dependence. Our results strongly support a spin-orbit coupled band-Mott scenario and explore in detail the nature of its exotic excitations. Guided by theoretical modeling, we interpret the low-energy excitations as a result of composite spin-orbital excitations. Their nature unveils the intricate interplay of crystal-field splitting and spin-orbit coupling in the band-Mott scenario. The high-energy excitations correspond to intra-atomic singlet-triplet transitions at an energy scale set by Hund’s coupling. Our findings give a unifying picture of the spin and orbital excitations in the band-Mott insulator Ca_{2}RuO_{4}.