We investigate the magnetic excitations of the hyperhoneycomb Kitaev magnet β-Li2IrO3 by means of inelastic Raman scattering. The spectra exhibit the coexistence of a broad scattering continuum and two sharp low-energy peaks at 2.5 and 3 meV, with a distinctive polarization dependence. While the continuum is suggestive of fractional quasiparticles emerging from a proximate quantum spin liquid phase, the sharp peaks provide the first experimental signature of the "non-Loudon- Fleury"one-magnon scattering processes proposed recently [Yang et al., Phys. Rev. B 104, 144412 (2021)2469-995010.1103/PhysRevB.104.144412]. The corresponding microscopic mechanism is similar to the one leading to the symmetric off-diagonal exchange interaction Γ (because it involves a combination of both direct and ligand-mediated exchange paths) but is otherwise completely unexpected within the traditional Loudon-Fleury theory of Raman scattering. The present experimental verification therefore calls for a drastic reevaluation of Raman scattering in similar systems with strong spin-orbit coupling and multiple exchange paths.
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Acknowledgments. We thank M. Li for earlier collaborations on related topics. Y.Y. and N.B.P. acknowledge the support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences. under Award No. DE-SC0018056. The work done by Y.W. was supported by the Office of Naval Research under Award No. N00014-20-1-2308. K.S.B. is grateful for the support of the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0018675. The work of I.R. was supported by the Engineering and Physical Sciences Research Council [Grant No. EP/V038281/1]. Work by J.G.A. and A.R. was supported by the Department of Energy Early Career Program, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under Contract No. DE-AC02-05CH11231 for crystal growth. A.R. also acknowledges support from the University of California President Postdoctoral Fellowship Program.
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