Observing spin fractionalization in the Kitaev spin liquid via temperature evolution of indirect resonant inelastic X-ray scattering

Motivated by the ongoing effort to search for high-resolution signatures of quantum spin liquids, we investigate the temperature dependence of the indirect resonant inelastic X-ray scattering (RIXS) response for the Kitaev honeycomb model. We find that, as a result of spin fractionalization, the RIXS response changes qualitatively at two well-separated temperature scales, TL and TH, which correspond to the characteristic energies of the two kinds of fractionalized excitations, Z2 gauge fluxes and Majorana fermions, respectively. While thermally excited Z2 gauge fluxes at temperature TL lead to a general broadening and softening of the response, the thermal proliferation of Majorana fermions at temperature TH∼10TL results in a significant shift of the spectral weight, both in terms of energy and momentum. Due to its exclusively indirect nature, the RIXS process we consider gives rise to a universal magnetic response and, from an experimental perspective, it directly corresponds to the K-edge of Ru3+ in the Kitaev candidate material α-RuCl3.