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Raman scattering has been employed to investigate lattice and magnetic excitations of the honeycomb Kitaev material α-RuCl3 and its Heisenberg counterpart CrCl3. Our phonon Raman spectra give evidence for a first-order structural transition from a monoclinic to a rhombohedral structure for both compounds. Significantly, only α-RuCl3 features a large thermal hysteresis, consistent with the formation of a wide phase of coexistence. In the related temperature interval of 70 − 170 K, we observe a hysteretic behavior of magnetic excitations as well. The stronger magnetic response in the rhombohedral compared to the monoclinic phase evidences a coupling between the crystallographic structure and low-energy magnetic response. Our results demonstrate that the Kitaev magnetism concomitant with fractionalized excitations is susceptible to small variations of bonding geometry.

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We examined the temperature (T ) evolution of the optical conductivity spectra of Sr 3 Ir 2 O 7 over a wide range of 10-400 K. The system was barely insulating, exhibiting a small indirect bandgap of 0.1 eV. The low-energy features of the optical d-d excitation ( ω < 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge x-ray-absorption spectra. This suggests that the T evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involved, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr 3 Ir 2 O 7 .