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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 .

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We investigated the metal-insulator transition (MIT) driven by all-in-all-out (AIAO) antiferromagnetic ordering in the 5d pyrochlore Cd 2 Os 2 O 7 using optical spectroscopy and first-principles calculations. We showed that the temperature evolution in the band-gap edge and free carrier density were consistent with rigid upward (downward) shifts of electron (hole) bands, similar to the case of Lifshitz transitions. The delicate relationship between the band gap and free carrier density provides experimental evidence for the presence of an AIAO metallic phase, a natural consequence of such MITs. The associated spectral weight change at high energy and first-principles calculations further support the origin of the MIT from the band shift near the Fermi level. Recently, MITs and antiferromagnetic ordering in 5d pyrochlores, A 2 B 2 O 7 (B ¼ Ir, Os), have received much attention due to their strong correlation and the key role they play in novel phases. The MITs in these compounds [3][4][5][6][7][8] reportedly are prompted by the emergence of magnetic ordering. Such magnetism-driven MITs are expected to induce exotic phenomena, including a Weyl semimetal [9][10][11][12] and quantum criticality [13] in rare-earth pyrochlore iridates. However, questions remain as to how magnetic ordering induces MITs in 5d pyrochlores.The peculiar antiferromagnetic structure in 5d pyrochlores makes the MITs distinct from the Slater type [14], a wellknown magnetic-driven MIT. In the original Slater picture, collinear antiferromagnetic ordering induces a doubling of the lattice periodicity, resulting in a BCS-like band gap at the folded zone boundaries. However, recent studies support that the magnetic ground states in 5d pyrochlores are noncollinear all-in-all-out (AIAO) magnetic orderings [9][10][11][15][16][17], where all spins in transition metal ions align inward or outward of tetrahedra. Since AIAO ordering maintains the unit cell of pyrochlores, a new paradigm beyond the Slater picture is essential to understanding MITs in 5d pyrochlores.Cd 2 Os 2 O 7 is a good candidate for the study of such MITs, in that it shows a continuous MIT at an antiferromagnetic ordering temperature T N (¼ 227 K) [18,19]. Until its magnetic ground state was revealed, this material was best known as one of a few examples of a Slater insulator [20]. However, recent experimental [19,21] and theoretical [22,23] results showed that the magnetic ground state in Cd 2 Os 2 O 7 is AIAO ordering. Investigations on Cd 2 Os 2 O 7 should provide an understanding of how AIAO ordering causes MITs in 5d pyrochlores.In this Letter, we report on the detailed temperature (T) evolution of the optical spectra of Cd 2 Os 2 O 7 to reveal how the emergence of AIAO ordering affects the electronic structure and induces the MITs. The absorption edge analyses suggest the Lifshitz-type MIT where a rigid shift of the bands away from the Fermi level E F gradually annihilates the Fermi surface [22,24]. The model based on the Lifshitz-type MIT reproduces the delicate relationship betw...

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Spin-phonon coupling mediated by single ion anisotropy was investigated using optical spectroscopy and first-principles calculations in the all-in-all-out pyrochlore magnet Cd_{2}Os_{2}O_{7}. Clear anomalies were observed in both the phonon frequencies and linewidths at the magnetic ordering temperature. The renormalization of the phonon modes was exceptionally large, signifying the presence of an unconventional magnetoelastic term from large spin-orbit coupling. In addition, the relative phonon frequency shifts show a strong correlation with the modulation of noncubic crystal field by the corresponding lattice distortion. Our observation establishes a new type of spin-phonon coupling through single ion anisotropy, a second-order spin-orbit coupling term, in Cd_{2}Os_{2}O_{7}.