transition metal oxides*

The spin-phonon (SP) coupling has long been a central theme in condensed matter physics. To date, most of these phenomena have been understood by considering the Heisenberg interaction between spins. However, recent studies on 4d/5d transition metal oxides (TMOs) have revealed that strong spin-orbit coupling (SOC) plays the crucial role in their magnetism. Since the orbitals are generically coupled to the lattice, the strong SOC present in 4d/5d orbitals creates strong and inevitable interactions between spin and lattice.
Using optical spectroscopy, we carried out comparative studies on the 5d TMOs. From infrared (IR) phonon spectra, we discovered unconventional SP coupling in Cd₂Os₂O₇ and Y₂Ir₂O₇. For Cd₂Os₂O₇, we observed large phonon frequency shifts below T_N. This is due to the SP coupling arising from single-ion anisotropy [1,2]. We also identified frequency shifts of Y₂Ir₂O₇ near T_N. However, in this case, the Dzyaloshinskii-Moriya interaction plays a crucial role in the phonon renormalization [3]. Using near-IR pump-probe technique, we investigated coherent phonon oscillations of 4d TMO. Recently, Ca₂RuO₄ has been of particular interest due to its exotic magnetic ground state driven by strong SOC. We observed that a coherent A_g phonon oscillation changes abruptly its amplitude and phase as the spin order develops. Density functional theory calculations revealed that the structural deformation by the magnetic order can induce such variations in coherent phonons, which we suggest as a unique signature of SP coupling [4].
Our comprehensive studies on unconventional SP coupling may lead to scientific advancement in 4d/5d TMOs, and possibly endow opening new research fields come across.

[1] C. H. Sohn et al., Phys. Rev. Lett. 118, 117201 (2017).
[2] T. M. H. Nguyen et al., Nat. Commun. 8, 251 (2017).
[3] J. Son et al., in preparation.
[4] M.-C. Lee et al., Phys. Rev. B 98, 161115 (2018).