Teruki Matsuzaki | Searcharxiv

arXiv2026

Coupled-cluster study of dynamic Jahn-Teller effect in a $5d^2$ W antifluorite

In correlated insulators, the interplay among coexisting charge, spin, orbital, and lattice degrees of freedom gives rise to rich quantum phenomena, while unraveling the interplay is not straightforward. In the family of cubic $5d^2$ double perovskites, the ground spin-orbit coupled electronic states of $5d$ metal sites are degenerate and couple to the Jahn-Teller active vibrations, whereas no experimental evidence of the symmetry-lowering in the low-temperature ordered phases has been reported. To quantitatively unravel the nature of $5d^2$ centers, we apply equation-of-motion coupled cluster (EOM-CC) theory to analyze the vibronic and magnetic properties of $5d^2$ W sites of Cs$_2$WCl$_6$. We derive the electronic and vibronic model Hamiltonians, calculate the W $L_3$ edge resonant inelastic x-ray scattering (RIXS) spectra, and determine the effective magnetic moment. The simulated RIXS spectra show that vibronic coupling makes several peaks asymmetric. The effective magnetic moments exhibit a temperature dependence similar to that observed experimentally, confirming the validity of the calculated distribution of low-energy levels. Our calculations indicate that the Jahn-Teller effect in Cs$_2$WCl$_6$ is in a weak regime, and noticeable deformation would not occur, whereas the dynamic Jahn-Teller effect modulates the shapes of the RIXS spectra and affects the magnetic moment. This work demonstrates the usefulness of the EOM-CC method for predicting physical phenomena on metal sites in correlated insulating materials.

cond-mat.str-el↗

arXiv2026

Coupled-cluster approach to vibronic effects in resonant inelastic x-ray scattering of quantum materials: Application to a $5d^1$ rhenium oxide

First-principles analysis of the spectroscopic signatures of correlated quantum materials poses significant challenges due to the interplay between spin-orbit and vibronic couplings, as well as the need to describe both dynamic and static electron correlation to reach decent accuracy. In this work, we apply the equation-of-motion coupled-cluster (EOM-CC) method to derive the spin-orbit-lattice entangled vibronic states and predict the Re $L_3$ edge resonant inelastic x-ray scattering (RIXS) spectra of Ba$_2$MgReO$_6$. The EOM-CC yields interaction parameters in close agreement with those extracted from RIXS spectra, with errors of less than 5\%. In particular, the EOM-CC method allowed us to determine the weak vibronic coupling to the $T_{2g}$ vibrations, which is difficult to address experimentally. The simulated spectra indicate that vibronic coupling to the $T_{2g}$ modes gives rise to a shoulder on the elastic peak. Going beyond the conventional treatment, which focuses solely on $E_g$ modes, we show that vibronic couplings to both $T_{2g}$ and $E_g$ modes are required to account for the fine structure of the RIXS spectra. This work demonstrates that the EOM-CC method is a powerful tool for accurately predicting the complex local states at metal sites and spectroscopic signatures of correlated insulating materials.

cond-mat.str-el↗