Cd₂Re₂O₇: a Strong Spin-Orbit Coupled Hund's Metal

A correlation-driven hidden order phase transition is found in a spin-orbit coupled pyrochlore metal Cd₂Re₂O₇ (5d²) at 200 K. In this work, we use both first principle DFT+DMFT calculations and optical conductivity measurements to study the strong correlation effects in this material. The calculated optical conductivity by DFT+DMFT in the normal state at large spin-orbit coupling (SOC) and Hund's coupling shows incoherent feature which is consistent with the experimental results, while it shows coherent feature at small Hund's coupling. The DFT+DMFT calculations at large Hund's coupling perfectly reproduce the complete saturation behavior of resistivity observed experimentally in the normal state down to very low temperature, which indicates that the coherent energy scale in this material is largely decreased by both SOC and Hund's coupling. We also find clear orbital-selective behavior between j_eff=1/2 and j_eff=3/2 orbitals, which is a result of strong SOC and Hund's coupling.
These results indicate that the normal state of Cd₂Re₂O₇ is a strong spin-orbit coupled Hund's metal. Furthermore, the measured optical conductivity in the hidden order phase shows spectral weight transfer to high frequency when decreasing temperature, which also indicates Hund's metal behavior.