Abstract:
The 5d1 ordered double perovskites have recently drawn significant interest as a fertile ground for the realization of novel multipolar physics. However, while these materials are theoretically described as having a null dipole magnetic moment due to large spin-orbit coupling, real materials manifest with a suppressed but non-zero dipole magnetic moment. Though this phenomenon has typically been attributed to orbital hybridization between the transition metal ions and ligand oxygens, recent studies have shown that coupling to lattice degrees of freedom via the dynamic Jahn-Teller (DJT) effect can also have a significant impact on the magnetic moment size in 5d1 systems. I will present Re L3 and O K-edge resonant inelastic X-ray scattering (RIXS) results that demonstrate the presence and impact of the DJT effect in the A2MgReO6 (A = Ca, Sr, Ba) family of 5d1 double perovskites. Initial Re L3-edge measurements revealed that the spin-orbit excitations in these materials show a strongly asymmetric lineshape indicative of dressing by lattice vibrations that broaden and shift to higher energy with temperature. These results were followed up by O K-edge RIXS measurements that leveraged Re-O orbital hybridization to probe the Re 5d1 excitations with higher resolution. This additional resolution allowed us to clearly resolve the individual harmonic modes of the spin-orbit excitation phonon sideband and track the temperature dependence more precisely. Our results are explained quantitatively using a vibronic RIXS model that yields an electronic ground state in which spin, orbit, and lattice degrees of freedom become entangled. Examining temperature and A site dependence additionally reveals that the spin-orbit lattice entangled state is robust against magnetic and structural phase transitions, as well as against significant static Jahn-Teller distortions.