Vibronic order and magnetism in cubic 5d 1 double perovskite compounds

Abstract

Abstract Cubic 5d 1 transition metal double perovskites exhibit various multipolar phases, which have not been unvailed within the spin-orbit based theories. A missing piece in the previous theories is the vibronic coupling on each site. In each 5d 1 transition metal sites, four-fold degenerate ground spin-orbit coupled states interact with Jahn-Teller active vibration, which gives rise to the development of the dynamic Jahn-Teller effect. Here, we incorporate the spin-orbit coupling and dynamic Jahn-Teller effect and describe all the ordered states reported for the 5d 1 double perovskites. We defined vibronic quadrupole moment and developed a spin-vibronic quadrupole model from the microscopic interactions in the double perovskites. The spin-vibronic model shows a rich mean-field phase diagram including the ordered phases that never appear in the conventional spin-orbit theories. Various ordered phases develop because the ordering of the vibronic states (vibronic order) gives strong impact on the magnetic orderings. The zero and finite temperature spin-vibronic ordered phases explain the puzzling orderings found in Cs2TaCl6 and Ba2MgReO6.