Spin-orbital-lattice entangled states in cubic d¹ double perovskites

The magnetism of cubic 4/5d¹ double perovskites has been intensively investigated due to their geometrical frustration and multipolar exchange interaction, whereas many puzzling phenomena related to the lattice degrees of freedom, e.g. “violation of the JT theorem” in structural data [1] and “breaking of local point symmetry” accompanying the ferromagnetic order [2], have not been understood. In this work, the interplay of spin-orbit coupling and vibronic coupling on the heavy d¹ site is investigated by ab initio calculations [3]. The stabilization energy of spin-orbital-lattice entangled states is found to be comparable to or larger than the exchange interactions, suggesting the presence of nonadiabatic Jahn-Teller dynamics in the systems. The entanglement of the spin and lattice degrees of freedom induces a strong magnetoelastic response. This multiferroic effect is at the origin of the recently reported breaking of local point symmetry accompanying the development of magnetic ordering in Ba₂NaOsO₆.
[1] A. S. Erickson et al., Phys. Rev. Lett. 99, 016404 (2007); T. Aharen et al., Phys. Rev. B 81, 224409 (2010).
[2] L. Lu et al., Nat. Commun. 8, 14407(2017).
[3] N. Iwahara, V. Vieru, and L. F. Chibotaru, Phys. Rev. B 98, 075138 (2018).