Pseudo-Jahn-Teller Effect and Magnetoelastic Coupling in Spin-Orbit Mott Insulators

The consequences of the Jahn-Teller (JT) orbital-lattice coupling for magnetism of pseudospin Jeff=1/2 and Jeff=0 compounds are addressed. In the former case, represented by Sr₂IrO₄, this coupling generates, through the so-called pseudo-JT effect, orthorhombic deformations of a crystal concomitant with magnetic ordering. The orthorhombicity axis is tied to the magnetization and rotates with it under magnetic field. The theory resolves a number of puzzles in Sr₂IrO₄ such as the origin of in-plane magnetic anisotropy and magnon gaps, metamagnetic transition, etc. In Jeff=0 systems, the pseudo-JT effect leads to spin-nematic transition well above magnetic ordering, which may explain the origin of "orbital order" in Ca₂RuO₄.