Prof. Nic Shannon
Okinawa Institute of Science and Technology, Japan

Abstract: Theory of spin nematic order in Sr2IrO4

Sr2IrO4, originally investigated as an iridum analogue to cuprate high-temperature superconductors, has proved to be a fascinating material in its own right.  Like La2CuO4, it is Mott insulator which exhibits strongly two-dimensional magnetism.  However, unlike La2CuO4, magnetism in Sr2IrO4 is governed by strong spin-orbit coupling, and built around j=1/2 moments with a relatively small Mott gap.   As a consequence, both anisotropic exchange interactions and strong quantum fluctuations come into play.

A recent surprise was the discovery of hidden, spin-nematic order in Sr2IrO4, coexisting with its Neel ground state [Kim et al, Nature 625, 264 (2024)].   The predominantly antiferromagnetic nature of exchange interactions in Sr2IrO4 mean that this result cannot be explained within existing theories of spin nematic order, which depend on frustrated ferromagnetic interactions.    

In this talk we use symmetry analysis to develop a minimal model of magnetism in Sr2IrO4, and DMRG simulations to solve this model for parameters consistent with experiment.  We find that spin nematic order follows from the interplay between strong cyclic exchange and anisotropic exchange interactions.   Along the way, we resolve a long-running dispute about the crystaline symmetry of Sr2IrO4, and enumerate the anisotropic exchange interactions permitted by the symmetry of the lattice.

These results provide an explanation for the spin nematic order observed in Sr2IrO4, and establish a robust mechanism for stabilizing spin nematic order in other Mott insulators with predominantly antiferromagnetic interactions.