Time-reversal symmetry breaking hidden order in Sr ₂(Ir,Rh)O ₄

Abstract

Layered 5 d transition iridium oxides, Sr ₂(Ir,Rh)O ₄, are described as unconventional Mott insulators with strong spin-orbit coupling. The undoped compound, Sr ₂IrO ₄, is a nearly ideal two-dimensional pseudospin-1/2 Heisenberg antiferromagnet, similarly to the insulating parent compound of high-temperature superconducting copper oxides. Using polarized neutron diffraction, we here report a hidden magnetic order in pure and doped Sr ₂(Ir,Rh)O ₄, distinct from the usual antiferromagnetic pseudospin ordering. We find that time-reversal symmetry is broken while the lattice translation invariance is preserved in the hidden order phase. The onset temperature matches that of the odd-parity hidden order recently highlighted using optical second-harmonic generation experiments. The novel magnetic order and broken symmetries can be explained by the loop-current model, previously predicted for the copper oxide superconductors.

Abstract

Exploring ordered states in similarly structured oxides aids the understanding of structure-property correlations. Here the authors discover an unconventional magnetic order in Sr ₂(Ir,Rh)O ₄, which is analogous to a loop-current phase in cuprate superconductors with the same lattice structure.