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

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 2 IrO 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.