In the 5d layered perovskite material, Sr2IrO4, spin orbit coupling and strong electron correlations combine to give rise to a spin-orbit coupled Mott insulator with a pseudo-spin J=1/2 antiferromagnetic (AFM) state . Owing to its close structural, electronic and magnetic similarities with the 3d layered perovskite material, La2CuO4, which evolves from a spin S=1/2 AFM Mott insulator to a high temperature superconductor upon doping, doped Sr2IrO4 has become a promising material to discover new states of matter, including unconventional superconductivity. Recently, optical second-harmonic generation (SHG) measurements in Sr2(Ir,Rh)O4 have highlighted an odd-parity hidden order, developing prior to its AFM state. This state possesses the same symmetries of a loop-current (LC) order, theoretically proposed to account for the pseudo-gap physics of superconducting cuprates . The existence of such a magneto-electric state has gained support from the detection in several cuprate families of its magnetic fingerprint by polarized neutron diffraction.  Using that technique for Sr2(Ir,Rh)O4, we have observed at the temperature of the hidden odd-parity order the appearance of a magnetic order which breaks time-reversal symmetry, while preserving lattice translation invariance. Among the magnetic point groups inferred from the SHG data, only the co-planar LC order produces a magnetic pattern consistent with our polarized neutron data. Our results suggest that exotic magnetic orders with the same symmetry properties as a LC phase exist in both iridates and cuprates.
Host : Prof. Je-Geun Park