  Our magnetic, electrical, and thermal measurements on single-crystals of the
novel Mott insulator, Sr2IrO4, reveal a novel giant magneto-electric effect
(GME) arising from a frustrated magnetic/ferroelectric state whose signatures
are: (1) a strongly enhanced electric permittivity that peaks near a newly
observed magnetic anomaly at 100 K, (2) a large (~100%) magneto-dielectric
shift that occurs near a metamagnetic transition, and (3) magnetic and electric
polarization hysteresis. The GME and electric polarization hinge on a
spin-orbit gapping of 5d-bands, rather than the magnitude and spatial
dependence of magnetization, as traditionally accepted.
