Superconductivity at 12 K in La${}_2$IOs${}_2$: a 5d metal with osmium honeycomb layer

We discovered superconductivity at $T_c$ = 12 K in a layered compound La${}_2$IOs${}_2$ with osmium honeycomb network. Despite heavy constituent elements unfavorable for phonon mediated mechanism, $T_c$ is the highest among lanthanoid iodides made of lighter elements such as La${}_2$IRu${}_2$ with $T_c$ = 4.8 K. Electronic anomalies are observed below 60 K similar to those observed in La${}_2$IRu${}_2$ below 140 K. La${}_2$IOs${}_2$ is a layered 5d electron system providing a platform to investigate the interplay between the electronic anomaly, superconductivity, and strong magnetic field.