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

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

Figure 1

Crystal structure of ${\mathrm{La}}_2{\mathrm{IOs}}_2$ seen from the in-plane direction (left) and from the $c$ axis (right) described by vesta [21]. Hexagonal and orthorhombic unit cells are shown by the black and purple lines, respectively. (b) X-ray diffraction patterns of powder (middle) and a pellet (top) of ${\mathrm{La}}_2{\mathrm{IOs}}_2$ compared with the calculated pattern (bottom). Triangles indicate the LaOI impurity phase. (c) Orbital projected density of states calculated on ${\mathrm{La}}_2{\mathrm{IRu}}_2$ (left) and ${\mathrm{La}}_2{\mathrm{IOs}}_2$ (right).

Figure 2

(a) Temperature dependences of electrical resistivity in the magnetic field perpendicular (left) and parallel (right) to the pellet: different pellets from the same powder sample are used. (b) Temperature dependence of $4\pi M/H$ of the powder sample of ${\mathrm{La}}_2{\mathrm{IOs}}_2$ at $H$ = 20 Oe. (c) Difference of the specific heat divided by temperature between the 0 and 9 T data (top): the dashed lines and the arrow are the eye guide. 9 T data is plotted against $T^2$ (bottom).

Figure 3

Temperature and magnetic field angle dependence of the magnetic torque of a single crystal of ${\mathrm{La}}_2{\mathrm{IOs}}_2$ at 6 (left) and 12 (right) T. The dashed lines are the eye guide. (b) $H-T$ phase diagram determined by the resistivity and torque data.

Figure 4

(a) Temperature dependence of the magnetic susceptibility at 6 T of the powder sample of ${\mathrm{La}}_2{\mathrm{IOs}}_2$. Electrical resistivity at 0 T is shown in the inset. (b) Powder x-ray diffraction pattern of ${\mathrm{La}}_2{\mathrm{IOs}}_2$ at selected temperatures compared with the simulation assuming the orthorohmbic unit cell (left). Temperature dependence of the lattice constant of ${\mathrm{La}}_2{\mathrm{IOs}}_2$ (right): values for hexagonal and orthorhombic unit cells are shown by the open and filled plots, respectively.