We report the unprecedented square-planar coordination of iridium in the iron iridium arsenide Ca 10 (Ir 4 As 8 )(Fe 2 As 2 ) 5 . This material experiences superconductivity at 16 K. X-ray photoemission spectroscopy and first-principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir 4 As 8 layers. Such metallic spacer layers are thought to enhance the interlayer coupling of Fe 2 As 2 , in which superconductivity emerges, thus offering a way to control the superconducting transition temperature.

To identify the contributions of the multiband nature and the anisotropy of a microscopic electronic structure to a macroscopic vortex lattice morphology, we develop a method based on the Eilenberger theory near H c2 combined with the first-principles band calculation to estimate the stable vortex lattice configuration. For a typical two-band superconductor MgB 2 , successive transitions of vortex lattice orientation that have been observed recently by small angle neutron scattering [Das, et al.: Phys. Rev. Lett. 108 (2012) 167001] are explained by the characteristic field-dependence of twoband superconductivity and the competition of sixfold anisotropy between the σand π-bands. The reentrant transition at low temperature reflects the Fermi velocity anisotropy of the σ-band.