Metallic Surface State in the Bulk Insulating Phase of Ca_2−xSr_xRuO₄ (x = 0.06) Studied by Photoemission Spectroscopy

We report the electronic structure of Ca2-xSrxRuO4 (CSRO) for x = 0.06 by means of angle-resolved photoemission spectroscopy (ARPES) and hard x-ray photoemission spectroscopy (HAXPES). The surface-sensitive ARPES results show the Fermi surfaces (FSs) consisting of the dxz/yz and dxy orbitals even in the low temperature bulk insulating phase, indicating the surface metallic state. In order to investigate the surface electronic structure as well as the bulk electronic structure, we have performed the angular dependent HAXPES and revealed the electronic structure depending on the probing depth. The HAXPES results suggest the bulk insulating state and the surface metallic state of CSRO (x = 0.06). Moreover, the observed band dispersion derived from the dxz/yz orbital exhibits kink structures with the energy scales of 35 and 60 meV. The distinct kink structures suggest the strong electron-phonon coupling compared with Sr2RuO4. Our results reveal that the surface metallic state with the large electron-lattice coupling is realized in the lightly Sr-doped region.