Spectroscopic Evidence for the Emergence of a Half-Metallic Surface State on the Bulk Insulator Sodium Cobaltate*

In recent years Na$_{x}$CoO$_{2}$ has attracted much attention for its unconventional superconductivity and antiferromagnetic phases. More recently, the stoichiometric compound NaCoO$_{2}$ has been proposed as a platform for achieving topological superconductivity through its predicted half-metallic surface state. We characterize this surface state and its relationship to local sodium concentration using low-temperature scanning tunneling spectroscopy (STS) and tuning fork-based atomic force microscopy. We also examine the magnetic moment of the surface state through temperature-dependent STS and Kerr rotation spectroscopy. These results are compared with density functional theory-calculated band structure and local density of states.