Hybrid STM/AFM study of half-metallic surface states in cobaltates*

Na$_{x}$CoO$_{2}$ is a well-known compound that has been studied at various Na concentrations, and has drawn much attention for its unconventional superconductivity and antiferromagnetic phase. In its stoichiometric concentration, NaCoO$_2$ has has recently been proposed as a system for observing topological superconductivity when mixed with a superconductor's electronic states through the proximity effect. We examine NaCoO$_2$ using an ultrahigh-vacuum low-temperature hybrid scanning tunneling and atomic force microscope at 4 K. We use the tuning-fork AFM mode to study the topography of this bulk insulating material when no tunneling is possible, and utilize a special electrical contact scheme to access the electronic surface states for spectroscopy.