Local electronic states of ordered and disordered Kondo insulator YbB 12 (001) surfaces

Local atomic and electronic structures of the YbB₁₂(001) surfaces are studied by scanning tunneling microscopy and spectroscopy at low temperatures for revealing the hybridized surface electronic states. The surface was prepared by annealing in an ultrahigh vacuum at 12000.16em0ex^∘C for one hour. The annealing process preferentially removed the surface Yb atoms, and the surface exhibits a homogeneous c(2×2) structure. On the inhomogeneous surface prepared by 14000.16em0ex^∘C annealing for several seconds, there are three types of local c(2×2) structures. The local tunneling spectra at 5 K near the Fermi energy on the surfaces exhibit characteristic features consisting of a co-tunneling lineshape due to the Kondo lattice formation and an additional central peak. On the homogeneous surface, the shape of the central peak depends on the tip-surface distance, suggesting the f-d hybridized nature of the surface electronic states.