Tunneling Evidence for the Quasiparticle Gap in Kondo Semiconductors CeNiSn and CeRhSb

The Kondo semiconductors CeNiSn and CeRhSb were investigated by tunneling spectroscopy using a break junction. The differential conductances at 2-4 K show energy gaps of 8-10 and 20-27 meV for CeNiSn and CeRhSb, respectively, which are comparable to the Kondo temperatures. The tunneling spectra give clear evidence for a strong gap anisotropy. With increasing temperature, the zero-bias conductance displays a crossover from a well-developed gap state to a partial-gap state, and to a Kondo-metallic state.