Semiconducting Ground State in the Kondo-Lattice Compounds

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We have studied the mechanism of gap formation in the Kondo semiconductors CeNiSn and CeRhSb by means of bulk and microscopic measurements.1. Single crystals of CeNin and CeRhSb were grown by the Czochralski and Bridgman methods, respectively. Purification of CeNiSn was performed by the solid-state electrotransport technique.2. For both the systems, the semiconducting behavior in the low-temperature resistivity is suppressed as the amount of impurities is decreased. This fact and the large enhancement of the carrier mobility support the anisotropic gap in the state density of heavy quasi particles.3. The common anisotropy in both the resistivity and magnetic susceptibility for the two systems indicates that the gap formation originates from a similar type of hybridization of 4f electronic states with conduction states.4. The spin excitation spectra obtained from neutron scattering show pronounced peaks at 4 and 2 meV,the former of which is quasi-one-dimensional along the b-axis.5. The energy gap determined by the tunneling measurements is 8-10 meV for CeNiSn and 20-27 meV for CeNiSn, which value is comparable to the Kondo temperature.