Anisotropic Gap in Quasiparticle Excitations in the Kondo-Lattice System

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We have studied the gap formation in the Kondo-lattice systems YbB12, Ce3Bi4Pt3, CeNiSn and CeRhSb by means of high-quality single crystals. New findings are as followings.1. Anisotropic Gap : Tunnel spectroscopy revealed the B SC-type gap in YbBI2 and Ce3Bi4Pt3. Substitution of Lu for Yb in YbB12 gives rise to a midgap state with keeping the gap width unchanged. By contrast, the gap in CeNiSn and CeRhSb is anisotropic with a residual density of states. Only 1% substitution in CeNiSn leads to a unitarity scattering of the residual carriers. The gap anisotropy is much weakened in CeRhSb due to the strong hybridization.2. Ground state of CeNiSn : It is a semimetallic state with an extremely low carrier density. A sort of magnetic polaron is indicated by the anomalous field dependence of the magnetoresistance and specific heat below 0.5 K.3. Effect of high magnetic field : The gap in CeNiSn is closed by application of field of 14 TAU along the a axis. However, a higher field of 30 T is required to close the gap in CeRhSb. For YbB12, a first order transition from a semiconductor to a metal is found near 50 TAU The sharp peaks in the magnetoresistance indicate the presence of fine structure in the gap.4. Effect of pressure : CeNiSn exhibits a pressure-induced magnetism, which is the first example among the Ce-based compounds.