Energy Gap Formation in the Kondo-Lattice Compounds

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The results of transport, thermal and magnetic measurements of CeNiSn characterize this compound as a rare example located near the crossover between the Kondo and VF regimes. The highly anisotropic behavior reflects the strong hybridization of the 4f states with the Sn-5p and Ni-3d states in the orthorhombic structure. The Kondo temperature of this system is about 28 K.At low temperatures below 12K,coherence of local spin fluctuations develops, as indicated by the peaking in both Pa(T) and Xa(T) at 12 K and in RH(T) at 9K.An anisotropic gap opens in the heavy-quasiparticle bands as temperature is reduced below 6 K. This semiconducting state with a charge excitation gap of several Kelvins is destructible by applying either hydrostatic pressure of 20 kbar or magnetic field of 13 T along the easy a axis.Substitutions of Co and Pt for a few percent of Ni in CeNiSn destroy the energy gap and shift the system toward the VF and the heavy-fermion regimes, respectively.The combined results indicate that the gap formation in CeNiSn is very sensitive to the degree of hybridization of the 4f and the conduction electron states. It is also emphasized that the development of coherence in the Kondo lattice is crucial for the gap formation.