Electric and magnetic properties of the Kondo-lattice compound CeCu2Si2

Abstract

Electric, magnetic, and thermoelectric properties of CexLa1−xCu2Si2 (0⩽x⩽1) compounds have been studied over a wide temperature interval 0.04 ⩽T⩽300 K in magnetic fieldsH⩽40 kOe. The paramagnetic-magnetic ordering transition temperatureTM is found to rise from ∼0.32 K for cerium concentrationx=0.2 to 1.6 K forx=0.6. A further increase inx from ∼0.8 to 1.0 leads to a decrease inTM. Simultaneously, the susceptibility kink is smeared out and atx≈1.0 it is transformed into temperature-independent enhanced Pauli paramagnetism. The magnetic phase diagram has been found to be similar to that proposed by Doniach for the one-dimensional Kondo-necklace model. The Kondo-lattice compound CeCu2Si2 exhibits a superconducting transition atTc⋍0.5 K. The variation of the magnetic properties of CexLa1−xCu2Si2 from magnetic ordering at 0.2≲x≲0.8 to the nonmagnetic superconducting state atx → 1.0 is caused by the crossover from the magnetic regimeTRKKY≫TK (in which the RKKY temperatureTRKKY exceeds the Kondo temperatureTK) to the nonmagnetic singlet ground state corresponding to the situation whenTK≫TRKKY. This crossover is accompanied by a sharp increase in the low-temperature Hall coefficientRH(T) in CexLa1−xCu2Si2 compounds atx → 1. At the same time, a minimum of the negative Seebeck coefficient with a high amplitude appears at 10 pKL, a continuous transition from the Kondo lattice to the intermediate valence state is observed, which is accompanied by a complete smearing out of the resonance near the Fermi level. Therefore the Kondo lattices represent a new class of solids, which can be characterized as the link between stable magnetism of metals with a deep 4f level and unstable magnetism associated with fluctuating valence. This novel state can be described by a set of anomalous low-temperature properties related to the giant Abrikosov-Suhl resonance near the Fermi level.