engleski

Theory of Yb- and Eu-based inter-metallic compounds with an unstable f-shell

The properties of a number of Yb- and Eu-based intermetallics which exhibit a valence-change transition, like YbInCu4, EuNi2(Si1?xGex)2 or Eu2(Pd1?xPtx)2Si2 are most unusual. At high temperatures, the magnetization indicates that the rare earth (RE) ions are in a stable configuration, with a local moments close to the free ion value, while transport properties correspond to a bad metal with suppressed Drude weight and temperature independent resistance and magneto-resistance. Surprisingly, the usual Kondo anomalies are not found. At temperature TV the valency of the RE ions changes, the local moments disappear and the band states reconstruct to give a good metal. In the low-temperature phase the susceptibility is Pauli like, the Drude peak is fully developed and the Hall data show a huge increase in the number of carriers. The entropy reduction across the transition is consistent with a complete loss of f-moments, which seem to be quenched by valence fluctuations present in the ground state. For T ? TV , the high- entropy (magnetic and semi-metallic) phase can be restored by a large enough magnetic field, H ? Hc. The values of Hc and TV are pressure- and doping-dependent but the the ratio g=kBTV / ?BHc seems to be determined by the degeneracy of the magnetic f-states only.

Heavy fermuions; valence fluctuators

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