Advances in the study of intermediate-valence and heavy-fermion effects in rare-earth and uranium systems

Summary Three intermetallic systems, Yb−Al, U−Au and Ce−Co, are studied as most typical examples of intermediate-valence, heavy-fermion and coexistence of both intermediate-valence and heavy-fermion effects, respectively. YbAl₂ and YbAl₃, the two phases stable at low temperatures in the equilibrium phase diagram of Yb−Al, are both intermediate-valence systems with Yb valence values of 2.33 and 2.79 at room temperature. The modulation of these values by variations of external parameters like temperature, pressure and chemical substitutions with proper atoms was studied by structural, magnetic and spectroscopical techniques. Two phases are given also by U with Au at low temperatures with compositions UAu₂ and U₁₄Au₅₁: both systems exhibit heavy-fermion properties although with different ordering mechanisms. U₁₄Au₅₁ is an antiferromagnetic heavy-fermion system with a Nèel temperature around 23 K, UAu₂ shows spin-fluctuating behaviour with a saturation in theT→0 K magnetic susceptibility. Effects of substitutions with thorium were investigated only in U₁₄Au₅₁: competition between strong antiferromagnetic coupling and Kondo-like behaviours is observed. The Ce-rich side of the Ce−Co phase diagram shows a single Ce₂₄Co₁₁ phase stable at room temperature interest in the present work but in CeCo₂, which was demonstrated to be intermediate valent by resonant photoemission and XAS experiments and to exibit low magnetic moment and superconducting transition. The coexistence of intermediate-valence and heavy-fermion behaviours observed in Ce₂₄Co₁₁ and of intermediate valence and superconductivity in CeCo₂ is shown by the analyses of XPS, XAS and magnetic experiments with suitable phenomenological models. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.