Pressure-Induced Valence Transition and Heavy-Fermion State in Europium Compounds

Abstract

Recent technological advances in growing single crystal samples of Eu compounds have boosted progress in the research field on valance fluctuation and /or transition of the Eu compounds. Here, we review the electronic states of single-crystalline EuT₂X₂ (T: transition metal, X: Si or Ge) and related compounds, together with the effect of pressure on these compounds. The rare-earth ions are usually trivalent, but a divalent (Eu²⁺) electronic state is more stable than a trivalent (Eu³⁺) one in the Eu-based metallic compounds. The energy difference between the Eu²⁺ and Eu³⁺ states is, however, not very large. Therefore, it is possible to change the Eu valence from nearly divalent (Eu²⁺) to nearly trivalent (Eu³⁺) by applying pressure. Here we show several characteristic features of Eu compounds under pressure: the magnetic order is suppressed, as revealed by the Doniach phase diagram, the magnetic order changes to the first-order valence transition above a critical pressure, and the moderate heavy-fermion state appears around the magnetic quantum critical point. Recently, a new phase diagram for the Eu compounds was found under pressure, which might be due to a sharp valence crossover induced by pressure.