Unique electronic states of Eu-based compounds

Pressure is a powerful tool for exploring novel phenomena in solid-state physics. 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. Eu ions in most of the Eu compounds are divalent (Eu2+), and the Eu2+ compounds order magnetically at low temperatures. The valence of the Eu ions in EuT2X2 and their related compounds can be, however, changed from divalent to nearly trivalent (Eu3+) by applying pressure. Several characteristic features were observed 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. Typical electronic properties for the Eu3+ state were observed in EuPd3. In addition, a skyrmion lattice state was found for the Eu-based cubic chiral antiferromagnet EuPtSi.