Heavy Fermion State Based on the Kondo Effect in EuNi2P2

EuNi 2 P 2 is known as a heavy fermion compound with an electronic specific heat coefficient $\gamma=100$ mJ/(K 2 $\cdot$mol). We grew single crystals and studied their electronic and magnetic properties by measuring the electrical resistivity, magnetic susceptibility, high-field magnetization, specific heat, and thermal expansion. The present heavy fermion state is clarified to be based on the Kondo effect as in CeRu 2 Si 2 , revealing an intensive shrinkage of the volume below about 100 K in the temperature dependence of thermal expansion. The temperature dependences of the $4f$-electron contribution to the volume thermal expansion $(\Delta V/V)_{4f}$ and the average Eu valence are found to show good scaling in EuNi 2 P 2 .