Antiferromagnetism, valence fluctuation, and heavy-fermion behavior in ${\mathrm{EuCu}}_2({\mathrm{Ge}}_{1-x}{\mathrm{Si}}_x{)}_2$

We have investigated the magnetic, thermodynamic, transport, and electronic properties across the transition from the divalent antiferromagnetic state to the valence fluctuating state of Eu in the alloy ${\mathrm{EuCu}}_2({\mathrm{Ge}}_{1-x}{\mathrm{Si}}_x{)}_2.$ The antiferromagnetic state is very stable from $x=0\mathrm{}$ up to $x=0.6\mathrm{}$ and disappears rather abruptly at $x_c\approx \mathrm{0.65.}$ Near the crossover, we confirmed a pronounced Kondo-like behavior in the resistivity and in the thermopower. Further on, for x slightly larger than $x_c,$ we observe the formation of a heavy Fermi liquid at low temperatures, as evidenced by a large linear coefficient of the specific heat $(\gamma =191\mathrm{}{\mathrm{m}\mathrm{J}/\mathrm{K}}^2\mathrm{mol}$ for $x=0.7),$ a large quadratic term in the resistivity and a strongly enhanced constant susceptibility ${\chi }_0.$ This is a unique observation of heavy fermion behavior in a Eu compound. The photoemission spectra of the Eu $4f$ and $3d$ core levels indicate significant valence fluctuation even for $x<\mathrm{}{x}_c$ in the magnetically ordered regime. The decrease of $T_N$ before reaching the critical point as well as the Kondo and the heavy fermion behavior found near $x_c$ are in strong contrast to the observation in typical Eu systems, which usually show a first-order transition from a (almost) divalent state to a strongly valence fluctuating Eu state.