Antiferromagnetism, valence fluctuation, and heavy-fermion behavior in EuCu2(Ge1-xSix)(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 EuCu2(Ge1-xSix)(2). The antiferromagnetic state is very stable from x = 0 up to x = 0.6 and disappears rather abruptly at x(c) approximate to 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 mJ/K-2 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