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Abstract

Increasing Si concentrations in the EuCu2(Ge1-xSix)(2) series tunes the divalent Eu antiferromagnetic (AF) compound EuCu2Ge2(T-N = 14 K) to a nonmagnetic intermediate valence (IV) system EuCu2Si2. There is a collapse of the magnetic state and heavy quasiparticles occur at x similar to 0.7 corresponding to a quantum critical point (QCP). We have systematically investigated the Eu-valence and magnetic states as well as the coupling to the lattice through the QCP in EuCu2(Ge1-xSix)(2). This involved the Eu-151 Mossbauer effect spectroscopy and angle-resolved x-ray diffraction measurements as a function of Si concentration (0 <= x <= 1) at variable temperatures in the range 300-4.2 K. The Eu-151 probe indicates that the divalent Eu AF state is stable up to x similar to 0.5, followed by a collapse of AF ordering for x > 0.6, which is associated with a simultaneous sharp change of the valence state towards a nonmagnetic IV state. The crossover from the AF ordered state to the nonmagnetic IV state is found at a QCP corresponding to x similar to 0.7, at which the nonmagnetic IV state is inhomogeneous and exhibits an enhanced Eu nu+ mean valence (nu similar to 2.5). We believe that the emergence of such an unusual valence state is related to the observed heavy quasiparticles at low temperatures near the QCP. Magnetic order and a nonmagnetic inhomogeneous IV state coexist in a narrow region 0.6 <= x < 0.7, which evolves to a homogeneous IV state above x - 0.8. The ThCrSi2-type tetragonal structure is maintained throughout the series, although there is a precipitous increase in the c/a ratio at 10 K when the valence fluctuations become enhanced at the critical concentration x = 0.7. X-ray diffraction temperature scans at the critical concentration indicate conspicuous changes to steep temperature dependences of decreasing (increasing) values of a (c) lattice parameters and decreasing unit-cell volume at T < 100 K, as the IV ground state become preferentially populated at low temperatures. Thus there is a clear manifestation of strong coupling between the lattice and the valence fluctuation process. A corresponding detailed phase diagram is constructed and compared with that obtained from recent external pressure studies on the system.