Interplay between valence fluctuations and lattice instabilities across the quantum phase transition in EuCu2(Ge1-xSix)2

Increasing Si concentrations in the EuCu2(Ge1-xSix)2 series tunes the divalent Eu antiferromagnetic (AF) compound EuCu2Ge2 (TN=14K) to a nonmagnetic intermediate valence (IV) system EuCu2Si2. There is a collapse of the magnetic state and heavy quasiparticles occur at x∼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 Eu151 Mössbauer 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 Eu151 probe indicates that the divalent Eu AF state is stable up to x∼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∼0.7, at which the nonmagnetic IV state is inhomogeneous and exhibits an enhanced Euν+ mean valence (ν∼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.