Rezensionen
Rezensionen
Mahmoud Abdelrazek Ahmida
EuCu2(Ge1–xSix)2: A Model System to Study Charge Fluctuations across a Quantum Phase Transition
Rezension
The intermetallic series EuCu{sub 2}(Ge{sub 1-x}Si{sub x}){sub 2} crystallizing in the tetragonal ThCr{sub 2}Si{sub 2}- type structure combines the antiferromagnetic (AF) compound EuCu{sub 2}Ge{sub 2} (T{sub N}=14 K) with the homogeneous intermediate valent (IV) compound EuCu{sub 2}Si{sub 2}. Thus, the system offers the opportunity to investigate charge fluctuations across a quantum phase transition (QPT) at {approx} 0.7. Such a study is of fundamental interest in view of the observation of pressure-induced superconductivity in CeCu{sub 2}Si{sub 2} and Eu metal, where superconductivity could be mediated by critical charge fluctuations. To gain a microscopic insight into the change of the magnetic and valence state and their interplay across the QPT, we have performed systematic {sup 151}Eu Moessbauer effect and X-ray diffraction measurements on EuCu{sub 2}(Ge{sub 1-x}Si{sub x}){sub 2} as a function of concentration (0{<=}x{<=}1) as well as external pressure at different temperatures (4.2 to 300 K). The analysis of the results shows that the collapse of AF ordering for x>0.5 is associated with a simultaneous sharp increase of the valence state of Eu towards a homogeneous intermediate valence state. Both values of the Eu mean valence ({nu}) at=near the QCP induced by doping or external pressure are found to be enhanced ({nu}{approx}2.45), indicating enhanced charge fluctuations. The nature of valence fluctuations at=near the QCP and its coupling to the lattice is discussed. (orig.)
