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In mixed-valence or heavy-fermion systems, the hybridization between local f orbitals and conduction band states can cause the suppression of long-range magnetic order, which competes with strong spin uctuations. Ce- and Yb-based systems have been found to exhibit fascinating physical properties (heavy-fermion superconductivity, non-Fermi-liquid states, etc.) when tuned to the vicinity of magnetic quantum critical points by use of various external control parameters (temperature, magnetic eld, chemical composition). Recently, similar effects (mixed-valence, Kondo uctuations, heavy Fermi liquid) have been reported to exist in some Eu-based compounds. Unlike Ce (Yb), Eu has a multiple electron (hole) occupancy of its 4f shell, and the magnetic Eu2+ state (4f7) has no orbital component in the usual LS coupling scheme, which can lead to a quite different and interesting physics. In the EuCu2(SixGe1-x)2 series, where the valence can be tuned by varying the Si/Ge ratio, it has been reported that a significant valence uctuation can exist even in the magnetic order regime. This paper presents a detailed study of the latter material using different microscopic probes (XANES, Mossbauer spectroscopy, elastic and inelastic neutron scattering), in which the composition dependence of the magnetic order and dynamics across the series is traced back to the changemore » in the Eu valence state. In particular, the results support the persistence of valence uctuations into the antiferromagnetic state over a sizable composition range below the critical Si concentration xc ≈ 0:65. In conclusion, the sequence of magnetic ground states in the series is shown to re ect the evolution of the magnetic spectral response.« less

Measurements of temperature and field dependent bulk properties have been carried out on ${{\mathrm{Ce},\mathrm{La}}}_{3}{\mathrm{Pd}}_{4}{\mathrm{Si}}_{4}$. Rietveld refinements of x-ray powder intensity data confirm the orthorhombic ${\mathrm{U}}_{3}{\mathrm{Ni}}_{4}{\mathrm{Si}}_{4}$-type structure for both compounds. While ${\mathrm{La}}_{3}{\mathrm{Pd}}_{3}{\mathrm{Si}}_{4}$ exhibits superconductivity below 2 K together with some indications of a two gap scenario, the former can be characterized as a Kondo lattice system in presence of crystalline electric field splitting. Due to the closeness of a quantum critical point, distinct non-Fermi-liquid features dominate the low temperature physics of ${\mathrm{Ce}}_{3}{\mathrm{Pd}}_{4}{\mathrm{Si}}_{4}$.

H. Kaldarar,1 E. Royanian,1 H. Michor,1 G. Hilscher,1 E. Bauer,1 A. Gribanov,2,3 D. Shtepa,2 P. Rogl,3 A. Grytsiv,3 Y. Seropegin,2 and S. Nesterenko2 1Institute of Solid State Physics, Vienna University of Technology, Wien A-1040, Austria 2Department of Chemistry, Moscow State University, Leninskie Gory, GSP-1, Moscow 119991, Russia 3Institute of Physical Chemistry, University of Vienna, Wahringerstrasse 42, Wien A-1090, Austria Received 13 November 2008; published 7 May 2009