Competing interactions in the presence of coupled spin, charge, and lattice
degrees of freedom in heavy-fermion materials lead to a near degeneracy of
ground states in some systems. A small perturbation in unit-cell volume or
composition subsequently can produce a qualitative change in the ground st
ate, for example, from magnetically ordered to superconducting, with, in so
me instances, the appearance of a non-Fermi-liquid (NFL) state near their b
oundary. We have studied two heavy-fermion materials, CeCu2Si2 and CeRh2Si2
, which illustrate these behaviors. Measurements of the atomic structure of
CeCu2+xSi2 as a function of x suggest that structural inhomogeneity may in
fluence the preferred ground state and the existence of NFL behavior in thi
s material. Thermal expansion measurements on CeRh2Si2 as a function of pre
ssure reveal the evolution of spin-lattice coupling as the balance between
RKKY and Kondo interactions is tuned by small volume changes. (C) 2000 Else
vier Science S.A. All rights reserved.