Competing ground states in heavy-fermion materials

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 state, for example, from magnetically ordered to superconducting, with, in some instances, the appearance of a non-Fermi-liquid (NFL) state near their boundary. 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 influence the preferred ground state and the existence of NFL behavior in this material. Thermal expansion measurements on CeRh2Si2 as a function of pressure reveal the evolution of spin-lattice coupling as the balance between RKKY and Kondo interactions is tuned by small volume changes.