Dozens of strongly correlated materials exhibit similar but by no means identical behavior patterns. Classifying the behaviors and the conditions under which they occur has been a high priority for more than two decades, and it may prove key to eventually fabricating useful materials including room-temperature superconductors. This project explores the role of magnetic frustration, where a system favors local magnetic configurations which are not mutually compatible. The system must satisfy some local constraints but not others, leading to a large number of states with the same or very similar energy. Squeezing a sample along a single direction can shift the energy levels to a broader range, thereby reducing the influence of frustration on the sample's behavior. This project applies such unidirectional pressure and measures how it changes a sample's low-temperature electrical and magnetic behavior. Understanding the effects of frustration may help in designing materials with specific desired properties in the future. The specialized cooling and pressure techniques provide excellent training for the graduate and undergraduate students who perform the experiments.
A major question in the low-temperature behavior of Yb-based heavy-fermion compounds is whether Kondo breakdown occurs, with a discontinuous jump in the size of the Fermi surface. More generally, the role of geometrical effects, such as geometic frustration and crystal dimensionality, should be ascertained. Uniaxial pressure is a natural technique for this purpose, since it affects both frustration and dimensionality more strongly than does hydrostatic pressure. This research involves specific heat, magnetic susceptibility, and transport measurements under uniaxial pressure up to 1 GPa, temperatures down to 30 mK, and magnetic fields up to 10 Tesla. These measurements establish a pressure-temperature-field phase diagram which reveals the importance of geometry among many competing influences. Of special note is the behavior of T*, the onset temperature for hybridization between the conduction electrons and local moments, under uniaxial pressure. T* is surprisingly stable to the related tuning techniques of hydrostatic pressure and chemical doping. One of the goals of this proposal is to test if the stability of the T* line occurs for uniaxial pressure as well.