Experimental Study of the Phase Diagrams of Heavy Fermion Superconductors with Multiple Transitions.

The usual BCS theory of superconductivity, with phonon-mediated, s-wave Cooper pairing of electrons, works very well for most superconductors. However, this theory fails to describe adequately superconductivity in some classes of materials, such as the high-T_ {rm c} ceramics as well as heavy fermion systems. In heavy fermion systems, superconductivity at low temperatures arises from highly correlated electrons with huge effective masses. These superconductors have properties that are difficult to understand in terms of the usual BCS theory and they are strong candidates for unconventional superconductivity with non-s-wave, or anisotropic, pairing states. This thesis concentrates on two examples of heavy fermion superconductivity--UPt_3 and thoriated UBe_{13}. Both of these materials have double transitions which can be seen in specific heat measurements C(T), where in addition to the specific heat jump at the superconducting transition temperature, a second jump occurs at a lower temperature. This second transition does not appear in measurements of the resistivity or magnetic susceptibility, both of which simply show that the material remains superconducting. While the nature of the lower transition is not yet perfectly clear, multiple superconducting states provide strong evidence of anisotropic superconductivity. We explore the phase diagrams of these two materials and probe the symmetry of the superconducting states with low temperature specific heat measurements. Specific heat reveals both the onset of superconductivity and the lower temperature transition, and has the advantage of being a thermodynamic measurement. In UPt_3, we study the response of the double transition to uniaxial stress. Stress, like magnetic field, is a symmetry-breaking field which couples to the superconductivity and therefore provides a valuable tool in the study of anisotropic superconductivity. In thoriated UBe _{13}, we look at the form of the low temperature specific heat C(T) in order to probe the structure of the superconducting gap. In addition, we use the stress techniques developed for our UPt _3 experiments to perform a high resolution study of the unusual phase diagram of the U_ {rm 1-x}Th_{ rm x}Be_{13} system.