Superconductivity in Ternary Thorium Transition-Metal Carbides: Th3Ni5C5, Th2NiC2, and ThCoC2

The superconducting phenomenon is one of the most extraordinary material properties observed in the laboratory. A century of scientific inquiry into the underlying physics of the phenomenon has been both one of the greatest triumphs and remaining conundrums in condensed matter physics. In this dissertation the implementation of a systematic experimental search of intermetallic compounds by magnetization, resistivity, and heat capacity measurements resulted in the discovery of new superconductors in a trio of ternary thorium transition-metal carbides: Th3Ni5C 5, Th2NiC2, and ThCoC2. Polycrystalline samples were synthesized by the arc melting method, and interestingly these compounds show evidence of bulk superconductivity despite containing the usually antagonistic ferromagnetic elements Ni and Co. The orthorhombic Th3Ni 5C5 compound forms in its own unique prototype crystal structure, with the results presented here showing evidence of bulk superconductivity with a critical temperature of Tc=5.0K and a upper critical field of Hc2(0)=5.8T. The Ni-based ternary carbide Th2NiC 2 forms in the simple body centered tetragonal U2IrC 2 prototype structure, same structure as the heavy-fermion superconductor U2PtC2, and is superconducting with a transition temperature ranging from Tc~8.5-10.6K depending on the sample quality. The Co-based ternary carbide ThCoC2 is a new example of a noncentrosymmetric superconductor with a critical temperature of Tc=2.65K. The superconductivity in ThCoC2 displays interesting behavior that could indicate unconventional superconductivity, including a deviation from exponential temperature dependence in the heat capacity and positive curvature in the upper critical field H c2 phase diagram. In addition, Ni substitution in the series of samples Th(Co1-xNix)C2 significantly increased the critical temperature up to Tc=12.1K at the nominal composition ThCo0.6Ni0.4C2.