Superconductivity and Metal Insulator Transitions in Layered Solids: High T(c) Oxides and Graphite Intercalation Compounds.

Field and temperature dependent levitation properties of high temperature superconducting copper oxide compounds were studied over the ranges 1 <=q H <=q 10kG, 0 <=q partialrm H/partial z <=q 0.77kG/mm and 4.2K <=q T<=q 300K. A novel cryogenic levitometer was developed for these experiments. For all of the specimens studied the levitation height shows a threshold temperature T _{rm L} above which the specimen will not levitate, and a dramatic slope discontinuity at a temperature T_{rm J} which is associated with the onset of the phase coherence between Josephson weakly linked grains. A linear relationship between T_sp{rm L}{2 } and the inverse field gradient was theoretically derived and found to be in excellent agreement with experiment for both the Y and Bi based compounds. The magnetization properties revealed by the levitation experiments were also examined using a SQUID susceptometer. A least squares fit using these data yielded parameters which agree well with a Ginzberg-Landau model of the temperature dependence of the penetration depth, lambda(T). We have also carried out systematic resistance measurements in order to study the kinetic and electronic properties of potassium-ammonia graphite intercalation compounds. The time dependence of c-axis resistance measurements reveals the kinetics of the ammoniation of KC_ {24} to form the ternary graphite intercalation compounds (GIC) K(NH_3) _{rm x}C_ {24}, 0 < x < 4.38. These results provide the first evidence of two dimensional diffusion limited intercalation kinetics that are governed by the growth of planar quasicommensurate domains. A least squares fit of our data, using a 2D model, yields a diffusion constant of the order 10^ {-9}cm^2/sec. This is in agreement with known values of gaseous diffusion into solids. The a-axis resistivity measurements exhibit an intriguing metal-nonmetal transition at x > 4. This transition is attributed to a 2D Mott type transition in the intercalated K-NH_3 liquid monolayer. The electrical anisotropy of the ternary GIC K(NH_3)_{rm x}C_{24} at x ~ 4 is found to be about 5 times 10^4 which is the highest yet reported for a stage-1 donor GIC. Other alkali metal ammonia intercalation GICs such as Rb(NH _3)_{rm x} C_{24} and Cs(NH _3)_{rm x}C_{24} were also studied in order to explore ion size effects. An empirical relationship between the saturation resistance and ionic parameters has been found.