Differing roles of disorder: Non-Fermi liquid behavior in uranium-copper-nickel and Curie temperature enhancement in uranium-copper-silicon-germanium

Disorder may be created by the substitution of one element for another where the elements are found on the periodic chart of elements. One compound that has structural disorder is UCu5- xNix where the transition element, copper, is replaced with another transition element, nickel. Another compound that has disorder is UCu2Si2-xGe x where silicon atoms are substituted with isovalent germanium elements. The disorder in these two compounds leads to interesting physics at low temperatures.; UCu5 is antiferromagnetic at around 16 K; however, the antiferromagnetism may be suppressed by doping nickel atoms onto the copper sites in the unit cell. The antiferromagnetic transition is suppressed to zero for the UCu 4Ni compound, leading to non-Fermi-liquid (NFL) behavior due to a quantum critical point. Multiple UCu5-xNi x compounds with small changes in the stoichiometry are synthesized around UCu4Ni in order to investigate if the cause of NFL behavior crosses over from a quantum critical point to rare strongly coupled magnetic clusters in a Griffiths phase scenario. The investigation is carried out by performing the following measurements on UCu5- xNix, compounds: (1) x-ray diffraction; (2) direct current (dc) magnetic susceptibility as a function of temperature in various magnetic fields; (3) alternating current (ac) susceptibility; (4) magnetization measurements as a function of magnetic fields; (5) low temperature dc resistivity; and (6) low temperature specific heat in zero and applied magnetic fields (up to 13 T).; UCu2Ge2 and UCu2Si2 are ferromagnets with ferromagnetic ordering temperatures, TCurie, occurring at around 108.5 K and 102.5 K respectively. Various compounds lying between the two aforementioned ferromagnets on the UCu2Si2- xGex phase diagram were synthesized in order to test theoretical predictions made concerning the nonmonotonic behavior of TCurie as a function of the amount of structural disorder. The nonmonotonic behavior was confirmed by measuring the dc magnetic susceptibility. As the amount of disorder is varied, the electron scattering crosses over from a ballistic regime for no structural disorder to a diffusive regime when disorder is present. This crossover is proven by dc resistivity measurements.