NMR studies of non-Fermi-liquid behavior in disordered Kondo systems

A number of heavy-fermion alloys have been discovered to have non-Fermi-liquid (NFL) properties in contrast to the Fermi-liquid behavior expected for normal metals. Since nuclear magnetic resonance (NMR) studies in the heavy-fermion UCu{dollar}sb{lcub}5-x{rcub}{dollar}Pd{dollar}sb{lcub}x{rcub}{dollar} by our group, the "Kondo disorder" model has been recognized as one of the possible origins of NFL behavior. This dissertation describes the use of NMR to study NFL behavior in the two heavy-fermion systems Ce(Ru{dollar}sb{lcub}1-x{rcub}{dollar}Rh{dollar}rmsb{lcub}x{rcub})sb2Sisb2{dollar} (x = 0.5) and U{dollar}sb{lcub}1-x{rcub}{lcub}rm Th{rcub}sb{lcub}x{rcub}{lcub}rm Pd{rcub}sb2{lcub}rm Al{rcub}sb3 (x > 0.6){dollar}. The cerium compound is disordered on non-f atoms (ligand disordered), whereas the uranium system is disordered on the f sublattice. Both exhibit complex phase diagrams and NFL behavior.; {dollar}sp{lcub}29{rcub}{dollar}Si powder-pattern NMR spectra from a randomly-oriented powder sample of CeRhRuSi{dollar}sb2{dollar} show broad linewidths at low temperature, consistent with disorder-induced NFL behavior. The spectra from a field-aligned sample further confirm that these broad linewidths are due to distributions of local susceptibilities. The NMR widths are in good agreement with the distribution {dollar}P(Tsb{lcub}K{rcub}){dollar} of Kondo temperatures {dollar}Tsb{lcub}K{rcub}{dollar} derived from the previous analysis of Graf et al., Phys. Rev. Lett. 78, 3769 (1997), including a "hole" in {dollar}P(Tsb{lcub}K{rcub}){dollar} for small {dollar}Tsb{lcub}K{rcub} lbrack P(Tsb{lcub}K{rcub} = 0) = 0rbrack{dollar} which describes the return to Fermi-liquid behavior below 1 K observed in the specific heat. The Kondo disorder model successfully explains the NMR linewidth and the NFL behavior in CeRhRuSi{dollar}sb2{dollar} either with or without consideration of RKKY interaction between Ce moments.; In U{dollar}sb{lcub}1-x{rcub}{lcub}rm Th{rcub}sb{lcub}x{rcub}{lcub}rm Pd{rcub}sb2{lcub}rm Al{rcub}sb3{dollar} (x = 0.7, 0.8, 0.9) the {dollar}sp{lcub}27{rcub}{dollar}Al NMR spectra in unaligned powders were initially thought to indicate a metallugical problem, namely, the existence of a second phase. After careful comparison of the behavior of Knight shifts in different concentrations, those extra lines were recognized as impurity satellites instead of coming from a second phase. These impurity satellites are due to specific U near-neighbor configurations to Al sites and appear clearly in the field-aligned spectra. The intensities of the impurity satellites are proportional to the probabilities of finding occupied U sites in specific near-neighbor shells around an Al site. Comparison of the calculated and observed satellite intensities allows us to reconstruct the spectra taken from field-aligned powders with the c axis both perpendicular and parallel to the external field. The narrow linewidths observed at low temperatures suggests that "Kondo disorder" is not the cause of NFL behavior in these alloys. Several theoretical models have been proposed to explain the source of the NFL behavior in U{dollar}sb{lcub}1-x{rcub}{lcub}rm Th{rcub}sb{lcub}x{rcub}{lcub}rm Pd{rcub}sb2{lcub}rm Al{rcub}sb3{dollar}.