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Register a new userDisorder, inhomogeneity and spin dynamics in f-electron non-Fermi liquid systems
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Douglas E. MacLaughlin
Publication date
2004
fields
Physics
and research's language is
English
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Muon spin rotation and relaxation ($mu$SR) experiments have yielded evidence that structural disorder is an important factor in many f-electron-based non-Fermi-liquid (NFL) systems. Disorder-driven mechanisms for NFL behaviour are suggested by the observed broad and strongly temperature-dependent $mu$SR (and NMR) linewidths in several NFL compounds and alloys. Local disorder-driven theories (Kondo disorder, Griffiths-McCoy singularity) are, however, not capable of describing the time-field scaling seen in muon spin relaxation experiments, which suggest cooperative and critical spin fluctuations rather than a distribution of local fluctuation rates. A strong empirical correlation is established between electronic disorder and slow spin fluctuations in NFL materials
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Magnetic resonance (muSR and NMR) studies of f-electron non-Fermi-liquid (NFL) materials give clear evidence that structural disorder is a major factor in NFL behavior. Longitudinal-field muSR relaxation measurements at low fields reveal a wide distribution of muon relaxation rates and divergences in the frequency dependence of spin correlation functions in the NFL systems UCu_{5-x}Pd_x and CePtSi_{1-x}Ge_x. These divergences seem to be due to slow dynamics associated with quantum spin-glass behavior, rather than quantum criticality as in a uniform system, for two reasons: the observed strong inhomogeneity in the muon relaxation rate, and the strong and frequency-dependent low-frequency fluctuation observed in U(Cu,Pd)_5 and CePt(Si,Ge). In the NFL materials CeCu_{5.9}Au_{0.1}, Ce(Ru_{0.5}Rh_{0.5})_2Si_2, CeNi_2Ge_2, and YbRh_2Si_2 the low-frequency weight of the spin fluctuation spectrum is much weaker than in the disordered NFL systems.
The non-Fermi-liquid (NFL) behavior observed in the low temperature specific heat $C(T)$ and magnetic susceptibility $chi(T)$ of f-electron systems is analyzed within the context of a recently developed theory based on Griffiths singularities. Measurements of $C(T)$ and $chi(T)$ in the systems $Th_{1-x}U_{x}Pd_{2}Al_{3}$, $Y_{1-x}U_{x}Pd_3$, and $UCu_{5-x}M_{x}$ (M = Pd, Pt) are found to be consistent with $C(T)/T propto chi(T) propto T^{-1+lambda}$ predicted by this model with $lambda <1$ in the NFL regime. These results suggest that the NFL properties observed in a wide variety of f-electron systems can be described within the context of a common physical picture.
Muon spin rotation experiments on a stoichiometric sample of the non-Fermi liquid (NFL) heavy-fermion compound UCu_4Pd, in which recent neutron Bragg scattering measurements are consistent with an ordered structure, indicate that the U-ion susceptibility is strongly inhomogeneous at low temperatures. This suggests that residual disorder dominates NFL behavior. The data also indicate a short correlation length (lesssim 1 lattice spacing) for this inhomogeneity and a rapid low-temperature U-moment relaxation rate (gtrsim 10^{12} s^{-1}), which constrain cluster-based models of NFL behavior.
We report measurements of the bulk magnetic susceptibility and ^{29}Si nuclear magnetic resonance (NMR) linewidth in the heavy-fermion alloy CeRhRuSi_2. The linewidth increases rapidly with decreasing temperature and reaches large values at low temperatures, which strongly suggests the wide distributions of local susceptibilities chi_j obtained in disorder-driven theories of non-Fermi-liquid (NFL) behavior. The NMR linewidths agree well with distribution functions P(chi) which fit bulk susceptibility and specific heat data. The apparent return to Fermi-liquid behavior observed below 1 K is manifested in the vanishing of P(chi) as chi to infty, suggesting the absence of strong magnetic response at low energies. Our results indicate the need for an extension of some current theories of disorder-driven NFL behavior in order to incorporate this low-temperature crossover.
Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu_{5-x}Pd_x, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function Gbar(t) indicates strongly inhomogeneous spin fluctuations, and exhibits the scaling Gbar(t,H) = Gbar(t/H^gamma) expected from glassy dynamics. At 0.05 K gamma(x=1.0) = 0.35 pm 0.1, but gamma(x=1.5) = 0.7 pm 0.1. This is in contrast to inelastic neutron scattering results, which yield gamma = 0.33 for both concentrations. There is no sign of static magnetism gtrsim 10^{-3} mu_B/U ion in either material above 0.05 K. Our results strongy suggest that both alloys are quantum spin glasses.
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