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Register a new usermuSR and NMR in f-electron non-Fermi liquid materials
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Added by
Douglas E. MacLaughlin
Publication date
2002
fields
Physics
and research's language is
English
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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.
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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
PbCuTe2O6 is a rare example of a spin liquid candidate featuring a three dimensional magnetic lattice. Strong geometric frustration arises from the dominant antiferromagnetic interaction which generates a hyperkagome network of Cu2+ ions although additional interactions enhance the magnetic lattice connectivity. Through a combination of magnetization measurements and local probe investigation by NMR and muSR down to 20 mK, we provide a robust evidence for the absence of magnetic freezing in the ground state. The local spin susceptibility probed by the NMR shift hardly deviates from the macroscopic one down to 1 K pointing to a homogeneous magnetic system with a low defect concentration. The saturation of the NMR shift and the sublinear power law temperature (T) evolution of the 1/T1 NMR relaxation rate at low T point to a non-singlet ground state favoring a gapless fermionic description of the magnetic excitations. Below 1 K a pronounced slowing down of the spin dynamics is witnessed, which may signal a reconstruction of spinon Fermi surface. Nonetheless, the compound remains in a fluctuating spin liquid state down to the lowest temperature of the present investigation.
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.
Non-Fermi-liquid (NFL) behavior in the f-sublattice-diluted alloy system U_{1-x}Th_xPd_2Al_3 has been studied using ^{27}Al nuclear magnetic resonance (NMR). Impurity satellites due to specific U near-neighbor configurations to ^{27}Al sites are clearly resolved in both random and field-aligned powder samples. The spatial mean Kbar and rms spread delta K of impurity satellite shifts, which are related to the mean chibar and rms spread delta chi of the inhomogeneous susceptibility, have been measured in field-aligned powders with the crystalline c axis both perpendicular and parallel to the external field. The relatively narrow lines observed at low temperatures suggest that disorder- induced inhomogeneity of the f-ion--conduction-electron hybridization is not the cause of NFL behavior in these alloys: at low temperatures the experimental values of delta chi(T)/chibar(T) are much smaller than required by disorder- driven models. This is in contrast to results in at least some alloys with disordered non-f-ion nearest neighbors to f ions (ligand disorder), where disorder-driven theories give good accounts of NFL behavior. Our results suggest that f-ion dilution does not produce as much inhomogeneity of the hybridization strength as substitution on ligand sites.
At certain quantum critical points in metals an entire Fermi surface may disappear. A crucial question is the nature of the electronic excitations at the critical point. Here we provide arguments showing that at such quantum critical points the Fermi surface remains sharply defined even though the Landau quasiparticle is absent. The presence of such a critical Fermi surface has a number of consequences for the universal phenomena near the quantum critical point which are discussed. In particular the structure of scaling of the universal critical singularities can be significantly modified from more familiar criticality. Scaling hypotheses appropriate to a critical fermi surface are proposed. Implications for experiments on heavy fermion critical points are discussed. Various phenomena in the normal state of the cuprates are also examined from this perspective. We suggest that a phase transition that involves a dramatic reconstruction of the Fermi surface might underlie a number of strange observations in the metallic states above the superconducting dome.
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