muSR and NMR in f-electron non-Fermi liquid materials

TL;DR

This paper reviews muSR and NMR studies on f-electron non-Fermi-liquid materials, highlighting the role of structural disorder and slow spin dynamics, with evidence pointing to quantum spin-glass behavior rather than quantum criticality.

Contribution

It provides experimental evidence linking structural disorder to NFL behavior and distinguishes between spin-glass dynamics and quantum criticality in these materials.

Findings

Disorder causes inhomogeneous muon relaxation rates.

Divergent low-frequency spin fluctuations suggest spin-glass behavior.

Less low-frequency fluctuation in more homogeneous NFL systems.

Abstract

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.