Radio-frequency muon spin resonance (RF mu SR) experiments on condensed matter

In this paper we present an overview of the radio-frequency muon spin resonance (RF mu SR) technique, an analogue to continuous-wave NMR, and an introduction to time-integral (TI) and time-differential (TD) RF mu SR on muons in diamagnetic or in paramagnetic environments. The general form of the resonance line for TI-RF mu SR as well as the expression for the time-dependence of the longitudinal muon spin polarization at resonance are given. Since RF mu SR does not require phase coherence of the muon spin ensemble, this technique allows us to investigate muon species that are generated by transitions from, or in the course of reactions of, a precursor muon species even if in transverse-field (TF) mu SR measurements the signal is lost due to dephasing. This ability of RF mu SR is clearly demonstrated by measurements on doped Si. Tn this example, at low temperatures, a very pronounced signal from a muon species in diamagnetic environment has been found in RF mu SR measurements, whereas in TF mu SR experiments only a very small signal from muons in diamagnetic environment could be detected and a large fraction of the implanted muons escaped detection. These findings could be interpreted in terms of the delayed formation of a diamagnetic muonium-dopant complex, and, due to the large diamagnetic RF mu SR signal, the RF mu SR technique is a unique tool to study how the variation of parameters and experimental conditions such as illumination affects formation and behavior of these complexes. First results obtained on illuminated boron doped Si are reported. However, as illustrated by the example of experiments on the muonated radical in solid C-60, results from conventional TI-RF mu SR cannot always be interpreted unambiguously since different parameters, namely the fraction of muons forming the investigated muon species, the longitudinal and the transverse relaxation rates, have similar effects on height and shape of the RF mu SR resonance line. These ambiguities, however, may be resolved by collecting time-differential data. With this extension RF mu SR becomes a very powerful complementary method to TF mu SR in the studies of dynamic effects.