A novel set-up for fast muon-spin–rotation experiments with an effective time resolution of (150±10) ps (full-width at half-maximum) of the ‘prompt peak’ is described. A new method of extending the histogram length of the hitherto existing data-acquisition system allows the use of the full time range of the clock with a histogram length of approximately 5μs and pile-up and second-muon gates with the same length. The whole electronics consists of only four NIM modules and an EG & G Ortec ‘Picosecond Time Analyzer’ mod. 9308 connected to a personal computer. The performance of the novel spectrometer is demonstrated in measurements of the hyperfine precession frequency of muonium in fused quartz, liquid water, and different silsesquioxanes at room temperature.

The hyperfine coupling constant A of a hydrogen isotope confined in a cage differs from its vacuum value, demonstrating that the atom acts as a probe of its environment. Room-temperature values of A for muonium in different cube-shaped Si8O12 units (T8 units or octasilsesquioxanes), in H2O, and in D2O were determined with an accuracy of about 1 MHz. The results are compared with those obtained from high-resolution ESR on H in H2O, and D in D2O, and in different silsesquioxanes in the temperature range 40–300 K. Both the strong isotope effect and the temperature dependence are well described by a single-oscillator model of Roduner et al. (J. Chem. Phys. 102 (1995) 5989). Striking differences seen between the different silsesquioxanes and between the porous Optipur and bulk Suprasil reflect varying spatial constraints and electronic interactions.

Results on the temperature dependence of the residual polarization of negative muons in silicon with phosphorus (4.5×1018,2.3×1015, and 3.2×1012cm−3) and aluminium (2.4×1018 and 2×1014cm−3) impurities are presented. The muon spin rotation (μSR) experiments were carried out in a magnetic field of 0.2 T and in the temperature range 4.2–300 K. In all investigated samples a relaxation of the muon spin and a shift of the spin-precession frequency were observed. The frequency shift (relative to the room-temperature value) amounts to 7×10−3 at 15 K. In the sample with a high concentration of phosphorus impurity (4.5×1018cm−3) damped and undamped components of the muon spin polarization were observed at T<30K. Hyperfine interaction between the magnetic moments of the muon and that of the electron shell of the muonic atom (acceptor centre – μAl) is estimated on the basis of the muon spin precession frequency shift data. The temperature dependence of the spin-lattice relaxation rate of the magnetic moment of the shallow acceptor centre in silicon in the absence of external stress is determined for the first time. It is found that the relaxation rate is well approximated by the power function ν(T)=CTq, where the parameter q lies between 2 and 3.

A new muonium centre, termed MuV, with weak hyperfine interaction has been discovered recently in crystalline silicon in longitudinal field quenching μSR (LFQ) experiments (Schefzik et al., Solid State Commun. 107 (1998) 395). The signatures of this species were found in intrinsic, but not in doped samples (dopant concentration larger than 1016cm−3). The centre is not formed promptly but results from a reaction in which normal muonium transforms into the novel species. Since from LFQ experiments one can obtain only a rough estimate of the hyperfine tensor of MuV it is now determined from zero-field (ZF) μSR experiments. The hyperfine interaction is found to be axially symmetric around the 〈110〉 crystallographic axis with small hyperfine parameters. Properly rescaled it essentially agrees with the hyperfine tensor of the hydrogen centre VH, which has been discovered recently by Bech Nielsen et al. (Phys. Rev. Lett. 79 (1997) 1507) and which has been attributed to hydrogen trapped in vacancies. Accordingly MuV is interpreted as muonium trapped in a vacancy, in agreement with the interpretation given on the basis of the earlier LFQ data.

Radio frequency (RF) muon-spin resonance (RFμSR) experiments on the paramagnetic muonium centres MuT (muonium located at tetrahedral sites) and MuBC (bond-centred muonium), present in crystalline silicon, were carried out at applied magnetic fields B0 up to 6mT and a constant irradiated frequency νRF of approximately 80 MHz. In the magnetic-field dependence of the RF asymmetry aRF, i.e. the RF induced change of the muon-decay asymmetry, two resonances were observed, one corresponding to MuT, the other to MuBC. Systematic RFμSR measurements were performed on intrinsic silicon single crystals grown by the Czochralski method (oxygen concentration 2×1017cm−3) and the float-zone method (oxygen concentration <1015cm−3). No MuT signal has been observed in transverse field (TF) μSR, below T=50K in the Czochralski-grown sample whereas a MuT signal was found with RFμSR. This RFμSR result proves that the MuT species in this sample undergo dynamic processes, in which presumably oxygen is involved.

A versatile time-differential μSR data-acquisition set-up for quasi-continuous muon beams is described, which is based on a recently available NIM time-to-digital converter. The set-up is very compact, easy to use and, in contrast to the traditional hardware method, the full data selection is performed by software. The set-up proved to be reliable and extremely stable, currently it is regularly used at the Paul Scherrer Institut by several groups.

A new muonium centre, termed MuX, with anisotropic hyperfine interaction has been discovered recently in crystalline silicon in transverse-field muon spin rotation experiments (Schefzik et al., Philos. Mag. B 79 (1999) 1561). Its hyperfine tensor is very close to that of bond centred muonium (MuBC). The signature of MuX is only observable in samples containing oxygen in a concentration which is higher than that of the donors or acceptors. This indicates that MuX is related to an oxygen–muonium centre. In the present work we have carried out a study of the temperature behaviour of the MuX signal on a sample showing a very pronounced MuX signature. The observations clearly show that the MuX signal, on the one hand, and the MuBC signals, on the other, have an identical temperature dependence.

The residual polarization of negative muons has been studied for phosphorus-doped and antimony-doped silicon crystals. The measurements were carried out in a transverse magnetic field of 0.1 T over the temperature region 4 K-300 K. The ionized and neutral states of the pseudo-acceptor were observed in antimony-doped silicon for the first time. The rate of transition from the neutral to the ionized state of the acceptor was found to be equal to over the temperature range 4 K-12 K. The estimated rates of relaxation of the magnetic moment of the acceptor-centre electron shell are and in phosphorus-doped silicon and and in antimony-doped silicon at 4 K and 15 K respectively. The experimental results obtained are interpreted in terms of spin-lattice relaxation of the acceptor magnetic moment and of the acceptor-donor pair formation.

A new muonium centre, termed Mu(X), with anisotropic hyperfine interaction has been discovered in crystalline silicon in transverse-field muon spin rotation experiments. Its hyperfine tensor is very close to that of bond centred muonium (Mu(BC)) The new muonium centre has been observed only in some of the many samples investigated. A correlation between dopant and oxygen concentration in the investigated samples and the observation of Mu(X) has been found, which indicates that Mu(X) might be related to a oxygen-muonium centre. According to the hyperfine tensor of Mu(X) the new muonium species is attributed to an oxygen-Mu(BC) complex. The Mu(X) signal is affected by applied electrostatic fields in contrast with the signal of all other known muon species.

The residual polarization of negative muons in crystal silicon samples with phosphorus (P: 1.6×1013 cm−3) and antimony (Sb: 2×1018 cm−3) impurities is investigated. The measurements are made in a 1000 G magnetic field oriented in a direction transverse to the muon spin in the temperature range 4–300 K. The relaxation rate and shift of the precession frequency in the silicon sample with the phosphorus impurity are measured more accurately than previously. It is found that in antimony-doped silicon the acceptor center µ A1 at temperatures below 30 K can be in both ionized and neutral states. The experimental data are interpreted on the basis of spin-lattice relaxation of the magnetic moment of an acceptor center, formation of acceptor-donor pairs, and recombination of charge carriers at the acceptor. Preliminary measurements showed a nonzero residual polarization of negative muons in germanium.

Investigations of positive muons (μ+) in crystalline silicon employing the longitudinal field-quenching (LFQ) technique give strong evidence for the existence of a novel paramagnetic muon species with a small anisotropic hyperfine interaction. It adds to the list of known muon species, i.e., normal and anomalous muonium and the diamagnetic muon species. The signatures of the novel species are found in intrinsic but not in doped samples (dopant concentration about 1016 cm−3). The novel species is not formed promptly but results from a reaction in which normal muonium transforms into the novel species. The reaction rate constant at 10 K was found to be about 106 s−1. The hyperfine coupling of the novel paramagnetic muon species corresponds, after rescaling, to that of a hydrogen center termed VH, which has been discovered recently by Bech Nielsen et al. (Phys. Rev. Lett., 79, 1997, 1507) and which has been attributed to hydrogen trapped in vacancies. The LFQ data are tentatively interpreted in terms of trapping of normal muonium in vacancies that are created during the slowing-down of implanted muons close to the end of their stopping tracks.