W. Staiger

μ− SR in semiconductorsSR in semiconductors

Abstractμ− SR experiments have been performed on Si between room temperature and 6 K. The amplitude of the muon spin precession signal in an applied magnetic field of 0.04 T decreased below 30 K. A zero-field measurement at 6 K revealed a μ− spin precession frequency of 650 MHz. The muonic atom represents an aluminium acceptor in the silicon matrix, its electronic state is responsible for the μSR signal. A possible influence of the γ recoil produced by the X-ray cascade is discussed.

μ+ SR studies of antiferromagnetic chromiumSR studies of antiferromagnetic chromium

Abstractμ+ SR measurements have been performed on Cr single crystals at temperatures 60 mK≤T≤295 K in applied magnetic fields 0≤Bappl≤1.5 T. The temperature dependence of the observed precession frequencies and transverse relaxation rates can be explained by the assumption that theμ+ are hopping between adjacent tetrahedral interstices. At temperaturesT≤11 K evidence for an interaction between theμ+ and the spin-density waves in Cr has been found. The directions and magnitudes of the lattice magnetic moments are unaffected by the applied magnetic fields.

Recent studies on π+/μ+ channelling

Abstract During the last few years the pion/muon channelling technique has been developed successfully as a tool for studying the sites of positive pions implanted into metal or semiconductor crystals. By this technique the behaviour of the positive pion — which may be considered as a light hydrogen isotope — can be studied in the temperature range from 1.5 K to 2200 K uninhibited by problems associated with hydrogen solubility, impurity accumulation and radiation damage. The implementation of a new pulsed pion beam at SIN permits the investigation of the site occupation as a function of time up to about 60 ns after pion implantation. Recent experiments have demonstrated that in Cu the stable π + sites are the octahedral interstices, whereas the tetrahedral interstices provide metastable sites. Surprisingly, it is found that the occupation of these metastable sites at elevated temperatures takes place through the stable sites only and not directly.

Phase transformation from quasicrystal to crystal

Several deformed samples of a quasicrystalline phase T2 of an Al-Li-Cu alloy were studied by transmission electron microscopy. A quasicrystalline state, a modulated structure as well as a crystalline phase were observed by means of high-resolution techniques in the same region of a thin foil. As there is no evidence of boundaries to separate them, this implies that a continuous transformation from the quasicrystalline to crystalline states can occur in this alloy through structural modulation.

Influence of elastic strain onμ+ SR in α-iron single crystalsSR in α-iron single crystals

The spin-precession frequencies and the transverse spin relaxation rates of positive mouns (μ+) have been measured on two elastically strained α-Fe single crystal platelets as well as on an unstrained reference α-Fe crystal at temperatures down to 2.7 K in applied magnetic field 0≤Bappl≤3 T. The drastic effects of the strains may be qualitatively understood in terms of their influence on both the magnetic domain structure and theμ+ energies at the various interstitial sites. This leads to the conclusion that at low temperaturesμ+ in α-Fe occupy configurations related to octahedral interstitials with dipolar fieldBdip=0.70 T.

Investigation of low-temperature quantum diffusion in α-iron byμ+ SR experiments on a single-crystal sphereSR experiments on a single-crystal sphere

The longitudinalμ+-spin relaxation rate has been measured on a high-purity spherical α-iron single crystal at temperaturesT down to 20 mK and in applied magnetic fieldsBappl parallel to 〈111〉 up to 3 T. Only above 1 K can the data be satisfactorily described by one rate constantГ. At 1 T≤Bappl≤2 T and 50 mK≤T≤300 mK, oscillations (“wiggles”) were in addition superimposed on the longitudinal relaxation. A qualitative understanding of the measurements may be achieved in terms of the increasing influence of internal stresses onμ+ diffusion as the temperature is lowered.

Transmission electron microscopy study of small-angle grain boundaries, dislocations and modulated phases in Al-Li-Cu quasicrystals

Abstract Quasicrystalline samples of the alloy Al-Li-Cu prepared by a conventional melting method have been studied by transmission electron microscopy. In conventional dark-field and in high-resolution images, modulations of the image contrast along threefold and fivefold axes with modulation periods of 2.7 and 0.99 nm have been found. This modulated icosahedral phase can be considered as an intermediate state between the quasicrystalline and a crystalline state and the image contrast modulations result probably from a local variation in the chemical composition. In our samples a very large density of small-angle grain boundaries have been found. From their image contrast it can be concluded that they are of twist boundary type. Single dislocations that occur in the same sample area have been studied with high-resolution microscopy. Their dislocation lines are situated in planes with fivefold symmetry and their Burgers vectors lie in planes with threefold symmetry.

Low-temperature π+/μ+ channelling in gold and α-ironchannelling in gold and α-iron

The lattice sites of positive pions in gold have been determined as a function of temperature by means of π+/μ+ channelling experiments. A blocking pattern in the <100> direction at low temperatures indicates that only octahedral interstitial sites are occupied. The vanishing of this pattern and the reduction of muon flux enhancement observed in <110> ifT is raised above the Debye temperature (165 K) may be interpreted in terms of muon dechannelling owing to the thermal vibrations of the lattice atoms.Time-differential π+/μ+ channelling studies on α-iron showed between 20 K and 70 K a strong dependence of the channelling profiles on the time the pions spent in the sample. The ‘young’ pions are well localized on O- or T-sites whereas the ‘old’ pions either become delocalized or occupy sites with strongly reduced symmetry. A model accounting for these observations is suggested. It is argued that there is a close relationship to earlierμ+ SR experiments.