[Show abstract][Hide abstract] ABSTRACT: The nature of hydrogen defect centres in II-VI semiconductors has been inferred by studying implanted positive muons, used to mimic interstitial protons. Neutral paramagnetic centres formed by electron capture fall into three distinct categories. CdS, CdSe, CdTe and ZnO show shallow-donor states, implying that hydrogen may act as an n-type dopant in these materials. ZnS, ZnSe, MgO, BeO and SrO show the more normal trapped-atom states, whilst HgO exhibits a state intermediate between the two extremes, reminiscent of the deep-donor hydrogen state in Si. Preliminary indications of shallow states in CdO and HgS and of a deep state in ZnTe remain to be clarified. In a search for a predictive model and in the light of recent theoretical notions of a common hydrogen pinning level, the systematics of the shallow-to-deep instability are correlated with the depths of the conduction-band minima.
[Show abstract][Hide abstract] ABSTRACT: The influence of the coherence breakdown in the Yb sublattice on the excitation spectra of YbB12-based systems and the characteristics of the crystal field potential have been studied by inelastic neutron scattering on Yb0.75Lu0.25B12 and Yb0.9Er0.1B12 samples. The crystal field potential has been determined by use of the Er3+ ions as a sensor. The spectral shape of the Yb magnetic response is considerably changed in the near-gap region due to Lu substitution with respect to the pure YbB12. The possible splitting of the Yb3+ ground-state multiplet due to the crystal field appears to be comparable with the gap energy as well as with the Kondo temperature and energy of the only inelastic peak in the YbB12 spectra at T>TKondo.
Applied Physics A 11/2002; 74. DOI:10.1007/s003390201615 · 1.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report muon spin rotation (muSR) measurements on the moving vortex lattice (VL) in the type-II superconductor Pb-In, backed up by small-angle neutron scattering (SANS) observations on the same sample. We observe a motional narrowing of the muSR lineshape p(B) and by SANS, alignment of the VL to the direction of vortex motion. We have calculated the muSR lineshape expected with a range of orientations of the moving VL. We demonstrate how the new muSR results give information on the moving VL which is complementary and consistent with the SANS data.
Physical Review B 08/2002; 66(5). DOI:10.1103/PhysRevB.66.054506 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A new type of muonium defect center has been observed in undoped CdS below 20 K. The hyperfine interaction amounts only to approximately 10-4 of the vacuum value, and is shown to have axial symmetry along the Cd-S bond direction. Results suggest that the muon is close to the sulfur antibonding site and the paramagnetic electron density is distributed over a large volume. In contrast to the behavior in other semiconductors, muonium forms a shallow center in CdS. By implication, analog isolated hydrogen impurity atoms act as electrically active shallow-level defect centers in CdS.
[Show abstract][Hide abstract] ABSTRACT: We have measured the zero-field muon spin relaxation (ZF mu +SR) spectra of the quasi-one-dimensional fulleride conductor CsC60. Static magnetic order of a random nature develops gradually below about 30 K. The local field distribution at the muon site has a Gaussian shape with a width of 48.3(7) G at 2.8 K. Muon spin rotation (TF mu +SR) measurements at an applied transverse field of 6 kG also show the onset of magnetic order in the same temperature range.
[Show abstract][Hide abstract] ABSTRACT: An unusual longitudinal-magnetic-field dependence of the muon polarization for anomalous muonium (Mu[sup *]) in polycrystalline semiconductor targets is predicted, and has been observed for Si. As a function of field, the experimentally measured muon polarization at 53 K exhibits a cusp, i.e., a discontinuous jump in the slope from negative to positive, at 340(5) mT, in good agreement with theory. This occurs because the effective field on the [mu][sup +] vanishes at this external field as the angle between it and the Mu[sup *] symmetry axis approaches 90[degree]. At 200 K no cusp is seen because Mu[sup *] has been thermally annihilated. Fractional populations of the different muonium species are derived from the 53-K data. All Mu[sup *] and other anisotropic centers, including molecular radicals, should exhibit such a cusp, allowing the possibility of observing such species even in disordered or amorphous solids where spectroscopic detection might be impossible. Integral counting (and therefore very high data rates) can be used for these observations.