W. Odermatt

Measurement of the London penetration depths in YBa2Cu3Ox by means of muon spin rotation (μSR) experiments

Abstract To get accurate values of the London penetration depths in YBa2Cu3O x , μSR measurements were performed on a high quality, sintered sample and a c-axis-oriented polycrystal. For the sintered sample the temperature dependence of the effective penetration depth λeff is well described by the two-fluid model, with λeff(0) = 155(10) nm. This behavior of λeff(T) is consistent with conventional s-wave pairing. The anisotropy ratio λ c λ ab ⋍ 5(1) was determined from measurements on the polycrystal. These results were used to calculate λab(0) = 130(10) nm and λ c (0) ⋍ 500 – 800 nm .

Internal magnetic fields in the high-temperature superconductor YBa2Cu3O7−δ from muon spin rotation experiments

Muon spin rotation (μSR) measurements of the internal magnetic fields in sintered YBa2Cu3O7−δ samples are reported. Pronounced differences between field-cooled (FC) and zero-field-cooled (ZFC) μSR signals, hysteresis effects and flux trapping are observed. The μSR results are compared with the results of macroscopic experiments (susceptibility, magnetization, resistivity) and with predictions obtained from models of weakly-coupled superconducting regions.

Muonium centers in the alkali halides

Muonium centers (Mu) in single crystals and powdered alkali halides have been studied using the high-timing-resolution transverse field μSR technique. Mu has been observed and its hyperfine parameter (HF) determined inevery alkali halide. For the rocksalt alkali halides, the HF parameter Aμ shows a systematic dependence on the host lattice constant. A comparison of the Mu HF parameter with hydrogen ESR data suggests that the Mu center is the muonic analogue of the interstitial hydrogenHi0-center. The rate of Mu diffusion can be deduced from the motional narrowing of the nuclear hyperfine interaction. KBr shows two different Mu states, a low-temperatureMuI-state and a high-temperatureMuII-state.

Internal magnetic fields in the high-Tc superconductors La1.8Sr0.2CuO4 and YBa2Cu3O7−δ from muon spin rotation experiments

Abstract Measurements of the internal magnetic field in the high- T c superconductors La 1.8 Sr 0.2 CuO 4 and YBa 2 Cu 3 O 7−δ have been performed after zero-field cooling using the muon spin rotation (μSR) technique. Temperature-dependent changes are observed below T c in both the magnitude of the internal field and the fraction of muons which experience this field.

Absolute sign of the Mu* hyperfine parameters in diamond

Standard μSR experiments in diamond have shown that the relative sign of the hyperfine parameters of the anisotropic Mu* state is negative (A‖/A⊥<0). We report an experimental determination of theabsolute sign of the Mu* hyperfine parameters by studying the transferred muon polarization during the thermally-activated transition from the isotropic Mu state to Mu*. The results demonstrate that the isotropic part of the Mu* hyperfine interaction is negative. In a nitrogen-poor diamond, both the Mu disappearance rate and the enhancement of the Mu* signals are well-described by a single Arrhenius law.

Muonium states in zincblende-structured compounds

The authors present a list of results of μSR experiments in materials with the diamond and zincblende structure. Besides the muonium hyperfine parameters, additional information is tabulated: the formation probability for the different muon states, the highest temperatures at which muonium states have been observed and the types of transitions found to occur between these states. The muonium hyperfine parameters show a linear rise as a function of host ionicity from Ge to GaAs to ZnSe followed by a sharp drop to CuBr.

Muonium states in silicon carbide

Implanted muons in samples of silicon carbide have been observed to form paramagnetic muonium centers (μ+ e−). Muonium precession signals in low applied magnetic fields have been observed at 22 K in a granular sample of cubic β-SiC, however it was not possible to determine the hyperfine frequency. In a single crystal sample of hexagonal 6H-SiC, three apparently isotropic muonium states were observed at 20 K and two at 300 K, all with hyperfine frequencies intermediate between those of the isotropic muonium centers in diamond and silicon. No evidence was seen of an anisotropic muonium state analogous to the Mu* state in diamond and silicon.

Temperature dependence of muon-decay positron channeling in semiconductors

Planar channeling data ofμ+-decay positrons in various semiconductors are reported. Together with the extensive spectroscopic data supplied by transverse μSR, the location of the different states of the hydrogen pseudo-isotopeμ+ e− (muonium) can be identified by means of planar simulations. In high purity silicon as well as in gallium arsenide a thermally activated site transition is observed which can be assigned to a transition between different muonium states.

Internal magnetic fields in YBa2Cu3O7−δ from Muon spin rotation experiments: Discussion in terms of a glasslike superconducting state

Muon spin rotation (μSR) measurements of the internal magnetic fields in YBa2Cu3O7−δ are reported. Pronounced differences between field-cooled (FC) and zero-field-cooled (ZFC) μSR signals, hysteresis effects and flux trapping are observed. The μSR depolarization rate provides information regarding the local magnetic field distribution in the superconducting state. The present results show similarities to calculations that model a glasslike superconducting state.

The transition from Mu to Mu* in diamond

Evidence is presented for a transition from the isotropic muonium state (Mu) to the [111] axially symmetric anomalous muonium state (Mu*) in diamond. Amplitude measurements for Mu* in a powder in zero field and with a single crystal oriented in a magnetic field indicate that such a transition occurs with a temperature-dependent rateΛ(T) and that the electron polarization is conserved during the transition. The possibility of determining the absolute sign of the Mu* hyperfine parameters is discussed.