H. Simmler

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.

Magnetic and electronic properties of YBa2Cu3Ox (6.5 < x < 7.0) studied by muon spin rotation

Abstract A systematic study of the local magnetic field distribution p(B) in the perovskite-type superconductor YBa2Cu3Ox is presented. Special attention is given to the second moment 〈 ΔB2 〉 of p(B) which is closely related to the superconducting carrier density ns and the effective mass m ∗ of the superconducting carriers. The influence of temperature, anisotropy and hole doping are investigated in detail. It is found that the temperature dependence of the magnetic penetration depth in YBa2Cu3Ox is in agreement with conventional s-wave pairing and suggests strong coupling. The anisotropic behavior of p(B) is well described by a theory for uniaxial type II superconductors, with an anisotropy ratio γ ∼- 5(1). Moreover, the zero-temperature values λab(0) = 130(10) nm and λc(0) ∼- 500–800 nm (penetration depth for screening currents flowing in the ab-planes and along the c-axis, respectively) were extracted from our data. μSR experiments performed on oxygen deficient samples indicate that the structural and electronic properties of YBa2Cu3Ox are strongly correlated via the oxygen content x.

μSR in oxygen deficient YBa2Cu3O x (6.5⩽x⩽7.0)

A systematic μSR study of the local magnetic field distribution in a series of oxygen deficient YBa2Cu3Ox samples with 6.5⩽x⩽7.0 is reported. Special attention was given to perform the experiments under the same conditions, so that the oxygen content of the measured samples was the only parameter varied. The behavior of the depolarization rate σ as a function of the oxygen contentx was found to have strong similarities with the behavior of the critical temperatureTc as a function ofx. In particular, two step-like increases of σ were observed abovex=6.7 and 6.9. The temperature dependence of the normalized depolarization rate σ(T)/σ(0) is well described by the two-fluid model forx⩾6.781(1) and clearly deviates from this behavior forx⩽6.704(1). Our results are compared to those obtained by other groups.

Muon-spin rotation (μSR) study of the temperature dependence of the London penetration depth in copper oxide superconductors

Abstract A μSR study of the temperature dependence of the London penetration depth λ in sintered samples of YBa 2 Cu 3 O z (with various oxygen contents x ), YBa 2 Cu 4 O 8 and Bi 2 Sr 2 CaCu 2 O 8 is presented. It is found that the temperature behavior of λ of all these cuprate superconductors is consistent with conventional s -wave pairing. However, there are significant differences concerning the exact temperature dependence of λ in these materials. In YBa 2 Cu 3 O z with high x , the behavior of λ( T ) is well described by the two-fluid model (strong coupling), whereas λ( T ) in YBa 2 Cu 3 O z with low x , YBa 2 Cu 4 O 8 and Bi 2 Sr 2 CaCu 2 O 8 is in better agreement with weak-coupling BCS theory. Possible reasons for the different temperature behavior of λ in these materials are discussed.

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.

Resolved nuclear hyperfine structure of muonium in CuCl by means of muon level-crossing resonance

Datailed muon level-crossing resonance measurements of Mu1 and Mu11 centres in single crystals of CuCl are presented. The hyperfine and nuclear hyperfine parameters of the closest two shells of nuclei are remarkably similar for the two centres, indicating that both are located at the same tetrahedral interstitial site with four Cu nearest neighbours and six Cl next-nearest neighbours. About 30% of the total unpaired-electron spin density is located on the muon, about 60% on the four nearest neighbours and the rest on the six next-nearest neighbours, with nothing observable for any other shell.

Resolved nuclear hyperfine structure of muonium centres in CuCl and GaAs by means of the avoided-level-crossing technique

Abstract Avoided-level-crossing resonances from isotropic muonium centres interacting with neighbouring nuclear spins in powdered CuCl are reported. The prominent resonances have a complex multiline structure and are strongly temperature-dependent. In addition, previously unobserved resonances in single-crystal GaAs from anomalous muonium interacting with a 71 Ga neighbour are presented.

Muon-decay positron channeling in semiconductors

Abstract The lattice positions of implanted positive muons (μ+) in intrinsic semiconductors (Si, GaAs, InP) have been investigated by μ-decay positron channeling at temperatures ranging from 95 K to 400 K. The positrons exhibit planar steering effects with a maximum amplitude of approximately 5% and a width of the order of 0.1°. In high purity float-zone (FZ) Si a metastable p. site is observed: Below 200 K, the pattern is consistent with a fraction of 40% near a BC (bond-center) site and 60% near a T (tetrahedral) site. Above 200 K, the T-like fraction undergoes a transition to the BC-like site, where virtually all muons are located above 300 K. By comparison with muon-spin-rotation (u.sr) measurements, these sites can be associated with the known paramagnetic muonium (μ+ e −) states observed in numerous semiconductors: the metastable site corresponds to the isotropic state (Mu), the BC-like configuration is the stable site for both the anisotropic state (Mu∗) at low temperatures as well as the final ionized state (“μ + )” at higher temperatures. In GaAs, there is evidence for a similar metastability. In InP, a near-BC configuration is observed throughout the temperature range investigated. There is no indication of a metastable site. Thus a BC-like configuration is found to be most stable in all measurements.