D.Ll. Williams

High temperature X-ray diffraction studies of the compound YBa2Cu3Ox

Ambient and high temperature X-ray diffraction methods were used to determine the unit cell parameters of the orthorhombic and tetragonal phases of the YBa2Cu3Ox compound, as a function of oxygen stoichiometry and temperature. The total oxygen content was obtained by the thermogravimetric method (TGA) by determining the weight loss in hydrogen up to 1000 degrees C. Reduction reactions in hydrogen were also performed in situ using a high temperature X-ray unit. Using the TGA method the oxygen content in the structure was obtained by weight loss in 100% oxygen. Thermal expansion coefficients of the lattice parameters were determined for both orthorhombic and tetragonal materials with different, constant x values.

Influence of oxygen ordering and sintering temperatures on the superconducting transition temperature of YBa2Cu3Ox compound at fixed oxygen content x higher than x=6.8

Abstract The transition temperatures from the normal to the superconducting state (Tc) of the high Tc superconducting compound YBa2Cu3Ox, have been measured by means of the four probe method. The oxygen content x has been determined with a thermogravimetric technique. The lattice parameters a, b, and c have been obtained on the basis of the x-ray powder diffraction spectra. At fixed x higher than 6.8, for samples obtained by the different final heat treatment, differences in Tc and in the unit cell parameters are observed. These differences indicate a dependence of Tc on ordering of the oxygen atoms and oxygen vacancies in the Cu-O basal plane and on sintering temperatures (Ts) at x=const..

Temperature dependence of the magnetic penetration depth in YBa2Cu3O7 measured by muon spin rotation

We report an improved measurement of the temperature dependence of the magnetic penetration depth in sintered YBa2Cu3O77 using muon spin rotation. The effective isotropic average of the magnetic penetration depth (λ) is estimated from the second moment of the field distribution in the vortex state in an external field of 15kG. The low temperature behaviour of λ is still consistent with an s-wave pairing mechanism although there are deviations from the conventional BCS-like behaviour for T/Tc > 0.2.

Single crystal studies of electric field gradients in dilute alloys. II. Copper based crystals

For Pt. I see abstr. A1732 of 1972. The electric field gradients at nearest and in some cases next nearest host Cu nuclei to various impurities in dilute Cu alloy single crystals have been determined by the quadrupole effects in nuclear magnetic resonance. The impurities are In, Sn, Sb and Ge. The field gradients at nearest neighbour sites are far from rotational symmetric around the direction towards the impurity and in the case of CuGe it does not even have the main component in that direction. These results question the validity of the theoretical models assuming spherical symmetry for the charge density around the impurity, at least in host metals like copper.

A point detector positron annihilation study of Cu85Zn15

A high resolution point detector geometry has been applied to the study of the angular correlation spectra from positron annihilation in Cu85Zn15 single crystals. Strong anisotropies are observed and some features are interpreted in terms of the alloy Fermi surface. A value for the [111] neck diameter is obtained which is 50% greater than that for pure copper. Considerable smearing of the expected slope discontinuity in the neighbourhood of the Fermi momentum is interpreted as evidence for Sterns theory of charging. A large anisotropy in the higher momentum contribution to the annihilation is also observed.

Final states in Si and GaAs via RF μSR spectroscopy

The ionization of muonium centers in Si and GaAs have been studied using radio frequency (RF) resonant techniques. In Si all three muonic centers are detectable by RF. No evidence was found for delayed Mu and Mu* states at any temperature. However, our results on the diamagnetic final state (μf+) show that it is composed of prompt fractions (as seen by conventional μSR) and delayed fractions arising from the ionization of Mu* and Mu. We observe a full μf+ fraction at 317 K when the Mu relaxation rate is above 10 μs−1. GaAs differs from the situation in Si in that we observed only a partial conversion of Mu* and Mu to a μ+ final state up to 310 K in spite of the fact that the transverse field relaxation rates become very high at 150 and 250 K respectively.

Magnetic ordering in YBa2Cu3Ox studied by μ+ spin rotation☆

YBa2Cu3Ox is known to be antiferromagnetic (AFM) for x ≲ 6.45 but the details of its sublattice magnetization(s) are still controversial. A zero-field μ+SR investigation of the x- and T-dependence of the local fields at several muon sites reveals sharp changes at 10K and 80K for an oriented powder sample with x ≈ 6.0, but other x ≈ 6.0 samples do not show the same features. The direction of the local field at one μ+ site is shown to be along the ĉ-axis. For x < 6.0 the samples still show local spin freezing up to 300K, but coherent AFM order is lost even at 3K.

Giant muon knight shifts in antimony and antimony alloys

Abstract The Knight shift Kμ at the positive muon has been measured as a function of magnetic field, temperature, crystal orientation and alloyed impurity (bismuth or tin) in antimony. The anomalously large and anisotropic Kμ in pure Sb at low temperature has been confirmed and shown to be independent of magnetic field up to 9 kG; its anisotropic part is found to have the same strong temperature dependence as its isotropic component. The addition of 6.3 at.% Bi significantly reduces both Kμ and its anisotropy, but enhances their temperature dependence. The addition of 12.5 at.% Bi, or, more dramatically, as little as 0.3 at.% Sn to antimony is sufficient to reduce Kμ to a small value, effectively eliminating the anomalous behaviour.

POSITIVE MUONS IN ANTIMONY BISMUTH ALLOYS

StroboscopicμSR and TD-μSR techniques were used to measure theμ+ Knight shiftKμ, and relaxation rateλ inSbBi alloys as functions of magnetic fieldH, temperatureT, the angleθ betweenH and the crystallineĉ axis, and the concentration [Bi] of alloyed Bi. In pure Sb and inSbBi (6.5%),Kμ (θ=0) andKμ (θ=π/2) both decrease linearly withT up to about 100 K, but bothKμ and its anisotropy are smaller in the 6.5% alloy, indicating a “dilution” effect. With 15 at % Bi,Kμ is reduced further but itsT-dependence and that ofλ are dramatically altered. At low temperaturesKμ (θ=0) inSbBi(15%) actually becomes negative and the sign of the anisotropy is reversed. In the same sample,λ is proportional toH at both 20 K and 150 K; at 120 Kλ is proportional toKμ ifθ is used as an implicit variable, but at 36 K this is not the case. A consistent phenomenological description is offered.

Coexistence of superconductivity and magnetic order in YBa2Cu3Ox

Abstract Resistivity, susceptibility and μ + SR measurements have been used to investigate the ( x , T ) phase diagram of YBa 2 Cu 3 O x . Carefully annealed samples are antiferromagnetic (AFM) insulators for 6.0 ≲ x ≲ 6.35 and nonmagnetic superconductors (SC) for 6.5 ≲ x ≲ 7.0; SC coexists with AFM or spin glass (SG) static magnetic order at low temperatures for 6.38 ≲ x ≲ 6.48. A sample with x = 6.462 is SC below 25 K and simultaneously magnetic below 10 K; the internal diamagnetism of the SC state is progressively reduced as local spin freezing sets in at low temperatures.