J. Oostens

Signature of twin planes in Mössbauer spectroscopy of cuprate superconductors

One of the four quadrupole doublets (A, B, C, and D) observed in Mössbauer spectra of orthorhombic (Y or RE)1Ba2(Cu1-xFex)307-δ(δ≈0) samples is identified with Fe dopant replacing a Cu site in twin planes. The site in question, labelled as D, is characterized by a quadrupole splitting ΔD in the range 1.6±0.1 mm/s. The D-site intensity is found to approach zero when the 1-2-3, material is driven tetragonal, either upon oxygen desorption, upon oxygen loading, or upon increasing the Fe-dopant concentration.

Superconductivity and magnetic ordering in YBa2(Cu1−x Fe x )3O7

Transverse-and zero-field μSR measurements have been made for YBa2(Cu1−xFex)3O7 withx=0.04, 0.08 and 0.12. The temperature range studied was from approximately 7.5 K to 100 K. The onset of magnetic ordering commences at about 7.5 K forx=0.04, 10 K forx=0.08 and 20 K forx=0.12. The Gaussian depolarization parameter, σ ofGx(t) = exp(−σ2t2/2), is depressed by a factor of about 0.6 forx=0.04, but for thex=0.08 sample σ is depressed by a factor of 10 and increasing suppression is seen as the temperature is lowered below 45 K. This decrease in σ is interpreted in terms of decreasing electronic mean free paths.

Magnetic ordering in Nd2-xCexCuO4-y

The n-type cuprate superconductor and closely related samples of the general form Nd2−xCexCuO4−y have been studied with the muon spin relaxation technique. Non-super-conducting samples display antiferromagnetism (AFM) withTN below 150 K, the value being dependent upon both Ce concentration and oxygen depletion. The results are interpreted in terms of carrier concentration.

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa2(Cu1−xNix)3O7−y withx=0.1 and 0.2, and YBa2(Cu1−xZnx)3O7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs−1.1.6μs−1, and 1 μs−1 forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs−1, 0.75 μs−1, 0.65 μs−1, and 0.4 μs−1 forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.

Fe Doping Effects in Aerosol Processed YBa 2 (Cu 1-x Fe x ) 3 0 7–δ Submicrocrystalline Grains

Magnetization and Mossbauer spectroscopy results on aerosol processed YBa 2 (Cu 1-x Fe x ) 3 0 7–δ samples are presented. Systematics of T c (x), x (x) and Fe dopant site occupancies I n /I(x) as a function of dopant concentration ‘x’ are deduced and compared to those established on analogous solid-state- reacted samples. The absence of twinning in the aerosol processed grains leads to a random replacement of Cu for Fe in chains and planes of the host structure.

Spin‐cluster effects in the spin glass Fe2−xTi1+xO5 as observed by transverse‐field muon‐spin relaxation (abstract)

Transverse‐field muon‐spin‐relaxation (μSR) experiments on the anisotropic spin glass Fe2−xTe1+xO5 were performed near and below the spin‐glass temperature (x=0.25; Tg=43.5 K). Earlier1 single‐crystal work has indicated that spin‐cluster effects are present in zero field slightly above the glass temperature. The effect of a transverse‐field (B⊥) upon the spin‐glass transition and state has been studied by measuring the temperature dependencies of the muon hyperfine parameters at B⊥=5 kOe, and the field dependencies at 7, 17, and 49 K. The B⊥ direction has been chosen to be perpendicular to the c axis. Spin‐cluster effects, as signaled by anomalous muon‐spin relaxation, have been observed above 44 K in 5 kOe transverse field; further, exponential muon‐spin relaxation above 60 K is observed as expected for a paramagnet. A substantial difference in the field dependence of the relaxation rate is seen between 49 K data and data taken at temperatures (7 and 17 K) well below Tg. A shift in Tg due to an applied m...

Muon Knight shift and zero-field relaxation in (U,Th)Be13

We report μ+ zero-field relaxation and Knight shift studies of the heavy-fermion superconductors U1−xThxBe13, x=0 and 0.033. The Knight shift in UBe13 shows a strong decrease as the temperature is reduced in the superconducting state, unlike U0.967Th0.033Be13 in which the shift remains at about the normal state value. If the superconducting state in UBe13 has odd-parity, the decrease of Kμ below Tc suggests that the order parameter is pinned to the lattice. Either spin-orbit scattering or the existence of two distinct superconducting states with different spin susceptibilities in the (U,Th)Be13 system would explain the differences observed in the Th-doped and pure UBe13 materials. The latter hypothesis would exclude conventional BCS superconductivity. No evidence for magnetic order is seen in the zero-field relaxation rate for either material down to 0.3 K.