Search Results - (Author, Cooperation:W. F. Lankford)

2015-05-23

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

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1572-9540

Springer Online Journal Archives 1860-2000

Physics

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract Transverse and zero-field μSR measurements were made on YBa2(Cu1−xNix)3O7−y withx=0.1 and 0.2, and YBa2(Cu1−x Zn x )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.

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract Transverse-and zero-field μSR measurements have been made for YBa2(Cu1−x Fe x )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 ofG x (t) = exp(−σ2 t 2/2), 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.

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract Precession of polarized positive muons is used to probe internal magnetic fields in a polycrystalline sample of high purity chromium. A depolarization, or damping in time, of the precession oscillation is observed in the antiferromagnetic states between 77 K and 312 K. The results are used to obtain the temperature dependence of the muon diffusion rates, with the assumption that the local fields are of dipolar origin.

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract The depolarization rate for spin polarized μ+ particles implanted into a high purity niobium crystal was studied as a function of temperature. The results were analyzed in terms of nuclear dipolar field inhomogeneity due to the host93Nb, and the local field averaging effect of the muon's motion. An analysis is presented in terms of the structure of the muon wavefunction and parameters characterizing the muon diffusion.

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract The n-type cuprate superconductor and closely related samples of the general form Nd2−x Ce x CuO4−y have been studied with the muon spin relaxation technique. Non-super-conducting samples display antiferromagnetism (AFM) withT N below 150 K, the value being dependent upon both Ce concentration and oxygen depletion. The results are interpreted in terms of carrier concentration.

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract Positive muon spin precession has been observed in various heavy-fermion systems in the transverse external magnetic field. In the superconductor CeCu2.1Si2, the relaxation rate of muon spins increases rapidly with decreasing temperature below TC. This is interpreted as the results of the inhomogeneous fields due to the imperfect penetration of the external field into the type-II superconducting state. The magnetic-field penetration depth λ is derived from the observed muon spin relaxation rate. λ is about 1200 ∢ at T∼0.5TC, and the temperature dependence of λ is consistent with the relation expected for a BCS superconductor. We have also measured the muon Knight shift Kμ in the normal (or paramagnetic) state of various heavy-fermion systems. Kμ is large and negative (about −1000∼−3000 ppm at T=10 K) for CeCu2Si2, UPt3 and CeAl3, while more complicated signals are measured in CePb3 and CeB6. The negative muon Knight shift in the non-magnetic heavy-fermion systems is discussed in terms of the Kondo-coupling between the conduction- and f-electrons.

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract We have studied Al(Cu), Al(Mg) and Al(Ag) alloys to determine the μ+ trapping sites, and how impurity distortions affect these sites. Cu (Monovalent) contracts the Al lattice, Mg (divalent) expands it, and Ag (monovalent) has almost no effect. Fitting to a two-state model we obtained a temperature exponent β of 0.93 (26 for zero field and 1.16(14) for a longitudinal, field where β=1 implies a one-phonon induced diffusion process. The measured secon moment in Al(Mg) indicates a changing trapping site, which was confirmed by zero and longitudinal field studies. Comparing the Al(Mg) results with earlier Al(Cu) data, two types of sites can be identified: those which are distant from the defect and depend on the magnitude but not the sign of the deformation, and those close to Mg. Ag produces no lattice distortion, and its weaker depolarization suggests a different trapping mechanism.

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract Low temperature sites for muons implanted in TiHx have been found to be a mixture of interstitial and substitutional sites, with substitutional occupancy determined by the probability that a muon in an interstitial site will have a vacant nearest neighbor substitutional site. As with ZrHx, activation from the interstitial site is observed below 300 K. From the depolarization rate in the substitutional site, the muon likely displaces the neighboring H atoms by about 0.1 A. Diffusion for the substitutional muons occurs above room temperature with an activation of about 0.38 eV, which is less than the 0.505 eV for hydrogen vacancy motion observed by NMR. To explain this the muon transition rate to a vacancy must be less than that of hydrogen.

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract We have studied the muon diffusion in various Nb−HX systems with 0.75〈x〈0.95, with special attention to the concentrations x〈0.9. For x〉0.9 the muon linewidth as function of temperature has a smooth behaviour and the muon mobility is strongly correlated to the hydrogen diffusion in the beta phase. The activation energy for the μ+ diffusion is 160 meV, which is lower than that for protons. At hydrogen concentrations below 0.9, the muon diffusion behaviour is more complicated, and the influence of Nb−H phase transitions is evident. The implications for the local environment of the muon are discussed.

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