Positive muon Knight shift in graphite and Grafoil

Positive muon Knight shifts and relaxation rates were measured at room temperature in a graphite crystal and in a stack of Grafoil sheets. The Knight shift was 500 ppm in the single crystal and reduced by 0.702 in Grafoil. Both have the same (large) fractional anisotropy relative to the axis or to the normal to the Grafoil sheets, respectively. The (isotropic) relaxation rates were 0.024(4)s^–1 in the crystal and 0.194(6) ^–1 in the Grafoil. Apparently the ⁺ in Grafoil sees highly aligned bulk crystalline graphite, and does not reach the surfaces of the sheets.We are grateful to Greg Dash, the owner of the graphite crystal, for lending it to Tony Arrott; and to Tony for lending it in turn to us.     

α-Chlor-nitrone III: Ein Weg zur indirekten «carboxolytischen» Spaltung von Olefin-Doppelbindungen. Über synthetische Methoden, 7. (vorläufige) Mitteilung

The enaminoid derivatives formed by deprotonation of the olefin-α-chloro-nitrone cycloadducts (see preceding communications) undergo a beautifully clean cycloreversion at ambient or slightly elevated temperatures. The sequence (2+4)-cycloaddition → deprotonation → (2′+4)-cycloreversion constitutes a potentially useful method for the oxidative cleavage of olefinic double bonds with concomitant extension of the carbon chain at one of the double bond termini (indirect «carboxolytic» cleavage of double bonds; see schemes 1 and 3).     

Time resolved fluorescence spectroscopy

Time correlated single photon counting was used to investigate solvent dynamics of laser dyes in a polar, viscous solvent. The resolved total luminescence spectra of oxazine 4 in 2-methyltetrahydrofuran (2-MTHF) have been measured and solvent relaxation observed.     

Observation of precipitates in a supersaturated Fe?Co alloy by muon spin rotation spectroscopy: a step towards understanding embrittlement of reactor pressure vessel steels

The technique of μ⁺SR spectroscopy was applied to detect precipitate formation due to ageing in the binary Fe-0.8 wt% Cu alloy. As compared to the homogenized specimen, the aged sample gives a μ⁺SR signal with considerably larger relaxation rate in the temperature interval 180 K < T < 350 K, having a maximum near T = 250 K with Λ = 14 μs⁻¹. Since precipitation of copper plays possibly an important role in the embrittlement of neutron irradiated ferritic steels, the signal from an irradiated steel specimen has also been investigated.     

Spin dynamics in CuGeO3 studied by muon spin rotation

We report a muon spin rotation (SR) study of the magnetic properties of the Cu²⁺ quasi-one-dimensional CuGeO₃ system and its lightly-doped derivative Cu_0.97Zn_0.03GeO₃. Susceptibility measurements on CuGeO₃ show a sudden change in the vicinity of 14 K that has been interpreted before as a magnetic transition to a spin-Peierls state. SR shows no evidence of spin freezing below 14 K, implying that the transition is to a magnetic state with no static (random or ordered) electronic moments. A modest slowing down of the electronic spin dynamics is also identified at this temperature. Similarly, no evidence of a transition to a static magnetic state is found for Cu_0.97Zn_0.03GeO₃ whose susceptibility shows hysteretic behaviour between zero-field and field cooled measurements at 4 K, previously ascribed to spinglass-like behaviour. Given the nature of the muon spin as a local magnetic probe, the present results necessitate a re-interpretation of the origin of the susceptibility anomaly observed in the doped system.     

A μ+SR study of the magnetic properties of ferritin

We present a study of the magnetic properties of the superparamagnetic ferritin system by employing zero‐field (ZF) and longitudinal‐field (LF) μ⁺SR measurements. Two μ⁺ fractions with different depolarisation behaviour are detected, one arising from muons stopped in the organic shell and one from muons stopped in the mineral core. The freezing of the superparamagnetic moments is evident in the temperature evolution of the ZF relaxation rates of both fractions. It occurs gradually below \approx 60\ K and is essentially complete at \approx 11\ K. The results are consistent with a distribution of blocking temperatures which in turn reflects the distribution of core sizes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Polarographie an Festelektroden V. Adsorptionskonkurrenz in binären metallischen Monoschichten an Festelektroden

The simultaneous reduction of binary mixtures of the depolarizers Tl⁺, Pb²⁺, and Bi³⁺ from aqueous solutions on the surface of Ag- and Cu-electrodes is studied chronoamperometrically in view of the behaviour of the metallic monolayer, using a chamber type cell. The current balance of the electrode reaction is verified by coulometrical evaluation of the chronoamperograms.     

Spin‐freezing in superconducting La2CuO4.03 single crystal

Electron spin‐freezing at T_f=8 K has been detected in superconducting (T_c=12\ K) single crystal La₂CuO_4+y (y\simeq0.03) by ZF‐μSR. According to diffraction data, the crystal is in Bmab phase without any traces of structural phase separation. TF‐μSR experiment has shown that no Abrikosov flux line lattice is formed below T_c. The data allow us to assume that the magnetic and superconducting regions in the crystal are space separated on the microscopic scales ~ 10² Å. The presence of large field induced broadening of the Knight shift distribution k_\sigma > 1000 ppm indicates that the crystal contains micro‐regions possessing enhanced magnetic susceptibility.     

Anisotropic μ+ shift in bismuthshift in bismuth

μSR studies in the semi metal Bi have revealed pronounced anisotropies in the μ⁺ Knightshift which vary strongly with temperature. Above 10 K the anisotropy is well described by a P₂(cosθ) distribution with a change of sign around 60 K. Below 10 K we find a very unusual angular dependence in that the anisotropy is described only by terms of fourth order in the direction cosines. Such a dependence is allowed by symmetry arguments, considering that the pointgroup of the crystalline μ⁺ site is D_3d, but requires that the electrons, responsible for the μ⁺ Knight shift, are subject to strong spin orbit coupling or involve effective, field induced moments of orbital origin. The various types of anisotropies seem to be associated with the different site occupations of the μ⁺. The total Knightshift is generally regative which is to be contrasted with the huge positive Knightshift found in the other semimetal Sb /1/.