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A Note on Methods for the Transformation of 1,2-Dicarboxylic Acids into α, β-Unsaturated Monocarboxylic Acids. A model study using the Diels-Alder adduct of maleic anhydride and butadiene (see scheme 3) exemplifies the conversion of the succinic acid type of structure into α, β-unsaturated carboxylic acids via two complementary fragmentations of 3,6-dioxo-perhydro-1,2-oxazine derivatives. 相似文献
35.
W. Studer F. N. Gygax A. Hintermann W. Rüegg A. Schenck A. J. van der Wal 《Hyperfine Interactions》1984,17(1-4):359-363
During the past much effort has been devoted to a systematic study of the muon Knight shiftK
in metallic environments and its implications on the local electronic structure of hydrogen in metals [1]. These measurements in simple metals were essentially all carried out in polycrystalline samples at room temperature. The present measurements in Cd in polycrystalline and single crystal samples cover a temperature range between 20 K and the melting point of this strongly anisotropic metal (hcp crystal structure,c/a ratio 1.89 — idealc/a ratio 1.63). These measurements add qualitatively new and interesting aspects and insights on the screening of a light hydrogen isotope in a metal as well as on certain properties of the host material itself. The outstanding features of the muon Knight shift in Cd are: (i) a strong intrinsic temperature dependence with an increase ofK
of more than 100% between 20 K and the melting point (T=593 K), (ii) an anomaly at 110 K in the form of a singularity in the isotropic part ofK
which is interpreted as a band structure effect, (iii) an anisotropic Knight shift contribution fitting the expressionK(T,)=K
iso(T)+K
ax(T) * (3 · cos2
–1)/2, where both, the isotropic and the axial contribution ofK
, are strongly temperature dependent. 相似文献
36.
F. N. Gygax E. Lippelt A. Schenck A. J. van der Wal S. Barth 《Hyperfine Interactions》1986,31(1-4):447-453
In continuation of our earlier studies of the anomalous temperature dependence of the μ+ Knight shift Kμ in Cd,CdHg (1.2 at %) andCdMg (3.38 at %) — interpreted as due to Van Hove-type singularities in the local density of electron states /1/ — we have studied
the temperature dependence of the μ+ Knight shift in polycrystallineCdMg (1.04 at %, 2.05 at %) and in a single crystal ofCdMg (2.5 at %). In contrast to pure Cd no anisotropies in Kμ could be detected. The temperature dependence of Kμ in aCdMg (2.05 at %) sample and in the monocrystallineCdMg (2.5 at %) sample essentially reproduces the one previously observed in polycrystallineCdMg (3.38 at %), showing a steplike discontinuity and a logarithmic singularity. A complete different behaviour is observed
inCdMg (1.04 at %), where no logarithmic singularity seems to show up and where a steplike discontinuity of opposite sign at around
90 K is clearly seen. If these singularities are still to be interpreted in terms of Van Hove singularities, the question
arises why there is such a nonlinear dependence on the Mg concentration. 相似文献
37.
M. Weber P. Birrer F. N. Gygax B. Hitti E. Lippelt H. Maletta A. Schenck 《Hyperfine Interactions》1991,63(1-4):93-101
The muon spin relaxation was measured in the high-T
c superconductors Bi2Sr1.3Ca0.7 CuO6.15, Bi2Sr2CaCu2O8 and Bi2Pb
x
Sr2Ca2Cu3O10. We present our method to determine the London penetration depth perpendicular toc-axis λ⊥ (0) taking also contributions from the anisotropic magnetization and nuclear dipoles to the muon frequency spectrum into
account. 相似文献
38.
We study Poincaré recurrence of chaotic attractors for regions of finite size. Contrary to the standard case, where the size of the recurrent regions tends to zero, the measure is no longer supported solely by unstable periodic orbits of finite length inside it, but also by other special recurrent trajectories, located outside that region. The presence of the latter leads to a deviation of the distribution of the Poincaré first return times from a Poissonian. Consequently, by taking into account the contribution of these special recurrent trajectories, a corrected estimate of the measure is obtained. This has wide experimental implications, as in the laboratory all returns can exclusively be observed for regions of finite size, and only unstable periodic orbits of finite length can be detected. 相似文献
39.
de Visser A Graf MJ Estrela P Amato A Baines C Andreica D Gygax FN Schenck A 《Physical review letters》2000,85(14):3005-3008
Transverse-field muon spin relaxation measurements have been carried out on the heavy-fermion superconductor UPt (3) doped with small amounts of Pd. We find that the critical Pd concentration for the emergence of the large-moment antiferromagnetic phase is approximately 0.6 at. %Pd. At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity, provides evidence for ferromagnetic spin-fluctuation mediated odd-parity superconductivity, which competes with antiferromagnetic order. 相似文献
40.