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991.
B. Kastelein van der Marel J. Andriessen H. Postma F. Pleiter 《Hyperfine Interactions》1992,73(3-4):357-369
The nuclear electric quadrupole interaction of111In ion-implanted in hightly oriented pyrolytic graphite has been observed by means of low-temperature nuclear orientation
and by means of perturbed angular correlations. From the first kind of experiment, it is concluded that a relatively large
number of indium nuclei experience a well-defined macroscopic orientation, which is partly lost after the radioactive decay
to cadmium. Indeed, the second kind of experiment revealed a broad distribution of electrid field gradients interacting with
the 245 keV Cd excited state, as well as a small faction experiencing a unique electric field gradient. The experimental results
are compared with theoretical calculations of the electric field gradient at various lattice positions, in which carbon and
indium electronic wavefunctions are allowed to hybridize. Lattice positions of the covalent indium atom between the graphite
layers can explain the measured electric field gradient ofV
22=+1.47(11)·1022 V/m2, directed parallel to the graphitec-axis. 相似文献
992.
It is shown how the self-consistent phonon Ansatz leads to a new class of exactly soluble models of a structural phase transition. Both nonpolynomial anharmonicity and disorder are analyzed in detail. In the classical limit, the thermodynamics is obtained and sufficient conditions on the anharmonicity are given to ensure a soft-mode phase transition. Diagonal disorder has been studied numerically. It is found that in three dimensions a pronounced mobility edge, separating localized and delocalized phonon states, may exist. 相似文献
993.
J. W. van Holten 《Zeitschrift fur Physik C Particles and Fields》1988,41(3):497-504
The classical and quantum mechanics of a free massless point-like fermion is presented. The action is invariant under local 1-dimensional reparametrizations and supersymmetry, as well as a large set of rigid symmetries, including conformal and chiral symmetries. An infinite set of rigid symmetries is derived starting from the chiral invariance, but only a finite subset does not vanish on shell. The BRST-transformations corresponding to thed=1 local symmetries are constructed and the quantization is performed. Finally, the BRST-cohomology is investigated and the conditions producing the physical states are derived. 相似文献
994.
Fast Field Echo imaging: an overview and contrast calculations 总被引:1,自引:0,他引:1
Current fast imaging techniques are based on gradient echo sequences with reduced flip angle excitation pulses and very short repetition times TR. Practical T2 values may be of the order of TR or longer. In this situation, a different image contrast can be obtained, depending on details of the sequence. Four essentially different versions of the basic Fast Field Echo (FFE) sequence can be distinguished and are described systematically in this article. For these sequences, image contrast formulas are presented. Practical imaging should tolerate small field inhomogeneities. This requirement can be satisfied by only three of the four versions. Numerical simulations are used to study the influence of a modified phase alternation scheme on image contrasts of two of the remaining sequences. The results of the calculations are verified by phantom studies on a 1.5-T whole-body imager. Implications for contrast in clinical images are discussed in relation to head images obtained on the same machine. 相似文献
995.
We analyze the free boson gas on a Cayley tree using two alternative methods. The spectrum of the lattice Laplacian on a finite tree is obtained using a direct iterative method for solving the associated characteristic equation and also using a random walk representation for the corresponding fermion lattice gas. The existence of the thermodynamic limit for the pressure of the boson lattice gas is proven and it is shown that the model exhibits boson condensation into the ground state. The random walk representation is also used to derive an expression for the Bethe approximation to the infinite-volume spectrum. This spectrum turns out to be continuous instead of a dense point spectrum, but there is still boson condensation in this approximation. 相似文献
996.
R. J. Vreeburg W. E. J. van Kooten O. L. J. Gijzeman J. W. Geus 《Surface science》1992,260(1-3):175-184
The reduction kinetics of oxidized Ni(111) surfaces are measured in situ with ellipsometry in a temperature range between 450 and 675 K. The reaction rate is proportional to the square root of the hydrogen pressure below reduction temperatures of 525 K. The rate limiting step is the reaction between chemisorbed oxygen and dissociated hydrogen and has an experimental activation energy of 57 ± 7 kJ/mol. This reaction takes only place on the oxide free part of the Ni surface. Above 600 K, the reaction rate is proportional to the hydrogen pressure. The rate limiting step is the formation of water and has no experimental activation energy. At temperatures above 600 K the distribution of oxygen throughout the Ni crystal has a large effect on the reduction curves. A new reduction model is proposed that describes all the observed curves satisfactorily. 相似文献
997.
Willem K. Klein Haneveld Leen Stougie Maarten H. van der Vlerk 《Annals of Operations Research》1996,64(1):67-81
We consider the objective function of a simple integer recourse problem with fixed technology matrix and discretely distributed right-hand sides. Exploiting the special structure of this problem, we devise an algorithm that determines the convex hull of this function efficiently. The results are improvements over those in a previous paper. In the first place, the convex hull of many objective functions in the class is covered, instead of only one-dimensional versions. In the second place, the algorithm is faster than the one in the previous paper. Moreover, some new results on the structure of the objective function are presented. 相似文献
998.
Fast and accurate calculation procedures for pH and redox potentials are required for optimum control of automatic titrations. The procedure suggested is based on a three-dimensional titration curve V = f(pH, redox potential). All possible interactions between species in the solution, e.g., changes in activity coefficients and influences of redox potential on pH variations, are taken into account. The number of titrant additions can be reduced considerably without loss of precision, by using the fact that the pH of a protolyte or mixture of protolytes at some fraction titrated does not depend strongly on the actual concentration. 相似文献
999.
P.M.A. de Bakker V. Slugen E. De Grave E. van Walle A. Fabry 《Hyperfine Interactions》1997,110(1-2):11-16
Mössbauer spectra (MS) at room temperature have been collected for non-irradiated Eastern- and Western-type nuclear reactor pressure vessel (RPV) steels. All samples showed a typical Mössbauer spectrum for steels with a low alloy-element concentration. Analysis with distributed hyperfine parameters revealed that the spectra consist of two magnetically split subspectra and that only for the Western-type RPV steels a small doublet is present. The analysis of the resulting Hhf-distribution profiles showed that for the Eastern-type steels the relative area for the ''perturbed'' component is more pronounced, and that it has a more complex structure than the corresponding profile for the Western-type steels. The additional doublet present in the MS of the Western-type steels could be assigned to Mn and/or Cr-substituted cementite, while no carbide doublet was observed for the Eastern-type RPV steel, Cr23C6, Cr7C3 and VC being the principal carbides. The distinctions between the two types of steel are due to compositional differences. The results further show that Mössbauer spectroscopy is sensitive to small changes in composition and hence is capable of distinguishing between different types of steel. 相似文献
1000.
A method is described for the preparation of N-substituted 2-(diphenylphosphinoyl) pyrrolidines. Application of these phosphine oxides in the Horner-Wittig reaction affords heterocyclic enamines and enamides in good yields. 相似文献