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121.
Sum rules derived from isospin current algebra are examined in perturbation theory for nucleons interacting with neutral pseudoscalar mesons and isovector pions. It is shown that the well-known sum rules for electromagnetic isovector form factors of nucleons and pions are correct in zero and second order of the meson-nucleon coupling constant. 相似文献
122.
Stating a concrete example, it is shown that the vacuum polarization develops anomalous singularities. The analytic structure of the vacuum polarization in these circumstances is discussed in some detail. It appears that its large q2 behaviour is not linked anymore to the asymptotic form of the e+e? total cross section. Since it is the large q2 behaviour of the vacuum polarization which solely can be calculated in asymptotically free gauge theories (giving in leading order the naive quark model result), this is a warning to using the naive quark model result for the e+e? total cross section as well in fitting the data. 相似文献
123.
Lorenz Kramer 《Zeitschrift für Physik A Hadrons and Nuclei》1973,258(5):367-380
The interaction between well-separated Abrikosov vortices is calculated using Eilenberger's version of the Gorkov theory. Let λ and ξ be the decay lengths determining the asymptotic behavior of the magnetic field and the order parameter for an isolated vortex. Then the onset of attraction in the limit of large vortex separation is either given by λ=ξ or by λ becoming complex, whichever occurs first. It is shown that in general a first order transition atH c1 takes place already in the region of asymptotic repulsion because of the appearance of a minimum in the interaction energy at finite vortex separation. 相似文献
124.
The kinetics of craze growth from sharp cracks in polystyrene (PS) and poly(methyl methacrylate) (PMMA) in contact with liquid methanol were measured with time-lapse photography as a function of the stress intensity factor KI. At high KI the craze length in both systems increases as √t if the sides of the craze are protected from methanol and as t if they are not, where t is the elapsed time after loading. If such a side-protected craze is dried under load and then methanol is reintroduced to the crack tip, the methanol front advances with the same kinetics as the original craze growth. This experiment Proves that solvent crazing velocities are limited by the hydrodynamic transport of solvent through the porous craze structure under a capillary pressure driving force (which can be as high as 100 atm). An improved model of fluid flow through the craze is developed and shown to predict craze growth kinetics in good agreement with those observed. The hydraulic permeability of methanol crazes in PS was found to be independent of craze length at small craze length and to be independent of KI except at very low KI. Although increases in molecular weight in the range Mw = 200,000 to Mw = 670,000 do not markedly affect the crazing kinetics, they greatly increase the time to fracture of the craze. 相似文献
125.
Thin films of ten glassy polymers are bonded to copper grids and strained in tension to produce crazes, which are then examined in the transmission electron microscope. The average craze fibril extension ratio λ for each polymer is determined from microdensitometer measurements of the mass thickness contrast of the crazes. The extension ratio λ is found to increase approximately linearly with the chain contour length le between entanglements, as determined from melt elasticity measurements of the entanglement molecular weight of these polymers. These results are analyzed by comparing them with λmax, the maximum extension ratio of an entanglement network in which polymer chains neither break nor reptate (i.e., permanent entanglement crosslinks are assumed). The values of λmax are given by le/d where d, the entanglement mesh spacing in the unoriented glass, is computed from d = k(Me)1/2 with k determined either from small-angle neutron scattering results on isolated chains in the glass or from coil size measurements in dilute solutions of a θ solvent. The craze extension ratios fall somewhat below λmax at low λ but increase to well above λmax for polymers with high le. This comparison suggests a significant contribution due to chain breakage (or reptation) in the higher-λ crazes of large-le polymers, which may arise from the higher true stresses in the craze fibrils (which for a given applied stress increase proportionally to λ). The results also imply that a useful way to increase the “brittle” fracture stress and decrease the ductile-to-brittle transition temperature of a glassy polymer is to decrease its entanglement contour length le. 相似文献
126.
P. Kramer 《Zeitschrift für Physik A Hadrons and Nuclei》1968,216(1):68-83
Recoupling coefficients of the symmetric group are defined from the reduction of irreducible representations of this group to direct product representations of particular subgroups. These recoupling coefficients are analysed and shown to be identical to the invariants of unitary groups arising from the reduction of Kronecker product representations. The recoupling coefficients have applications in configurations of several shells or clusters. 相似文献
127.
Brown has shown that low-angle electron diffraction (LAED) may be used to determine fibril diameters D and spacings D0 of crazes in thin polymer films. He found, however, that the D and D0 determined for air crazes in polystyrene (PS) thin films were larger by about a factor of 3 than those in PS bulk crazes determined by using small-angle x-ray scattering (SAXS). We have repeated Brown's LAED experiments and find that the discrepancy may be caused by an aging effect. Our fresh crazes have D and D0 values from LAED that are comparable to those of bulk PS crazes determined by SAXS. As the craze ages, however, fibrils retract and coalesce in wide regions of the craze, leading eventually to an observable “skin.” Aged crazes thus have much larger D and D0 values than do fresh crazes. The large molecular mobility of the PS molecules in the fibrils necessary for this aging to occur at room temperature has important implications for fibril failure. 相似文献
128.
Michael Klasen Gustav Kramer 《The European Physical Journal C - Particles and Fields》2010,70(1-2):91-106
Now that new publications of H1 data for the diffractive photoproduction of dijets, which overlap with the earlier published H1 data and the recently published data of the ZEUS collaboration, have appeared, we have recalculated the cross sections for this process in next-to-leading order (NLO) of perturbative QCD to see whether they can be interpreted consistently. The results of these calculations are compared to the data of both collaborations. We find that the NLO cross sections disagree with the data, showing that factorization breaking occurs at that order. If direct and resolved contributions are both suppressed by the same amount, the global suppression factor depends on the transverse-energy cut. However, by suppressing only the resolved contribution, also reasonably good agreement with all the data is found with a suppression factor independent of the transverse-energy cut. 相似文献
129.
In?situ x-ray diffraction (XRD) coupled with molecular dynamics (MD) simulations have been used to quantify antisite defect trapping during crystallization. Rietveld refinement of the XRD data revealed a marked lattice distortion which involves an a axis expansion and a c axis contraction of the stable C11b phase. The observed lattice response is proportional in magnitude to the growth rate, suggesting that the behavior is associated with the kinetic trapping of lattice defects. MD simulations demonstrate that this lattice response is due to incorporation of 1% to 2% antisite defects during growth. 相似文献
130.