11 research outputs found
The contributions of components to the axial charges of proton and its resonances
We calculate the axial charges of the proton and its resonances in the
framework of the constituent quark model, which is extended to include the
components. If 20% admixtures of the components in
the proton are assumed, the theoretical value for the axial charge in our model
is in good agreement with the empirical value, which can not be well reproduced
in the traditional constituent quark model even though the symmetry breaking or relativistic effect is taken into account. We also
predict an unity axial charge for with 30%
components constrained by the strong and electromagnetic decays.Comment: 4 pages, 4 table
Band-filling effects on electron-phonon properties of normal and superconducting state
We address the effect of band filling on the effective electron mass
and the superconducting critical temperature in a electron-phonon system.
We compare the vertex corrected theory with the non-crossing approximation of
the Holstein model within a local approximation. We identify two regions of the
electron density where and are enhanced or decreased by the
inclusion of the vertex diagrams. We show that the crossover between the
enhancement at low density and the decrease towards half filling is almost
independent of the microscopic electron-phonon parameters. These different
behaviors are explained in terms of the net sign of the vertex diagrams which
is positive at low densities and negative close to half filling. Predictions of
the present theory for doped MgB, which is argued to be in the low density
regime, are discussed.Comment: 13 revtex pages, figures eps include