43 research outputs found
Anisotropic optical response of the diamond (111)-2x1 surface
The optical properties of the 21 reconstruction of the diamond (111)
surface are investigated. The electronic structure and optical properties of
the surface are studied using a microscopic tight-binding approach. We
calculate the dielectric response describing the surface region and investigate
the origin of the electronic transitions involving surface and bulk states. A
large anisotropy in the surface dielectric response appears as a consequence of
the asymmetric reconstruction on the surface plane, which gives rise to the
zigzag Pandey chains. The results are presented in terms of the reflectance
anisotropy and electron energy loss spectra. While our results are in good
agreement with available experimental data, additional experiments are proposed
in order to unambiguously determine the surface electronic structure of this
interesting surface.Comment: REVTEX manuscript with 6 postscript figures, all included in uu file.
Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.html Submitted to
Phys. Rev.
Electromagnetic form factors of the nucleon in a covariant diquark model
We present a simple covariant constituent diquark-quark model for the
nucleon. The nucleon is assumed to be composed of a scalar diquark and a quark
which interact via a quark exchange. Starting from the Bethe-Salpeter equation,
the instantaneous approximation leads to a diquark-quark Salpeter equation. In
the Mandelstam formalism, the electromagnetic form factors of the nucleon are
calculated for momentum transfers up to q^2 = - 3 \; (\mbox{GeV/c})^2. A
remarkable description of the experimental data is obtained. Especially, the
model gives nearly the right values for the proton and (negative) neutron
charge radii, and a qualitative description of the magnetic form factors.Comment: 17 pages, revtex, 8 figures in additional fil
1/m_Q Corrections to the Bethe-Salpeter Equation for \Lambda_{Q} in the Diquark Picture
Corrections of order 1/m_Q (Q=b or c) to the Bethe-Salpeter (B-S) equation
for \Lambda_Q are analyzed on the assumption that the heavy baryon \Lambda_Q is
composed of a heavy quark and a scalar, light diquark. It is found that in
addition to the one B-S scalar function in the limit m_Q -> \infty, two more
scalar functions are needed at the order 1/m_Q. These can be related to the B-S
scalar function in the leading order. The six form factors for the weak
transition \Lambda_b -> \Lambda_c are expressed in terms of these wave
functions and the results are consistent with heavy quark effective theory to
order 1/m_Q. Assuming the kernel for the B-S equation in the limit m_Q ->
\infty to consist of a scalar confinement term and a one-gluon-exchange term we
obtain numerical solutions for the B-S wave functions, and hence for the
form factors to order 1/m_Q. Predictions are given
for the differential and total decay widths for \Lambda_b -> \Lambda_c l
\bar{\nu}, and also for the nonleptonic decay widths for \Lambda_b -> \Lambda_c
plus a pseudoscalar or vector meson, with QCD corrections being also included.Comment: Latex, 24 pages, two figure
Lumbar Discectomy Outcomes Vary by Herniation Level in the Spine Patient Outcomes Research Trial
Background: The Spine Patient Outcomes Research Trial showed an overall advantage for operative compared with nonoperative treatment of lumbar disc herniations. Because a recent randomized trial showed no benefit for operative treatment of a disc at the lumbosacral junction (L5-S1), we reviewed subgroups within the Spine Patient Outcomes Research Trial to assess the effect of herniation level on outcomes of operative and nonoperative care