462 research outputs found
Binding of holons and spinons in the one-dimensional anisotropic t-J model
We study the binding of a holon and a spinon in the one-dimensional
anisotropic t-J model using a Bethe-Salpeter equation approach, exact
diagonalization, and density matrix renormalization group methods on chains of
up to 128 sites. We find that holon-spinon binding changes dramatically as a
function of anisotropy parameter \alpha=J_\perp/J_z: it evolves from an exactly
deducible impurity-like result in the Ising limit to an exponentially shallow
bound state near the isotropic case. A remarkable agreement between the theory
and numerical results suggests that such a change is controlled by the
corresponding evolution of the spinon energy spectrum.Comment: 4 pages, 5 figures, published versio
Spinon-holon interactions in an anisotropic t-J chain: a comprehensive study
We consider a generalization of the one-dimensional t-J model with
anisotropic spin-spin interactions. We show that the anisotropy leads to an
effective attractive interaction between the spinon and holon excitations,
resulting in a localized bound state. Detailed quantitative analytic
predictions for the dependence of the binding energy on the anisotropy are
presented, and verified by precise numerical simulations. The binding energy is
found to interpolate smoothly between a finite value in the t-Jz limit and zero
in the isotropic limit, going to zero exponentially in the vicinity of the
latter. We identify changes in spinon dispersion as the primary factor for this
non-trivial behavior.Comment: 12 pages, 13 figures, long story. The short story is
cond-mat/0702213. Published versio
Casimir Densities for a Massive Fermionic Quantum Field in a Global Monopole Background with Spherical Boundary
We investigate the vacuum expectation value of the energy-momentum tensor
associated with a massive fermionic field obeying the MIT bag boundary
condition on a spherical shell in the global monopole spacetime. The asymptotic
behavior of the vacuum densities is investigated near the sphere center and
surface, and at large distances from the sphere. In the limit of strong
gravitational field corresponding to small values of the parameter describing
the solid angle deficit in global monopole geometry, the sphere-induced
expectation values are exponentially suppressed.Comment: 8 pages, 4 figures, 6th Alexander Friedmann International Seminar on
Gravitation and Cosmolog
Pion Photoproduction Amplitude Relations in the 1/N_c Expansion
We derive expressions for pion photoproduction amplitudes in the 1/N_c
expansion of QCD, and obtain linear relations directly from this expansion that
relate electromagnetic multipole amplitudes at all energies. The leading-order
relations in 1/N_c compare favorably with available data, while the
next-to-leading order relations seem to provide only a small improvement.
However, when resonance parameters are compared directly, the agreement at
O(1/N_c) or O(1/N_c^2) is impressive.Comment: 19 pages, ReVTeX, 50 eps files combine into 5 compound figure
On aspects of self-consistency in the Dyson-Schwinger approach to QED and \lambda (\phi^\star \phi)^2 theories
We investigate some aspects of the self-consistency in the Dyson-Schwinger
approach to both the QED and the self-interacting scalar field theories. We
prove that the set of the Dyson-Schwinger equations, together with the
Green-Ward-Takahashi identity, is equivalent to the analogous set of integral
equations studied in condensed matter, namely many-body perturbation theory,
where it is solved self-consistently and iteratively. In this framework, we
compute the non-perturbative solution of the gap equation for the
self-interacting scalar field theory.Comment: 9 pages, to appear on Phys. Rev.
Proton spin polarizabilities from polarized Compton scattering
Polarized Compton scattering off the proton is studied within the framework
of subtracted dispersion relations for photon energies up to 300 MeV. As a
guideline for forthcoming experiments, we focus the attention on the role of
the proton's spin polarizabilities and investigate the most favorable
conditions to extract them with a minimum of model dependence. We conclude that
a complete separation of the four spin polarizabilities is possible, at photon
energies between threshold and the region, provided one can
achieve polarization measurements with an accuracy of a few percent.Comment: 26 pages, 7 figures
Lamb shift in muonic deuterium atom
We present new investigation of the Lamb shift (2P_{1/2}-2S_{1/2}) in muonic
deuterium (mu d) atom using the three-dimensional quasipotential method in
quantum electrodynamics. The vacuum polarization, nuclear structure and recoil
effects are calculated with the account of contributions of orders alpha^3,
alpha^4, alpha^5 and alpha^6. The results are compared with earlier performed
calculations. The obtained numerical value of the Lamb shift 202.4139 meV can
be considered as a reliable estimate for the comparison with forthcoming
experimental data.Comment: 24 pages, 11 figures. arXiv admin note: text overlap with
arXiv:hep-ph/061229
Vacuum polarization radiative correction to the parity violating electron scattering on heavy nuclei
The effect of vacuum polarization on the parity violating asymmetry in the
elastic electron-nucleus scattering is considered. Calculations are performed
in the high-energy approximation with an exact account for the electric field
of the nucleus. It is shown that the radiative correction to the parity
violating asymmetry is logarithmically enhanced and the value of the correction
is about -1%.Comment: 6 pages, 3 figures, REVTex
A teleparallel model for the neutrino
The main result of the paper is a new representation for the Weyl Lagrangian
(massless Dirac Lagrangian). As the dynamical variable we use the coframe, i.e.
an orthonormal tetrad of covector fields. We write down a simple Lagrangian -
wedge product of axial torsion with a lightlike element of the coframe - and
show that variation of the resulting action with respect to the coframe
produces the Weyl equation. The advantage of our approach is that it does not
require the use of spinors, Pauli matrices or covariant differentiation. The
only geometric concepts we use are those of a metric, differential form, wedge
product and exterior derivative. Our result assigns a variational meaning to
the tetrad representation of the Weyl equation suggested by J.B.Griffiths and
R.A.Newing.Comment: 4 pages, REVTe
Quantum Mechanical Properties of Bessel Beams
Bessel beams are studied within the general framework of quantum optics. The
two modes of the electromagnetic field are quantized and the basic dynamical
operators are identified. The algebra of these operators is analyzed in detail;
it is shown that the operators that are usually associated to linear momentum,
orbital angular momentum and spin do not satisfy the algebra of the translation
and rotation group. In particular, what seems to be the spin is more similar to
the helicity. Some physical consequences of these results are examined.Comment: 17 pages, no figures. New versio
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