11,996 research outputs found
Constraints on Dark Energy Models Including Gamma Ray Bursts
In this paper we analyze the constraints on the property of dark energy from
cosmological observations. Together with SNe Ia Gold sample, WMAP, SDSS and
2dFGRS data, we include 69 long Gamma-Ray Bursts (GRBs) data in our study and
perform global fitting using Markov Chain Monte Carlo (MCMC) technique. Dark
energy perturbations are explicitly considered. We pay particular attention to
the time evolution of the equation of state of dark energy parameterized as
with the scale factor of the universe, emphasizing
the complementarity of high redshift GRBs to other cosmological probes. It is
found that the constraints on dark energy become stringent by taking into
account high redshift GRBs, especially for , which delineates the
evolution of dark energy.Comment: 7 pages and 3 figures. Replaced with version accepted for publication
in Phys. Lett.
Schwinger boson mean field theory of the Heisenberg Ferrimagnetic Spin Chain
The Schwinger boson mean field theory is applied to the quantum ferrimagnetic
Heisenberg chain. There is a ferrimagnetic long range order in the ground
state. We observe two branches of the low lying excitation and calculate the
spin reduction, the gap of the antiferromagnetic branch, and the spin
fluctuation at . These results agree with the established numerical
results quite well. At finite temperatures, the long range order is destroyed
because of the disappearance of the Bose condensation. The thermodynamic
observables, such as the free energy, magnetic susceptibility, specific heat,
and the spin correlation at , are calculated. The has a
minimum at intermediate temperatures and the spin correlation length behaves as
at low temperatures. These qualitatively agree with the numerical
results and the difference is small at low temperatures.Comment: 15 pages, 5 figures. Accepted by Phys. Rev.
The Fermi surface of underdoped high-T_c superconducting cuprates
The coexistence of -flux state and d-wave RVB state is considered in
this paper within the slave boson approach. A critical value of doping
concentration is found, below which the coexisting -flux and
d-wave RVB state is favored in energy. The pseudo Fermi surface of spinons and
the physical electron spectral function are calculated. A clear Fermi-level
crossing is found along the (0,0) to (, ) direction, but no such
crossing is detected along the (, 0) to (, ) direction. Also, an
energy gap of d-wave symmetry appears at the Fermi level in our calculation.
The above results are in agreement with the angle-resolved photoemission
experiments which indicate at a d-wave pseudo-gap and a half-pocket-like Fermi
surface in underdoped cuprates.Comment: 18 pages RevTex, 6 figures in PS file
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