67,640 research outputs found
Unified phase diagram of models exhibiting neutral-ionic transition
We have studied the neutral-ionic transition in organic mixed-stack
compounds. A unified model has been derived which, in limiting cases, is
equivalent to the models proposed earlier, the donor-acceptor model and the
ionic Hubbard model. Detailed numerical calculations have been performed on
this unified model with the help of the density-matrix renormalization-group
(DMRG) procedure calculating excitation gaps, ionicity, lattice site entropy,
two-site entropy, and the dimer order parameter on long chains and the unified
phase diagram has been determined.Comment: 11 pages, 8 figure
Viscous dark fluid universe
We investigate the cosmological perturbation dynamics for a universe
consisting of pressureless baryonic matter and a viscous fluid, the latter
representing a unified model of the dark sector. In the homogeneous and
isotropic background the \textit{total} energy density of this mixture behaves
as a generalized Chaplygin gas. The perturbations of this energy density are
intrinsically non-adiabatic and source relative entropy perturbations. The
resulting baryonic matter power spectrum is shown to be compatible with the
2dFGRS and SDSS (DR7) data. A joint statistical analysis, using also
Hubble-function and supernovae Ia data, shows that, different from other
studies, there exists a maximum in the probability distribution for a negative
present value of the deceleration parameter. Moreover,
while previous descriptions on the basis of generalized Chaplygin gas models
were incompatible with the matter power spectrum data since they required a
much too large amount of pressureless matter, the unified model presented here
favors a matter content that is of the order of the baryonic matter abundance
suggested by big-bang nucleosynthesis.Comment: 19 pages, 6 figure
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