44,980 research outputs found
Metal-Insulator Transition in a Generalized Hubbard Model with Correlated Hopping at Half-Filling
In the present paper metal-insulator transition is studied in a generalized
Hubbard model with correlated hopping at half-filling and zero temperature.
Single-particle Green function and energy spectrum of electron system are
calculated. The expressions for energy gap width and the concentration of polar
states (holes or doublons) are obtained. The conditions for metallic and
insulating states are found.Comment: 11 pages, 2 eps figures, Latex 2.09, submitted to Phys. Stat. Sol.
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Comment on "Observation of neutronless fusion reactions in picosecond laser plasmas"
The paper by Belyaev et al. [Phys. Rev. E {\bf 72}, 026406 (2005)] reported
the first experimental observation of alpha particles produced in the
thermonuclear reaction B()Be induced by
laser-irradiation on a B polyethylene (CH) composite target. The
laser used in the experiment is characterized by a picosecond pulse duration
and a peak of intensity of 2 W/cm. We suggest that both the
background-reduction method adopted in their detection system and the choice of
the detection energy region of the reaction products are possibly inadequate.
Consequently the total yield reported underestimates the true yield. Based on
their observation, we give an estimation of the total yield to be higher than
their conclusion, i.e., of the order of 10 per shot.Comment: 3 figures, accepted for publication in the Comment section of
Physical Review
Star-Formation in Low Radio Luminosity AGN from the Sloan Digital Sky Survey
We investigate faint radio emission from low- to high-luminosity Active
Galactic Nuclei (AGN) selected from the Sloan Digital Sky Survey (SDSS). Their
radio properties are inferred by co-adding large ensembles of radio image
cut-outs from the FIRST survey, as almost all of the sources are individually
undetected. We correlate the median radio flux densities against a range of
other sample properties, including median values for redshift, [OIII]
luminosity, emission line ratios, and the strength of the 4000A break. We
detect a strong trend for sources that are actively undergoing star-formation
to have excess radio emission beyond the ~10^28 ergs/s/Hz level found for
sources without any discernible star-formation. Furthermore, this additional
radio emission correlates well with the strength of the 4000A break in the
optical spectrum, and may be used to assess the age of the star-forming
component. We examine two subsamples, one containing the systems with emission
line ratios most like star-forming systems, and one with the sources that have
characteristic AGN ratios. This division also separates the mechanism
responsible for the radio emission (star-formation vs. AGN). For both cases we
find a strong, almost identical, correlation between [OIII] and radio
luminosity, with the AGN sample extending toward lower, and the star-formation
sample toward higher luminosities. A clearer separation between the two
subsamples is seen as function of the central velocity dispersion of the host
galaxy. For systems with similar redshifts and velocity dispersions, the
star-formation subsample is brighter than the AGN in the radio by an order of
magnitude. This underlines the notion that the radio emission in star-forming
systems can dominate the emission associated with the AGN.Comment: Accepted for publication in Astronomical Journal; 15 pages, 8 color
figure
Critical slowing down near the multiferroic phase transition in MnWO
By using broadband dielectric spectroscopy in the radiofrequency and
microwave range we studied the magnetoelectric dynamics in the multiferroic
chiral antiferromagnet MnWO. Above the multiferroic phase transition at
K we observe a critical slowing down of the corresponding
magnetoelectric fluctuations resembling the soft-mode behavior in canonical
ferroelectrics. This electric field driven excitation carries much less
spectral weight than ordinary phonon modes. Also the critical slowing down of
this mode scales with an exponent larger than one which is expected for
magnetic second order phase transition scenarios. Therefore the investigated
dynamics have to be interpreted as the softening of an electrically active
magnetic excitation, an electromagnon.Comment: 5 pages, 4 figures, appendi
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