236 research outputs found
Recent progress in silica aerogel Cherenkov radiator
In this paper, we present recent progress in the development of hydrophobic
silica aerogel as a Cherenkov radiator. In addition to the conventional method,
the recently developed pin-drying method for producing high-refractive-index
aerogels with high transparency was studied in detail. Optical qualities and
large tile handling for crack-free aerogels were investigated. Sufficient
photons were detected from high-performance aerogels in a beam test.Comment: Proceedings of 2nd International Conference on Technology and
Instrumentation in Particle Physics (TIPP 2011), to be published in Physics
Procedia, 8 pages, 7 figure
Optical and radiographical characterization of silica aerogel for Cherenkov radiator
We present optical and X-ray radiographical characterization of silica
aerogels with refractive index from 1.05 to 1.07 for a Cherenkov radiator. A
novel pin-drying method enables us to produce highly transparent hydrophobic
aerogels with high refractive index by shrinking wet-gels. In order to
investigate the uniformity in the density (i.e., refractive index) of an
individual aerogel monolith, we use the laser Fraunhofer method, an X-ray
absorption technique, and Cherenkov imaging by a ring imaging Cherenkov
detector in a beam test. We observed an increase in density at the edge of the
aerogel tiles, produced by pin-drying.Comment: To be published in IEEE Trans. Nucl. Sci., 7 pages, 9 figures, 1
tabl
Comparative study of hybrid functionals applied to structural and electronic properties of semiconductors and insulators
We present a systematic study that clarifies validity and limitation of
current hybrid functionals in density functional theory for structural and
electronic properties of various semiconductors and insulators. The three
hybrid functionals, PBE0 by Perdew, Ernzerhof, and Becke, HSE by Heyd,
Sucseria, and Ernzerhof, and a long-range corrected (LC) functional, are
implemented in a well-established plane-wave-basis-set scheme combined with
norm-conserving pseudopotentials, thus enabling us to assess applicability of
each functional on equal footing to the properties of the materials. The
materials we have examined in this paper range from covalent to ionic materials
as well as a rare-gas solid whose energy gaps determined by experiments are in
the range of 0.6 eV - 14.2 eV: i.e., Ge, Si, BaTiO, -GaN, diamond,
MgO, NaCl, LiCl, Kr, and LiF. We find that the calculated bulk moduli by the
hybrid functionals show better agreement with the experiments than the
generalized gradient approximation (GGA) provides, whereas the calculated
lattice constants by the hybrid functionals and GGA show comparable accuracy.
The calculated energy band gaps and the valence-band widths for the ten
prototype materials show substantial improvement using the hybrid functional
compared with GGA. In particular, it is found that the band gaps of the ionic
materials as well as the rare-gas solid are well reproduced by the LC-hybrid
functional, whereas those of covalent materials are well described by the HSE
functional. We also examine exchange effects due to short-range and long-range
components of the Coulomb interaction and propose an optimum recipe to the
short-range and long-range separation in treating the exchange energy.Comment: 13pages, 4 figures, 4 table
Linear-response theory of spin Seebeck effect in ferromagnetic insulators
We formulate a linear response theory of the spin Seebeck effect, i.e., a
spin voltage generation from heat current flowing in a ferromagnet. Our
approach focuses on the collective magnetic excitation of spins, i.e., magnons.
We show that the linear-response formulation provides us with a qualitative as
well as quantitative understanding of the spin Seebeck effect observed in a
prototypical magnet, yttrium iron garnet.Comment: 6 pages, 3 figures. Added references and revised argument on the
length scales at the end of Sec.
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