23,110 research outputs found
Gamma-Ray Burst Spectral Features: Interpretation as X-ray Emission From A Photoionized Plasma
Numerous reports have been made of features, either in emission or
absorption, in the 10 - 1000 keV spectra of some gamma-ray bursts. Originally
interpreted in the context of Galactic neutron star models as cyclotron line
emission and annihilation features, the recent demonstration that
the majority of GRBs lie at cosmological distances make these explanations
unlikely. In this letter, we adopt a relativistic fireball model for
cosmological GRBs in which dense, metal rich blobs or filaments of plasma are
entrained in the relativistic outflow. In the context of this model, we
investigate the conditions under which broadband features, similar to those
detected, can be observed. We find a limited region of parameter space capable
of reproducing the observed GRB spectra. Finally, we discuss possible
constraints further high-energy spectral observations could place on fireball
model parameters.Comment: Accepted for publication in Astrophysical Journal Letters Four pages,
2 figure
Multi-Orbital Molecular Compound (TTM-TTP)I_3: Effective Model and Fragment Decomposition
The electronic structure of the molecular compound (TTM-TTP)I_3, which
exhibits a peculiar intra-molecular charge ordering, has been studied using
multi-configuration ab initio calculations. First we derive an effective
Hubbard-type model based on the molecular orbitals (MOs) of TTM-TTP; we set up
a two-orbital Hamiltonian for the two MOs near the Fermi energy and determine
its full parameters: the transfer integrals, the Coulomb and exchange
interactions. The tight-binding band structure obtained from these transfer
integrals is consistent with the result of the direct band calculation based on
density functional theory. Then, by decomposing the frontier MOs into two
parts, i.e., fragments, we find that the stacked TTM-TTP molecules can be
described by a two-leg ladder model, while the inter-fragment Coulomb energies
are scaled to the inverse of their distances. This result indicates that the
fragment picture that we proposed earlier [M.-L. Bonnet et al.: J. Chem. Phys.
132 (2010) 214705] successfully describes the low-energy properties of this
compound.Comment: 5 pages, 4 figures, published versio
Structures and Electromagnetic Properties of New Metal-Ordered Manganites; RBaMn_{2}O_{6} (R = Y and Rare Earth Elements)
New metal-ordered manganites RBaMn_{2}O_{6} have been synthesized and
investigated in the structures and electromagnetic properties. RBaMn_{2}O_{6}
can be classified into three groups from the structural and electromagnetic
properties. The first group (R = La, Pr and Nd) has a metallic ferromagnetic
transition, followed by an A-type antiferromagnetic transition in
PrBaMn_{2}O_{6}. The second group (R = Sm, Eu and Gd) exhibits a charge-order
transition, followed by an antiferromagnetic long range ordering. The third
group (R = Tb, Dy and Ho) shows successive three phase transitions, the
structural, charge/orbital-order and magnetic transitions, as observed in
YBaMn_{2}O_{6}. Comparing to the metal-disordered manganites
(R^{3+}_{0.5}A^{2+}_{0.5})MnO_{3}, two remarkable features can be recognized in
RBaMn_{2}O_{6}; (1) relatively high charge-order transition temperature and (2)
the presence of structural transition above the charge-order temperature in the
third group. We propose a possible orbital ordering at the structural
transition, that is a possible freezing of the orbital, charge and spin degrees
of freedom at the independent temperatures in the third group. These features
are closely related to the peculiar structure that the MnO_{2} square-lattice
is sandwiched by the rock-salt layers of two kinds, RO and BaO with extremely
different lattice-sizes.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp
Direct observation of the proliferation of ferroelectric loop domains and vortex-antivortex pairs
We discovered "stripe" patterns of trimerization-ferroelectric domains in
hexagonal REMnO3 (RE=Ho, ---, Lu) crystals (grown below ferroelectric
transition temperatures (Tc), reaching up to 1435 oC), in contrast with the
vortex patterns in YMnO3. These stripe patterns roughen with the appearance of
numerous loop domains through thermal annealing just below Tc, but the stripe
domain patterns turn to vortex-antivortex domain patterns through a freezing
process when crystals cross Tc even though the phase transition appears not to
be Kosterlitz-Thouless-type. The experimental systematics are compared with the
results of our six-state clock model simulation and also the Kibble-Zurek
Mechanism for trapped topological defects
Frustrated Spin System in theta-(BEDT-TTF)_2RbZn(SCN)_4
The origin of the spin gap behavior in the low-temperature dimerized phase of
theta-(BEDT-TTF)_2RbZn(SCN)_4 has been theoretically studied based on the
Hartree-Fock approximation for the on-site Coulomb interaction at absolute
zero. Calculations show that, in the parameter region considered to be relevant
to this compound, antiferromagnetic ordering is stabilized between dimers
consisting of pairs of molecules coupled with the largest transfer integral.
Based on this result an effective localized spin 1/2 model is constructed which
indicates the existence of the frustration among spins. This frustration may
result in the formation of spin gap.Comment: 4 pages, 5 figures, to be published in J. Phys. Soc. Jpn. 67 (1998)
no.
Dissipative bracing-based seismic retrofit hypothesis of an elevated water storage tank with R/C frame staging structure
A seismic performance assessment and supplemental damping-based retrofit study on a heritage reinforced concrete (R/C) elevated water storage tank is presented. The structure was built in the early 1930s as water supplier for the coal power plant of Santa Maria Novella Station in Florence, and is still in service. The tank has a R/C frame supporting structure and is currently used as water supplier for trains and platform services.
The dynamic behavior of the fluid is simulated by a classical convective plus impulsive mass model, for which a discrete three-dimensional schematization is originally implemented in the finite element analysis.
The time-history assessment enquiry highlights a remarkable plastic response of the frame structure under seismic action scaled at the maximum considered earthquake level.
Based on these results, a retrofit hypothesis is proposed, consisting in the installation in the staging structure of a dissipative bracing (DB) system incorporating pressurized fluid viscous spring-dampers. The DB technology, studied by the first two authors during the last two decades by focusing on the numerical and analytical modeling, the experimental characterization and verification, the definition of design procedures, and the development of several applications to R/C and steel frame building structures, is explored for the first time within the study reported here for the seismic retrofit of elevated tanks.
The mechanical parameters, design criteria and technical implementation details of the rehabilitation strategy are illustrated. The verification time-history analyses in protected conditions show that a substantial enhancement of the seismic response capacities of the structure is attained as compared to its original configuration, with little architectural intrusion, quick installation works and low costs
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