18,663 research outputs found
Role of c-axis pairs in V2O3 from the band-structure point of view
The common interpretation of the LDA band structure of VO is that
the apparent splitting of the band into a low intensity structure deep
below the Fermi energy and a high intensity feature above it, is due to the
bonding-antibonding coupling of the vertical V-V pair. Using tight-binding
fitting to --as well as first-principles NMTO downfolding of-- the spin-up
LDA+U band, we show that there are other hopping integrals which are
equally important for the band shape as the integral for hopping between the
partners of the pair
Optical properties of random alloys : Application to Cu_{50}Au_{50} and Ni_{50}Pt_{50}
In an earlier paper [K. K. Saha and A. Mookerjee, Phys. Rev. B 70 (2004) (in
press) or, cond-mat/0403456] we had presented a formulation for the calculation
of the configuration-averaged optical conductivity in random alloys. Our
formulation is based on the augmented-space theorem introduced by one of us [A.
Mookerjee, J. Phys. C: Solid State Phys. 6, 1340 (1973)]. In this communication
we shall combine our formulation with the tight-binding linear muffin-tin
orbitals (TB-LMTO) technique to study the optical conductivities of two alloys
Cu_{50}Au_{50} and Ni_{50}Pt_{50}.Comment: 5 pages, 7 figure
Study of Phase Stability in NiPt Systems
We have studied the problem of phase stability in NiPt alloy system. We have
used the augmented space recursion based on the TB-LMTO as the method for
studying the electronic structure of the alloys. In particular, we have used
the relativistic generalization of our earlier technique. We note that, in
order to predict the proper ground state structures and energetics, in addition
to relativistic effects, we have to take into account charge transfer effects
with precision.Comment: 22 pages, 7 figures. Accepted for publication in JPC
Origin of gamma-ray emission in the shell of Cassiopeia A
Non-thermal X-ray emission from the shell of Cassiopeia A (Cas A) has been an
interesting subject of study, as it provides information about relativistic
electrons and their acceleration mechanisms in the shocks. Chandra X-ray
observatory revealed the detailed spectral and spatial structure of this SNR in
X-rays. The spectral analysis of Chandra X-ray data of Cas A shows unequal flux
levels for different regions of the shell, which can be attributed to different
magnetic fields in those regions. Additionally, the GeV gamma-ray emission
observed by Large Area Telescope on board Fermi Gamma Ray Space Telescope
showed that the hadronic processes are dominating in Cas A, a clear signature
of acceleration of protons. In this paper we aim to explain the GeV-TeV
gamma-ray data in the context of both leptonic and hadronic scenario. We
modeled the multi-wavelength spectrum of Cas A. We use synchrotron emission
process to explain the observed non-thermal X-ray fluxes from different regions
of the shell. These result in estimation of the model parameters, which are
then used to explain TeV gamma-ray emission spectrum. We also use hadronic
scenario to explain both GeV and TeV fluxes simultaneously. We show that a
leptonic model alone cannot explain the GeV-TeV data. Therefore, we need to
invoke a hadronic model to explain the observed GeV-TeV fluxes. We found that
although pure hadronic model is able to explain the GeV-TeV data, a
lepto-hadronic model provides the best fit to the data.Comment: Accepted in A&
Microelectromechanical systems vibration powered electromagnetic generator for wireless sensor applications
This paper presents a silicon microgenerator, fabricated using standard silicon micromachining techniques, which converts external ambient vibrations into electrical energy. Power is generated by an electromagnetic transduction mechanism with static magnets positioned on either side of a moving coil, which is located on a silicon structure designed to resonate laterally in the plane of the chip. The volume of this device is approximately 100 mm3. ANSYS finite element analysis (FEA) has been used to determine the optimum geometry for the microgenerator. Electromagnetic FEA simulations using Ansoftâs Maxwell 3D software have been performed to determine the voltage generated from a single beam generator design. The predicted voltage levels of 0.7â4.15 V can be generated for a two-pole arrangement by tuning the damping factor to achieve maximum displacement for a given input excitation. Experimental results from the microgenerator demonstrate a maximum power output of 104 nW for 0.4g (g=9.81 m s1) input acceleration at 1.615 kHz. Other frequencies can be achieved by employing different geometries or material
Optical properties of perovskite alkaline earth titanates : a formulation
In this communication we suggest a formulation of the optical conductivity as
a convolution of an energy resolved joint density of states and an
energy-frequency labelled transition rate. Our final aim is to develop a scheme
based on the augmented space recursion for random systems. In order to gain
confidence in our formulation, we apply the formulation to three alkaline earth
titanates CaTiO_3, SrTiO_3 and BaTiO_3 and compare our results with available
data on optical properties of these systems.Comment: 19 pages, 9 figures, Submitted to Journal of Physics: Condensed
Matte
Space-time evolution induced by spinor fields with canonical and non-canonical kinetic terms
We study spinor field theories as an origin to induce space-time evolution.
Self-interacting spinor fields with canonical and non-canonical kinetic terms
are considered in a Friedman-Robertson-Walker universe. The deceleration
parameter is calculated by solving the equation of motion and the Friedman
equation, simultaneously. It is shown that the spinor fields can accelerate and
decelerate the universe expansion. To construct realistic models we discuss the
contributions from the dynamical symmetry breaking.Comment: 16 pages, 19 figure
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