18,430 research outputs found
Laser heterodyne system for obtaining height profiles of minor species in the atmosphere
An infrared laser heterodyne system for obtaining height profiles of minor constituents of the atmosphere was developed and erected. A brief discription of the system is given. The system consists of a tunable CO2 waveguide laser in the 9 to 11 micrometer band, that is used as a local oscillator and a heliostat that follows the sun and brings in solar radiation, that is mixed with the laser beam in a high speed liquid nitrogen cooled mercury cadmium telluride detector. The detected signal is analysed in a RF spectrum analyser that allows tracing absorption line profiles. Absorption lines of a number of minor constituents in the troposphere and stratosphere, such as O3, NH3, H2O, SO2, ClO, N2O, are in the 9 to 11 micrometer band and overlap with that of CO2 laser range. The experimental system has been made operational and trial observations taken. Current measurements are limited to ozone height profiles. Results are presented
Valley-Selective Landau-Zener Oscillations in Semi-Dirac p-n Junctions
We study transport across p-n junctions of gapped two-dimensional semi-Dirac
materials: nodal semimetals whose energy bands disperse quadratically and
linearly along distinct crystal axes. The resulting electronic properties ---
relevant to materials such as TiO/VO multilayers and
-(BEDT-TTF)I salts --- continuously interpolate between those
of mono- and bi-layer graphene as a function of propagation angle. We
demonstrate that tunneling across the junction depends on the orientation of
the tunnel barrier relative to the crystalline axes, leading to strongly
non-monotonic current-voltage characteristics, including negative differential
conductance in some regimes. In multi-valley systems these features provide a
natural route to engineering valley-selective transport.Comment: 7 pages, 7 figures, appendice
Ultracold bosons in a synthetic periodic magnetic field: Mott phases and re-entrant superfluid-insulator transitions
We study Mott phases and superfluid-insulator (SI) transitions of ultracold
bosonic atoms in a two-dimensional square optical lattice at commensurate
filling and in the presence of a synthetic periodic vector potential
characterized by a strength and a period , where is an integer
and is the lattice spacing. We show that the Schr\"odinger equation for the
non-interacting bosons in the presence of such a periodic vector potential can
be reduced to an one-dimensional Harper-like equation which yields energy
bands. The lowest of these bands have either single or double minima whose
position within the magnetic Brillouin zone can be tuned by varying for a
given . Using these energies and a strong-coupling expansion technique, we
compute the phase diagram of these bosons in the presence of a deep optical
lattice. We chart out the and dependence of the momentum distribution
of the bosons in the Mott phases near the SI transitions and demonstrate that
the bosons exhibit several re-entrant field-induced SI transitions for any
fixed period . We also predict that the superfluid density of the resultant
superfluid state near such a SI transition has a periodicity () in
real space for odd (even) and suggest experiments to test our theory.Comment: 8 pages, 11 figures, v
Augmented space recursion for partially disordered systems
Off-stoichiometric alloys exhibit partial disorder, in the sense that only
some of the sublattices of the stoichiometric ordered alloy become disordered.
This paper puts forward a generalization of the augmented space recursion (ASR)
(introduced earlier by one of us (Mookerjee et al 1997(*))) for systems with
many atoms per unit cell. In order to justify the convergence properties of ASR
we have studied the convergence of various moments of local density of states
and other physical quantities like Fermi energy and band energy. We have also
looked at the convergence of the magnetic moment of Ni, which is very sensitive
to numerical approximations towards the k-space value 0.6 with the
number of recursion steps prior to termination.Comment: Latex 2e, 21 Pages, 13 Figures, iopb style file attache
Interacting spinor and scalar fields in Bianchi type-I Universe filled with viscous fluid: exact and numerical solutions
We consider a self-consistent system of spinor and scalar fields within the
framework of a Bianchi type I gravitational field filled with viscous fluid in
presence of a term. Exact self-consistent solutions to the
corresponding spinor, scalar and BI gravitational field equations are obtained
in terms of , where is the volume scale of BI universe. System of
equations for and \ve, where \ve is the energy of the viscous fluid,
is deduced. Some special cases allowing exact solutions are thoroughly studied.Comment: 18 pages, 6 figure
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
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