447 research outputs found
Understanding the Fano Resonance : through Toy Models
The Fano Resonance, involving the mixing between a quasi-bound `discrete'
state of an inelastic channel lying in the continuum of scattering states
belonging to the elastic channel, has several subtle features. The underlying
ideas have recently attracted attention in connection with interference effects
in quantum wires and mesoscopic transport phenomena. Simple toy models are
provided in the present study to illustrate the basics of the Fano resonance in
a simple and tractable setting.Comment: 17 pages, 1 figur
Phase dealy time and superluminal propagation in barrier tunneling
In this work we study the behaviour of Wigner phase delay time for tunneling in the reflection mode. Our system consists of a circular loop connected to a single wire of semi-infinite length in the presence of Aharonov-Bohm flux. We calculate the analytical expression for the saturated delay time. This saturated delay time is independent of Aharonov- Bohm flux and the width of the opaque barrier thereby generalizing the Hartman effect. This effect implies superluminal group velocities as a consequence. We also briefly discuss the concept called "space collapse or space destroyer"
The conductance of a multi-mode ballistic ring: beyond Landauer and Kubo
The Landauer conductance of a two terminal device equals to the number of
open modes in the weak scattering limit. What is the corresponding result if we
close the system into a ring? Is it still bounded by the number of open modes?
Or is it unbounded as in the semi-classical (Drude) analysis? It turns out that
the calculation of the mesoscopic conductance is similar to solving a
percolation problem. The "percolation" is in energy space rather than in real
space. The non-universal structures and the sparsity of the perturbation matrix
cannot be ignored.Comment: 7 pages, 8 figures, with the correct version of Figs.6-
Scattering phase shifts in quasi-one-dimension
Scattering of an electron in quasi-one dimensional quantum wires have many
unusual features, not found in one, two or three dimensions. In this work we
analyze the scattering phase shifts due to an impurity in a multi-channel
quantum wire with special emphasis on negative slopes in the scattering phase
shift versus incident energy curves and the Wigner delay time. Although at
first sight, the large number of scattering matrix elements show phase shifts
of different character and nature, it is possible to see some pattern and
understand these features. The behavior of scattering phase shifts in
one-dimension can be seen as a special case of these features observed in
quasi-one-dimensions. The negative slopes can occur at any arbitrary energy and
Friedel sum rule is completely violated in quasi-one-dimension at any arbitrary
energy and any arbitrary regime. This is in contrast to one, two or three
dimensions where such negative slopes and violation of Friedel sum rule happen
only at low energy where the incident electron feels the potential very
strongly (i.e., there is a very well defined regime, the WKB regime, where FSR
works very well). There are some novel behavior of scattering phase shifts at
the critical energies where -matrix changes dimension.Comment: Minor corrections mad
The evolution of stellar triples: The most common evolutionary pathways
Many stars do not live alone, but instead have one or more stellar
companions. Observations show that these binaries, triples and higher-order
multiples are common. Whereas the evolution of single stars and binaries have
been studied extensively, the same is not true for the evolution of stellar
triples. To fill this gap in our general understanding of stellar lives, we aim
to systematically explore the long-term evolution of triples and to map out the
most common evolutionary pathways that triples go through. We quantitatively
study how triples evolve, which processes are most relevant, and how this
differs from binary evolution. We simulate the evolution of several large
populations of triples with a population synthesis approach. We make use of the
triple evolution code TRES to simulate the evolution of each triple in a
consistent way; including three-body dynamics (based on the secular approach),
stellar evolution and their mutual influences. We simulate the evolution of the
system up until mass transfer starts, the system becomes dynamically unstable,
or a Hubble time has passed. We find that stellar interactions are common in
triples. Compared to a binary population, we find that the fraction of systems
that can undergo mass transfer is about 2 to 3 times larger in triples.
Moreover, whereas in binaries the orbits typically reach circularisation before
Roche-lobe overflow, this is not true anymore in triples. In our simulations,
about 40% of systems retain an eccentric orbit. Additionally, we discuss
various channels of triple evolution in detail such as those where the
secondary or the tertiary is the first star to initiate a mass transfer event.Comment: updated version: accepted for publication in A&A 18 pages, 16
figures, 2 table
Novel geopolymeric building materials through synergistic utilisation of industrial waste
Synergistic utilisation of major industrial wastes generated in India, namely fly ash, blast furnace slag and red mud, has been explored to develop novel building components using geopolymerisation. These include: (a) high strength cements (b) self glazed wall tiles, and (c) pavement tiles. Fly ash was used as main source of silico-aluminate for geopolymerisation. Granulated blast furnace slag (GBFS) and red mud were used individually or in combination with fly ash to tailor properties of the developed components. Chemical and mechanical activation have been judiciously incorporated in the processing schemes through an understanding of processing-structure-property relationships. Improvement in the reactivity of fly ash by mechanical activation using highenergy
mills was found to results in the formation of a compact microstructure during geopolymerisation leading to high compressive strength (above 100 MPa) in geopolymer cements. The cements also exhibited improved setting time and a very low autoclave expansion. In self-glazed wall tiles, the hard impervious glazed surface was obtained at temperature
lower than 150°C by controlling the particle size distribution of solid reactants, viscosity of slurry and reaction atmosphere. The self-glazed surface showed the presence of gismodine (Na-plagioclase) phase which
was absent in the main body of the tiles. In pavement tiles, fly ash and granulated blast furnace slag were used to give structural framework, whereas red mud was used to supplement the iron oxide for colouring effect and alkalis. The setting and hardening occurred due to formation of
cementitious A-S-H and C-S-H gel (A = Al2O3, S = SiO2, C = CaO, H = H2O). The technologies have been developed at bench scale and efforts are underway for scaling up to pilot plant level
Recommended from our members
Oxygen Transport Ceramic Membranes
A non-agglomerated and nanocrystalline-sized powder was successfully produced using ethylene glycol nitrate methods. The LSFT powder prepared using this method exhibits well dispersed and nano-sized particles about 100-200 nm. The density of LSFT sintered at 1300 C was about 90% of the theoretical density at which is 100 C less than that of the previous LSFT which was sintered at 1400 C. The sample sintered at 1400 C exhibited the evidence of a liquid phase at the grain boundaries and 2nd phase formation which probably caused low mechanical stability. The electrical conductivity and Seebeck coefficient were measured as a function of temperature. The LSFT-CGO specimens were cut from the as sintered bars and used for the evaluation of Mechanical Properties after polishing. The effect of strain rate on the flexural strength of the LSFT-CGO test specimens was studied. Three strain rates 6, 60 and 600 {micro}m/ min were chosen for this study. It is observed from the results that with increasing cross head speed the membrane takes higher loads to fail. A reduction in the strength of the membrane was observed at 1000 C in N{sub 2}. Two different routes were investigated to synthesis GDC using either formate or carbonate precursors. The precursor and CGO particle morphologies were examined by scanning electron microscopy. The thermal decomposition behaviors of Ce(Gd)(HCOO){sub 3} and Ce(Gd)(CO{sub 3})(OH) were determined by thermogravimetric analysis (TGA) at a rate of 3 C/min in air. The X-ray powder diffraction patterns of the precursor and CGO were collected and nitrogen adsorption isotherms were measured. Conductivity measurements were made by AC impedance spectroscopy on sintered disks in air using platinum electrodes
- …