16,117 research outputs found
Enhancement of magnetoresistance in manganite multilayers
Magnanite multilayers have been fabricated using La0.67Ca0.33MnO3 as the
ferromagnetic layer and Pr0.7Ca0.3MnO3 and Nd0.5Ca0.5MnO3 as the spacer layers.
All the multilayers were grown on LaAlO3 (100) by pulse laser deposition. An
enhanced magnetoresistnace (defined (RH- R0)/R0) of more than 98% is observed
in these multilayers. Also a low field magnetoresistance of 41% at 5000 Oe is
observed in these multilayer films. The enhanced MR is attributed to the
induced double exchange in the spacer layer, which is giving rise to more
number of conducting carriers. This is compared by replacing the spacer layer
with LaMnO3 where Mn exists only in 3+ state and no enhancement is observed in
the La0.67Ca0.33MnO3 / LaMnO3 multilayers as double exchange mechanism can not
be induced by external magnetic fields.Comment: 13 pages, 5 Figure
Dynamics of circular arrangements of vorticity in two dimensions
The merger of two like-signed vortices is a well-studied problem, but in a
turbulent flow, we may often have more than two like-signed vortices
interacting. We study the merger of three or more identical co-rotating
vortices initially arranged on the vertices of a regular polygon. At low to
moderate Reynolds numbers, we find an additional stage in the merger process,
absent in the merger of two vortices, where an annular vortical structure is
formed and is long-lived. Vortex merger is slowed down significantly due to
this. Such annular vortices are known at far higher Reynolds numbers in studies
of tropical cyclones, which have been noticed to a break down into individual
vortices. In the pre-annular stage, vortical structures in a viscous flow are
found here to tilt and realign in a manner similar to the inviscid case, but
the pronounced filaments visible in the latter are practically absent in the
former. Interestingly at higher Reynolds numbers, the merger of an odd number
of vortices is found to proceed very differently from that of an even number.
The former process is rapid and chaotic whereas the latter proceeds more slowly
via pairing events. The annular vortex takes the form of a generalised
Lamb-Oseen vortex (GLO), and diffuses inwards until it forms a standard
Lamb-Oseen vortex. For lower Reynolds number, the numerical (fully nonlinear)
evolution of the GLO vortex follows exactly the analytical evolution until
merger. At higher Reynolds numbers, the annulus goes through instabilities
whose nonlinear stages show a pronounced difference between even and odd mode
disturbances. It is hoped that the present findings, that multiple vortex
merger is qualitatively different from the merger of two vortices, will
motivate studies on how multiple vortex interactions affect the inverse cascade
in two-dimensional turbulence.Comment: Abstract truncated. Paper to appear in Physical Review
Geomagnetic storm effects on GPS based navigation
The energetic events on the sun, solar wind and subsequent effects on the Earth's geomagnetic field and upper atmosphere (ionosphere) comprise space weather. Modern navigation systems that use radio-wave signals, reflecting from or propagating through the ionosphere as a means of determining range or distance, are vulnerable to a variety of effects that can degrade the performance of the navigational systems. In particular, the Global Positioning System (GPS) that uses a constellation of earth orbiting satellites are affected due to the space weather phenomena. <br><br> Studies made during two successive geomagnetic storms that occurred during the period from 8 to 12 November 2004, have clearly revealed the adverse affects on the GPS range delay as inferred from the Total Electron Content (TEC) measurements made from a chain of seven dual frequency GPS receivers installed in the Indian sector. Significant increases in TEC at the Equatorial Ionization anomaly crest region are observed, resulting in increased range delay during the periods of the storm activity. Further, the storm time rapid changes occurring in TEC resulted in a number of phase slips in the GPS signal compared to those on quiet days. These phase slips often result in the loss of lock of the GPS receivers, similar to those that occur during strong(&gt;10 dB) L-band scintillation events, adversely affecting the GPS based navigation
Giant Meterwave Radio Telescope observations of an M2.8 flare: insights into the initiation of a flare-coronal mass ejection event
We present the first observations of a solar flare with the GMRT. An M2.8
flare observed at 1060 MHz with the GMRT on Nov 17 2001 was associated with a
prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the
initiation of a fast partial halo CME observed with the LASCO C2 coronograph.
Towards the start of the eruption, we find evidence for reconnection above the
prominence. Subsequently, we find evidence for rapid growth of a vertical
current sheet below the erupting arcade, which is accompanied by the flare and
prominence eruption.Comment: Accepted for publication in Solar Physic
Remote surface inspection system
This paper reports on an on-going research and development effort in remote surface inspection of space platforms such as the Space Station Freedom (SSF). It describes the space environment and identifies the types of damage for which to search. This paper provides an overview of the Remote Surface Inspection System that was developed to conduct proof-of-concept demonstrations and to perform experiments in a laboratory environment. Specifically, the paper describes three technology areas: (1) manipulator control for sensor placement; (2) automated non-contact inspection to detect and classify flaws; and (3) an operator interface to command the system interactively and receive raw or processed sensor data. Initial findings for the automated and human visual inspection tests are reported
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