2,301 research outputs found
Singularity: Raychaudhuri Equation once again
I first recount Raychaudhuri's deep involvement with the singularity problem
in general relativity. I then argue that precisely the same situation has
arisen today in loop quantum cosmology as obtained when Raychaudhuri discovered
his celebrated equation. We thus need a new analogue of the Raychaudhuri
equation in quantum gravity.Comment: 11 pages, Contribution to Special issue of Pramana on Raychaudhuri
Equation at Cross-roads, edited by Naresh Dadhich, Pankaj Joshi and Probir
Ro
Temperature dependence of the resistance of metallic nanowires (diameter 15 nm): Applicability of Bloch-Gr\"{u}neisen theorem
We have measured the resistances (and resistivities) of Ag and Cu nanowires
of diameters ranging from 15nm to 200nm in the temperature range 4.2K-300K with
the specific aim to assess the applicability of the Bloch-Gr\"{u}neisen formula
for electron phonon resistivity in these nanowires. The wires were grown within
polymeric templates by electrodeposition. We find that in all the samples the
resistance reaches a residual value at T=4.2K and the temperature dependence of
resistance can be fitted to the Bloch-Gr\"{u}neisen formula in the entire
temperature range with a well defined transport Debye temperature
(). The value of Debye temperature obtained from the fits lie
within 8% of the bulk value for Ag wires of diameter 15nm while for Cu
nanowires of the same diameter the Debye temperature is significantly lesser
than the bulk value. The electron-phonon coupling constants (measured by
or ) in the nanowires were found to have the same
value as that of the bulk. The resistivities of the wires were seen to increase
as the wire diameter was decreased. This increase in the resistivity of the
wires may be attributed to surface scattering of conduction electrons. The
specularity p was estimated to be about 0.5. The observed results allow us to
obtain the resistivities exactly from the resistance and gives us a method of
obtaining the exact numbers of wires within the measured array (grown within
the template).Comment: 9 pages, 10 figure
A Minimal Model of Signaling Network Elucidates Cell-to-Cell Stochastic Variability in Apoptosis
Signaling networks are designed to sense an environmental stimulus and adapt
to it. We propose and study a minimal model of signaling network that can sense
and respond to external stimuli of varying strength in an adaptive manner. The
structure of this minimal network is derived based on some simple assumptions
on its differential response to external stimuli. We employ stochastic
differential equations and probability distributions obtained from stochastic
simulations to characterize differential signaling response in our minimal
network model. We show that the proposed minimal signaling network displays two
distinct types of response as the strength of the stimulus is decreased. The
signaling network has a deterministic part that undergoes rapid activation by a
strong stimulus in which case cell-to-cell fluctuations can be ignored. As the
strength of the stimulus decreases, the stochastic part of the network begins
dominating the signaling response where slow activation is observed with
characteristic large cell-to-cell stochastic variability. Interestingly, this
proposed stochastic signaling network can capture some of the essential
signaling behaviors of a complex apoptotic cell death signaling network that
has been studied through experiments and large-scale computer simulations. Thus
we claim that the proposed signaling network is an appropriate minimal model of
apoptosis signaling. Elucidating the fundamental design principles of complex
cellular signaling pathways such as apoptosis signaling remains a challenging
task. We demonstrate how our proposed minimal model can help elucidate the
effect of a specific apoptotic inhibitor Bcl-2 on apoptotic signaling in a
cell-type independent manner. We also discuss the implications of our study in
elucidating the adaptive strategy of cell death signaling pathways.Comment: 9 pages, 6 figure
Using Jet Substructure at the LHC to Search for the Light Higgs Bosons of the CP-Violating MSSM
The CP-violating version of the Minimal Supersymmetric Standard Model (MSSM)
is an example of a model where experimental data do not preclude the presence
of light Higgs bosons in the range around 10 -- 110 GeV. Such light Higgs
bosons, decaying almost wholly to b-bbar pairs, may be copiously produced at
the LHC, but would remain inaccessible to conventional Higgs searches because
of intractable QCD backgrounds. We demonstrate that a significant number of
these light Higgs bosons would be boosted strongly enough for the pair of
daughter -jet pairs to appear as a single `fat' jet with substructure.
Tagging such jets could extend the discovery potential at the LHC into the
hitherto-inaccessible region for light Higgs bosons.Comment: LaTeX, 33 pages, 5 eps figures and 5 tables embedded. minor changes,
to appear in Physical Review
On the dual interpretation of zero-curvature Friedmann-Robertson-Walker models
Two possible interpretations of FRW cosmologies (perfect fluid or dissipative
fluid)are considered as consecutive phases of the system. Necessary conditions
are found, for the transition from perfect fluid to dissipative regime to
occur, bringing out the conspicuous role played by a particular state of the
system (the ''critical point '').Comment: 13 pages Latex, to appear in Class.Quantum Gra
Temperature dependence of transport spin polarization in NdNi5 measured using Point Contact Andreev reflection
We report a study in which Point contact Andreev reflection (PCAR)
spectroscopy using superconducting Nb tip has been carried out on NdNi5, a
ferromagnet with a Curie temperature of TC~7.7K. The measurements were carried
out over a temperature range of 2-9K which spans across the ferromagnetic
transition temperature. From an analysis of the spectra, we show that (i) the
temperature dependence of the extracted value of transport spin polarization
closely follows the temperature dependence of the spontaneous magnetization;
(ii) the superconducting quasiparticle lifetime shows a large decrease close to
the Curie temperature of the ferromagnet. We attribute the latter to the
presence of strong ferromagnetic spin fluctuations in the ferromagnet close to
the ferromagnetic transition temperature.Comment: pdf file including figures-Typographical error and errors in
references correcte
Investigation of the effect of microstructure and grain boundaries in nanostructured CMR thin films using scanning tunneling microscopy (STM) and local conductance map (LCMAP)
We have investigated the spatially resolved local electronic properties of a nanostructured film of a colossal magnetoresistive (CMR) material by local conductance mapping (LCMAP) using a variable temperature scanning tunneling microscope (STM) operating in a magnetic field. The nanostructured thin films (thickness ≈500 nm) of the CMR material La0.67Sr0.33MnO3 (LSMO) on silicon substrates were prepared using chemical solution deposition (CSD) process. These films have a large density of natural incoherent grain boundaries (GBs) which leads to significantly different behavior compared to oriented and epitaxial films of the same composition. Due to the presence of the GBs, these films show substantial low field magnetoresistance (LFMR) followed by a slower and almost linear decrease at higher fields and this is found to be strictly dependent on particle size. Most of the mechanisms proposed to explain the LFMR in the GB are based on tunneling through the GB. The purpose of this study is to use different STM based techniques to image these inhomogeneities and quantify them to the extent possible. In particular, we study the effect of grain size and the grain boundaries and their role in the electrical transport in nanostructured films of CMR materials
Magnetoresistance of metallic perovskite oxide LaNiO
We report a study of the magnetoresistance (MR) of the metallic perovskite
oxide LaNiO as a function of the oxygen stoichiometry
( 0.14), magnetic field (H ) and temperature (1.5K T 25K). We find a strong dependence of the nature of MR on the oxygen
stoichiometry. The MR at low temperatures change from positive to negative as
the sample becomes more oxygen deficient (i.e, increases). Some of the
samples which are more resistive, show a resistivity minima at
20K. We find that in these samples the MR is positive at T >
and negative for T < . We conclude that in the absence of
strong magnetic interaction, the negative MR in these oxides can arise from
weak localisation effects.Comment: 10 pages in REVTeX format, 4 eps fig
A GEANT-based study of atmospheric neutrino oscillation parameters at INO
We have studied the dependence of the allowed space of the atmospheric
neutrino oscillation parameters on the time of exposure for a magnetized Iron
CALorimeter (ICAL) detector at the India-based Neutrino Observatory (INO). We
have performed a Monte Carlo simulation for a 50 kTon ICAL detector generating
events by the neutrino generator NUANCE and simulating the detector response by
GEANT. A chi-square analysis for the ratio of the up-going and down-going
neutrinos as a function of is performed and the allowed regions at 90%
and 99% CL are displayed. These results are found to be better than the current
experimental results of MINOS and Super-K. The possibilities of further
improvement have also been discussed.Comment: 8 pages, 13 figures, a new figure added, version accepted in IJMP
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