7,642 research outputs found
Studies of structural, magnetic, electrical and photoconducting properties of BiCaMnO epitaxial thin films
The dynamics of the charge ordered (CO) state under non-equilibrium
conditions created by strong dc-electric field (~106 V/cm) and
photo-illumination with short (~ 6 ns) laser pulses is investigated in
Bi1-xCaxMnO3 (x > 0.5) epitaxial films. A pulsed laser deposition method was
used to synthesize films on (100) LaAlO3 (LAO) and (100) SrTiO3 (STO)
substrates. The crystallographic structure, temperature dependence of
electrical resistivity and magnetization of the samples of different
composition prepared under different oxygen partial pressure (pO2) and
deposition temperature (TD) are studied. For the x = 0.6 sample grown on LAO, a
clear signature of charge ordering at ~275 K is seen in the magnetization and
at ~ 260 K in the resistivity data. The same sample grown on STO revealed a
complex behavior, which entails charge ordering at ~300 K, a Neel order at ~150
K and finally a weak ferromagnetic phase below 50 K. A strong correlation
between charge ordering temperature (TCO) and the c-axis lattice parameter (c)
of the type (dTCO/dc ~-350 K/A) imerges from measurements on films deposited
under different growth conditions. Since the out of plane lattice parameter (c)
increases with in plane compressive strain, this effect directly show a
compressive strain induced suppression of the TCO. The current (I)- voltage (V)
characteristics of the samples at T < TCO show hysteresis due to a compound
effect of Joule heating and collapse of the CO state. Transient changes in
conductivity of lifetime ranging from nano to microseconds are seen at T < TCO
on illumination with pulsed UV (355 nm) radiation. These observations are
explained on the basis of the topological and electronic changes in the charge
ordered phase.Comment: 19 figures, 34 page
Complementarity of perturbations driving insulator-to-metal transition in a charge ordered manganite
Modulation of charge carrier dynamics and hence electrical conductivity of
solids by photoexcitation has been a rich field of research with numerous
applications. Similarly, electric and magnetic field assisted enhancement of
conductivity are of fundamental importance and technological use. Hole doped
manganites of the type (ABMnO, where A and B are rare and
alkaline earth metals respectively have the distinction of showing all three
effects. Here we establish the complementarity of the electric, magnetic and
photon fields in driving an insulator-metal transition in epitaxial thin films
of LaPrCaMnO whose electrical ground state
is insulating. Both pulsed and CW lasers cause a giant photon flux dependent
enhancement of conductivity. It is further observed that electric and magnetic
fields trigger the persistent enhancement of conductivity whose magnitude can
be accentuated by application of these fields in parallel.Comment: 17 pages 6 figure
A probabilistic approach to model-based adaptive control for damping of interarea oscillations
Published versio
Spectral Representation of Thermal OTO Correlators
We study the spectral representation of finite temperature, out of time
ordered (OTO) correlators on the multi-time-fold generalised Schwinger-Keldysh
contour. We write the contour-ordered correlators as a sum over time-order
permutations acting on a funda- mental array of Wightman correlators. We
decompose this Wightman array in a basis of column vectors, which provide a
natural generalisation of the familiar retarded-advanced basis in the finite
temperature Schwinger-Keldysh formalism. The coefficients of this de-
composition take the form of generalised spectral functions, which are Fourier
transforms of nested and double commutators. Our construction extends a variety
of classical results on spectral functions in the SK formalism at finite
temperature to the OTO case.Comment: 19 pages+appendices, references adde
Out of Time Ordered Quantum Dissipation
We consider a quantum Brownian particle interacting with two harmonic baths,
which is then perturbed by a cubic coupling linking the particle and the baths.
This cubic coupling induces non-linear dissipation and noise terms in the
influence functional/master equation of the particle. Its effect on the
Out-of-Time-Ordered Correlators (OTOCs) of the particle cannot be captured by
the conventional Feynman-Vernon formalism.We derive the generalised influence
functional which correctly encodes the physics of OTO fluctuations, response,
dissipation and decoherence. We examine an example where Markovian
approximation is valid for the OTO dynamics. If the original cubic coupling has
a definite time-reversal parity, the leading order OTO influence functional is
completely determined by the couplings in the usual master equation via OTO
generalisation of Onsager-Casimir relations. New OTO fluctuation-dissipation
relations connect the non-Gaussianity of the thermal noise to the thermal
jitter in the damping constant of the Brownian particle.Comment: 46 pages+appendices, typos corrected, minor changes, references
update
Scaled frequency-dependent transport in the mesoscopically phase-separated colossal magnetoresistive manganite La_{0.625-y}Pr_yCa_{0.375}MnO_3
We address the issue of massive phase separation (PS) in manganite family of
doped Mott insulators through ac conductivity measurements on
LaPrCaMnO (0.375 y 0.275), and
establish applicability of the scaling theory of percolation in the critical
regime of the PS. Measurements of dc resistivity, magnetization (M(T)) and
electron diffraction show incomplete growth of a ferromagnetic (FM) metallic
component on cooling the high temperature charge ordered (CO) phase well below
Curie temperature. The impedance Z(T,f) measured over a frequency
(f) range of 10 Hz to 10 MHz in the critical regime follows a universal scaling
of the form R(T,0)g(f) with 0.86
and the normalized correlation length varying from 1 to 4, suggesting anomalous
diffusion of holes in percolating FM clusters.Comment: 12 pages and 5 figure
A Session based Multiple Image Hiding Technique using DWT and DCT
This work proposes Steganographic technique for hiding multiple images in a
color image based on DWT and DCT. The cover image is decomposed into three
separate color planes namely R, G and B. Individual planes are decomposed into
subbands using DWT. DCT is applied in HH component of each plane. Secret images
are dispersed among the selected DCT coefficients using a pseudo random
sequence and a Session key. Secret images are extracted using the session key
and the size of the images from the planer decomposed stego image. In this
approach the stego image generated is of acceptable level of imperceptibility
and distortion compared to the cover image and the overall security is high.Comment: 4 pages,16 figures, "Published with International Journal of Computer
Applications (IJCA)
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