22,215 research outputs found
A study of disordered systems with gain: Stochastic Amplification
A study of statistics of transmission and reflection from a random medium
with stochastic amplification as opposed to coherent amplification is
presented. It is found that the transmission coefficient , for sample length
less than the critical length grows exponentially with . In the
limit transmission decays exponentially as \avg{lnt} = -L/\xi
where is the localization length. In this limit reflection coefficient
saturates to a fixed value which shows a monotonic increase as a function
of strength of amplification . The stationary distribution of
super-reflection coefficient agrees well with the analytical results obtained
within the random phase approximation (RPA). Our model also exhibits the well
known duality between absorption and amplification. We emphasize the major
differences between coherent amplification and stochastic amplification
where-ever appropriate.Comment: 7 pages RevTex, two column format, 9 eps figures included mpeg
simulations at http://www.iopb.res.in/~joshi/mpg.htm
Dephasing of Aharonov-Bohm oscillations in a mesoscopic ring with a magnetic impurity
We present a detailed analysis of the Aharonov-Bohm interference oscillations
manifested through transmission of an electron in a mesoscopic ring with a
magnetic impurity atom inserted in one of its arms. The electron interacts with
the impurity through the exchange interaction leading to exchange spin-flip
scattering. Transmission in the spin-flipped and spin-unflipped channels are
explicitly calculated. We show that the spin-flipper acts as a dephasor in
spite of absence of any inelastic scattering. The spin-conductance (related to
spin-polarized transmission coefficient) is asymmetric in the flux reversal as
opposed to the two probe conductance which is symmetric under flux reversal.Comment: 4 pages RevTex, 6 figures, brief repor
Modelling of Stochastic Absorption in a Random Medium
We report a detailed and systematic study of wave propagation through a
stochastic absorbing random medium. Stochastic absorption is modeled by
introducing an attenuation constant per unit length in the free
propagation region of the one-dimensional disordered chain of delta function
scatterers. The average value of the logarithm of transmission coefficient
decreases linearly with the length of the sample. The localization length is
given by , where and
are the localization lengths in the presence of only disorder and
of only absorption respectively. Absorption does not introduce any additional
reflection in the limit of large , i.e., reflection shows a monotonic
decrease with and tends to zero in the limit of , in
contrast to the behavior observed in case of coherent absorption. The
stationary distribution of reflection coefficient agrees well with the
analytical results obtained within random phase approximation (RPA) in a larger
parameter space. We also emphasize the major differences between the results of
stochastic and coherent absorption.Comment: 7 pages RevTex, 9 eps figures included, modified version of
cond-mat/9909327, to appear in PRB, mpeg simulations at
http://www.iopb.res.in/~joshi/mpg.htm
Quantum Stochastic Absorption
We report a detailed and systematic study of wave propagation through a
stochastic absorbing random medium. Stochastic absorption is modeled by
introducing an attenuation constant per unit length in the free
propagation region of the one-dimensional disordered chain of delta function
scatterers. The average value of the logarithm of transmission coefficient
decreases linearly with the length of the sample. The localization length is
given by , where and
are the localization lengths in the presence of only disorder and
of only absorption respectively. Absorption does not introduce any additional
reflection in the limit of large , i.e., reflection shows a monotonic
decrease with and tends to zero in the limit of , in
contrast to the behavior observed in case of coherent absorption. The
stationary distribution of reflection coefficient agrees well with the
analytical results obtained within random phase approximation (RPA) in a larger
parameter space. We also emphasize the major differences between the results of
stochastic and coherent absorption.Comment: RevTex, 6 pages,2 column format, 9 .eps figures include
Loss of interference in an Aharonov-Bohm ring
We study a simple model of dephasing of Aharonov-Bohm oscillations in the
transmission of an electron across a mesoscopic ring. A magnetic impurity in
one of the arms of the ring couples to the electron spin via an exchange
interaction. This interaction leads to spin flip scattering and induces
dephasing via entanglement. This is akin to the models evoked earlier to
explain destruction of interference due to which-path information in
double-slit experiments. Total transmission is found to be symmetric under flux
reversal but not the spin polarization.Comment: 4 pages, latex/revtex, 4 eps figures. Proceedings of CMDAYS2K, held
at Guru Ghasidas University, Bilaspur, Chattisgarh, India, Aug 29-31, 2
Multi-frequency scatter broadening evolution of pulsars - I
We present multi-wavelength scatter broadening observations of 47 pulsars,
made with the Giant Metre-wave Radio Telescope (GMRT), Ooty Radio Telescope
(ORT) and Long Wavelength Array (LWA). The GMRT observations have been made in
the phased array mode at 148, 234, and 610 MHz and the ORT observations at 327
MHz. The LWA data sets have been obtained from the LWA pulsar data archive. The
broadening of each pulsar as a function of observing frequency provides the
frequency scaling index, . The estimations of have been
obtained for 39 pulsars, which include entirely new estimates for 31 pulsars.
This study increases the total sample of pulsars available with
estimates by 50\%. The overall distribution of with the
dispersion measure (DM) of pulsar shows interesting variations, which are
consistent with the earlier studies. However, for a given value of DM a range
of values are observed, indicating the characteristic turbulence along
each line of sight. For each pulsar, the estimated level of turbulence,
, has also been compared with and DM. Additionally, we
compare the distribution of with the theoretically predicated model to
infer the general characteristics of the ionized interstellar medium (ISM).
Nearly 65\% of the pulsars show a flatter index (i.e., ) than
that is expected from the Kolmogorov turbulence model. Moreover, the group of
pulsars having flatter index is typically associated with an enhanced value of
than those with steeper index.Comment: 13 pages, 4 figures, 3 tables. Accepted for publication in Ap
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