1,971 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
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
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
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
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
Wave propagation through a coherently amplifying random medium
We report a detailed and systematic numerical study of wave propagation
through a coherently amplifying random one-dimensional medium. The coherent
amplification is modeled by introducing a uniform imaginary part in the site
energies of the disordered single-band tight binding Hamiltonian. Several
distinct length scales (regimes), most of them new, are identified from the
behavior of transmittance and reflectance as a function of the material
parameters. We show that the transmittance is a non-self-averaging quantity
with a well defined mean value. The stationary distribution of the super
reflection differs qualitatively from the analytical results obtained within
the random phase approximation in strong disorder and amplification regime. The
study of the stationary distribution of the phase of the reflected wave reveals
the reason for this discrepancy. The applicability of random phase
approximation is discussed. We emphasize the dual role played by the lasing
medium, as an amplifier as well as a reflector.Comment: 33 pages RevTex, 14 EPS figures included, Accepted for publication in
IJMP-
Role of quantum entanglement due to a magnetic impurity on current magnification effect in mesoscopic open rings
We study the current magnification effect in presence of exchange scattering
of electron from a magnetic impurity placed in one arm of an open mesoscopic
ring. The exchange interaction causes entanglement of electron spin and
impurity spin. Earlier studies have shown that such an entanglement causes
reduction or loss of interference in the Aharonov-Bohm oscillations leading to
decoherence. We find however, that this entanglement, in contradiction to the
naive expectation of a reduction of current magnification, leads to enhancement
as well as suppression of the effect. We also observe additional novel features
like new resonances and current reversals.Comment: 5 pages RevTex, 5 figures include
Wigner delay time from a random passive and active medium
We consider the scattering of electron by a one-dimensional random potential
(both passive and active medium) and numerically obtain the probability
distribution of Wigner delay time (). We show that in a passive medium
our probability distribution agrees with the earlier analytical results based
on random phase approximation. We have extended our study to the strong
disorder limit, where random phase approximation breaks down. The delay time
distribution exhibits the long time tail () due to resonant states,
which is independent of the nature of disorder indicating the universality of
the tail of the delay time distribution. In the presence of coherent absorption
(active medium) we show that the long time tail is suppressed exponentially due
to the fact that the particles whose trajectories traverse long distances in
the medium are absorbed and are unlikely to be reflected.Comment: 13 pages RevTex, 5 EPS figures included, communicated to PR
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