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 $t$, for sample length $L$ less than the critical length $L_c$ grows exponentially with $L$. In the limit $L \to \infty$ transmission decays exponentially as \avg{lnt} = -L/\xi where $\xi$ is the localization length. In this limit reflection coefficient $r$ saturates to a fixed value which shows a monotonic increase as a function of strength of amplification $\alpha$. 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

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 $\alpha$ 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 $\xi ~ = ~ \xi_w \xi_\alpha / (\xi_w + \xi_\alpha)$, where $\xi_w$ and $\xi_\alpha$ 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 $\alpha$, i.e., reflection shows a monotonic decrease with $\alpha$ and tends to zero in the limit of $\alpha\to\infty$, 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 $\alpha$ 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 $\xi ~ = ~ \xi_w \xi_\alpha / (\xi_w + \xi_\alpha)$, where $\xi_w$ and $\xi_\alpha$ 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 $\alpha$, i.e., reflection shows a monotonic decrease with $\alpha$ and tends to zero in the limit of $\alpha\to\infty$, 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

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

Evaluation of Naked Barley Landraces for Agro-morphological Traits

Naked barley (Hordeum vulgare var. nudum L.) is a traditional, culturally important, climate-resilient winter cereal crop of Nepal. Evaluation of the naked barely genotypes for yield and disease is fundamental for their efficient utilization in plant breeding schemes and effective conservation programs. Therefore, to identify high yielding and yellow rust resistant landraces of naked barley for hilly and mountainous agro-ecosystem, twenty naked barley landraces collected from different locations of Nepal, were evaluated in randomized complete block design (RCBD) with three replications during winter season of 2016 and 2017 at Khumaltar, Lalitpur, Nepal. Combined analysis of variances revealed that NGRC04902 (3.46 t/ha), NGRC00886 (3.28 t/ha), NGRC02309 (3.21 t/ha) and NGRC06026 (3.10 t/ha) were the high yielding landraces and statistically at par with the released variety 'Solu Uwa' (3.15 t/ha). The landraces namely NGRC00837 (ACI Value: 1.86) was found resistant to yellow rust diseases. Landraces NGRC06034 (131.7 days) and NGRC02363 (130.8 days) were found early maturing and NGRC02306 (94.36 cm) was found dwarf landraces among tested genotypes. These landraces having higher yield and better resistance to yellow rust need to be deployed to farmers' field to diversify the varietal options and used in resistant breeding program to improve the productivity of naked barley for Nepalese farmers

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, $\alpha$. The estimations of $\alpha$ have been obtained for 39 pulsars, which include entirely new estimates for 31 pulsars. This study increases the total sample of pulsars available with $\alpha$ estimates by $\sim$50\%. The overall distribution of $\alpha$ 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 $\alpha$ values are observed, indicating the characteristic turbulence along each line of sight. For each pulsar, the estimated level of turbulence, $C^{2}_{n_e}$, has also been compared with $\alpha$ and DM. Additionally, we compare the distribution of $\alpha$ 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., $\alpha < 4.4$) 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 $C^{2}_{n_e}$ than those with steeper index.Comment: 13 pages, 4 figures, 3 tables. Accepted for publication in Ap

Thermal and Transport Behavior of Single Crystalline R2CoGa8 (R = Gd, Tb, Dy, Ho, Er, Tm, Lu and Y) Compounds

The anisotropy in electrical transport and thermal behavior of single crystalline R$_{2}$CoGa$_{8}$ series of compounds is presented. These compounds crystallize in a tetragonal structure with space gropup P4/mmm. The nonmagnetic counterparts of the series namely Y$_{2}$CoGa$_{8}$ and Lu$_{2}$CoGa$_{8}$show a behavior consistent with the low density of states at the fermi level. In Y$_{2}$CoGa$_{8}$, a possibility of charge density wave transition is observed at $\approx$ 30 K. Gd$_{2}$CoGa$_{8}$ and Er$_{2}$CoGa$_{8}$ show a presence of short range correlation above the magnetic ordering temperature of the compound. In case of Gd$_{2}$CoGa$_{8}$, the magnetoresistance exhibits a significant anisotropy for current parallel to {[}100{]} and {[}001{]} directions. Compounds with other magnetic rare earths (R = Tb, Dy, Ho and Tm) show the normal expected magnetic behavior whereas Dy$_{2}$CoGa$_{8}$ exhibits the possibility of charge density wave (CDW) transition at approximately same temperature as that of Y$_{2}$CoGa$_{8}$. The thermal property of these compounds is analysed on the basis of crystalline electric field (CEF) calculations.Comment: 10 Pages 14 Figures. Submitted to PR

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-