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Sadhana Singh

Publications and source records attributed to Sadhana Singh.

17 recordsLinked to original sources

DHARA: Data Handling and Automated Reduction pipeline for AIMPOL

We present an automated data-reduction and analysis for optical linear polarimetric data obtained from a dual-beam polarimeter. The pipeline is optimized for observations acquired with the ARIES Imaging POLarimeter mounted at the Cassegrain focus of the 1.04-m Sampurnanand Telescope at ARIES. It is implemented using interactive Python routines that process raw images to derive the Stokes parameters, from which the degree of polarization and polarization angle are computed along with their uncertainties. The pipeline framework is designed to handle the reduction of both single-source and crowded-field observations. The identification of extraordinary (e-ray) and ordinary (o-ray) image pairs is a crucial step and is performed using different strategies for single-source and crowded-field data, while subsequent stages, such as photometric and polarimetric analysis, follow a common procedure. We validate the pipeline using observations of polarized standard stars acquired over multiple epochs between 2017 to 2025, and compare the results with values reported in the literature. To demonstrate the applicability of the pipeline to crowded-field observations, we apply it to polarimetric data of the Alessi~1 open cluster and compare with previously published results derived using traditional reduction methods. In both cases, the polarization parameters derived using the pipeline agree with literature values within $2σ$ uncertainties. Although developed for AIMPOL, the pipeline is readily adaptable to any dual-channel imaging polarimeter in which the e-ray and o-ray images are recorded in a FITS image from a single CCD.

astro-ph.IM

Non-equilibrium dynamics of disordered fractal spring network with active forces

We investigate the non-equilibrium dynamics of active bead-spring critical percolation clusters under the action of monopolar and dipolar forces. Previously, Langevin dynamics simulations of Rouse-type dynamics were performed on a deterministic fractal -- the Sierpinski gasket -- and combined with analytical theory [Chaos {\bf 34}, 113107 (2024)]. To study disordered fractals, we use here the critical (bond) percolation infinite cluster of square and triangular lattices, where beads (occupying nodes) are connected by harmonic springs. Two types of active stochastic forces, modeled as random telegraph processes, are considered: force monopoles, acting on individual nodes in random directions, and force dipoles, where extensile or contractile forces act between pairs of nodes, forming dipole links. A dynamical steady state is reached where the network is dynamically swelled for force monopoles. The time-averaged mean square displacement (MSD) shows sub-diffusive behavior at intermediate times longer than the force correlation time, whose anomalous exponent is solely controlled by the spectral dimension $(d_s)$ of the fractal network yielding MSD $\sim t^ν$, with $ν=1-\frac{d_s}{2}$, similar to the thermal system and in accord with the general analytic theory. In contrast, dipolar forces require a diverging time to reach a steady state, depending on the fraction of dipoles, and lead to network shrinkage. Within a quasi-steady-state assumption, we find a saturation behavior at the same temporal regime. Thereafter, a second ballistic-like rise is observed for networks with a low fraction of dipole forces, followed by a linear, diffusive increase. The second ballistic rise is, however, absent in networks fully occupied with force dipoles. These two behaviors are argued to result from local rotations of nodes, which are either persistent or fluctuating.

cond-mat.soft

Buried Interfaces and Spin Orientation in [Co/Pt]10 /Fe multilayer with Orthogonal Magnetic Anisotropy: Effect of Fe Thickness

In the present work, spin orientation and variation of the strength of coupling in [Co/Pt]ML/Fe multilayer have been investigated as a function of the thickness of the Fe layer. [Co/Pt]ML/Fe multilayer has orthogonal anisotropy with the Fe layer, and [Co/Pt]ML has in-plane magnetic anisotropy and perpendicular anisotropy, respectively. Measurements are performed using in-situ magneto-optical Kerr effect (MOKE) and isotope-sensitive depth-resolved nuclear resonance scattering (NRS) technique. Real-time in-situ MOKE measurement during Fe growth reveals that with an increase in thickness of the Fe layer, moments of Fe layer reorientation from out of the plane to in-plane direction. This is attributed to the decrease in the coupling between the [Co/Pt]ML and Fe layer. For the depth-dependent study, two [Co/Pt]ML/Fe multilayers having the same thickness but different positions of the Fe57 marker layer ([Co/Pt]MLFe/Fe57) and [Co/Pt]MLFe57/Fe ) were studied using the NRS technique. Films with varying external magnetic fields were also studied to investigate coupling strength. Measurements were performed under the x-ray standing wave conditions to enhance resonance yield. It is observed that for the 75 Å Fe in [Co/Pt]ML/Fe multilayer, the coupling varies along the depth of the Fe layer. The coupling is strong at [Co/Pt]ML and Fe interface with spins of the Fe layer aligned in the out-of-plane direction, whereas moments away from the interface are weakly coupled and aligned in-plane along the magnetic easy axis. Due to this gradient in strength of coupling along the depth, a large magnetic field is required to reorient spins at the interface along the magnetic hard axis of the Fe layer; however, spins away from the interface can rotate freely even in the low magnetic field.

cond-mat.mtrl-sci

Active fractal networks with stochastic force monopoles and force dipoles: Application to subdiffusion of chromosomal loci

Motivated by the well-known fractal packing of chromatin, we study the Rouse-type dynamics of elastic fractal networks with embedded, stochastically driven, active force monopoles and force dipoles that are temporally correlated. We compute, analytically -- using a general theoretical framework -- and {\it via} Langevin dynamics simulations, the mean square displacement (MSD) of a network bead. Following a short-time superdiffusive behavior, force monopoles yield anomalous subdiffusion with an exponent identical to that of the thermal system. In contrast, force dipoles do not induce subdiffusion, and the early superdiffusive MSD crosses over to a relatively small, system-size-independent saturation value. In addition, we find that force dipoles may lead to "crawling" rotational motion of the whole network, reminiscent of that found for triangular micro-swimmers and consistent with general theories of the rotation of deformable bodies. Moreover, force dipoles lead to network collapse beyond a critical force strength, which persists with increasing system size, signifying a true first-order dynamical phase transition. We apply our results to the motion of chromosomal loci in bacteria and yeast cells' chromatin, where anomalous sub-diffusion, MSD$\sim t^ν$ with $ν\simeq 0.4$, was found in both normal and ATP-depleted cells, albeit with different apparent diffusion coefficients. We show that the combination of thermal, monopolar, and dipolar forces in chromatin is typically dominated by the active monopolar and thermal forces, explaining the observed normal cells vs the ATP-depleted cells behavior.

cond-mat.soft

Foreground Dust Properties towards the Cluster NGC 7380

Using starlight polarization, we present the properties of foreground dust towards cluster NGC 7380 embedded in H{\sc ii} region Sh 2-142. Observations of starlight polarization are carried out in four filters using an imaging polarimeter equipped with a 104-cm ARIES telescope. Polarization vectors of stars are aligned along the Galactic magnetic field. Towards the east and southeast regions, the dust structure appears much denser than in other regions (inferred from extinction contours and colour composite image) and is also reflected in polarization distribution. We find that the polarization degree and extinction tend to increase with distance and indication for the presence of a dust layer at a distance of around 1.2 $kpc$. We have identified eight potential candidates exhibiting intrinsic polarization by employing three distinct criteria to distinguish between stars of intrinsic polarization and interstellar polarized stars. For interstellar polarized stars, we find that the maximum polarization degree increases with the color excess and has a strong scatter, with the mean value of 1.71$\pm$0.57$\%$. The peak wavelength spans $0.40-0.88μ$m with the mean value of 0.56$\pm$0.07 $μm$, suggesting similar grain sizes in the region as the average diffuse interstellar medium. The polarization efficiency is also found to decrease with visual extinction as $P_{max}/A_{V}\propto A_{V}^{-0.61}$. Our observational results are found to be consistent with the predictions by the radiative torque alignment theory.

astro-ph.GA

X-ray observations of the Intermediate Polar TX Col

We present the timing analysis of the intermediate polar TX Col in the X-ray band using the observations made by Chandra, Swift, and Suzaku during the years 2000, 2007, and 2009, respectively. The spin, orbital, and beat periods derived from these data are consistent with the earlier findings. We found that the spin modulation was dominant during the Chandra observation, whereas both orbital and beat modulations were dominant during the Swift and Suzaku observations. These findings and past X-ray observations indicate that TX Col is changing its accretion geometry from disc dominance to stream dominance and vice versa.

astro-ph.HE

Polarimetric Approach Towards Membership Probability of Open Star Cluster NGC 2345

Using the linear polarimetric observations, we present a method to derive the membership probability of stars in cluster NGC 2345. The polarimetric observations of cluster NGC 2345 are performed using the instrument ARIES IMaging POLarimeter (AIMPOL) mounted as a backend of the 104-cm telescope of ARIES. Members of the cluster should exhibit comparable polarization since they are located nearly at the same distance. This concept is used to extract the membership probability of known member stars of cluster NGC 2345. The membership probability estimated using the polarimetric data for cluster NGC 2345 agrees with the membership probability derived from the proper motion method in the previous studies.

astro-ph.SR

Asymmetric magnetism at the interfaces of MgO/FeCoB bilayers by exchanging the order of MgO and FeCoB

Interfaces in FeCoB/MgO/FeCoB magnetic tunnel junction play a vital role in controlling their magnetic and transport properties for various applications in spintronics and magnetic recording media. In this work, interface structures of a few nm thick FeCoB layers in FeCoB/MgO and MgO/FeCoB bilayers are comprehensively studied using x-ray standing waves (XSW) generated by depositing bilayers between Pt waveguide structures. High interface selectivity of nuclear resonance scattering (NRS) under the XSW technique allowed measuring structure and magnetism at the two interfaces, namely FeCoB-on-MgO and MgO-on-FeCoB, yielding an interesting result that electron density and hyperfine fields are not symmetric at both interfaces. The formation of a high-density FeCoB layer at the MgO/FeCoB (FeCoB-on-MgO) interface with an increased hyperfine field (~34.65 T) is attributed to the increasing volume of FeCo at the interface due to boron diffusion from 57FeCoB to the MgO layer. Furthermore, it caused unusual angular-dependent magnetic properties in MgO/FeCoB bilayer, whereas FeCoB/MgO is magnetically isotropic. In contrast to the literature, where the unusual angular dependent in FeCoB based system is explained in terms of in-plane magnetic anisotropy, present findings attributed the same to the interlayer exchange coupling between bulk and interface layer within the FeCoB layer.

cond-mat.mtrl-sci

Escape of a lamb to safe haven in pursuit by a lion under restarts

We study the escape behavior of a lamb to safe haven pursued by a hungry lion. Identifying the system with a pair of vicious Brownian walkers we evaluate the probability density function for the vicious pair and from there we estimate the distribution of first passage times. The process ends in two ways: either the lamb makes it to the safe haven (success) or is captured by the lion (failure). We find that the conditional distribution for both success and failure possesses a finite mean, but no higher moments exist. This makes it interesting to study these first passage properties of this Bernoulli process under restarts, which we do via Poissonian and sharp restart protocols. We find that under both restart protocols the probability of success exhibits a monotonic dependence on the restart parameters, however, their approach to the case without restarts is completely different. The distribution of first passage times exhibits an exponential decay for the two restart protocols. In addition, the distribution under sharp resetting also exhibits a periodic behavior, following the periodicity of the sharp restart protocol itself.

cond-mat.stat-mech

Capture of a diffusing lamb by a diffusing lion when both return home

A diffusing lion pursues a diffusing lamb when both of them are allowed to get back to their homes intermittently. Identifying the system with a pair of vicious random walkers, we study their dynamics under Poissonian and sharp resetting. In absence of any resets, the location of intersection of the two walkers follows a Cauchy distribution. In presence of resetting, the distribution of the location of annihilation is composed of two parts: one in which the trajectories cross without being reset (center) and the other where trajectories are reset at least once before they cross each other (tails). We find that the tail part decays exponentially for both the resetting protocols. The central part of the distribution, on the other hand, depends on the nature of the restart protocol, with Cauchy for Poisson resetting and Gaussian for sharp resetting. We find good agreement of the analytical results with numerical calculations.

cond-mat.stat-mech

Study of Asymmetric Magnetization Reversal and Exchange Bias in FePt(L10)/FeCo/CoO/FeCo Magnetic Multilayer

The effect of the saturation field on the magnetization reversal of FePt(L10)/FeCo/CoO/FeCo multilayer (ML) has been investigated to understand the origin of asymmetric magnetization reversal and its correlation with exchange bias (EB). In the ML structure, the bottom FeCo layer is coupled to the hard FePt(L10) layer, and the top FeCo layer is comparatively free due to the relatively more distance from it. The ML has been deposited under UHV conditions and characterized at each stage of growth using magneto-optical Kerr effect and x-ray reflectivity techniques. Magnetization reversal is further studied through domain imaging using the Kerr microscopy technique. The experimental findings reveal that ML exhibits asymmetrical magnetization reversal for a certain range of azimuthal angles for both 1.5kOe and 50kOe saturation fields; however, this angular range of asymmetry decreases with the increase in the saturation field. Furthermore, EB was absent at the low saturation field, whereas, EB, in addition to asymmetry, is observed at the large saturation field. The origin of asymmetry is attributed to non-collinearity between magnetic anisotropy axes of both FeCo layers. It results from the proximity effect through short-range Heisenberg exchange interaction via the CoO barrier layer. On the other hand, EB arises due to unidirectional anisotropy induced in the FePt layer due to the high saturation field. It is further proposed that asymmetry would disappear when unidirectional anisotropy is strong enough to align both the FeCo layers in the saturation direction leading to loss of the non-collinearity between them.

cond-mat.mtrl-sci

Origin of exchange bias in [Co/Pt]ML/Fe multilayer with orthogonal magnetic anisotropies

Magnetization reversal of soft ferromagnetic Fe layer, coupled to [Co/Pt]ML multilayer [ML] with perpendicular magnetic anisotropy (PMA), has been studied in-situ with an aim to understand the origin of exchange bias (EB) in orthogonal magnetic anisotropic systems. The interface remanant state of the ML is modified by magnetic field annealing, and the effect of the same on the soft Fe layer is monitored using the in-situ magneto-optical Kerr effect (MOKE). A considerable shift in the Fe layer hysteresis loop from the centre and an unusual increase in the coercivity, similar to exchange bias phenomena, is attributed to the exchange coupling at the [Co/Pt]ML and Fe interface. The effect of the coupling on spin orientation at the interface is further explored precisely by performing an isotope selective grazing incident nuclear resonance scattering (GINRS) technique. Here, the interface selectivity is achieved by introducing a 2 nm thick Fe57 marker between [Co/Pt]ML and Fe layers. Interface sensitivity is further enhanced by performing measurements under the x-ray standing wave conditions. The combined MOKE and GINRS analysis revealed the unidirectional pinning of the Fe layer due to the net in-plane magnetic spin at the interface caused by magnetic field annealing. Unidirectional exchange coupling or pinning at the interface, which may be due to the formation of asymmetrical closure domains, is found responsible for the origin of EB with an unusual increase in coercivity.

cond-mat.mtrl-sci

Polarization, Polarizing Efficiency, and Grain alignment towards the direction of the cluster NGC 2345

We have investigated the grain alignment and dust properties towards the direction of the cluster NGC 2345 using the multi-band optical polarimetric observations. For the majority of the stars, the observed polarization is found to be due to the interstellar medium with average values of maximum polarization and wavelength corresponding to it as 1.55% and 0.58 $μm$, respectively. This reveals a similar size distribution of dust grains to that in the general interstellar medium in the direction of NGC 2345. Alteration of dust properties near the distance of 1.2 kpc towards the direction of NGC 2345 has been noticed. The dust grains located beyond this distance are found to be aligned with the Galactic magnetic field, whereas a dispersion in orientation of the dust grains lying in the foreground of this distance is found. Polarizing efficiency of grains in this direction is found to be close to the average efficiency for our Galaxy. The decreased grain size along with the increased polarizing efficiency towards the core region of the cluster indicates the local radiation field is higher within the cluster which is responsible for the increased alignment efficiency of small grains. The wavelength of maximum polarization (associated with the average size of aligned grains) is also found to increase with extinction and reduces with the increase in polarizing efficiency, which can be explained by the radiative torque alignment mechanism.

astro-ph.GA

Broadband Linear Polarization in the Region of the Open Star Cluster NGC 1817

Multi-band linear polarimetric observations of 125 stars in the region of the cluster NGC 1817 have been carried out intending to study properties of interstellar dust and grains in that direction. The polarization is found to be wavelength-dependent, being maximum in the V band with an average value of 0.95%. The foreground interstellar dust grains appear to be the main source of linear polarization of starlight toward the direction of NGC 1817. The average value of the position angle in the V band of 119.2 degree is found to be less than the direction of the Galactic parallel in the region, indicating that the dust grains in the direction are probably not yet relaxed. Spatial distribution of dust appears to be more diverse in the coronal region than the core region of the cluster. The maximum value of the degree of polarization is estimated to be 0.93% for members of the cluster using the Serkowski relation. The average value of wavelength corresponding to the maximum polarization of 0.54+/-0.02 micron indicates that the size distribution of dust grains in the line of sight is similar to that of the general interstellar medium. Several variable stars in the cluster were also observed for polarimetrically and pulsating variables appear to have a slightly lower value of polarization from other nonvariable member stars of the cluster. There are indications of the existence of dust layers existence in front of those clusters which are located close to galactic plane while for clusters located away from galactic plane no major dust layers are observed.

astro-ph.SR

Transport and tumbling of polymers in viscoelastic shear flow

Polymers in shear flow are ubiquitous and we study their motion in a viscoelastic fluid under shear. Employing dumbbells as representative, we find that the center of mass motion follows: $\langle x^2_c(t) \rangle \sim \dotγ^2 t^{α+2}, ~0< α<1$, generalizing the earlier result: $\langle x^2_c(t) \rangle \sim \dotγ^2t^3 ~(α= 1)$. Motion of the relative coordinate, on the other hand, is quite intriguing in that $\langle x^2_r(t) \rangle \sim t^β$ with $β= 2(1-α)$ for small $α$. This implies nonexistence of the steady state. We remedy this pathology by introducing a nonlinear spring with FENE-LJ interaction and study tumbling dynamics of the dumbbell. The overall effect of viscoelasticity is to slow down the dynamics in the experimentally observed ranges of the Weissenberg number. We numerically obtain the characteristic time of tumbling and show that small changes in $α$ result in large changes in tumbling times.

cond-mat.stat-mech

Optical Linear Polarization toward the Open Star Cluster Casado Alessi 1

We present B-, V-, R-, and I-bands linear polarimetric observations of 73 stars in the direction of open star cluster Casado Alessi 1 (hereafter Alessi 1). We aim to use polarimetry as a tool to investigate the properties and distribution of dust grains toward the direction of the cluster. The polarimetric observations were carried out using the ARIES IMaging POLarimeter mounted at the 104 cm telescope of ARIES, Nainital (India). Using the Gaia photometric data the age and distance of the cluster are estimated to be $0.8\pm0.1$ Gyr and $673\pm98$ pc, respectively. A total of 66 stars with a 26 arcmin radius from the cluster are identified as members of the cluster using the astrometric approach. Out of these 66 members, 15 stars were observed polarimetrically and found to have the same value of polarization. The majority of the stars in the region follow the general law of the polarization for the interstellar medium, indicating that polarization toward the cluster Alessi 1 is dominated by foreground dust grains. The average values of the maximum polarization ($P_{max}$) and the wavelength corresponding to the maximum polarization ($λ_{max}$) toward the cluster are found to be $0.83\pm0.03$% and $0.59\pm0.04$ $μ$m, respectively. Also, dust grains toward the cluster appear to be aligned, possibly due to the galactic magnetic field.

astro-ph.GA

Dynamics of a polymer under multi-gradient fields

Effects of multi-gradient fields on the transport of a polymer chain have been investigated by using generalized Langevin dynamics simulations. We observe that the natural frequency of tumbling follows $Wi^{0.66}$ scaling, where $Wi$ is the Weissenberg number. Analysis of angular tumbling time distribution reveals that the tail of distribution follows exponential distribution and at high Weissenberg number, deviates from Poisson behaviour. Competition between velocity gradient which results shear flow in the system, and solvent quality gradient arising due to the interaction among monomers revealed that there is another scaling associated with the angular tumbling time distribution. Moreover, at low temperature, we observe unusual behaviour that at intermediate shear rates, decay rate $ν$ decreases with $Wi$.

cond-mat.soft