arXiv subjects

Vijay Singh

Publications and source records attributed to Vijay Singh.

35 records · Page 2Linked to original sources

arXiv2019

Universal properties of concentration sensing in large ligand-receptor networks

Cells estimate concentrations of chemical ligands in their environment using a limited set of receptors. Recent work has shown that the temporal sequence of binding and unbinding events on just a single receptor can be used to estimate the concentrations of multiple ligands. Here, for a network of many ligands and many receptors, we show that such temporal sequences can be used to estimate the concentration of a few times as many ligand species as there are receptors. Crucially, we show that the spectrum of the inverse covariance matrix of these estimates has several universal properties, which we trace to properties of Vandermonde matrices. We argue that this can be used by cells in realistic biochemical decoding networks.

q-bio.MN↗

arXiv2019

A competitive binding model predicts nonlinear responses of olfactory receptors to complex mixtures

In color vision, the quantitative rules for mixing lights to make a target color are well understood. By contrast, the rules for mixing odorants to make a target odor remain elusive. A solution to this problem in vision relied on characterizing receptor responses to different wavelengths of light and subsequently relating these responses to perception. In olfaction, experimentally measuring receptor responses to a representative set of complex mixtures is intractable due to the vast number of possibilities. To meet this challenge, we develop a biophysical model that predicts mammalian receptor responses to complex mixtures using responses to single odorants. The dominant nonlinearity in our model is competitive binding (CB): only one odorant molecule can attach to a receptor binding site at a time. This simple framework predicts receptor responses to mixtures of up to twelve monomolecular odorants to within 15\% of experimental observations and provides a powerful method for leveraging limited experimental data. Simple extensions of our model describe phenomena such as synergy, overshadowing, and inhibition. We demonstrate that the presence of such interactions can be identified via systematic deviations from the competitive binding model.

q-bio.NC↗

arXiv2018

Magnetic Order and Lattice Instabilities in Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$ Heusler based Magnetic Shape-Memory Alloys

The magnetic correlations in the austenite phase and the consequent martensitic transition in inverse magnetocaloric alloys, Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$, have been a matter of debate for decades. We conclusively establish using {\it ab initio} phonon calculations that the spin alignment of excess Mn at the Sn site (Mn$_{Sn}$) with the existing Mn in the unit cell in the high temperature cubic phase of Ni-Mn-Sn alloy is ferromagnetic (FM), and not ferrimagnetic (FI), resolving a long lasting controversy. Using first principles density functional perturbation theory (DFPT), we observe an instability of the TA$_{2}$ mode along the $Γ$-M direction in the FM phase, very similar to that observed in the prototypical ferromagnetic shape memory alloy (FSMA) Ni$_{2}$MnGa. This specific instability is not observed in the FI phase. Further finite temperature first principles lattice dynamics calculations reveal that at 300 K the FM phase becomes mechanically stable, while the FI phase continue to remain unstable providing credence to the fact that the high-temperature phase has FM order. These results will be primordial to understand the magneto-structural properties of this class of compounds.

cond-mat.str-el↗

arXiv2018

Topological Nontrivial Phase in Hexagonal Antiperovskites A3BiB (A=Ba,Sr; B=P,N)

In this article, we predict the occurrence of topological non-trivial phase in hexagonal antiperovskite systems. By carefully investigating the evolution of band structure, we have studied the pressure induced topological phase transition in Ba$_{3}$BiP, Ba$_{3}$BiN, and Sr$_{3}$BiN compounds using the hybrid functional calculations. The non-trivial topology has been verified by computing Dirac like surface dispersion and topological invariant {\it Z$_2$} index via parity analysis. The unconventional spin texture has been analyzed which guaranteed the absence of impurity induced backscattering on boundary of the sample while respecting the time reversal symmetry. Our simulation confirms the chemical and mechanical stability of all the three compounds. The present study introduces an important new class of hexagonal antiperovskite compounds in the topological regime and are believed to capture ample of attention both from theoretical as well as experimental front.

cond-mat.mtrl-sci↗

arXiv2018

A Moving-Trap Zeeman Decelerator

We present a moving-trap Zeeman decelerator (MTZD) for use in molecular beam manipulation and magnetic-trapping experiments of paramagnetic atoms and molecules. The 0.49 m MTZD consists of a combination of a 2D magnetic quadrupole guide and deceleration coils, which together produce an array of 3D magnetic traps. The design is based on that of Trimeche et al. with significant modifications. The 2D quadrupole is driven by fast rising and falling square pulses of current (up to 700 A) of arbitrary lengths of time. The traps can be made to move at velocities from ca. 370 m/s down to zero, which is done by driving through the deceleration coils a sinusoidal current with frequencies ranging from zero to ca. 9 kHz and peak currents up to 1000 A. The trap velocity is proportional to the current frequency. The MTZD manipulates the velocities of molecular beams by traveling initially at the same velocity as the beam. 3D guiding is achieved with a constant current frequency and deceleration is achieved with a downward chirp of the current frequency at a rate corresponding to the desired deceleration. We explain the technical design and operation principles of the MTZD and we detail the custom power electronics that drive the 2D quadrupole guide and the decelerator coils. We present extensive Monte-Carlo trajectory simulations to demonstrate the properties of the MTZD and we conclude that decelerations should be kept below 30000 m/s/s to maintain a good 6D phase-space acceptance. In proof-of-principle experiments, with the deceleration coils operating with a reduced current in the range 100-200 A, we demonstrate the 3D guiding of a beam of metastable argon atoms at 373 m/s and deceleration at 25000 m/s/s from 342 m/s to 304 m/s. The deceleration corresponds to the removal of 21% of the kinetic energy of the beam.

physics.atom-ph↗

arXiv2018

Optimized cell geometry for buffer-gas-cooled molecular-beam sources

We have designed, constructed, and commissioned a cryogenic helium buffer-gas source for producing a cryogenially-cooled molecular beam and evaluated the effect of different cell geometries on the intensity of the produced molecular beam, using ammonia as a test molecule. Planar and conical entrance and exit geometries are tested. We observe a three fold enhancement in the NH$_3$ signal for a cell with planar-entrance and conical-exit geometry, compared to that for a typically used `box'-like geometry with planar entrance and exit. These observations are rationalized by flow-field simulations for the different buffer-gas cell geometries. The full thermalization of molecules with the helium buffer-gas is confirmed through rotationally-resolved REMPI spectra yielding a rotational temperature of 5 K.

physics.chem-ph↗

arXiv2016

Extrinsic and intrinsic correlations in molecular information transmission

Cells measure concentrations of external ligands by capturing ligand molecules with cell surface receptors. The numbers of molecules captured by different receptors co-vary because they depend on the same extrinsic ligand fluctuations. However, these numbers also counter-vary due to the intrinsic stochasticity of chemical processes because a single molecule randomly captured by a receptor cannot be captured by another. Such structure of receptor correlations is generally believed to lead to an increase in information about the external signal compared to the case of independent receptors. We analyze a solvable model of two molecular receptors and show that, contrary to this widespread expectation, the correlations have a small and negative effect on the information about the ligand concentration. Further, we show that measurements that average over multiple receptors are almost as informative as those that track the states of every individual one.

q-bio.NC↗

arXiv2015

Accurate sensing of multiple ligands with a single receptor

Cells use surface receptors to estimate the concentration of external ligands. Limits on the accuracy of such estimations have been well studied for pairs of ligand and receptor species. However, the environment typically contains many ligands, which can bind to the same receptors with different affinities, resulting in cross-talk. In traditional rate models, such cross-talk prevents accurate inference of individual ligand concentrations. In contrast, here we show that knowing the precise timing sequence of stochastic binding and unbinding events allows one receptor to provide information about multiple ligands simultaneously and with a high accuracy. We argue that such high-accuracy estimation of multiple concentrations can be realized by the familiar kinetic proofreading mechanism.

q-bio.MN↗

arXiv2014

From explosive to infinite-order transitions on a hyperbolic network

We analyze the phase transitions that emerge from the recursive design of certain hyperbolic networks that includes, for instance, a discontinuous ("explosive") transition in ordinary percolation. To this end, we solve the $q$-state Potts model in the analytic continuation for non-integer $q$ with the real-space renormalization group. We find exact expressions for this one-parameter family of models that describe the dramatic transformation of the transition. In particular, this variation in $q$ shows that the discontinuous transition is generic in the regime $q<2$ that includes percolation. A continuous ferromagnetic transition is recovered in a singular manner only for the Ising model, $q=2$. For $q>2$ the transition immediately transforms into an infinitely smooth order parameter of the Berezinskii-Kosterlitz-Thouless (BKT) type.

cond-mat.stat-mech↗

arXiv2014

Director Field Model of the Primary Visual Cortex for Contour Detection

We aim to build the simplest possible model capable of detecting long, noisy contours in a cluttered visual scene. For this, we model the neural dynamics in the primate primary visual cortex in terms of a continuous director field that describes the average rate and the average orientational preference of active neurons at a particular point in the cortex. We then use a linear-nonlinear dynamical model with long range connectivity patterns to enforce long-range statistical context present in the analyzed images. The resulting model has substantially fewer degrees of freedom than traditional models, and yet it can distinguish large contiguous objects from the background clutter by suppressing the clutter and by filling-in occluded elements of object contours. This results in high-precision, high-recall detection of large objects in cluttered scenes. Parenthetically, our model has a direct correspondence with the Landau - de Gennes theory of nematic liquid crystal in two dimensions.

q-bio.NC↗

arXiv2014

Friedmann cosmology with particle creation in modified $f(R,T)$ gravity

The theoretical and observational consequences of thermodynamics of open systems, which allow particle creation are investigated in modified $f(R,T)$ ($R$ is the Ricci scalar and $T$ is the trace of energy-momentum tensor) theory of gravity within the framework of a flat Friedmann-Robertson-Walker line element. A simplest particular model $f(R,T)=R+2f(T)$ and "gamma-law" equation of state $p=(γ-1)ρ$ are assumed to explore the role of particle creation in the early and late time evolution of the universe. A power-law expansion model is proposed for $0\leq γ\leq 2$ by considering the natural phenomenological particle creation rate $ψ=3βnH$, where $β$ is a pure number of the order of unity. A Big Rip singularity is observed for $γ<0$, which describes the phantom cosmology. We observe that the accelerated expansion of the universe is driven by the particle creation without any exotic "quintessence" component or a cosmological constant. It is also found by density parameter that the model becomes of negative curvature due to particle creation and the entropy increases with the evolution of the universe. Some kinematics tests such as lookback time, luminosity distance, proper distance, angular diameter versus redshift are discussed in detail to observe the role of particle creation in early and late time evolution of the universe.

gr-qc↗

arXiv2014

Scaling of Clusters near Discontinuous Percolation Transitions in Hyperbolic Networks

We investigate the onset of the discontinuous percolation transition in small-world hyperbolic networks by studying the systems-size scaling of the typical largest cluster approaching the transition, $p\nearrow p_{c}$. To this end, we determine the average size of the largest cluster $\left\langle s_{\rm max}\right\rangle \sim N^{Ψ\left(p\right)}$ in the thermodynamic limit using real-space renormalization of cluster generating functions for bond and site percolation in several models of hyperbolic networks that provide exact results. We determine that all our models conform to the recently predicted behavior regarding the growth of the largest cluster, which found diverging, albeit sub-extensive, clusters spanning the system with finite probability well below $p_{c}$ and at most quadratic corrections to unity in $Ψ\left(p\right)$ for $p\nearrow p_{c}$. Our study suggest a large universality in the cluster formation on small-world hyperbolic networks and the potential for an alternative mechanism in the cluster formation dynamics at the onset of discontinuous percolation transitions.

cond-mat.stat-mech↗

arXiv2013

String Cosmology with Magnetic field in Anisotropic Space-time

In this paper we study the effect of the magnetic field in string cosmology for a spatially homogenous and anisotropic Bianchi type -V space-time model. In order to study the effect of magnetic field, the standard form of the energy momentum tensor for cosmic strings is modified by including an additional term for magnetic field. The magnetic field is due to an electric current produced along the x-axis with infinite electrical conductivity. The field equations are solved for different string models such as geometric string (Nambu string), Takabayashi string (p-string) and Reddy string using string equation of state. We also find the solution for string models with uniform energy density. The physical and geometrical properties of each string model with and without magnetic field are discussed in detail.

gr-qc↗

arXiv2012

Some Exact Solutions of Magnetized viscous model in String Cosmology

In this paper, we study anisotropic Bianchi-V universe with magnetic field and bulk viscous fluid in string cosmology. Exact solutions of the field equations are obtained by using the equation of state for a cloud of strings, and a relationship between bulk viscous coefficient and scalar expansion. The bulk viscous coefficient is assumed to be inversely proportional to the expansion scalar. It is interesting to examine the effects of magnetized bulk viscous string model in early and late stages of the evolution of the universe. This paper presents the different string models like geometrical (Nambu string), Takabayashi (p-string) and Reddy string models by taking certain physical conditions. We discuss the nature of the classical potential for viscous fluid with and without magnetic field. We find that the introduction of bulk viscosity with and without magnetic field results in rapid change in density parameters and in the classical potential. The presence of bulk viscosity prevents the universe to be empty in its future evolution. The other physical and geometrical aspects of each string model are discussed in detail.

gr-qc↗

arXiv2012

LaSrVMoO$_6$: A case study for $A$-site covalency-driven local cationic order in double perovskites

An unusual atomic scale chemical fluctuation in LaSrVMoO$_6$, in terms of narrow patches of La,V and Sr,Mo-rich phases, has been probed in detail to understand the origin of such a chemical state. Exhaustive tuning of the equilibrium synthesis parameters showed that the extent of phase separation can never be melted down below an unit cell dimension making it impossible to achieve the conventional $B$-site ordered structure, which establishes that the observed `inhomogeneous' patch-like structure with minimum dimension of few angstroms is a reality in LaSrVMoO$_6$. Therefore, another type of local chemical order, hitherto unknown in double perovskites, gets introduced here. X-ray diffraction, electron microscopy elemental mapping, magnetic, and various spectroscopic studies have been carried out on samples, synthesized under different conditions. These experimental results in conjunction with {\it ab-initio} electronic structure calculation revealed that it is the energy stability, gained by typical La-O covalency as in LaVO$_3$, that leads to the preferential La,V and Sr,Mo ionic proximity, and the consequent patchy structure.

cond-mat.mtrl-sci↗

arXiv2012

An Intelligent Mobile-Agent Based Scalable Network Management Architecture for Large-Scale Enterprise System

Several Mobile Agent based distributed network management models have been proposed in recent times to address the scalability and flexibility problems of centralized (SNMP or CMIP management models) models. Though the use of Mobile Agents to distribute and delegate management tasks comes handy in dealing with the previously stated issues, many of the agent-based management frameworks like initial flat bed models and static mid-level managers employing mobile agents models cannot efficiently meet the demands of current networks which are growing in size and complexity. Moreover, varied technologies, such as SONET, ATM, Ethernet, DWDM etc., present at different layers of the Access, Metro and Core (long haul) sections of the network, have contributed to the complexity in terms of their own framing and protocol structures. Thus, controlling and managing the traffic in these networks is a challenging task. This paper presents an intelligent scalable hierarchical agent based model for the management of large-scale complex networks to address aforesaid issues. The cost estimation, carried out with a view to compute the overall management cost in terms of management data overhead, is being presented. The results obtained thereafter establish the usefulness of the presented architecture as compare to centralized and flat bed agent based models.

cs.NI↗

arXiv2011

Atomic scale chemical fluctuation in LaSrVMoO6: A proposed halfmetallic antiferromagnet

Half metallic antiferromagnets (HMAFM) have been proposed theoretically long ago but have not been realized experimentally yet. Recently, a double perovskite compound, LaSrVMoO6, has been claimed to be an almost real HMAFM system. Here, we report detailed experimental and theoretical studies on this compound. Our results reveal that the compound is neither a half metal nor an ordered antiferromagnet. Most importantly, an unusual chemical fluctuation is observed locally, which finally accounts for all the electronic and magnetic properties of this compound.

cond-mat.mtrl-sci↗