arXiv subjects

A. K. Das

Publications and source records attributed to A. K. Das.

At least 55 records · Page 3Linked to original sources

arXiv2019

Magnetic glassy state at low spin state of Co3+ in EuBaCo2O5+δ (δ = 0.47) cobaltite

The magnetic glassy state is a fascinating phenomenon, which results from the kinetic arrest of the first order magnetic phase transition. Interesting properties, such as metastable magnetization and nonequilibrium magnetic phases, are naturally developed in the magnetic glassy state. Here, we report magnetic glass property in the low spin state of Co3+ in EuBaCo2O5+δ (δ = 0.47) cobaltite at low temperature (T < 60 K). The measurements of magnetization under the cooling and heating in unequal fields, magnetization relaxation and thermal cycling of magnetization show the kinetic arrest of low magnetization state below 60 K. The kinetically arrested low temperature magnetic phase is further supported through the study of isothermal magnetic entropy, which shows the significant entropy change. The present results will open a new window to search the microscopic relation between the spin state transitions and the kinetic arrest induced magnetic glassy phenomena in complex materials.

cond-mat.mtrl-sci↗

arXiv2019

Size-Dependent Structural and Magnetic Properties of Disordered Co2FeAl Heusler Alloy Nanoparticles

Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by chemical route. The effect of the size of NPs upon the structure and magnetization compared to its bulk counterpart was investigated. The structure and composition were determined from X-ray diffraction (XRD) and electron microscopy. XRD analysis shows that the samples are having single (A2-type) disordered phase. Magnetization measurements suggest that the samples are soft ferromagnetic in nature with very low coercivity. Enhanced magnetic properties like saturation magnetization, coercive force, retentivity, and Curie-temperature are observed with a decrease in particle size. The effect of particle size on hysteresis losses is also discussed. The smallest particles of size 16 nm exhibited the highest saturation magnetization and transition temperature of 180.73 emu/g and 1261 K, respectively. The origin of enhancement in the magnetization of Co2FeAl nano-alloy is attributed to the strong Co-Co exchange interaction due to disorder present in the systems.

cond-mat.str-el↗

arXiv2019

On Semimonotone Star Matrices and Linear Complementarity Problem

In this article, we introduce the class of semimonotone star ($E_0^s$) matrices. We establish the importance of the class of $E_0^s$-matrices in the context of complementarity theory. We show that the principal pivot transform of $E_0^s$-matrix is not necessarily $E_0^s$ in general. However, we prove that $\tilde{E_0^s}$-matrices, a subclass of the $E_0^s$-matrices with some additional conditions, is in $E_0^f$ by showing this class is in $P_0.$ We prove that LCP$(q, A)$ can be processable by Lemke's algorithm if $A\in \tilde{E_0^s}\cap P_0.$ We find some conditions for which the solution set of LCP$(q, A)$ is bounded and stable under the $\tilde{E^s_0}$-property. We propose an algorithm based on an interior point method to solve LCP$(q, A)$ given $A \in \tilde{E^{s}_{0}}.$

math.OC↗

arXiv2019

Development of an Advanced Force Field for Water using Variational Energy Decomposition Analysis

Given the piecewise approach to modeling intermolecular interactions for force fields, they can be difficult to parameterize since they are fit to data like total energies that only indirectly connect to their separable functional forms. Furthermore, by neglecting certain types of molecular interactions such as charge penetration and charge transfer, most classical force fields must rely on, but do not always demonstrate, how cancellation of errors occurs among the remaining molecular interactions accounted for such as exchange repulsion, electrostatics, and polarization. In this work we present the first generation of the (many-body) MB-UCB force field that explicitly accounts for the decomposed molecular interactions commensurate with a variational energy decomposition analysis, including charge transfer, with force field design choices that reduce the computational expense of the MB-UCB potential while remaining accurate. We optimize parameters using only single water molecule and water cluster data up through pentamers, with no fitting to condensed phase data, and we demonstrate that high accuracy is maintained when the force field is subsequently validated against conformational energies of larger water cluster data sets, radial distribution functions of the liquid phase, and the temperature dependence of thermodynamic and transport water properties. We conclude that MB-UCB is comparable in performance to MB-Pol, but is less expensive and more transferable by eliminating the need to represent short-ranged interactions through large parameter fits to high order polynomials.

physics.chem-ph↗

arXiv2019

Electron doping induced magnetic glassy state in phase separated YBaCo$_2$O$_{5.5-δ}$

The structural, magnetic and transport properties of the layered RBaCo$_2$O$_{5.5}$ cobaltites are sensitive to the oxygen stoichiometry. In this present study, we report the presence of a low-temperature magnetic glassy state in electron-doped polycrystalline YBaCo$_2$O$_{5.5}$ cobaltite. The ac magnetization studies show the absence of conventional spin glass features, while the various dc magnetization studies demonstrate the presence of non-equilibrium magnetic glassy state at low temperature. The magnetic glassy state of this sample results from the kinetic arrest of the first order ferro (or ferri) to antiferromagnetic transition. The role of electron doping in the occurrence of magnetic glassy phenomena is discussed in terms of magnetic phase separation involving the Co3+/Co2+ clusters of the ferrimagnetic phase in the Co3+/Co3+ antiferromagnetic matrix.

cond-mat.str-el↗

arXiv2019

Effect of L21 and XA ordering on phase stability, half-metallicity and magnetism of Co2FeAl Heusler Alloy: GGA and GGA+U approach

The generalized gradient approximation (GGA) scheme in the first-principles calculations are used to study the effect of L21 and XA ordering on the phase stability, half-metallicity and magnetism of Co2FeAl (CFA) Heusler alloy. Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II were prepared under the conventional L21 and inverse XA phases by altering the atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of CFA is the most stable phase energetically among all the structures. The electronic structure calculations without U show the presence of half-metallic (HM) ground state only in L21-1 structure and the other structures are found to be metallic. However, the electronic structures of CFA are significantly modified in the presence of U, although the total magnetic moments per cell remained the same and consistent with the Slater-Pauling (SP) rule. The metallic ground states of CFA in L21-II and XA-II structures are converted into the half-metallic ground states in presence of U but remained the same (metallic) in XA-I structure. The results indicate that the electronic structures are not only dependent on the L21 and XA ordering of the atoms but also depend on the choice of U values. So experiments may only verify the superiority of GGA+U to GGA.

cond-mat.str-el↗

arXiv2019

More On Hidden $Z$-Matrices and Linear Complementarity Problem

In this article we study linear complementarity problem with hidden $Z$-matrix. We extend the results of Fiedler and Pt{á}k for the linear system in complementarity problem using game theoretic approach. We establish a result related to singular hidden $Z$-matrix. We show that for a non-degenerate feasible basis, linear complementarity problem with hidden $Z$-matrix has unique non-degenerate solution under some assumptions. The purpose of this paper is to study some properties of hidden $Z$-matrix.

math.OC↗

arXiv2018

Iterative Descent Method for Generalized Leontief Model

In this paper we consider generalized Leontief model. We show that under certain condition the generalized Leontief model is solvable by iterative descent method based on infeasible interior point algorithm. We prove the convergence of the method from strictly positive starting point. A numerical example is presented to demonstrate the performance of the algorithm

math.OC↗

arXiv2018

Some Aspects on Solving Transportation Problem

In this paper, we consider a class of transportation problems which arises in sample surveys and other areas of statistics. The associated cost matrices of these transportation problems are of special structure. We observe that the optimality of North West corner solution holds for the general problem where cost component is replaced by a convex function. We revisit assignment problem and present a weighted version of K$\ddot{o}$nig-Egerv$\acute{a}$ry theorem and Hungarian method. The weighted Hungarian method proposed in the paper can be used for solving transportation problem.

math.OC↗

arXiv2016

Low field magnetoelectric effect in Fe substituted Co4Nb2O9

Co4Nb2O9 (CNO) having α-Al2O3 crystal structure with Co chains along c-direction shows gigantic magnetoelelctric coupling below antiferromagnetic ordering temperature of 27 K but above a spin flop field of 1.6 T. We have investigated structural, magnetic and magnetoelectric properties of Fe substituted (10% and 20%) samples and compared with the parent one. In fact magnetic and specific heat measurements have revealed an additional magnetic transition below 10 K and presence of short range magnetic ordering above ~ 50 K in parent as well as in Fe substituted samples. Linear magnetoelelctric and ferroelectric behaviours are evidenced in the Fe substituted samples where an electric field of 5 kV/m is sufficient to align the dipoles and the magnetoelelctric coupling is ensured for magnetic fields as low as 0.25 T, far below the spin flop field.

cond-mat.mtrl-sci↗

arXiv2016

Generalized Dirac duality and CP violation in a two photon theory

A kinetic mixing term, which generalizes the duality symmetry of Dirac, is studied in a theory with two photons (visible and hidden). This theory can be either CP conserving or CP violating depending on the transformation of fields in the hidden sector. However if CP is violated, it necessarily occurs in the hidden sector. This opens up an interesting possibility of new sources of CP violation.

hep-ph↗

arXiv2013

A non-scanning atomic magnetometer based on coherent population trapping

The method of operation of an atomic magnetometer based on coherent population trapping (CPT) without any requirement of radio frequency scanning is demonstrated. Using a hybrid approach comprising of polarization rotation and tailored transmission due to CPT states, the simultaneous operation of the device as atomic frequency standard as well as atomic magnetometer is investigated. The magnetometer sensitivity can be further improved by utilizing the inherent option of magnetic field assisted synchronized radio frequency scanning.

physics.atom-ph↗

arXiv2012

Non-linear magneto-optic and self polarization rotation by superposition of states

We report the observation of enhanced magneto-optic rotation as the coherent superposition of different hyperfine states is established in an atomic sample. The polarization rotation near the two photon Raman resonance condition appears to have an analogous characteristic to the well established Faraday rotation observed in the vicinity of a single photon resonance, however it contains sharp features arising from coherent population trapping state. The profile of the two photon rotation signal exhibits interesting features for slightly imbalanced circular polarization component of the laser field as well as for on and away from the single photon resonance. The investigation can be used to explore the effect of superposition states generated by coherent population trapping on optical activity. A complete density matrix based numerical simulation that consistently captures all the relevant features of the experiment is presented.

physics.optics↗

arXiv2011

Large magnetodielectric response in Pr0.6Ca0.4MnO3/ polyvinylidene fluoride nanocomposites

We have studied the magnetic field effect on low frequency dielectric properties of Pr0.6Ca0.4MnO3/polyvinylidene fluoride nanocomposite with 22.5% volume fraction of Pr0.6Ca0.4MnO3 nanoparticles. A strong magnetodielectric response was observed below 120 K where Pr0.6Ca0.4MnO3 nanoparticles show the magnetic phase transition indicating a direct correlation between magnetism and dielectric properties. A large change of the dielectric permittivity ~ 30% has been observed in a magnetic field of 4.6 T with loss as low as 0.17 at 70 K. The observed magnetodielectric response has been attributed to the decrement of polaron activation barrier of Pr0.6Ca0.4MnO3 nanoparticles with the increase of magnetic field.

cond-mat.mtrl-sci↗

arXiv2010

Multi-Objective Geometric Programming Problem Being Cost Coefficients as Continous Function with Weighted Mean Method

Geometric programming problems occur frequently in engineering design and management. In multiobjective optimization, the trade-off information between different objective functions is probably the most important piece of information in a solution process to reach the most preferred solution. In this paper we have discussed the basic concepts and principles of multiple objective optimization problems and developed a solution procedure to solve this optimization problem where the cost coefficients are continuous functions using weighted method to obtain the non-inferior solutions.

cs.DS↗

arXiv2010

Geometric Programming Problem with Co-Efficients and Exponents Associated with Binary Numbers

Geometric programming (GP) provides a power tool for solving a variety of optimization problems. In the real world, many applications of geometric programming (GP) are engineering design problems in which some of the problem parameters are estimating of actual values. This paper develops a solution procedure to solve nonlinear programming problems using GP technique by splitting the cost coefficients, constraint coefficients and exponents with the help of binary numbers. The equivalent mathematical programming problems are formulated to find their corresponding value of the objective function based on the duality theorem. The ability of calculating the cost coefficients, constraint coefficients and exponents developed in this paper might help lead to more realistic modeling efforts in engineering design areas. Standard nonlinear programming software has been used to solve the proposed optimization problem. Two numerical examples are presented to illustrate the method.

math.NA↗

arXiv2009

Interaction of fast charged projectiles with two-dimensional electron gas: Interaction and disorder effects

The results of a theoretical investigation on the stopping power of ions moving in a disordered two-dimensional degenerate electron gas are presented. The stopping power for an ion is calculated employing linear response theory using the dielectric function approach. The disorder, which leads to a damping of plasmons and quasiparticles in the electron gas, is taken into account through a relaxation time approximation in the linear response function. The stopping power for an ion is calculated in both the low- and high-velocity limits. In order to highlight the effects of damping we present a comparison of our analytical and numerical results, in the case of point-like ions, obtained for a non-zero damping with those for a vanishing damping. It is shown that the equipartition sum rule first formulated by Lindhard and Winther for three-dimensional degenerate electron gas does not necessarily hold in two-dimensions. We have generalized this rule introducing an effective dielectric function. In addition some new results for two-dimensional interacting electron gas have been obtained. In this case the exchange-correlation interactions of electrons are considered via local-field-corrected dielectric function.

physics.plasm-ph↗

arXiv2008

DC transferred arc thermal plasma assisted growth of nanoparticles with different crystalline phases

The control of the crystalline phases of the nanoparticles grown in a direct-current transferred-arc plasma-assisted reactor is reported. The crystalline phases of the as synthesized nanoparticles are shown to critically depend on the operating gas pressure. The paper reports about the change in the crystalline phases of three distinct compounds namely aluminium oxide (Al2O3), aluminium nitride (AlN) and iron oxide (FexOy). The major outcome of the present work is that the phases having higher defect densities are more probable to form at the sub-atmospheric operating pressure. The variations in the crystalline structures are discussed on the basis of the equilibrium defect density formed during the homogeneous nucleation. The as synthesized nanoparticles were examined by X-ray diffraction analysis and transmission electron microscopy. In addition, the confirmatory analysis for the crystalline phases of the as synthesized iron oxides was carried out with the help of Mossbauer spectroscopy.

cond-mat.mtrl-sci↗