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P. Devi

Publications and source records attributed to P. Devi.

At least 19 recordsLinked to original sources
arXiv2025

The Birth of a Major Coronal Mass Ejection with Intricate Magnetic Structure from Multiple Active Regions

Coronal mass ejections (CMEs) are the eruptions of magnetised plasma from the Sun and are considered the main driver of adverse space weather events. Hence, undrstanding its formation process, particularly the magnetic topology, is critical for accurate space weather prediction. Here, based on imaging observations and three-dimensional (3D) data-constrained thermodynamic magnetohydrodynamical (MHD) simulation in spherical coordinates, we exhibit the birth of a CME with intricate magnetic structure from multiple active regions (ARs) due to 3D magnetic reconnection. It is observed as a coronal jet between active regions, accompanied by the back-flowing of filament materials along the jet spine after the passage of the eruptive filament. This jet connects two dimming regions within different active regions. This is an observational proxy of 3D magnetic reconnection between the CME flux rope and the null-point magnetic field lines crossing active regions. Hereafter, the thermodynamic data-constrained MHD simulation successfully reproduces the observed jet and the reconnection process that flux ropes partake in, leading to a CME flux rope with a complex magnetic structure distinct from its progenitor. The generality of this scenario is then validated by data-inspired MHD simulations in a simple multipolar magnetic configuration. This work demonstrates the role of multiple active regions in forming CMEs with intricate magnetic structures. On the one hand, a non-coherent flux rope where not all twisted magnetic field lines wind around one common axis is naturally formed. On the other hand, our findings suggest that the topology of a real CME flux rope may not be solely determined by a single active region, particularly during periods of solar maximum.

astro-ph.SR↗
arXiv2024

Filament eruption deflection and associated CMEs

We present the observations of a quiescent filament eruption and its deflection from the radial direction. The event occurred in the southern solar hemisphere on 2021 May 9 and was observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO), by the STEREO A Observatory and GONG. Part of the filament erupted in the west direction, while major part of the filament deviated towards east direction. LASCO observed a very weak CME towards the west direction where it faded quickly. Moreover, the eruption was associated with CME observed by STEREO A COR1 and COR2. Our observations provide the evidence that the filament eruption was highly non-radial in nature.

astro-ph.SR↗
arXiv2022

Thermoelectric properties of Fe$_{2}$VAl at high temperature region: A combined experimental and theoretical study

Heusler type compounds have long been recognized as potential thermoelectric (TE) materials. Here, the experimentally observed TE properties of Fe$_{2}$VAl are understood through electronic structure calculations in the temperature range of $300-800$ K. The observed value of $S$ is $\sim-$138 $\mu$V/K at 300 K. Then, the $|S|$ decreases with increase in temperature up to the highest temperature with the value of $\sim-$18 $\mu$V/K at 800 K. The negative sign of $S$ in the full temperature window signifies the dominating $n$-type character of the compound. The temperature dependent of electrical conductivity, $\sigma$ (thermal conductivity, $\kappa$) exhibits the increasing (decreasing) trend with the values of $\sim$1.2 $\times$ 10$^{5}$ $\Omega^{-1}$m$^{-1}$ ($\sim$23.7 W/m-K) and $\sim$2.2 $\times$ 10$^{5}$ $\Omega^{-1}$m$^{-1}$ ($\sim$15.3 W/m-K) at 300 K and 800 K, respectively. In order to understand these transport properties, the DFT based semi-classical Boltzmann theory is used. The contributions of multi-band electron and hole pockets are found to be mainly responsible for the temperature dependent trend of these properties. The decrement of $|S|$ and increment of $\sigma/\tau$ $\&$ $\kappa_{e}/\tau$ ($\tau$ is relaxation time) with temperature is directly related with the contribution of multiple hole pockets. Present study suggests that DFT based electronic calculations provide reasonably good explanations of experimental TE properties of Fe$_{2}$VAl in the high temperature range of $300-800$ K.

cond-mat.mtrl-sci↗
arXiv2020

Study of single crystalline SrAgSb and SrAuSb semimetals

Given renewed interest in the electronic properties of semimetallic compounds with varying degrees of spin orbit coupling we have grown single crystals of SrAgSb and SrAuSb, measured their temperature and field dependent electrical resistivity and magnetization and performed density functional theory (DFT) band structure calculations. Magnetization measurements are consistent with a diamagnetic host with a small amount of local moment bearing impurities. Although the residual resistivity ratio (RRR) for all samples studied was relatively low, ranging between 2.4 and 3.4, the compounds had non-saturating magnetoresistance (MR), reaching values of $\sim$ 17% and $\sim$ 70% at 4 K and 9 T for SrAgSb and SrAuSb respectively. Band structure calculations, using the experimentally determined Wyckoff positions for the Sr, Ag/Au, and Sb atoms, show that whereas SrAgSb is a topologically trivial, but compensated, semimetal; SrAuSb is a topologically non-trivial, Dirac semimetal.

cond-mat.mtrl-sci↗
arXiv2020

Effect of chemical and hydrostatic pressure on the coupled magnetostructural transition of Ni-Mn-In Heusler alloys

Ni-Mn-In magnetic shape-memory Heusler alloys exhibit generally a large thermal hysteresis at their first-order martensitic phase transition which hinder a technological application in magnetic refrigeration. By optimizing the Cu content in Ni$_2$Cu$_x$Mn$_{1.4-x}$In$_{0.6}$, we obtained a thermal hysteresis of the martensitic phase transition in Ni$_{2}$Cu$_{0.2}$Mn$_{1.2}$In$_{0.6}$ of only 6 K. We can explain this very small hysteresis by an almost perfect habit plane at the interface of martensite and austenite phases. Application of hydrostatic pressure does not reduce the hysteresis further, but shifts the martensitic transition close to room temperature. The isothermal entropy change does not depend on warming or cooling protocols and is pressure independent. Experiments in pulsed-magnetic fields on Ni$_{2}$Cu$_{0.2}$Mn$_{1.2}$In$_{0.6}$ find a reversible magnetocaloric effect with a maximum adiabatic temperature change of -13 K.

cond-mat.mtrl-sci↗
arXiv2020

Improved crystallographic compatibility and magnetocaloric reversibility in Pt substituted Ni2Mn1.4In0.6 magnetic shape memory Heusler alloy

We present here the improved crystallographic/geometric compatibility and magnetocaloric reversibility by measurement of magnetic entropy change using different protocols in 10% Pt substituted Ni2Mn1.4In0.6 magnetic shape memory alloy. The substitution of Pt reduces the thermal hysteresis about 50% to the Ni2Mn1.4In0.6. The origin of the reduced thermal hysteresis is investigated by the crystallographic compatibility of the austenite and martensite phases. The calculated middle eigenvalue of the transformation matrix turned out to be 0.9982, which is very close to 1 (deviation is only 0.18%) suggests for the crystallographic compatibility between the austenite and martensite phases in Ni1.9Pt0.1Mn1.4In0.6. A very small thermal hysteresis and crystallographic compatibility between two phases in this alloy system indicate a stress-free transition layer (i.e. perfect habit plane) between the austenite and martensite phase, which is expected to give reversible martensite phase transition and therefore reversible magnetocaloric effect (MCE) as well. The calculated value of the isothermal entropy change ({\Delta}Siso) using the magnetization curve under three different measurement protocols (i.e. isothermal, loop, and isofield measurement protocol) is found to be nearly same indicating a reversible MCE in the present alloy system. Our work provides a path to design new magnetic shape memory Heusler alloys for magnetic refrigeration and also suggest that any of the above measurement protocol can be used for the calculation of {\Delta}Siso for materials satisfying geometrical compatibility condition.

cond-mat.mtrl-sci↗
arXiv2020

Improved magnetostructural and magnetocaloric reversibility in magnetic Ni-Mn-In shape-memory Heusler alloy by optimizing the geometric compatibility condition

We report an improved reversibility of magnetostriction and inverse magnetocaloric effect (MCE) for the magnetic shape-memory Heusler alloy Ni$_{1.8}$Mn$_{1.8}$In$_{0.4}$. We show that the magnetostriction and MCE crucially depends on the geometrical compatibility of the austenite and martensite phases. Detailed information on the compatibility of both phases has been obtained from the transformation matrix calculated from x-ray diffraction data. The uniqueness of the lattice parameters results in an improved reversibility of the magnetostriction and the MCE. In the thermal hysteresis region of the martensitic transformation, the maximum relative length change is 0.3% and the adiabatic temperature change $\Delta T_{ad}\approx -10$ K in pulsed magnetic fields. Our results reveal that the approach of geometric compatibility will allow one to design materials with reversible magnetostriction and reversible inverse MCE at a first-order magnetostructural phase transition in shape-memory Heusler alloys.

cond-mat.mtrl-sci↗
arXiv2020

Reversible adiabatic temperature change in the shape memory Heusler alloy Ni2.2Mn0.8Ga: An effect of structural compatibility

The large magnetocaloric effect (MCE) observed in Ni-Mn based shape-memory Heusler alloys put them forward to use in magnetic refrigeration technology. It is associated with a first-order magnetostructural (martensitic) phase transition. We conducted a comprehensive study of the MCE for the off-stoichiometric Heusler alloy Ni$_{2.2}$Mn$_{0.8}$Ga in the vicinity of its first-order magnetostructural phase transition. We found a reversible MCE under repeated magnetic field cycles. The reversible behavior can be attributed to the small thermal hysteresis of the martensitic phase transition. Based on the analysis of our detailed temperature dependent X-ray diffraction data, we demonstrate the geometric compatibility of the cubic austenite and tetragonal martensite phases. This finding directly relates the reversible MCE behavior to an improved geometric compatibility condition between cubic austenite and tetragonal martensite phases. The approach will help to design shape-memory Heusler alloys with a large reversible MCE taking advantage of the first-order martensitic phase transition.

cond-mat.mtrl-sci↗
arXiv2019

Bounds on Spreads of Matrices related to Fourth Central Moment. II

We derive some inequalities involving first four central moments of discrete and continuous distributions. Bounds for the eigenvalues and spread of a matrix are obtained when all its eigenvalues are real. Likewise, we discuss bounds for the roots and span of a polynomial equation.

math.ST↗
arXiv2018

Exploring the best scenario for understanding the high temperature thermoelectric behaviour of Fe2VAl

Heusler-type Fe2VAl compound is a promising thermoelectric candidate with non-magnetic ground state. The present work investigates the Seebeck coefficient (S) of Fe2VAl in the temperature region 300 to 620 K with the help of experimental and theoretical tools. The experimental value of S is observed -130 {\mu}V/K at 300 K. Afterthat, the magnitude of S decreases gradually as the temperature increases. At T = 620 K, the value of S is found to be -26 {\mu}V/K. In order to understand the behaviour of the experimentally observed S value, the band-structure and density of states calculations are performed by using LDA, PBE, PBEsol, mBJ and SCAN within density functional theory. All the above mentioned exchange-correlation (XC) functionals (except mBJ) predict the semi-metal like behaviour of the compound, whereas the mBJ gives the indirect band gap of 0.22 eV having the well agreement with experimentally observed value. The temperature dependence of S for Fe2VAl is also calculated with the help of all the five mentioned functionals individually. The best XC functional is investigated for searching the new thermoelectric materials by taking Fe2VAl as a case example through this study. The best matching between experimental and calculated values of S as a function of temperature is observed by setting the mBJ band gap with the band-structure of PBEsol or SCAN. Therefore, the present study suggests that the band-structure of PBEsol or SCAN with mBJ band gap can be used for searching the new thermoelectric materials.

cond-mat.mtrl-sci↗
arXiv2018

Gradual pressure-induced change in the magnetic structure of the non-collinear antiferromagnet Mn$_3$Ge

By means of powder neutron diffraction we investigate changes in the magnetic structure of the coplanar non-collinear antiferromagnet Mn$_3$Ge caused by an application of hydrostatic pressure up to 5\phantom{ }GPa. At ambient conditions the kagom\'e layers of Mn atoms in Mn$_3$Ge order in a triangular 120$^{\circ}$ spin structure. Under high pressure the spins acquire a uniform out-of-plane canting, gradually transforming the magnetic texture to a non-coplanar configuration. With increasing pressure the canted structure fully transforms into the collinear ferromagnetic one. We observed that magnetic order is accompanied by a noticeable magnetoelastic effect, namely, spontaneous magnetostriction. The latter induces an in-plane magnetostrain of the hexagonal unit cell at ambient pressure and flips to an out-of-plane strain at high pressures in accordance with the change of the magnetic structure.

cond-mat.str-el↗
arXiv2017

Co$_{2}$PtGa: A promising magnetic shape memory alloy with high martensite transition temperature

In the present work, a combined theoretical and experimental study on Co$_{2}$PtGa Heusler alloy shows that it exhibits a martensite transition around 1320 K with a small thermal hysteresis (10 K). Dynamical stability of Co$_2$PtGa in the tetragonal phase has been established by the theoretically calculated phonon dispersion curves. Magnetization measurements suggest that this alloy is a ferromagnetic material with a saturation magnetic moment of 2.83 $\mu_B$/f.u. at 2 K, which is in excellent agreement with the value obtained from \textit{ab-initio} calculations (2.87 $\mu_B$/f.u.). Our present study demonstrates that Co$_{2}$PtGa is a promising material for high temperature magnetic shape memory application.

cond-mat.mtrl-sci↗
arXiv2016

Robust Bain distortion in the premartensite phase of platinum substituted Ni2MnGa magnetic shape memory alloy

The premartensite phase of shape memory and magnetic shape memory alloys (MSMAs) is believed to be a precursor state of the martensite phase with preserved austenite phase symmetry. The thermodynamic stability of the premartensite phase and its relation to the martensitic phase is still an unresolved issue, even though it is critical to the understanding of the functional properties of MSMAs. We present here unambiguous evidence for macroscopic symmetry breaking leading to robust Bain distortion in the premartensite phase of 10% Pt substituted Ni2MnGa. We show that the robust Bain distorted premartensite (T2) phase results from another premartensite (T1) phase with preserved cubic-like symmetry through an isostructural phase transition. The T2 phase finally transforms to the martensite phase with additional Bain distortion on further cooling. Our results demonstrate that the premartensite phase should not be considered as a precursor state with the preserved symmetry of the cubic austenite phase.

cond-mat.mtrl-sci↗
arXiv2016

Adaptive modulation in Ni2Mn1.4In0.6 magnetic shape memory Heusler alloy

The origin of incommensurate structural modulation in Ni-Mn based Heusler type magnetic shape memory alloys (MSMAs) is still an unresolved issue inspite of intense focus on this due to its role in the magnetic field induced ultra-high strains. In the archetypal MSMA Ni2MnGa, the observation of non-uniform displacement of atoms from their mean positions in the modulated martensite phase, premartensite phase and charge density wave as well as the presence of phason broadening of satellite peaks have been taken in support of the electronic instability model linked with a soft acoustic phonon. We present here results of a combined high resolution synchrotron x-ray powder diffraction (SXRPD) and neutron powder diffraction (NPD) study on Ni2Mn1.4In0.6 using (3+1)D superspace group approach, which reveal not only uniform atomic displacements in the modulated structure of the martensite phase with physically acceptable ordered magnetic moments in the antiferromagnetic phase at low temperatures but also the absence of any premartensite phase and phason broadening of the satellite peaks. Our HRTEM studies and first principles calculations of the ground state also support uniform atomic displacements predicted by powder diffraction studies. All these observations suggest that the structural modulation in the martensite phase of Ni2Mn1.4In0.6 MSMA can be explained in terms of the adaptive phase model. The present study underlines the importance of superspace group analysis using complimentary SXRPD and NPD in understanding the physics of the origin of modulation as well as the magnetic and the modulated ground states of the Heusler type MSMAs. Our work also highlights the fact that the mechanism responsible for the origin of modulated structure in different Ni-Mn based MSMAs may not be universal and it must be investigated thoroughly in different alloy compositions.

cond-mat.mtrl-sci↗
arXiv2016

Inclusion graph of subgroups of a group

For a finite group $G$, we define the inclusion graph of subgroups of $G$, denoted by $\mathcal I(G)$, is a graph having all the proper subgroups of $G$ as its vertices and two distinct vertices $H$ and $K$ in $\mathcal I(G)$ are adjacent if and only if either $H \subset K$ or $K \subset H$. In this paper, we classify the finite groups whose inclusion graph of subgroups is one of complete, bipartite, tree, star, path, cycle, disconnected, claw-free. Also we classify the finite abelian groups whose inclusion graph of subgroups is planar. For any given finite group, we estimate the clique number, chromatic number, girth of its inclusion graph of subgroups and for a finite abelian group, we estimate the diameter of its inclusion graph of subgroups. Among the other results, we show that some groups can be determined by their inclusion graph of subgroups

math.GR↗
arXiv2015

Coprime graph of subgroups of a group

Let $G$ be a group. We define the coprime graph of subgroups of $G$, denoted by $\mathcal P(G)$, is a graph whose vertex set is the set of all proper subgroups of $G$, and two distinct vertices are adjacent if and only if the order of the corresponding subgroups are coprime. In this paper, we study some connections between algebraic properties of a group and graph theoretic properties of its coprime graph.

math.GR↗
arXiv2015

Intersection graph of cyclic subgroups of groups

Let $G$ be a group. The intersection graph of cyclic subgroups of $G$, denoted by $\mathscr I_c(G)$, is a graph having all the proper cyclic subgroups of $G$ as its vertices and two distinct vertices in $\mathscr I_c(G)$ are adjacent if and only if their intersection is non-trivial. In this paper, we classify the finite groups whose intersection graph of cyclic subgroups is one of totally disconnected, complete, star, path, cycle. We show that for a given finite group $G$, $girth(\mathscr I_c (G)) \in \{3, \infty\}$. Moreover, we classify all finite non-cyclic abelian groups whose intersection graph of cyclic subgroups is planar. Also for any group $G$, we determine the independence number, clique cover number of $\mathscr I_c (G)$ and show that $\mathscr I_c (G)$ is weakly $\alpha$-perfect. Among the other results, we determine the values of $n$ for which $\mathscr I_c (\mathbb{Z}_n)$ is regular and estimate its domination number.

math.GR↗