arXiv subjects

S. K. Srivastava

Publications and source records attributed to S. K. Srivastava.

At least 37 records · Page 2Linked to original sources

arXiv2016

Magnetic instability and $f-d$ hybridization in CeFe$_2$ on substituting Cr, Ag and Au for Fe

Hybridization between Ce $f$ and conduction $d$ states has been speculatively known to be one of the mechanisms responsible for magnetic instability of the ferromagnetic ground state of CeFe$_2$. Substituting Fe by small amounts of certain elements stabilizes it to an antiferromagnetic state below the Curie temperature via a first-order second phase transition. In the present work, we seek any direct relation between the $f-d$ hybridization and the second transition by measuring primarily dc magnetization and Ce M$_{4,5}$ edge x-ray absorption spectra of Ce(Fe$_{1-x}$M$_x$)$_2$ pseudobinaries, with M = Cr, Ag and Au. X-ray diffraction and x-ray photoelectron spectroscopy measurements are also performed essentially to monitor the quality of the samples. Whereas Cr impurity is found to cause the second transition, Ag and Au apparently do not induce any. In the former, the Curie and second transition temperatures vary systematically, but differently, with $x$. Our results imply that there is a definite proportionality between the $x$ dependences of the second transition temperature and the $f-d$ hybridization strength estimated qualitatively from the absorption spectra.

cond-mat.str-el↗

arXiv2016

Geometry of pointwise pseudo-slant warped product submanifolds in a Kähler manifold

The purpose of this paper is to study pointwise pseudo-slant warped product submanifolds of a Kähler manifold $\widetilde{M}$. We derive the conditions of integrability and totally geodesic foliation for the distributions allied to the characterization of a pointwise pseudo-slant submanifold of $\widetilde{M}$. The necessary and sufficient conditions for isometrically immersed pointwise pseudo-slant submanfold of $\widetilde{M}$ to be a pointwise pseudo-slant warped product and a locally Riemannian product are obtained.

math.DG↗

arXiv2016

Harmonic maps and para-Sasakian geometry

The purpose of this paper is to study the harmonicity of maps to or from para-Sasakian manifolds. We derive the condition for the tension field of paraholomorphic map between almost para-Hermitian manifold and para-Sasakian manifold. The necessary and sufficient condition for a paraholomorphic map between para-Sasakian manifolds to be parapluriharmonic are shown and a non-trivial example is presented for its illustrations.

math.DG↗

arXiv2016

$\mathcal{P}\mathcal{R}$-anti-slant warped product submanifold of a nearly paracosymplectic manifold

In this paper, we study $\mathcal{P}\mathcal{R}$-anti-slant warped product submanifold of a nearly paracosymplectic manifold $\widetilde{M}$. The necessary and sufficient condition is obtained for the distributions allied to the characterization of a $\mathcal{P}\mathcal{R}$-anti-slant submanifold being integrable and totally geodesic foliation. In addition, we have defined $\mathcal{P}\mathcal{R}$-anti-slant warped product submanifold of $\widetilde{M}$ and gave some illustrations. Finally, we extracted the constraints for a submanifold of $\widetilde{M}$ to be a $\mathcal{P}\mathcal{R}$-anti-slant warped product of the form $F\times_{f}N_λ$.

math.DG↗

arXiv2015

Geometry of $\mathcal{P}\mathcal{R}$-semi-invariant warped product submanifolds in paracosymplectic manifold

The purpose of this paper is to study $\mathcal{P}\mathcal{R}$-semi-invariant warped product submanifolds of a paracosymplectic manifold $\widetilde{M}$. We prove that the distributions associated with the definition of $\mathcal{P}\mathcal{R}$-semi-invariant warped product submanifold $M$ are always integrable. A necessary and sufficient condition for an isometrically immersed $\mathcal{P}\mathcal{R}$-semi-invariant submanifold of $\widetilde{M}$ to be a $\mathcal{P}\mathcal{R}$-semi-invariant warped product submanifold is obtained in terms of the shape operator.

math.DG↗

arXiv2015

Magnetic Property of Rutile Ti0.94A0.06O2 (A=Li, Mg, K) Compounds

Our purpose is to study role of ionic radii of non-magnetic dopants; Li+ (0.68 Å), Mg2+ (0.72 Å) and K+ (1.38 Å) on the magnetic property of rutile TiO2 compound. The Ti0.94A0.06O2 (A=Li, Mg, K) compounds have been synthesized via solid state route method at equilibrium. The structural analyses of X-ray diffraction pattern reveals that the doping of Li and Mg lead to Ti site substitution and K doping lead to core shell kind of structure. The magnetic property measurement by SQUID magnetometer indicate that all compounds exhibit weak paramagnetism with highest paramagnetic moment of ~ 0.3 MuB / ion for K doped compound but no long-range ferromagnetic ordering. We have discussed the observed magnetism in correlation with the nature of substitution.

cond-mat.mtrl-sci↗

arXiv2014

On a class of paracontact metric 3-manifolds

The purpose of this paper is to classify paracontact metric $3$-manifolds $M^3$ such that the Ricci operator $S$ commutes with the endomorhism $ϕ$ of its tangent bundle $Γ(TM^3)$.

math.DG↗

arXiv2014

On a class of $α$-para Kenmotsu manifolds

The purpose of this paper is to classify $α$-para Kenmotsu manifolds $M^3$ such that the projection of the image of concircular curvature tensor $L$ in one-dimensional linear subspace of $T_{p}(M^{3})$ generated by $ξ_{p}$ is zero.

math.DG↗

arXiv2013

Almost contact curves in trans-Sasakian 3-manifolds

This paper is devoted to the study of curvature and torsion of almost contact curves in trans-Sasakian 3-Manifolds. The conditions for the frenet curves to be almost contact curves in trans-Sasakian 3-manifolds have been obtained.

math.DG↗

arXiv2013

Non-magnetic doping induced magnetism in Li doped SnO2 nanoparticles

We address the possibility of non-magnetic doping induced magnetism, in Li doped SnO2 nano-particles. The compounds have been prepared by solid state route at equilibrium and were found to be crystallized in single rutile phase. The magnetization measurements have shown that Li-doping induces magnetism in SnO2 for a particular range of Li concentration. However, for other Li concentrations, including pure SnO2, the samples exhibit diamagnetism. To investigate the possible origin of the induced magnetism, we have studied the variation of the magnetization as a function of the average nano-particle radius. Possible scenarios for the appearance of magnetism in these compounds are discussed.

cond-mat.mtrl-sci↗

arXiv2011

Absence of Ferromagnetism in Mn-doped Tetragonal Zirconia

In a recent letter, it has been predicted within first principle studies that Mn-doped ZrO2 compounds could be good candidate for spintronics application because expected to exhibit ferromagnetism far beyond room temperature. Our purpose is to address this issue experimentally for Mn-doped tetragonal zirconia. We have prepared polycrystalline samples of Y0.15(Zr0.85-yMny)O2 (y=0, 0.05, 0.10, 0.15 & 0.20) by using standard solid state method at equilibrium. The obtained samples were carefully characterized by using x-ray diffraction, scanning electron microscopy, elemental color mapping, X-ray photoemission spectroscopy and magnetization measurements. From the detailed structural analyses, we have observed that the 5% Mn doped compound crystallized into two symmetries (dominating tetragonal & monoclinic), whereas higher Mn doped compounds are found to be in the tetragonal symmetry only. The spectral splitting of the Mn 3s core-level x-ray photoelectron spectra confirms that Mn ions are in the Mn3+ oxidation state and indicate a local magnetic moment of about 4.5 μB/Mn. Magnetic measurements showed that compounds up to 10% of Mn doping are paramagnetic with antiferromagnetic interactions. However, higher Mn doped compound exhibits local ferrimagnetic ordering. Thus, no ferromagnetism has been observed for all Mn-doped tetragonal ZrO2 samples.

cond-mat.mtrl-sci↗

arXiv2010

Is room temperature ferromagnetism possible in K-doped SnO2?

Ab initio studies have theoretically predicted room temperature ferromagnetism in crystalline SnO2, ZrO2 and TiO2 doped with non magnetic element from the 1A column as K and Na. Our purpose is to address experimentally the possibility of magnetism in both Sn1-xKxO2 and Sn1-xCaxO2 compounds. The samples have been prepared using equilibrium methods of standard solid state route. Our study has shown that both Sn1-xCaxO2 and Sn1-xKxO2 structure is thermodynamically unstable and leads to a phase separation, as shown by X-ray diffraction and detailed micro-structural analyses with high resolution transmission electron microscopy (TEM). In particular, the crystalline SnO2 grains are surrounded by K-based amorphous phase. In contrast to Ca: SnO2 samples we have obtained a magnetic phase in K: SnO2 ones, but no long range ferromagnetic order. The K: SnO2 samples exhibit a moments of the order of 0.2 μB/K /ion, in contrast to ab-initio calculations which predict 3μB, where K atoms are on the Sn crystallographic site. The apparent contradictions between our experiments and first principle studies are discussed.

cond-mat.mtrl-sci↗

arXiv2008

A Different Approach to $f(R)$-Cosmology

Here $f(R)$-cosmology is discussed using a different approach. This model explains early-inflation, emergence of cosmic background radiation at the exit from inflation, cosmic deceleration during radiation-dominance followed by deceleration due to curvature-induced matter and acceleration in the very late universe due to curvature-induced phantom dark energy. This model predicts collapse in the future universe. Further, a possible avoidence of collapse as well as revival of very early universe is suggested.

gr-qc↗

arXiv2008

Varying Gravitational Constant as Well as Cosmology from the Early Inflation to Late Acceleration and Future Universe

Here, cosmology is obtained from the variable gravitational constant $ G \propto ϕ^{-2}$ with $ ϕ(x) $ being a scalar and its fluctuations around the ground state. The gravitational action contains Einstein-Hilbert like term with variable $ G $, kinetic energy and self-interaction potential for $ ϕ(x) $. Two phase transitions take place in this model. The first one takes place at the GUT (grand unified theory) scale $ \sim 2.45 \times 10^{14}{\rm GeV} $, when the early universe exits the inflationay phase and the second one at the electro-weak scale. Spontaneous symmetry breaking takes place around this scale As a consequence, variable $ G $ acquires constant value $G_N$ (the Newtonian gravitational constant).The standard model of cosmology is obtained in the post-second phase transition era. Interestingly, the dark matter and quintessence dark energy are created from the gravitational sector as a combined effect of the linear term of scalar curvature and $ ϕ(x) $ without using non-linear terms of curvature or any other matter fields. It is interesting to find that fluctuations in $ ϕ(x) $ cause the phantom dark energy after the second phase transition, which grows later in the expanding universe. It is also found that the phantom era is free from big-rip singularity.

gr-qc↗

arXiv2008

$f(R)$-Cosmolgy from Early Universe Upto Future Cosmic Collapse and its Avoidance

In the present model, a unified picture of cosmology from early inflation to late acceleration is obtained from $ f(R)-$ gravity with non-linear terms $ R^2 $ and $ R^5 $ of scalar curvature $R$. It is discussed that elementary particles and radiation are produced during early inflation. The emitted radiation thermalizes the universe to a very high temperature $ \sim 10^{18}{\rm GeV}$. The exit of the universe from inflationary phase is followed by deceleration due to radiation-dominance heralding the standard cosmology with background radiation having the initial temperature $ \sim 10^{18}{\rm GeV}$. It is found that dark matter is induced by curvature and baryonic matter is produced during inflation. Radiation-dominated phase is followed by deceleration due to matter-dominance. The curvature-induced phantom dark energy dominates at the red-shift $ z = 0.303 $ causing the late acceleration. It is found that the universe will collapse in future. Further, it is investigated that back-reaction of quantum particles produced near the collapse time can avoid cosmic collapse and the universe will escape to revival of the state of early universe.

gr-qc↗

arXiv2008

Non-linear Equation of State, COSMIC Acceleration and Deceleration During Phantom-Dominance

Here, RS-II model of brane-gravity is considerd for phantom universe using non-linear equation of stste. Phantom fluid is known to violate the weak energy condition. In this paper, it is found that this characteristic of phantom energy is affected drastically by the negative brane-tension $λ$ of the RS-II model. It is interesting to see that upto a certain value of energy density $ρ$ satisfying $ρ/λ< 1$, weak energy condition is violated and universe super-accelerates. But as $ρ$ increases more, only strong energy condition is violated and universe accelerates. When $1 < ρ/λ<2$, even strong energy condition is not violated and universe decelerates. Expansion of the universe stops, when $ρ= 2 λ$. This is contrary to earlier results of phantom universe exhibiting acceleration only.

gr-qc↗

arXiv2008

Cosmic Evolution with Early and Late Acceleration Inspired by Dual Nature of the Ricci Scalar Curvature

In the present paper, it is found that dark energy emerges spontaneously from the modified gravity. According to cosmological scenario, obtained here, the universe inflates for $\sim 10^{-37}$ sec. in the beginning and late universe accelerates after 8.58 Gyrs. During the long intermediate period, it decelerates driven by radiation and subsequently by matter. Emerged gravitational dark energy mimics quintessence and its density falls by 115 orders from its initial value $2.58\times 10^{68} {\rm GeV}^4$ to its current value $2.19\times 10^{-47} {\rm GeV}^4$ .

astro-ph↗

arXiv2008

Acceleration and Deceleration in Curvature Induced Phantom Model of the Late and Future Universe, Cosmic Collapse as Well as its Quantum Escape

Here, cosmology of the late and future universe is obtained from $f(R)$-gravity with non-linear curvature terms $R^2$ and $R^3$ ($R$ being the Ricci scalar curvature). It is different from $f(R)$-dark enrgy models, where non-linear curvature terms are taken as gravitational alternative of dark energy. In the present model, neither linear nor no-linear curvature terms are taken as dark energy. Rather, dark energy terms are induced by curvature terms in the Friedmann equation derived from $f(R)$-gravitational equations. It has advantage over $f(R)$- dark energy models in the sense that the present model satisfies WMAP results and expands as $\sim t^{2/3}$ during matter-dominance. So, it does not have problems due to which $f(R)$-dark energy models are criticized. Curvature-induced dark energy, obtained here, mimics phantom. Different phases of this model, including acceleration and deceleration during phantom phase, are investigated here.It is found that expansion of the universe will stop at the age $(3.87 t_0 + 694.4 {\rm kyr})$ ($t_0$ being the present age of the universe) and after this epoch, it will contract and collapse by the time $(336.87 t_0 + 694.4 {\rm kyr})$. Further,it is shown that universe will escape predicted collapse (obtained using classical mechanics) on making quantum gravity corrections relevant near collapse time due to extremely high energy density and large curvature analogous to the state of very early universe. Interestingly, cosmological constant is also induced here, which is very small in classical domain, but very high in quantum domain.

gr-qc↗