arXiv subjects

S. K. Mishra

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

At least 19 recordsLinked to original sources

arXiv2026

Hopf bifurcation and stochastic spiking in an antiferromagnetic FitzHugh--Nagumo normal form

Antiferromagnets offer ultrafast, stray-field-free dynamics that are attractive for neuromorphic spintronic devices. Here we analyze an antiferromagnetic spin-Hall nano-oscillator in the overdamped regime and derive a reduced set of equations for the Néel-vector dynamics constrained to the unit sphere. For spin polarization along the easy axis, the model reduces to an asymmetric rotator, for which analytic solutions and the associated spin-pumping signal are obtained in selected limits. We further show that near a suitable operating point, the projected dynamics can be transformed into a local FitzHugh-Nagumo normal form. The resulting mapping identifies the effective fast variable, recovery variable, bias current, and Hopf condition in terms of magnetic material parameters. We finally extend the reduced model to an Itô stochastic FitzHugh-Nagumo equation driven by spin-pumping input and additive thermal or electronic fluctuations. The stochastic phase portrait shows that the deterministic nullcline geometry organizes noisy spike cycles and produces controlled spike-time variability. These results provide a minimal analytic framework for AFM-based spiking-neuron elements and suggest design criteria for future neuromorphic spintronic devices.

nlin.CD↗

arXiv2026

Precursor Solitons in Plasma Flow Past Charged Obstacles: Role of Obstacle Bias and Ion Temperature Anisotropy

We investigate electrostatic ion-acoustic precursor solitons in a plasma flow past an absorbing charged obstacle using two-dimensional (2D) electrostatic PIC simulations. A key outcome of the present formulation is that ion-temperature anisotropy can enable precursor formation even in regimes where isotropic plasmas, due to Landau damping, cannot sustain such structures. Specifically, temperature anisotropy in the 2D drifting flow along the x-direction, arising from a reduction in the transverse thermal velocity (y-direction) relative to the parallel thermal velocity (x-direction), favors the generation of coherent upstream structures whose effectiveness increases with stronger anisotropy. Both positive and negative obstacle polarities are considered to identify the conditions for upstream nonlinear structure formation. A negatively biased plate produces only a wake-like response, whereas a positively biased plate generates upstream density pulses. This study offers physical insight into nonlinear wave formation in streaming plasmas over charged objects and could be useful for plasma-based debris detection in the low Earth orbit (LEO) region.

physics.plasm-ph↗

arXiv2025

Cataloging the nonlinear waves excited by moving a charged body in the dusty plasma medium

The nonlinear waves excited by the movement of a charged body in the dusty plasma medium are studied. A charged body moving through a dusty plasma medium can generate diverse nonlinear waves, such as precursors and pinned solitons. These wave excitations under weakly nonlinear and dispersive limits are described theoretically by the forced Korteweg-de Vries (fKdV) type equation. We have examined the role of the driver in shaping and evolving these wave excitations. In particular we studied the effect of primarily three source parameters, namely, amplitude, width, and flow speed, on the evolution of nonlinear structures. The driver generates a perturbation in the stable system configuration, which couples with medium characteristics and eventually evolves into propagating excitations. Our finding shows that the excitation of nonlinear structure by a moving body in a plasma medium is not just dictated by the mach number but also the features of the source such as amplitude and width. As a novel finding apart from pinned and precursor solitons, we observe another nonlinear structure that lags behind the source term, maintaining its shape and speed as it propagates. These features are the first ever theoretical depiction of such lagging structures.

physics.plasm-ph↗

arXiv2025

Entanglement properties of photon-magnon crystal from nonlinear perspective

Quantifying the entanglement between two continuous bosonic modes, such as magnons and photons, is not trivial. The logarithmic negativity, calculated through the quantum Langevin equations is subjected to thermal noise. However this method requires further approximation. The phase space of a generic nonlinear system contains topologically different regions, and the steady state may correspond to the different types of fixed points, such as Saddle Points, Stable or unstable Spirals, and Nodes. In the present work, we propose a new procedure. Namely, we derived the complete set of nonlinear equations, which includes equations for the magnon and photon number operators and phases. We show that not only number operators but also phases are important for exploring the character of the fixed point, and magnon-photon entanglement. We showed that methods of the qualitative theory of nonlinear differential equations are also relevant for photon-magnon entanglement problems. Our main finding is that entanglement is not defined in the Saddle Point region. On the other hand, the maximum of the entanglement corresponds to the region near the border between the Stable node and Stable spiral regions. Our approach is quite general. However, we did calculations for a particular system: photon-magnon crystal based on the yttrium iron garnet (YIG) film with the periodic air holes drilled in the film. Our interest focuses on magnons with a particular wavelength and frequency corresponding to the magnon condensate. Those magnons couple strongly with the photons of similar frequency. We discuss in detail the interaction between magnons and photons originating from the magneto-electric coupling and the effective Dzyaloshinskii-Moriya interaction. We show that this interaction is responsible for the robust photon-magnon entanglement in the system.

cond-mat.mes-hall↗

arXiv2023

Plasmonic skyrmion quantum thermodynamics

The primary obstacle in the field of quantum thermodynamics revolves around the development and practical implementation of quantum heat engines operating at the nanoscale. One of the key challenges associated with quantum working bodies is the occurrence of "quantum friction," which refers to irreversible wasted work resulting from quantum inter-level transitions. Consequently, the construction of a reversible quantum cycle necessitates the utilization of adiabatic shortcuts. However, the experimental realization of such shortcuts for realistic quantum substances is exceedingly complex and often unattainable. In this study, we propose a quantum heat engine that capitalizes on the plasmonic skyrmion lattice. Through rigorous analysis, we demonstrate that the quantum skyrmion substance, owing to its topological protection, exhibits zero irreversible work. Consequently, our engine operates without the need for adiabatic shortcuts. We checked by numerical calculations and observed that when the system is in the quantum skyrmion phase, the propagated states differ from the initial states only by the geometricl and dynamical phases. The adiabacit evoluation leads to the zero transition matrix elements and zero irreversible work. By employing plasmonic mods and an electric field, we drive the quantum cycle. The fundamental building blocks for constructing the quantum working body are individual skyrmions within the plasmonic lattice. As a result, one can precisely control the output power of the engine and the thermodynamic work accomplished by manipulating the number of quantum skyrmions present.

quant-ph↗

arXiv2023

Topological entanglement entropy to identify topological order in quantum skyrmions

We study the topological entanglement entropy and scalar chirality of a topologically ordered skyrmion formed in a two-dimensional triangular lattice. Scalar chirality remains a smooth function of the magnetic field in both helical and quantum skyrmion phases. In contrast, topological entanglement entropy remains almost constant in the quantum skyrmion phase, whereas it experiences enhanced fluctuations in the helical phase. Therefore, topological entanglement entropy is an effective tool to distinguish between the two phases and pinpoint the quantum phase transition in the system.

cond-mat.str-el↗

arXiv2023

Deleterious satellite charging and possible mitigation schemes

Electrostatic charge dissipation is one of the major concerns for satellites operating in the Earth's orbits. Under energetic plasma conditions, they may acquire very high negative potential (up to 10's of kV) due to the collection of energetic plasma constituents - resulting in temporary outages and permanent damages to onboard equipment. This study proposes and discusses a couple of physics-based schemes capable of mitigating/ minimizing the excessive charging effects over satellites under extreme plasma conditions in LEO/ GEO. An estimate of charge build-up on the space objects based on the charging dynamics as a function of ambient plasma parameters has been made. Our calculations illustrate that in the absence of a significant charge dissipation mechanism, a severe charging (10's kV) in the dark/ shadowed at GEO and high latitude LEO regions. We propose that installing a suitable UV lamp and micro/nano-structuring of the surface fabric can induce an efficient dissipation mechanism and effectively prevent the surface from deleterious charging effects during satellite operation. We demonstrate that the UV illumination may maintain the satellite surface at quite a small positive potential (~ 2 V) while the surface nanofabrication sustains it at a sufficiently low negative potential (~ 10 V). Both concepts are shown to work efficiently in mitigating the potential threat of massive charging and safely performing the satellite operation.

physics.plasm-ph↗

arXiv2023

Magnetoelectric fractals, Magnetoelectric parametric resonance and Hopf bifurcation

In the present work, we study the dynamics of a magnetic nanoparticle coupled through the magnetoelectric coupling to the ferroelectric crystal. The model of our interest is nonlinear, and we explore the problem under different limits of weak and strong linearity. By applying two electric fields with different frequencies, we control the form of the confinement potential of the ferroelectric subsystem and realize different types of dynamics. We proved that the system is more sensitive to magnetoelectric coupling in the case of double-well potential. In particular, in the case of strong nonlinearity, arbitrary small values of magnetoelectric coupling lead to chaotic dynamics. In essence, magnetoelectric coupling plays a role akin to the small perturbations destroying invariant tors according to the KAM theorem. We showed that bifurcations in the system are of Hopf's type. We observed the formation of magnetoelectric fractals in the system. In the limit of weak nonlinearity, we studied a problem of parametric nonlinear resonance and enhancement of magnetic oscillations through magnetoelectric coupling.

cond-mat.mes-hall↗

arXiv2023

Topological dynamical quantum phase transition in a quantum skyrmion phase

Quantum skyrmionic phase is modelled in a 2D helical spin lattice. This topological skyrmionic phase retains its nature in a large parameter space before moving to a ferromagnetic phase. Next nearest-neighbour interaction improves the stability and it also causes a shift of the topological phase in the parameter space. Nonanalytic behaviour of the rate function observed, when the system which is initially in a quantum skyrmion phase is quenched to a trivial quantum ferromagnetic phase, indicates a dynamical quantum phase transition. Dynamical quantum phase transition is absent when the system initially in a skyrmion phase is quenched to a helical phase.

cond-mat.str-el↗

arXiv2023

Charge transfer and disorder-induced spin relaxation in La2NiMnO6 crystallites

Investigation of the electronic and spin structure in double perovskites is recently attracting significant attention, mainly driven by their unique multifunctional properties and other underlying charge and spin dynamics. Herein, using X-ray photoelectron spectroscopy (XPS), we explore the influence of variable fractions of Mn3+/Mn4+ cation in different crystallite sizes of La2NiMnO6 that control the various completing exchange interactions of Ni/Mn cations responsible for multiple magnetic transitions. The enhanced itinerant electron due to Mn4+ + Ni2+ to Mn3+ + Ni3+ charge transfer emerged as a shoulder like characteristics at the low binding energy in the Mn-2P core-level spectrum. The various approaches such as difference in saturation magnetization, presence of multiple charge valance, and magnetic entropy calculations confirm the presence of antisites disorder and it varies as a function of milling. As milling provides excess energy that helps with nucleation or cation ordering. Competing magnetic interactions driven by mixed valences and disorder were established across a cluster glassy phase in the crystallites. Electron spin resonance spectroscopy (ESR) was utilized to probe the temperature-driven ferromagnetic-cluster spin-glass transition with modified g-factor ranging from 2.050 to 2.037. The line width of the ESR signals increases across the ferromagnetic to cluster-glass phase transition due to spin freezing. This phase transition is further characterized by temperature-dependent ac-magnetic susceptibility measurements. Argand diagram for the ac-susceptibility of the interacting crystallites suggests a collective magnetization relaxation dynamic in the proximity of spin-glass freezing temperature of La2NiMnO6.

cond-mat.mtrl-sci↗

arXiv2022

Element-specific probe of quantum criticality in $\mathrm{CeCoIn_{5}}$

Employing the elemental sensitivity of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), we study the valence and magnetic order in the heavy fermion superconductor CeCoIn$_5$. We probe spin population of the f-electrons in Ce and d-electrons in Co as a function of temperature (down to 0.1 K) and magnetic field (up to 6 T). From the XAS we find a pronounced contribution of Ce$^{4+}$ component at low temperature and a clear temperature dependence of the Ce valence below 5 K, suggesting enhanced valence fluctuations, an indication for the presence of a nearby quantum critical point (QCP). We observe no significant corresponding change with magnetic field. The XMCD displays a weak signal for Ce becoming clear only at 6 T. This splitting of the Kramers doublet ground state of Ce$^{3+}$ is significantly smaller than expected for independent but screened ions, indicating strong antiferromagnetic pair interactions. The unconventional character of superconductivity in CeCoIn$_5$ is evident in the extremely large specific heat step at the superconducting transition.

cond-mat.str-el↗

arXiv2022

Hybrid quantum-classical chaotic NEMS

We present an exactly solvable model of a hybrid quantum-classical system of a Nitrogen-Vacancy (NV) center spin (quantum spin) coupled to a nanocantilever (classical) and analyze the enforcement of the regular or chaotic classical dynamics onto the quantum spin dynamics. The main problem we focus in this paper is whether the classical dynamical chaos may induce chaotic effects in the quantum spin dynamics or not. We explore several characteristic criteria of the quantum chaos, such as quantum Poincaré recurrences, generation of coherence and energy level distribution and observe interesting chaotic effects in the spin dynamics. Dynamical chaos imposed in the cantilever dynamics through the kicking pulses induces the stochastic dynamics on the quantum subsystem. We consider a quantum system of two and three levels and show that in a two-level case, type of stochasticity is not conforming all the characteristic features of the quantum chaos and is distinct from it. We also explore the effect of quantum feedback on dynamics of the cantilever and the entire system.

quant-ph↗

arXiv2022

Quasi-periodic spicule-like cool jets driven by Alfvén pulses

We perform a 2.5 dimensional magnetohydrodynamic (MHD) simulation to understand a comprehensive view of the formation of spicule-like cool jets due to initial transverse velocity pulses akin to Alfvén pulses in the solar chromosphere. We invoke multiple velocity ($V_{z}$) pulses between 1.5 and 2.0 Mm in the solar atmosphere, which create the initial transverse velocity perturbations. These pulses transfer energy non-linearly to the field aligned perturbations due to the ponderomotive force. This physical process further creates the magnetoacoustic shocks followed by quasi-periodic plasma motions in the solar atmosphere. The field aligned magnetoacoustic shocks move upward which subsequently cause quasi-periodic rise and fall of the chromospheric plasma into the overlying corona as a thin and cool spicule-like jets. The magnitude of the initial applied transverse velocity pulses are taken in the range of 50-90 km $s^{-1}$. These pulses are found to be strong enough to generate the spicule-like jets. We analyze the evolution, kinematics and energetics of these spicule-like jets. We find that the transported mass flux and kinetic energy density are substantial in the localized solar-corona. These mass motions generate $\it in$ $situ$ quasi-periodic oscillations on the scale of $\simeq$ 4.0 min above the transition region.

astro-ph.SR↗

arXiv2020

Generation of coherence in an exactly solvable nonlinear nanomechanical system

This study is focused on the quantum dynamics of a nitrogen-vacancy (NV) center coupled to a nonlinear, periodically driven mechanical oscillator. For a continuous periodic driving that depends on the position of the oscillator, the mechanical motion is described by Mathieu elliptic functions. This solution is employed to study the dynamics of the quantum spin system including environmental effects and to evaluate the purity and the von Neumann entropy of the NV-spin. The unitary generation of coherence is addressed. We observe that the production of coherence through a unitary transformation depends on whether the system is prepared initially in mixed state. Production of coherence is efficient when the system initially is prepared in the region of the separatrix (i.e., the region where classical systems exhibit dynamical chaos). From the theory of dynamical chaos, we know that phase trajectories of the system passing through the homoclinic tangle have limited memory, and therefore the information about the initial conditions is lost. We proved that quantum chaos and diminishing of information about the mixed initial state favors the generation of quantum coherence through the unitary evolution. We introduced quantum distance from the homoclinic tangle and proved that for the initial states permitting efficient generation of coherence, this distance is minimal.

quant-ph↗

arXiv2020

Anharmonic Phonons and Anomalous Thermal Expansion of Graphite

We have investigated the anisotropic thermal expansion of graphite using ab-initio calculation of lattice dynamics and anharmonicity of the phonons, which reveal that the negative thermal expansion (NTE) in the a-b plane below 600 K and very large positive thermal expansion along the c-axis up to high temperatures arise due to various phonons polarized along the c-axis. While the NTE arises from the anharmonicity of transverse phonons over a broad energy range up to 60 meV, the large positive expansion along the c-axis occurs largely due to the longitudinal optic phonon modes around 16 meV and a large linear compressibility along the c-axis. The hugely anisotropic bonding in graphite is found to be responsible for wide difference in the energy range of the transverse and longitudinal phonon modes polarized along the c-axis, which are responsible for the anomalous thermal expansion behavior. This behaviour is in contrast to other nearly isotropic hexagonal structures like water-ice, which show anomalous thermal expansion in a small temperature range arising from a narrow energy range of phonons.

cond-mat.mtrl-sci↗

arXiv2020

Charge transfer induced interfacial ferromagnetism in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$

Charge transfer induced interfacial ferromagnetism and its impact on the exchange bias effect in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni$_3^+$ cations which are naturally part of the NdNiO$_3$ layer, Ni$_2^+$ ions are formed at the interface due to a charge transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the Mn$_4^+$ ions in the La$_{0.7}$Sr$_{0.3}$MnO$_3$ layer. This can be seen as detrimental to the strength of the unidirectional anisotropy since a significant part of the interface does not contribute to the pinning of the ferromagnetic layer. By analyzing the line shape changes of the x-ray absorption at the Ni L$_{2,3}$ edges, the metal-insulator transition of the NdNiO$_3$ layer is resolved in an element specific manner. This phase transition is initiated at about 120 K, way above the paramagnetic to antiferromagnetic transition of NdNiO$_3$ layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Neel temperature of the NdNiO$_3$ layer. Different from La$_{0.7}$Sr$_{0.3}$MnO$_3$/LaNiO$_3$, the exchange bias observed in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ is due to the antiferromagnetism of NdNiO$_3$ and the frustration at the interface. These results suggest that reducing the interfacial orbital hybridization may be used as a tunable parameter for the strength of the exchange bias effect in all-oxide heterostructures which exhibit a charge transfer mechanism.

cond-mat.mtrl-sci↗

arXiv2019

Structural, magnetic and electric polarization properties of geometrically frustrated YBaCo4O7 and DyBaCo4O7 cobaltites

In RBaCo4O7 (R = Ca, Y and Rare earth) cobaltite family, only CaBaCo4O7 shows 3D long range ferrimagnetic and spin driven electric polarization properties. In the present study, we have investigated the structural, magnetic and electric polarization properties in YBaCo4O7 and DyBaCo4O7 members of this family and the obtained results are compared with the CaBaCo4O7. The compound YBaCo4O7 showed a series of magnetic transitions in agreement with reported literature, additionally, a cluster glass behavior below 5 K is observed. Powder neutron diffraction studies on DyBaCo4O7 cobaltite showed an orthorhombic Pbn21 symmetry and signature of short-range 120 degree magnetic correlations of kagome layer and a spin glass behavior below 65 K. Dielectric measurements on both the samples showed strong frequency dispersion at high temperature and frequency independent behavior at low temperature without dielectric anomaly. Pyroelectric current measurement has showed a broad peak around 50 K in both the samples; however, a careful analysis relates the peak to thermally stimulated depolarization current. This study signifies that the giant structural distortions and cobalt charge ordering at kagome layer are the key factors to drive both long range magnetic ordering and spin driven electric polarization in this cobaltite family.

cond-mat.str-el↗

arXiv2019

Probing of Structural Phase Transitions in Barium Titanate Modified Sodium Niobate using Raman Scattering

Raman Spectroscopic measurements are carried out to investigate the structural phase transitions as a function of composition in modified sodium niobate [(1-x) NaNbO3-xBaTiO3:NNBTx] for x=0.0 to 0.15 at room temperature. The characteristic antiferroelectric modes at around 93.4 and 123.6cm-1alongwith a mode at 155.5 cm-1were found to disappear across the structural phase transition from antiferroelectric orthorhombic phase (Pbcm) to ferroelectric orthorhombic phase (Pmc21) phase for x>0.02. The redistribution of intensities and positions of the Raman lines in bending (150-350 cm-1) and stretching modes (>550cm-1) on increasing the concentration x>0.05 also confirms the occurrence of another phase transition from ferroelectric orthorhombic phase (Pmc21) to another ferroelectric orthorhombic phase (Amm2) phase across x~0.10.The phase transitions as observed from Raman measurements are consistent with previous x-ray diffraction study.

cond-mat.mtrl-sci↗