arXiv subjects

Jayanta K Bhattacharjee

Publications and source records attributed to Jayanta K Bhattacharjee.

13 recordsLinked to original sources

arXiv2023

Emergence of a Non-van der Waals Magnetic Phase in a van der Waals Ferromagnet

Manipulation of long-range order in two-dimensional (2D) van der Waals (vdW) magnetic materials (e.g., CrI$_3$, CrSiTe$_3$ etc.), exfoliated in few-atomic layer, can be achieved via application of electric field, mechanical-constraint, interface engineering, or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition and hydrolysis in the presence of water/moisture causes degradation in magnetic nanosheets which, in turn, affects the nanoelectronic/spintronic device performance. Counterintuitively, our current study reveals that exposure to the air at ambient atmosphere results in advent of a stable nonlayered secondary ferromagnetic phase in the form of Cr$_2$Te$_3$ (T$_{C2}$ ~ 160 K) in the parent vdW magnetic semiconductor Cr$_2$Ge$_2$Te$_6$ (T$_{C1}$ ~ 69 K). In addition, the magnetic anisotropy energy (MAE) enhances in the hybrid by an order from the weakly anisotropic pristine Cr$_2$Ge$_2$Te$_6$ crystal, increasing the stability of the FM ground state with time. Comparing with the freshly prepared Cr$_2$Ge$_2$Te$_6$, the coexistence of the two ferromagnetic phases in the time elapsed bulk crystal is confirmed through systematic investigation of crystal structure along with detailed dc/ac magnetic susceptibility, specific heat, and magnetotransport measurement. To capture the concurrence of the two ferromagnetic phases in a single material, Ginzburg-Landau theory with two independent order parameters (as magnetization) with a coupling term can be introduced. In contrast to rather common poor environmental stability of the vdW magnets, our results open possibilities of finding air-stable novel materials having multiple magnetic phases.

cond-mat.mtrl-sci↗

arXiv2022

Universal scaling regimes in rotating fluid turbulence

We analyse the scaling properties of the energy spectra in fully developed incompressible turbulence in forced, rotating fluids in three dimensions (3D), which are believed to be characterised by universal scaling exponents in the inertial range. To elucidate the scaling regimes, we set up a scaling analysis of the 3D Navier-Stokes equation for a rotating fluid that is driven by large-scale external forces. We use scaling arguments to extract the scaling exponents, which characterise the different scaling regimes of the energy spectra. We speculate on the intriguing possibility of two-dimensionalisation of 3D rotating turbulence within our scaling theory. Our results can be tested in large scale simulations and relevant laboratory-based experiments.

cond-mat.stat-mech↗

arXiv2020

A Henon map for the transmission dynamics of COVID-19: The role of asymptomatic transmitters and delayed symptoms

We consider the transmission dynamics of COVID-19 which is characterized by two distinct features. One is the existence of asymptomatic carriers which is a hidden variable in the problem. The other is the issue of latency which means that among the symptomatic carriers there could be a fraction whose symptoms develop after a couple of days. Our modelling is restricted to what we call the Phase -1 of the disease. During this phase the disease sets in and the number of infected people starts growing fast ( the number of new cases per day keeps growing on an average ) and then it slows down ( the number of new cases per day starts decreasing ) with the number of new cases decreasing to about one tenth of its peak value or even smaller). We define Phase-1 to be over when the daily cases start rising once again. We write down a Henon-like map to take various effects into account for this first phase.

q-bio.PE↗

arXiv2020

When Hopf meets saddle: bifurcations in the diffusive Selkov model for glycolysis

We study the linear instabilities and bifurcations in the Selkov model for glycolysis with diffusion. We show that this model has a zero wave-vector, finite frequency Hopf bifurcation to a growing oscillatory but spatially homogeneous state and a saddle-node bifurcation to a growing inhomogeneous state with a steady pattern with a finite wavevector. We further demonstrate that by tuning the relative diffusivity of the two concentrations, it is possible to make both the instabilities to occur at the same point in the parameter space, leading to an unusual type of codimension-two bifurcation. We then show that in the vicinity of this bifurcation the initial conditions decide whether a spatially uniform oscillatory or a spatially periodic steady pattern emerges in the long time limit.

nlin.PS↗

arXiv2019

Instability Zones in the Dynamics of a Quantum Mechanical Quasiperiodic Parametric Oscillator

Quasi-periodically driven quantum parametric oscillators have been the subject of several recent investigations. Here we show that for such oscillators, the instability zones of the mean position and variance (alternatively the mean energy) for a time developing wave packet are identical for the strongest resonance in the three-dimensional parameter space of the quasi-periodic modulation as it is for the two-dimensional parameter space of the periodic modulations.

quant-ph↗

arXiv2019

Kolmogorov or Bolgiano-Obukhov: Universal scaling in stably stratified turbulent fluids

We set up the scaling theory for stably stratified turbulent fluids. For a system having infinite extent in the horizontal directions, but with a finite width in the vertical direction, this theory predicts that the inertial range can display three possible scaling behaviour, which are essentially parametrised by the buoyancy frequency $N$, or dimensionless horizontal Froude number $F_h$, and the vertical length scale $l_v$ that sets the scale of variation of the velocity field in the vertical direction, for a fixed Reynolds number. For very low $N$ or very high $Re_b$ or $F_h$, and with $l_v$ being of the same order as $l_h$, the typical horizontal length scale, buoyancy forces are irrelevant and hence, unsurprisingly, the kinetic energy spectra shows the well-known K41 scaling in the inertial range. In this regime, the local temperature behaves as a passively advected scalar, without any effect on the flow fields. For intermediate ranges of values of $N,\,F_h\sim {\cal O}(1)$, corresponding to moderate stratification, buoyancy forces are important enough to affect the scaling. This leads to the Bolgiano-Obukhov scaling which is isotropic, when $l_v\sim l_h$. Finally, for very large $N$ or equivalently for vanishingly small $F_h,\,L_o$, corresponding to strong stratification, together with a very small $l_v$, the system effectively two-dimensionalise; the kinetic energy spectrum is predicted to be anisotropic with only the horizontal part of the kinetic energy spectra follows the K41 scaling, suggesting an intriguing {\em re-entrant} K41 scaling, as a function of stratification, for ${\bf v}_\perp$ in this regime. The scaling theory further predicts the scaling of the thermal energy in each of these three scaling regimes. Our theory can be tested in large scale simulations and appropriate laboratory-based experiments.

cond-mat.stat-mech↗

arXiv2018

Varieties of scaling regimes in hydromagnetic turbulence

We revisit the scaling properties of the energy spectra in fully developed incompressible homogeneous turbulence in forced magnetofluids (MHD) in three dimensions (3D), which are believed to be characterised by {\em universal scaling exponents} in the inertial range. Enumerating these universal scaling exponents that characterise the energy spectra remains a theoretical challenge. To study this, we set up a scaling analysis of the 3D MHD equations, driven by large-scale external forces and with or without a mean magnetic field. We use scaling arguments to bring out various scaling regimes for the energy spectra. We obtain a variety of scaling in the inertial range, ranging from the well-known Kolmogorov spectra in the isotropic 3D ordinary MHD to more complex scaling in the anisotropic cases that depend on the magnitude of the mean magnetic field. We further dwell on the possibility that the energy spectra scales as $k^{-2}$ in the inertial range, where $k$ is a wavevector belonging to the inertial range, and also speculate on unequal scaling by the kinetic and magnetic energy spectra in the inertial range of isotropic 3D ordinary MHD. We predict the possibilities of {\em scale-dependent anisotropy} and intriguing {\em weak dynamic scaling} in the Hall MHD and electron MHD regimes of anisotropic MHD turbulence. Our results can be tested in large scale simulations and relevant laboratory-based and solar wind experiments.

cond-mat.stat-mech↗

arXiv2011

Fluctuating hydrodynamics and turbulence in a rotating fluid: Universal properties

We analyze the statistical properties of three-dimensional ($3d$) turbulence in a rotating fluid. To this end we introduce a generating functional to study the statistical properties of the velocity field $\bf v$. We obtain the master equation from the Navier-Stokes equation in a rotating frame and thence a set of exact hierarchical equations for the velocity structure functions for arbitrary angular velocity $\mathbf Ω$. In particular we obtain the {\em differential forms} for the analogs of the well-known von Karman-Howarth relation for $3d$ fluid turbulence. We examine their behavior in the limit of large rotation. Our results clearly suggest dissimilar statistical behavior and scaling along directions parallel and perpendicular to $\mathbf Ω$. The hierarchical relations yield strong evidence that the nature of the flows for large rotation is not identical to pure two-dimensional flows. To complement these results, by using an effective model in the small-$Ω$ limit, within a one-loop approximation, we show that the equal-time correlation of the velocity components parallel to $\mathbf Ω$ displays Kolmogorov scaling $q^{-5/3}$, where as for all other components, the equal-time correlators scale as $q^{-3}$ in the inertial range where $\bf q$ is a wavevector in $3d$. Our results are generally testable in experiments and/or direct numerical simulations of the Navier-Stokes equation in a rotating frame.

cond-mat.stat-mech↗

arXiv2005

Universal properties of three-dimensional magnetohydrodynamic turbulence: Do Alfvén waves matter?

We analyse the effects of the propagating Alfvén waves, arising due to non-zero mean magnetic fields, on the nonequilibrium steady states of three-dimensional (3d) homogeneous Magnetohydrodynamic (MHD) turbulence. In particular, the effects of Alfvén waves on the universal properties of 3dMHD turbulence are studied in a one-loop self-consistent mode-coupling approach. We calculate the kinetic- and magnetic energy-spectra. We find that {\em even} in the presence of a mean magnetic field the energy spectra are Kolmogorov-like, i.e., scale as $k^{-5/3}$ in the inertial range where $\bf k$ is a Fourier wavevector belonging to the inertial range. We also elucidate the multiscaling of the structure functions in a log-normal model by evaluating the relevant intermittency exponents, and our results suggest that the multiscaling deviations from the simple Kolmogorov scaling of the structure functions decrease with increasing strength of the mean magnetic field. Our results compare favourably with many existing numerical and observational results.

cond-mat.stat-mech↗

arXiv2003

Scaling in a temperature quench in systems with a Lifshitz point: Nonconserved and conserved order parameters

We study the growth of an N-component (including N=1) order parameter when a system with a Lifshitz point is quenched from the homogeneous disordered state to the ordered states. We study the scaling behaviours of the structure factors for both the non-conserved and conserved order parameters in the long time limit after a quench through the Lifshitz point by using the large-N and renormalisation group (RG) methods. We construct the analogues of Allen-Cahn and Lifshitz-Slyzov growth laws for nonconserved and conserved order parameters which agree with our RG results for N=1. By extending our large-N methods to the anisotropic Lifshitz point we show that the anisotropy is relevant for the growth of the nonconserved order parameter, but irrelevant for the conserved case in the large-N limit. We discuss the effects of mode coupling terms on scaling of the structure factor for the conserved order parameter case. We also consider the ordering dynamics after a quench through an off-Lifshitz point. In a large-N set up we calculate the form of the structure factors of a non-conserved order parameter when a system is quenched from the homogeneous paramegnetic to the modulated phase. We show that after a quench from the paramegnetic to the modulated phase the form of the structure factor violates the standard form of dynamical scaling. Our results compare favourably with the available numerical results.

cond-mat.stat-mech↗

arXiv2002

The Hydraulic Jump In Two Dimensions

The first quantitative calculation of the hydraulic jump was carried out only a few years ago by Bohr et al. Since it is the only calculation of the jump, we have analysed it from a slightly different point of view.Our results are similar to those of Bohr et.al.

physics.flu-dyn↗

arXiv1998

The dynamo effect - A dynamic renormalisation group approach

The Dynamo effect is used to describe the generation of magnetic fields in astrophysical objects. However, no rigorous derivation of the dynamo equation is available. We justify the form of the equation using an Operator Product Expansion (OPE) of the relevant fields. We also calculate the coefficients of the OPE series using a dynamic renormalisation group approach and discuss the time evolution of the initial conditions on the initial seed magnetic field.

cond-mat↗

arXiv1998

Mean Magnetic Field and Noise Cross-Correlation in Magnetohydrodynamic Turbulence: Results from a One-Dimensional Model

We show that a recently proposed [J. Fleischer and P.H. Diamond, {\em Phys. Rev. E}{\bf 58}, R2709 (1998)] one-dimensional Burgers-like model for magnetohydrodynamics (MHD) is in effect identical to existing models for drifting lines and sedimenting lattices. We use the model to demonstrate, contrary to claims in the literature, that the energy spectrum of MHD turbulence should be independent of mean magnetic field and that cross-correlations between the noise sources for the velocity and magnetic fields cannot change the structure of the equations under renormalisation. We comment on the scaling and the multiscaling properties of the stochastically forced version of the model.

cond-mat↗