arXiv subjects

P. Chatterjee

Publications and source records attributed to P. Chatterjee.

7 recordsLinked to original sources

arXiv2024

Challenges and Insights in Growing Epitaxial FeSn Thin Films on GaAs(111) substrate Using Molecular Beam Epitaxy

FeSn is a room-temperature antiferromagnet composed of alternating Fe3Sn kagome layers and honeycomb Sn layers. Its distinctive lattice allows the formation of linearly dispersing Dirac bands and topological flat bands in its electronic band structure, positioning FeSn as an ideal candidate for investigating the interplay between magnetism and topology. In this study, we investigate the epitaxial growth of FeSn thin films on GaAs(111) substrates by molecular beam epitaxy. A significant challenge in this growth process is the diffusion of Ga and As from the substrate into the deposited films and the diffusion of Fe into the substrate. This diffusion complicates the formation of a pure FeSn phase. Through a comprehensive analysis-including reflection high energy electron diffraction, high-resolution X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometry-we demonstrate that the Sn evaporation temperature plays a critical role in influencing the crystallinity, surface morphology, and magnetic behaviour of the films. Our results show that while it is difficult to grow a single-phase FeSn film on GaAs due to diffusion, optimizing the Sn evaporation temperature can enhance the dominance of the FeSn phase, partially overcoming these challenges.

cond-mat.mtrl-sci↗

arXiv2020

The Pencil Code, a modular MPI code for partial differential equations and particles: multipurpose and multiuser-maintained

The Pencil Code is a highly modular physics-oriented simulation code that can be adapted to a wide range of applications. It is primarily designed to solve partial differential equations (PDEs) of compressible hydrodynamics and has lots of add-ons ranging from astrophysical magnetohydrodynamics (MHD) to meteorological cloud microphysics and engineering applications in combustion. Nevertheless, the framework is general and can also be applied to situations not related to hydrodynamics or even PDEs, for example when just the message passing interface or input/output strategies of the code are to be used. The code can also evolve Lagrangian (inertial and noninertial) particles, their coagulation and condensation, as well as their interaction with the fluid.

astro-ph.IM↗

arXiv2018

Strong nonlocality variations in a spherical mean-field dynamo

To explain the large-scale magnetic field of the Sun and other bodies, mean-field dynamo theory is commonly applied where one solves the averaged equations for the mean magnetic field. However, the standard approach breaks down when the scale of the turbulent eddies becomes comparable to the scale of the variations of the mean magnetic field. Models showing sharp magnetic field structures have therefore been regarded as unreliable. Our aim is to look for new effects that occur when we relax the restrictions of the standard approach, which becomes particularly important at the bottom of the convection zone where the size of the turbulent eddies is comparable to the depth of the convection zone itself. We approximate the underlying integro-differential equation by a partial differential equation corresponding to a reaction-diffusion type equation for the mean electromotive force, making an approach that is nonlocal in space and time feasible under conditions where spherical geometry and nonlinearity are included. In agreement with earlier findings, spatio-temporal nonlocality lowers the excitation conditions of the dynamo. Sharp structures are now found to be absent. However, in the surface layers the field remains similar to before.

astro-ph.SR↗

arXiv2014

The Sun's interior structure and dynamics, and the solar cycle

The Sun's internal structure and dynamics can be studied with helioseismology, which uses the Sun's natural acoustic oscillations to build up a profile of the solar interior. We discuss how solar acoustic oscillations are affected by the Sun's magnetic field. Careful observations of these effects can be inverted to determine the variations in the structure and dynamics of the Sun's interior as the solar cycle progresses. Observed variations in the structure and dynamics can then be used to inform models of the solar dynamo, which are crucial to our understanding of how the Sun's magnetic field is generated and maintained.

astro-ph.SR↗

arXiv2010

Reynolds stress and heat flux in spherical shell convection

(abridged) Context. Turbulent fluxes of angular momentum and heat due to rotationally affected convection play a key role in determining differential rotation of stars. Here we perform a systematic comparison between Cartesian and spherical geometries as a function of the rotation rate. Aims. We extend the earlier studies by using spherical wedges to obtain turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare results from spherical and Cartesian models in the same parameter regime. In particular, we want to clarify whether the sharp equatorial profile of the horizontal Reynolds stress found in earlier Cartesian models is reproduced in spherical models. Methods. We employ direct numerical simulations of turbulent convection. In order to reach as high spatial resolution as possible in the spherical runs, we model only parts of the latitude and longitude. The rotational influence, measured by the Coriolis number, is varied from zero to roughly seven, which is the regime that is likely to be realised in the solar convection zone. Cartesian simulations are performed in overlapping parameter regimes. Results. For slow rotation we find that the radial and latitudinal turbulent angular momentum fluxes are directed inward and equatorward, respectively. In the rapid rotation regime the radial flux changes sign in contradiction with theory. The latitudinal flux remains mostly equatorward and develops a maximum close to the equator. In Cartesian simulations this peak can be explained by the strong `banana cells'. The latitudinal heat flux is mostly equatorward for slow rotation but changes sign for rapid rotation. The rotation profiles vary from anti-solar (slow equator) for slow and intermediate rotation to solar-like (fast equator) for rapid rotation. The solar-like profiles are dominated by the Taylor--Proudman balance.

astro-ph.SR↗

arXiv2010

Turbulent transport in hydromagnetic flows

The predictive power of mean-field theory is emphasized by comparing theory with simulations under controlled conditions. The recently developed test-field method is used to extract turbulent transport coefficients both in kinematic as well as nonlinear and quasi-kinematic cases. A striking example of the quasi-kinematic method is provided by magnetic buoyancy-driven flows that produce an alpha effect and turbulent diffusion.

astro-ph.SR↗

arXiv2005

Development of twist in an emerging magnetic flux tube by poloidal field accretion

Following an earlier proposal by Choudhuri (2003) for the origin of twist in the magnetic fields of solar active regions, we model the penetration of a wrapped up background poloidal field into a toroidal magnetic flux tube rising through the solar convective zone. The rise of the straight, cylindrical flux tube is followed by numerically solving the induction equation in a comoving Lagrangian frame, while an external poloidal magnetic field is assumed to be radially advected onto the tube with a speed corresponding to the rise velocity. One prediction of our model is the existence of a ring of reverse current helicity on the periphery of active regions. On the other hand, the amplitude of the resulting twist depends sensitively on the assumed structure (diffuse vs. concentrated/intermittent) of the active region magnetic field right before its emergence, and on the assumed vertical profile of the poloidal field. Nevertheless, in the model with the most plausible choice of assumptions a mean twist comparable to the observations can result rather naturally. Our results indicate that the contribution of this mechanism to the twist can be quite significant, and under favourable circumstances it can potentially account for most of the current helicity observed in active regions.

astro-ph↗