16 research outputs found
Three-dimensional structure and stability of discontinuities between unmagnetized pair plasma and magnetized electron-proton plasma
We study with a 3D PIC simulation discontinuities between an
electron-positron pair plasma and magnetized electrons and protons. A pair
plasma is injected at one simulation boundary with a speed 0.6 along its
normal. It expands into an electron-proton plasma and a magnetic field that
points orthogonally to the injection direction. Diamagnetic currents expel the
magnetic field from within the pair plasma and pile it up in front of it. It
pushes electrons, which induces an electric field pulse ahead of the magnetic
one. This initial electromagnetic pulse (EMP) confines the pair plasma
magnetically and accelerates protons electrically. The fast flow of the
injected pair plasma across the protons behind the initial EMP triggers the
filamentation instability. Some electrons and positrons cross the injection
boundary and build up a second EMP. Electron-cyclotron drift instabilities
perturb the plasma ahead of both EMPs seeding a Rayleigh-Taylor-type
instability. Despite equally strong perturbations ahead of both EMPs, the
second EMP is much more stable than the initial one. We attribute the rapid
collapse of the initial EMP to the filamentation instability, which perturbed
the plasma behind it. The Rayleigh-Taylor-type instability transforms the
planar EMPs into transition layers, in which magnetic flux ropes and
electrostatic forces due to uneven numbers of electrons and positrons slow down
and compress the pair plasma and accelerate protons. In our simulation, the
expansion speed of the pair cloud decreased by about an order of magnitude and
its density increased by the same factor. Its small thickness implies that it
is capable of separating a relativistic pair outflow from an electron-proton
plasma, which is essential for collimating relativistic jets of pair plasma in
collisionless astrophysical plasma.Comment: 25 pages, 12 figures, provisionally accepted for publication by the
New Journal of Physic
Nitrogen vacancy, self-interstitial diffusion, and Frenkel-pair formation/dissociation in B1 TiN studied by ab initio and classical molecular dynamics with optimized potentials
We use ab initio and classical molecular dynamics (AIMD and CMD) based on the modified embedded-atom method (MEAM) potential to simulate diffusion of N vacancy and N self-interstitial point defects in B1 TiN. TiN MEAM parameters are optimized to obtain CMD nitrogen point-defect jump rates in agreement with AIMD predictions, as well as an excellent description of TiNx (∼0.7 < x 1) elastic, thermal, and structural properties. We determine N dilute-point-defect diffusion pathways, activation energies, attempt frequencies, and diffusion coefficients as a function of temperature. In addition, the MD simulations presented in this paper reveal an unanticipated atomistic process, which controls the spontaneous formation of N self-interstitial/N vacancy (NI/NV) pairs (Frenkel pairs), in defect-free TiN. This entails that the N lattice atom leaves its bulk position and bonds to a neighboring N lattice atom. In most cases, Frenkel-pair NI and NV recombine within a fraction of ns; ∼50% of these processes result in the exchange of two nitrogen lattice atoms (N−NExc). Occasionally, however, Frenkel-pair N-interstitial atoms permanently escape from the anion vacancy site, thus producing unpaired NI and NV point defects
Recent progress in simulations of the paramagnetic state of magnetic materials
We review recent developments in the field of first-principles simulations of magnetic materials above the magnetic order–disorder transition temperature, focusing mainly on 3d-transition metals, their alloys and compounds. We review theoretical tools, which allow for a description of a system with local moments, which survive, but become disordered in the paramagnetic state, focusing on their advantages and limitations. We discuss applications of these theories for calculations of thermodynamic and mechanical properties of paramagnetic materials. The presented examples include, among others, simulations of phase stability of Fe, Fe–Cr and Fe–Mn alloys, formation energies of vacancies, substitutional and interstitial impurities, as well as their interactions in Fe, calculations of equations of state and elastic moduli for 3d-transition metal alloys and compounds, like CrN and steels. The examples underline the need for a proper treatment of magnetic disorder in these systems
Finite temperature, magnetic,and many-body effects in Ab initio simulations of alloy thermodynamics
Ab initio electronic structure theory is known as a useful tool for prediction of materials properties. However, majority of simulations still deal with calculations in the framework of density functional theory with local or semi-local functionals carried out at zero temperature. We present new methodological solutions, which go beyond this approach and explicitly take finite temperature, magnetic, and many-body effects into account. Considering Ti-based alloys, we discuss limitations of the quasiharmonic approximation for the treatment of lattice vibrations, and present an accurate and easily extendable method to calculate free energies of strongly anharmonic solids. We underline the necessity to going beyond the state-of-the-art techniques for the determination of effective cluster interactions in systems exhibiting metal-to-insulator transition, and describe a unified cluster expansion approach developed for this class of materials. Finally, we outline a first-principles method, disordered local moments molecular dynamics, for calculations of thermodynamic properties of magnetic alloys, like Cr1-xAl xN, in their high-temperature paramagnetic state. Our results unambiguously demonstrate importance of finite temperature effects in theoretical calculations of thermodynamic properties of materials