New type of stable particle like states in chiral magnets (Chiral bobbers)

We present a new type of a thermodynamically stable magnetic state at interfaces and surfaces of chiral magnets. The state is a soliton solution of micromagnetic equations localized in all three dimensions near a boundary and contains a singularity, but nevertheless has a finite energy. Both features combine to a quasi-particle state for which we expect unusual transport and dynamical properties. It exhibits high thermal stability and thereby can be considered as promising object for fundamental research and practical applications in spintronic devices. We provide arguments that such a state can be found in different B20-type alloys e.g. Mn$_{1-x}$Fe$_x$Ge, Mn$_{1-x}$Fe$_x$Si, Fe$_{1-x}$Co$_x$Si.Comment: accepted in PR

QuantumATK: An integrated platform of electronic and atomic-scale modelling tools

QuantumATK is an integrated set of atomic-scale modelling tools developed since 2003 by professional software engineers in collaboration with academic researchers. While different aspects and individual modules of the platform have been previously presented, the purpose of this paper is to give a general overview of the platform. The QuantumATK simulation engines enable electronic-structure calculations using density functional theory or tight-binding model Hamiltonians, and also offers bonded or reactive empirical force fields in many different parametrizations. Density functional theory is implemented using either a plane-wave basis or expansion of electronic states in a linear combination of atomic orbitals. The platform includes a long list of advanced modules, including Green's-function methods for electron transport simulations and surface calculations, first-principles electron-phonon and electron-photon couplings, simulation of atomic-scale heat transport, ion dynamics, spintronics, optical properties of materials, static polarization, and more. Seamless integration of the different simulation engines into a common platform allows for easy combination of different simulation methods into complex workflows. Besides giving a general overview and presenting a number of implementation details not previously published, we also present four different application examples. These are calculations of the phonon-limited mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model simulation of lithium ion drift through a battery cathode in an external electric field, and electronic-structure calculations of the composition-dependent band gap of SiGe alloys.Comment: Submitted to Journal of Physics: Condensed Matte

Сomputational modeling of a charge transport problem in MOSFET transistor

We propose and describe in detail a new effective numerical algorithm for finding the stationary solutions of charge transport problem in MOSFET transistor. For mathematical description of charge transport process a hydrodynamical MEP model is used. It is worth noting that this model is a set of nonlinear PDE's with small parameters and specific boundary conditions corresponding to MOSFET. It makes the computational process much more complicated. The proposed algorithm is based on the stabilization method, the application of regularized smoothing operators and ideas of schemes without saturation

Advanced Multi-phase Flow CFD Model Development for Solid Rocket Motor Flowfield Analysis

A Navier-Stokes code, finite difference Navier-Stokes (FDNS), is used to analyze the complicated internal flowfield of the SRM (solid rocket motor) to explore the impacts due to the effects of chemical reaction, particle dynamics, and slag accumulation on the solid rocket motor (SRM). The particulate multi-phase flowfield with chemical reaction, particle evaporation, combustion, breakup, and agglomeration models are included in present study to obtain a better understanding of the SRM design. Finite rate chemistry model is applied to simulate the chemical reaction effects. Hermsen correlation model is used for the combustion simulation. The evaporation model introduced by Spalding is utilized to include the heat transfer from the particulate phase to the gase phase due to the evaporation of the particles. A correlation of the minimum particle size for breakup expressed in terms of the Al/Al2O3 surface tension and shear force was employed to simulate the breakup of particles. It is assumed that the breakup occurs when the Weber number exceeds 6. A simple L agglomeration model is used to investigate the particle agglomeration. However, due to the large computer memory requirements for the agglomeration model, only 2D cases are tested with the agglomeration model. The VOF (Volume of Fluid) method is employed to simulate the slag buildup in the aft-end cavity of the redesigned solid rocket motor (RSRM). Monte Carlo method is employed to calculate the turbulent dispersion effect of the particles. The flowfield analysis obtained using the FDNS code in the present research with finite rate chemical reaction, particle evaporation, combustion, breakup, agglomeration, and VOG models will provide a design guide for the potential improvement of the SRM including the use of materials and the shape of nozzle geometry such that a better performance of the SRM can be achieved. The simulation of the slag buildup in the aft-end cavity can assist the designer to improve the design of the RSRM geometry

On an approach to the construction of difference schemes for the moment equations of charge transport in semiconductors

We discuss the construction of a class of difference schemes for a hydrodynamical model of charge transport in semiconductors

Design and development of solid-state functional materials for Na-ion batteries

This Thesis addresses new functional materials for Na-ion battery (NIB) applications. Since the breakthrough of Li-ion battery (LIB), extensive research has been focusing on alternatives to Lithium, based on cheaper and widespread elements for sustainable energy storage solutions. In this context, the effective large-scale deployment of NIB requires great efforts in the development of good Na+ host anodes, high-energy cathodes and safe electrolytes. New components must ensure enhanced efficiency in the NIB operating processes (i.e., Na+ insertion/extraction at the electrode/electrolyte interface and Na+ transport through the electrolyte) for empowering high energy density and long-term cycle stability. Here, we present NIB materials optimization through an innovative approach, based on computational methods that are directly related to experiments. Our aim is to unveil the most important features that can affect the material capabilities towards Na+ uptake, transport and storage. During the research activity at Università di Napoli Federico II, state-of-the-art DFT methods have been employed to investigate the structure-property relationship of solid-state nanoelectrodes. Our studies on TiO2 anatase and MoS2/graphene 2D-heterostructure reveals that sodiation mechanisms are driven by intrinsic structural features. Migration barriers are directly correlated to structure-dependent descriptors, such as the accessible area for the intercalating Na+ at TiO2 surfaces, and the S coordination around the migrating Na+ within MoS2/graphene interface. From these outcomes, we provide new design strategies to improve the electrode efficiency upon sodiation, for example suggesting the preferential growth of TiO2 along the (001) direction or the introduction of S vacancies in MoS2 monolayers. On the cathode side, we unveil the charge compensation mechanism occurring in NaxNi0.25Mn0.68O2 upon desodiation, with a major focus on the O-redox chemistry at very low Na loads. Molecular O2 is predicted to be released from Mn-deficient sites in the bulk cathode via formation of superoxo-species and preferential breaking of labile Ni-O bonds. We prove that increasing M-O covalency via suitable doping would prevent O2 loss and allows to fully recover a reversible process. Research stages at ENS de Lyon and the R&D laboratory of Lithops s.r.l. have been dedicated to the optimization of electrolyte materials. By development and application of polarizable force fields in molecular dynamics simulations, we report reliable predictions of Na+ diffusion and solvation properties into the PyrFSI room-temperature ionic liquid (RT-IL). We combine RT-ILs with cross-linked PEO matrix to obtain highly conductive polymeric membranes. Galvanostatic cycling of Na metal based cells containing these innovative polymer electrolytes and state-of-the-art electrodes shows promising performances and paves the route to further assessment of efficient cells. The foreseen integration of these studies will provide new understanding on the complex charge transfer processes occurring at the electrode/electrolyte interface during battery functioning. The new knowledge on electrochemical behavior of advanced materials will be key for boosting the NIB technology in the near future

Hydrodynamic modeling of electron transport in silicon quantum wires

In questa tesi un modello idrodinamico per il trasporto di elettroni nei fili quantici al silicio (SiNW) è presentato. Tale modello è stato formulato considerando i momenti derivanti dall'equazione cinetica di Boltzmann e chiudendo il sistema dei momenti ottenuto per mezzo del Principio di Massima Entropia della Termodinamica Estesa. Questo modello di trasporto include necessariamente l'equazione di Schroedinger bidimensionale accoppiata all'equazione di Poisson 3D. Al fine di testare il modello, simulazioni numeriche per SiNW transistors sono eseguite. Importanti proprietà in tali dispositivi, come la mobilità elettronica, sono studiate e i risultati ottenuti sono confrontati con quelli conseguiti usando approcci diversi nella simulazione di dispositivi elettronici basati sui nanowires