Calculation Of Transport Properties Research Articles

A theoretical study for electronic and transport properties of covalent functionalized MoS2 monolayer

The geometries, electronic and electron transport properties of a series of functionalized MoS2 monolayers were investigated using density-functional theory (DFT) and the non-equilibrium Green’s function (NEGF) methods. n-Propyl, n-trisilicyl, phenyl, p-nitrophenyl and p-methoxyphenyl are chosen as electron-donating groups. The results show covalent functionalization with electron-donating groups could make a transformation from typical semiconducting to metallic properties for appearance of midgap level across the Fermi level (Ef). The calculations of transport properties for two-probe devices indicate that conductivities of functionalized systems are obviously enhanced relative to pristine MoS2 monolayer. Grafted groups contribute to the major transport path and play an important role in enhancing conductivity. The NDR effect is found. The influence of grafted density is also studied. Larger grafted density leads to wider bandwidth of midgap level, larger current response of I-V curves and larger current difference between peak and valley.

A numerical perspective on the relation between particle rotational inertia and the equilibrium magnetization of a ferrofluid

This work explores, from a numerical perspective, the role of particle rotational inertia on the magnetization dynamics of ferrofluids. A robust numerical method is used for this purpose. The numerical research code is based on the use of a convergent long range dipolar interactions technique. These interactions are computed through a sophisticated Ewald summation procedure. The balance of linear and angular momentum is solved for N ensembles containing N particles each. Long range dipolar magnetic torques are solved in a periodic system of Lattices, spread in physical and reciprocal spaces to assure the convergence on the calculation of the suspension transport properties. A small effect of particle rotational inertia is considered. The system of equations of N particles distributed randomly in space is solved simultaneously for different parallel realizations in order to achieve a meaningful statistics of our many-body system. The results are focused on the behavior of the suspension magnetization for different particle concentrations and intensities of rotational inertia. The physical parameter used to express this effect is the particle rotational Stokes number. The simulations indicate that, from a numerical perspective, rotational inertia may induce a relevant, but often neglected, effect on the magnetization equilibrium of a ferrofluid. This finding is relevant for the community of numericists interested in using Langevin Dynamics applied to dipolar suspensions. We propose an expression with a correction on the effect of the particle rotational inertia to compute the magnetization of a magnetic fluid. The results obtained in this work are compatible with some insights previously pointed out in former scientifical works.

Self-energy matrices for electron transport calculations within the real-space finite-difference formalism.

The self-energy term used in transport calculations, which describes the coupling between electrode and transition regions, is able to be evaluated only from a limited number of the propagating and evanescent waves of a bulk electrode. This obviously contributes toward the reduction of the computational expenses in transport calculations. In this paper, we present a mathematical formula for reducing the computational expenses further without using any approximation and without losing accuracy. So far, the self-energy term has been handled as a matrix with the same dimension as the Hamiltonian submatrix representing the interaction between an electrode and a transition region. In this work, through the singular-value decomposition of the submatrix, the self-energy matrix is handled as a smaller matrix, whose dimension is the rank number of the Hamiltonian submatrix. This procedure is practical in the case of using the pseudopotentials in a separable form, and the computational expenses for determining the self-energy matrix are reduced by 90% when employing a code based on the real-space finite-difference formalism and projector-augmented wave method. In addition, this technique is applicable to the transport calculations using atomic or localized basis sets. Adopting the self-energy matrices obtained from this procedure, we present the calculation of the electron transport properties of C_{20} molecular junctions. The application demonstrates that the electron transmissions are sensitive to the orientation of the molecule with respect to the electrode surface. In addition, channel decomposition of the scattering wave functions reveals that some unoccupied C_{20} molecular orbitals mainly contribute to the electron conduction through the molecular junction.

Density-functional calculations of transport properties in the nondegenerate limit and the role of electron-electron scattering.

We compute electrical and thermal conductivities of hydrogen plasmas in the nondegenerate regime using Kohn-Sham density functional theory (DFT) and an application of the Kubo-Greenwood response formula, and demonstrate that for thermal conductivity, the mean-field treatment of the electron-electron (e-e) interaction therein is insufficient to reproduce the weak-coupling limit obtained by plasma kinetic theories. An explicit e-e scattering correction to the DFT is posited by appealing to Matthiessen's Rule and the results of our computations of conductivities with the quantum Lenard-Balescu (QLB) equation. Further motivation of our correction is provided by an argument arising from the Zubarev quantum kinetic theory approach. Significant emphasis is placed on our efforts to produce properly converged results for plasma transport using Kohn-Sham DFT, so that an accurate assessment of the importance and efficacy of our e-e scattering corrections to the thermal conductivity can be made.

Using BIB-SEM Imaging for Permeability Prediction in Heterogeneous Shales

Organic-rich shale samples from a lacustrine sedimentary sequence of the Newark Basin (New Jersey, USA) are investigated by combining Broad Ion Beam polishing with Scanning Electron Microscopy (BIB-SEM). We model permeability from this 2D data and compare our results with measured petrophysical properties. Three samples with total organic carbon (TOC) contents ranging from 0.7% to 2.9% and permeabilities ranging from 4 to 160 nD are selected. Pore space is imaged at high resolution (at 20,000x magnification) and segmented from representative BIB-SEM maps. Modeled permeabilities, derived using the capillary tube model (CTM) on segmented pores, range from 2.3 nD to 310 nD and are relatively close to measured intrinsic permeabilities. SEM-visible porosities range from 0.1% to 1.8% increasing with TOC, in agreement with our measurements. The CTM predicts permeability correctly within one order of magnitude. The results of this work demonstrate the potential of 2D BIB-SEM for calculating transport properties of heterogeneous shales.

Neutrino emissivities and bulk viscosity in neutral two-flavor quark matter

We study thermodynamic and transport properties for the isotropic color-spin-locking (iso-CSL) phase of two-flavor superconducting quark matter under compact star constraints within a Nambu-Jona-Lasinio-type chiral quark model. Chiral symmetry breaking and the phase transition to superconducting quark matter leads to a density dependent change of quark masses, chemical potentials, and diquark gap. A self-consistent treatment of these physical quantities influences the microscopic calculations of transport properties. We present results for the iso-CSL direct URCA emissivities and bulk viscosities, which fulfil the constraints on quark matter derived from cooling and rotational evolution of compact stars. We compare our results with the phenomenologically successful, but yet heuristic $2\mathrm{SC}+\mathrm{X}$ phase. We show that the microscopically founded iso-CSL phase can replace the purely phenomenological $2\mathrm{SC}+\mathrm{X}$ phase in modern simulations of the cooling evolution for compact stars with color-superconducting quark matter interior.

Regional analysis techniques for integrating experimental and numerical measurements of transport properties of reservoir rocks

Assessing the mechanisms of micro-structural change and their effect on transport properties using digital core analysis requires balancing field of view and resolution. This typically leads to the compromise of working with relatively small samples, where boundary effects can be substantial. A direct comparison with experiment, as e.g. desirable to eliminate unknown parameters and integrate numerical and physical experiments, needs to consider these boundary effects. Here we develop a workflow to define measuring windows within a sample where these boundary effects are minimised allowing the integration of physical and numerical experiment. We consider in particular sleeve leakage and use a radial partitioning of the solutions to various transport equations to derive relevant regional measures, which may be used for the development of cross-correlations between physical properties.Samples of Bentheimer and Castlegate sandstone as well as Mt. Gambier limestone and a sucrosic dolomite are considered. The sample plugs are encased in rubber sleeves and micro-CT images acquired at ambient conditions. Using these high-resolution images we calculate transport properties, namely permeability and electrical conductivity, and analyse the resulting field solutions with regard to flux across different regions of interest. The latter are selected on the basis of distance to the sample sleeve inner surface. Clear bypassing at the sleeve-sample interface in terms of elevated fluxes is observed for all samples, although to different extent. We consider different sleeve boundary conditions to define a measuring window minimising these effects, use the procedure to compare flux averages defined over these measuring windows with conventional choices of simulation domains, and compare resulting physical cross-correlations.

Calculation of effective transport properties of partially saturated gas diffusion layers

Abstract A large number of currently available Computational Fluid Dynamics numerical models of Polymer Electrolyte Membrane Fuel Cells (PEMFC) are based on the assumption that porous structures are mainly considered as thin and homogenous layers, hence the mass transport equations in structures such as Gas Diffusion Layers (GDL) are usually modelled according to the Darcy assumptions. Application of homogenous models implies that the effects of porous structures are taken into consideration via the effective transport properties of porosity, tortuosity, permeability (or flow resistance), diffusivity, electric and thermal conductivity. Therefore, reliable values of those effective properties of GDL play a significant role for PEMFC modelling when employing Computational Fluid Dynamics, since these parameters are required as input values for performing the numerical calculations. The objective of the current study is to calculate the effective transport properties of GDL, namely gas permeability, diffusivity and thermal conductivity, as a function of liquid water saturation by using the Lattice-Boltzmann approach. The study proposes a method of uniform water impregnation of the GDL based on the “Fine-Mist” assumption by taking into account the surface tension of water droplets and the actual shape of GDL pores.

CFD simulations of turbulent nonreacting and reacting flows for rocket engine applications

A consistent real gas framework based on a preconditioning scheme is implemented into the CFD code TASCOM3D to simulate nonreacting and reacting turbulent flows with transcritical injection. All thermodynamic properties and partial derivatives are rigorously evaluated from the reduced residual Helmholtz energy by use of fundamental thermodynamic relations. The Soave–Redlich–Kwong equation of state with an optional volume translation is employed in combination with high-pressure models for the calculation of fluid transport properties. Three test cases are simulated to validate the implemented real gas framework using different turbulence modeling strategies. A good match between numerical and experimental data is obtained.

Engineering the electronic structure of zigzag graphene nanoribbons with periodic line defect

By using first principle calculations we have studied the magnetic, electronic and transport properties of zigzag-graphene nanoribbon (zGNR) with a topological line defect (LD) composed of pentagons and heptagons (5-7). We show that one can engineer the magnetic and electronic properties of the edge passivated zGNR with 5-7 LD through the variation of either the width of the zGNR or the position of the LD. Thus, one can have ferromagnetic behaviour in zGNR by introducing 5-7 LD close to one edge of the ribbon. One can tune the zGNR with 5-7 LD from semi-metallic to semi-metallic semiconductor either by increasing the width of the ribbon or by changing the position of the LD. We have also studied the effect of the doping on the degeneracy of the spin states of 4-4-LD-zGNR. The calculation of transport properties of N-doped 4-4-LD-zGNR reveals that it has high spin filtering efficiencies. The tuning of the spin polarization through the formation of 5-7 LD in zGNR holds a promise for its application in spintronic devices.

Importance of the Hubbard correction on the thermal conductivity calculation of strongly correlated materials: a case study of ZnO.

The wide bandgap semiconductor, ZnO, has gained interest recently as a promising option for use in power electronics such as thermoelectric and piezoelectric generators, as well as optoelectronic devices. Though much work has been done to improve its electronic properties, relatively little is known of its thermal transport properties with large variations in measured thermal conductivity. In this study, we examine the effects of a Hubbard corrected energy functional on the lattice thermal conductivity of wurtzite ZnO calculated using density functional theory and an iterative solution to the Boltzmann transport equation. Showing good agreement with existing experimental measurements, and with a detailed analysis of the mode-dependence and phonon properties, the results from this study highlight the importance of the Hubbard correction in calculations of thermal transport properties of materials with strongly correlated electron systems.

Calculated electronic, transport, and related properties of zinc blende boron arsenide (zb-BAs)

We present the results from ab-initio, self-consistent density functional theory (DFT) calculations of electronic, transport, and bulk properties of zinc blende boron arsenide. We utilized the local density approximation potential of Ceperley and Alder, as parameterized by Vosko and his group, the linear combination of Gaussian orbitals formalism, and the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF), in carrying out our completely self-consistent calculations. With this method, the results of our calculations have the full, physical content of density functional theory (DFT). Our results include electronic energy bands, densities of states, effective masses, and the bulk modulus. Our calculated, indirect band gap of 1.48 eV, from Γ to a conduction band minimum close to X, for the room temperature lattice constant of 4.777 Å, is in an excellent agreement with the experimental value of 1.46 ± 0.02 eV. We thoroughly explain the reasons for the excellent agreement between our findings and corresponding, experimental ones. This work provides a confirmation of the capability of DFT to describe accurately properties of materials, if the computations adhere strictly to the conditions of validity of DFT, as done by the BZW-EF method.

Quantum chemical calculation of hole transport properties in crystalline polyethylene

Hole mobility in crystalline polyethylene (PE) is evaluated from an atomistic point of view, with the combination of quantum chemical calculation and kinetic Monte Carlo simulations. Hole hopping rate between PE chains are computed by Fermi’s golden rule and Marcus rate expression. It turns out that hole transfer in PE occurs in a hopping regime rather than in a band regime even for crystalline structure without any inherent structural disorders. The results indicate that in crystalline PE, hole hops between localized states that are created by polaronic effect. The hole mobility is strongly dependent on the chain length, and increases with increasing chain-length. When the chain length was comparable to the thickness of lamellar crystals, i.e., the characteristic length scale of crystal in the direction of c axis, hole mobility and activation energy for hole transfer were in reasonable agreement with experimental values. Simulated results support the experimental prediction that fast and slow mobility are due to charge transfer in the crystalline region and amorphous region, respectively. Our findings show that high hole mobility observed in crystalline PE, regardless of the small inter-molecular electronic couplings, is due to the small reorganization energy, which is realized through the intra-molecular hole delocalization, and due to quantum tunneling effects owing to the small energetic disorder and high-frequency intra-molecular phonon modes.

Quantum Scattering Calculations of Transport Properties for the H-N2 and H-CH4 Collision Pairs.

Transport properties for collisions of hydrogen atoms with molecular nitrogen and methane were calculated through close-coupling quantum scattering calculations. For these calculations, potential energy surfaces for the interaction of H atoms with these molecules, with their geometries fixed at the respective equilibrium structures, were obtained with a coupled cluster method that included single, double, and (perturbatively) triple excitations [RCCSD(T)]. The computed transport properties for H-N2 were found to be similar in magnitude to those computed in a previous study but significantly different from those obtained through the conventional approach that employs isotropic Lennard-Jones (LJ) (12-6) potentials. The differences in the transport properties for H-CH4 computed in this work and those estimated with isotropic LJ potentials are somewhat smaller.

Quantum close coupling calculation of transport and relaxation properties for Hg-H2 system

Quantum mechanical close coupling calculation of the state-to-state transport and relaxation cross sections have been done for Hg-H2 molecular system using a high-level ab initio potential energy surface. Rotationally averaged cross sections were also calculated to obtain the energy dependent Senftleben-Beenakker cross sections at the energy range of 0.005–25,000cm−1. Boltzmann averaging of the energy dependent Senftleben-Beenakker cross sections showed the temperature dependency over a wide temperature range of 50–2500K. Interaction viscosity and diffusion coefficients were also calculated using close coupling cross sections and full classical Mason-Monchick approximation. The results were compared with each other and with the available experimental data. It was found that Mason-Monchick approximation for viscosity is more reliable than diffusion coefficient. Furthermore, from the comparison of the experimental diffusion coefficients with the result of the close coupling and Mason-Monchick approximation, it was found that the Hg-H2 potential energy surface used in this work can reliably predict diffusion coefficient data.

First principles study and empirical parametrization of twisted bilayer MoS2 based on band-unfolding

We explore the band structure and ballistic electron transport in twisted bilayer MoS2 using the density functional theory. The sphagetti like bands are unfolded to generate band structures in the primitive unit cell of the original 2H MoS2 bilayer and projected onto the original bands of an individual layer. The corresponding twist angle dependent bandedges are extracted from the unfolded band structures. Based on a comparison within the same primitive unit cell, an efficient two band effective mass model for indirect ΓV and ΛC valleys is created and parametrized by fitting to the unfolded band structures. With the two band effective mass model, we calculate transport properties—specifically, the ballistic transmission in arbitrarily twisted bilayer MoS2.

Simulation Study on Understanding the Spin Transport in MgO Adsorbed Graphene Based Magnetic Tunnel Junction

First principles calculations of spin-dependent electronic transport properties of magnetic tunnel junction (MTJ) consisting of MgO adsorbed graphene nanosheet sandwiched between two CrO2 half-metallic ferromagnetic (HMF) electrodes is reported. MgO adsorption on graphene opens bandgap in graphene nanosheet which makes it more suitable for use as a tunnel barrier in MTJs. It was found that MgO adsorption suppresses transmission probabilities for spin-down channel in case of parallel configuration (PC) and also suppresses transmission in antiparallel configuration (APC) for both spin-up and spin-down channel. Tunnel magneto-resistance (TMR) of 100% is obtained at all bias voltages in MgO adsorbed graphene-based MTJ which is higher than that reported in pristine graphene-based MTJ. HMF electrodes were found suitable to achieve perfect spin filtration effect and high TMR. I–V characteristics for both parallel and antiparallel magnetization states of junction are calculated. High TMR suggests its usefulness in spin valves and other spintronics-based applications.

Molecular Modeling of Energy Barrier of Li Ion Transport from Negative to Positive Electrodes

Lithium ion battery is used as a main power supply of a small portable device and large power supplies for electric cars. For further applications, however, an improvement in battery performance from materials level based on rational design of Li ion transport in the battery is needed. In this study, we carried out systematic analysis of energy barrier of Li ion transport from anode to cathode materials using first-principles molecular dynamics and classical molecular dynamics. First, we calculated the energy barrier of diffusion of Li ion through several kinds of electrolyte. To validate the accuracy of calculated transport properties, diffusion coefficients and activation energies for diffusion of ions in LiPF6-DEC-EC at different temperature (-20 ℃ – 20 ℃) and ratio of DEC/EC are compared with experimental data and confirmed the reasonable agreement. Furthermore, the solvation energy of Li ion in electrolytes as well as the salvation structure were investigated by density functional theory and classical molecular dynamics. First principles molecular dynamics and density functional theory were used to calculate the adsorption energy and activation energy fir diffusion of Li ion in cathode materials such as LiCoO2. From these analysis, we conclude that the solvation energy, which is more than 50 kcal/mol for EC/DEC system, is the highest energy barrier for transport of Li ion of which would be the limited step for Li ion transport although it depends on the condition. We also investigated the influence of these transport properties to the battery performance (I-V curve). We will present more concrete data regarding Li ion transport at the conference.

Comparative calculations of electron transport properties in 6H–SiC using three and five valley models

Steady-state electron properties are investigated in 6H–SiC at various temperatures, using Monte Carlo simulation where the band structure model is a major part when dealing with high fields. The aim of this work is to optimize the number of valleys involved in the simulation program in order to obtain accurate results while improving the calculation efficiency. For high fields, a five valley model was found to be more accurate than a three valley model and as efficient as the full band method though much less computer time-consuming.

Ab initio electron scattering cross-sections and transport in liquid xenon

Ab initio fully differential cross-sections for electron scattering in liquid xenon are developed from a solution of the Dirac–Fock scattering equations, using a recently developed framework (Boyle et al 2015 J. Chem. Phys. 142 154507) which considers multipole polarizabilities, a non-local treatment of exchange, and screening and coherent scattering effects. A multi-term solution of Boltzmann’s equation accounting for the full anisotropic nature of the differential cross-section is used to calculate transport properties of excess electrons in liquid xenon. The results were found to agree to within 25% of the measured mobilities and characteristic energies over the reduced field range of 10−4–1 Td. The accuracies are comparable to those achieved in the gas phase. A simple model, informed by highly accurate gas-phase cross-sections, is presented to improve the liquid cross-sections, which was found to enhance the accuracy of the transport coefficient calculations.