No Arabic abstract
In this work we present a new method for the calculation of the electrostrictive properties of materials using density functional theory. The method relies on the thermodynamical equivalence, in a dielectric, of the quadratic mechanical responses (stress or strain) to applied electric stimulus (electric or polarisation fields) to the strain or stress dependence of its dielectric susceptibility or stiffness tensors. Comparing with current finite-field methodologies for the calculation of electrostriction, we demonstrate that our presented methodology offers significant advantages of efficiency, robustness, and ease of use. These advantages render tractable the highthroughput theoretical investigation into the largely unknown electrostrictive properties of materials.
The electronic transport behaviour of materials determines their suitability for technological applications. We develop an efficient method for calculating carrier scattering rates of solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 16 semiconductors and comparing the results against experimental measurements. The present work is amenable for use in high-throughput computational workflows and enables accurate screening of carrier mobilities, lifetimes, and thermoelectric power.
The bulk photovoltaic effect (BPVE) has attracted an increasing interest due to its potential to overcome the efficiency limit of traditional photovoltaics, and much effort has been devoted to understanding its underlying physics. However, previous work has shown that theoretical models of the shift current and the phonon-assisted ballistic current in real materials do not fully account for the experimental BPVE photocurrent, and so other mechanisms should be investigated in order to obtain a complete picture of BPVE. In this Letter, we demonstrate two approaches that enable the ab initio calculation of the ballistic current originating from the electron-hole interaction in semiconductors. Using BaTiO$_3$ and MoS$_2$ as two examples, we show clearly that for them the asymmetric scattering from electron-hole interaction is less appreciable than that from electron-phonon interaction, indicating more scattering processes need to be included to further improve the BPVE theory. Moreover, our approaches build up a venue for predicting and designing materials with larger ballistic current due to electron-hole interactions.
We use state-of-the art GW-BSE formalism to study electronic structure and optical properties of oxygen vacancies in $alpha$-alumina. Many body perturbation theory within GW approximation in recent years have been used extensively to study excited state properties of a wide range of systems. Moreover, solving Bethe-Salpeter equation (BSE) enable us to capture excitonic effects in a material. We compute the charge transition levels (CTLs) for oxygen vacancies using DFT+GW formalism. We propose an alternative approach to calculate these CTLs, which provides a more efficient way to perform electrostatic correction required because of finite supercell sizes and periodic boundary condition used in first principles calculations. We find that oxygen vacancy in this material has deep donor levels, (+2/+1) at 2.5 eV and a (+1/0) level at 3.8 eV above the VBM. We also study F-center absorption and emission processes using constrained--DFT and BSE. Our calculated absorption and emission energies are in excellent agreement with experimental results.
We calculate the density of states (DOS) and the Mulliken population of the diamond and the co-doped diamonds with different concentrations of lithium (Li) and phosphorus (P) by the method of the density functional theory, and analyze the bonding situations of the Li-P co-doped diamond thin films and the impacts of the Li-P co-doping on the diamond conductivities. The results show that the Li-P atoms can promote the split of the diamond energy band near the Fermi level, and improve the electron conductivities of the Li-P co-doped diamond thin films, or even make the Li-P co-doped diamond from semiconductor to conductor. The effect of Li-P co-doping concentration on the orbital charge distributions, bond lengths and bond populations is analyzed. The Li atom may promote the split of the energy band near the Fermi level as well as may favorably regulate the diamond lattice distortion and expansion caused by the P atom.
Inelastic scattering experiments are key methods for mapping the full dispersion of fundamental excitations of solids in the ground as well as non-equilibrium states. A quantitative analysis of inelastic scattering in terms of phonon excitations requires identifying the role of multi-phonon processes. Here, we develop an efficient first-principles methodology for calculating the {it all-phonon} quantum mechanical structure factor of solids. We demonstrate our method by obtaining unprecedented agreement between measurements and calculations of the diffuse diffraction patterns of black phosphorus, showing that multi-phonon scattering plays a substantial role. The present approach constitutes a pivotal advancement in the interpretation of static and time-resolved electron, X-ray, and neutron inelastic scattering data.