X-ray Absorption Near-Edge Structure calculations with pseudopotentials. Application to K-edge in diamond and alpha-quartz

We present a reciprocal-space pseudopotential scheme for calculating X-ray absorption near-edge structure (XANES) spectra. The scheme incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole interaction through large supercells (hundreds of atoms). The method is compared with recently developed Bethe-Salpeter approach. The method is applied to the carbon K-edge in diamond and to the silicon and oxygen K-edges in alpha-quartz for which polarized XANES spectra were measured. Core-hole effects are investigated by varying the size of the supercell, thus leading to information similar to that obtained from cluster size analysis usually performed within multiple scattering calculations.Comment: 11 pages, 4 figure

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

Electron energy-loss spectroscopy: DFT modelling and application to experiment

The all-electron density functional theory (DFT) code Wien2k has an established track record of modelling energy-loss near-edge structure (ELNES). The pseudopotential DFT code CASTEP can reproduce results found using Wien2k. A methodology was developed for DFT code parameter selection, based on converging parameters to the ELNES prediction. Various aluminium systems were studied; aluminium, aluminium nitride and aluminium oxide. Uniquely for aluminium metal, a ground state calculation provided strong agreement with experiment, as the core-hole is well screened. It was quantitatively demonstrated that the core-hole causes ionisation edge peaks to shift towards the Fermi level, and increases the intensity of those peaks - effects found to be larger for the cationic species. Group 4 and 5 transition metal carbides were modelled using CASTEP. Systems with vacancies were considered; TiC0.79, TiC0.58N0.30, TiC0.45N0.43, TiC0.19N0.65 and TiN0.82. By comparison with experimental data, structures for these systems were proposed. CASTEP was used to model oxygen K edges in various systems. For bulk MgO, acceptable experimental agreement was found using a ground state calculation. This was rationalised by observing that the introduction of a corehole had relatively little effect on the p orbital DOS prediction for oxygen. For the interface of Fe (001) / MgO (001), it was demonstrated by careful comparison of theory and experiment that some degree of oxidation was present. Nanoscale analysis of multilayered CrAlYN/CrN coatings was performed. Experimentally observed ELNES was reproduced using ground state Wien2k calculations. Combined experimental and theoretical analysis indicated that the nominal CrN layers were close to stoichiometric CrN, the Cr/N ratio being 1.05 ± 0.1. For the CrAlYN layers, the theoretical system showing the best agreement was Cr0.5Al0.5N. This thesis has established methodologies for utilising DFT codes, illustrating how links between experimental and theoretical ELNES can be used in the nanoscale characterisation of technologically important materials

Vibrational anharmonicity of small gold and silver clusters using the VSCF method

We study the vibrational spectra of small neutral gold (Au2–Au10) and silver (Ag2–Au5) clusters using the vibrational self-consistent field method (VSCF) in order to account for anharmonicity. We report harmonic, VSCF, and correlation-corrected VSCF calculations obtained using a vibrational configuration interaction approach (VSCF/VCI). Our implementation of the method is based on an efficient calculation of the potential energy surfaces (PES), using periodic density functional theory (DFT) with a plane-wave pseudopotential basis. In some cases, we use an efficient technique (fast-VSCF) assisted by the Voter–Chen potential in order to get an efficient reduction of the number of pair-couplings between modes. This allows us to efficiently reduce the computing time of 2D-PES without degrading the accuracy. We found that anharmonicity of the gold clusters is very small with maximum rms deviations of about 1 cm−1, although for some particular modes anharmonicity reaches values slightly larger than 2 cm−1. Silver clusters show slightly larger anharmonicity. In both cases, large differences between calculated and experimental vibrational frequencies (when available) stem more likely from the quality of the electronic structure method used than from vibrational anharmonicity. We show that noble gas embedding often affects the vibrational properties of these clusters more than anharmonicity, and discuss our results in the context of experimental studies

Modelling of semiconductor nanostructures : electronic properties and simulated optical spectra

III-V semiconductor nanostructures are widely used in optoelectronic devices (e.g. lasers and detectors) in the visible (0.4-0.8 μm), near-infrared (0.8-3 μm), mid-infrared (3-5 μm) and far-infrared (> 8 μm) wavelength ranges, with great potential for high performance and high temperature operation. As well as simple designs, complex structures incorporating low dimensional components (e.g. quantum wells and quantum dots) are not unusual. Often, the optical and electronic characteristics of these structures are altered significantly as compared to bulk material. As a prerequisite to design for different applications, the study of their electronic and optical properties is essential. With the increasing computational power of modern personal computers, computational modelling becomes viable and more efficient. Indeed, it has become routine to follow (or to precede) experimental studies with computational modelling of good interpretive and predictive power. Combined with experimental studies, this is a powerful tool to provide insight into new devices. This research work is primarily based on calculations of the electronic band structure of various semiconductor nanostructures, followed by modelling of optical transitions and optical spectra. All numerical calculations use a cost effective computational method. The applicability of the model to ultra-thin structures of short period InAs/GaSb superlattices is investigated. The work is then extended to study complex quantum-dot-in-well structures. Finally, the attempt to extract the structural parameters of quantum dots by a combination of modelling and optical spectroscopy is presented