Simplified pseudopotential problems for the classroom

Ab initio methods have been used for many decades to accurately predict properties of solids such as the physical, electronic, optical, magnetic, and elastic properties. A generation ago, many research groups developed their own in-house codes to perform ab initio calculations. In doing so, research students were intimately involved in many aspects of the coding, such as developing the theoretical framework, and algorithmic and programming details. Over time however, collaborations between various research groups within academia and in industry have resulted in the creation of more than 50 large opensource and commercial electronic structure packages. These software packages are widely used today for condensed matter research by students who, unfortunately, often have very little understanding of the fundamental aspects of these codes. To address this shortcoming, we have embarked on a program at the University of Pretoria to devise a range of simplified, easily programmable computational problems appropriate for the classroom, which can be used to teach advanced undergraduate students about particular theoretical and computational aspects of the electronic structure method. In this paper, we focus on the pseudopotential, which is a centrally important concept in many modern ab initio methods. Whereas the full implementation of the pseudopotential construct in a real electronic structure code requires complex numerical methods, e.g. accelerated convergence to self-consistency including the interactions between all the electrons in the system, we show that the essential principles of the pseudopotential can, nevertheless, be presented in a simpler class of problems, which can readily be coded by students.National Research Foundation and the University of Pretoria.http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=5992hb201

Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials

Quantum ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn Source Package for Research in Electronic Structure, Simulation, and Optimization". It is freely available to researchers around the world under the terms of the GNU General Public License. Quantum ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively-parallel architectures, and a great effort being devoted to user friendliness. Quantum ESPRESSO is evolving towards a distribution of independent and inter-operable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte

Ab-initio design of bulk materials assembled with silicon clusters

Tese de doutoramento, Física, Universidade de Lisboa, Faculdade de Ciências, 2011The fact that the founding papers of Density Functional Theory are among the most cited papers ever, testi es for the importance of Quantum Mechanics and its (often) counter intuitive features in characterizing many-particle systems at a nano and sub-nano scale. Density Functional Theory has enabled one to use the computer to predict quantitatively several of the properties of the aforementioned many-particle systems. The prediction of new materials, often exhibiting meta-stability, is one of its distinctive features. In this work we will discuss a new class of meta-materials which, being silicon based, exhibit properties which in no way resemble those of its main constituent. We investigate the feasibility of assembling the exceptionally stable isovalent X@Si16 (X=Ti, Zr and Hf) nanoparticles to form new bulk materials. We use rst principles density functional theory. Our results predict the formation of stable, wide band-gap materials crystallizing in HCP structures in which the cages bind weakly, similar to fullerite. The present study suggests new pathways through which endohedral cage clusters may constitute viable means toward the production of synthetic materials with pre-de ned physical and chemical properties. Within the same rst-principles framework we will investigate the vibrational modes and infrared spectra of the isovalent X@Si16 (X=Ti, Zr and Hf) nanoparticles. Our results predict the existence of high-intensity modes of low frequency. An estimate of the electron-phonon coupling strength is also provided based on a single-molecule method introduced recently. The large value of combined with predicted stability of bulk materials assembled with these nanoparticles suggest that these new materials, when appropriately doped, may exhibit high-temperature superconducting properties

ATOMISTIC AND EXPERIMENTAL DETERMINATION OF THE STRUCTURAL AND THERMOPHYSICAL PROPERTIES OF THE ACCIDENT TOLERANT FUEL MATERIALS

The tragic nuclear accident at the Fukushima-Daiichi power station in Japan brought in to our attention the risk associated with the current design of reactors based on uranium dioxide (UO2) fuel and zirconium cladding. As an offshoot, the research towards accident tolerant nuclear fuel (ATF) that can withstand the loss of coolant for a long time while improving thermal efficiency has gained momentum. Most desirable thermophysical properties expected of an ATF is high thermal conductivity, the lack of which leads to the poor dissipation and rapid meltdown at the core of the fuel pellet during the loss of coolant. Several approaches are being considered by researchers across the world to improve the thermal conductivity of nuclear fuels. Apart from the state of art of uranium-based fuels, there is a renewed interest in thorium-based fuels (especially thorium dioxide (ThO2) and thorium nitride (ThN)) in the quest of ATF. This thesis focuses on evolutionary fuel concepts based on thoria fuels. Unlike UO2, the information regarding the thermophysical properties of ThO2 fuels, and the additive materials under the normal operating conditions and the extreme accident conditions are not well known. Therefore, in this thesis, the computational techniques such as density functional theory (DFT) and classical molecular dynamics (MD) are used to determine the thermophysical properties of the thoria fuel, surrogate of thoria CeO2 and additive materials such as SiC and BeO. One of the significant limitations in the front end of the thoria fuel cycle has the difficulty of fabricating dense pellets by conventional sintering techniques. Hence the processing of thoria fuels by the spark plasma sintering (SPS) was proposed, and the effect of the sintering parameters on the density, microstructure and the thermal conductivity of ThO2 fuel was established. Finally, using SPS, a novel composite fuel of ThO2-SiC has been fabricated with the enhanced thermal conductivity

Aqueous systems from first-principles : structure, dynamics and electron-transfer reactions

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006.Includes bibliographical references (p. 127-141).In this thesis, we show for the first time how it is possible to calculated fully from first-principles the diabatic free-energy surfaces of electron-transfer reactions. The excitation energy corresponding to the transfer of an electron at any given ionic configuration (the Marcus energy gap) is accurately assessed within ground-state density-functional theory via a novel penalty functional for oxidation-reduction reactions that appropriately acts on the electronic degrees of freedom alone. The self-interaction error intrinsic to common exchange-correlation functionals is also corrected by the same penalty functional. The diabatic free-energy surfaces are then constructed from umbrella sampling on large ensembles of configurations. As a paradigmatic case study, the self-exchange reaction between ferrous and ferric ions in water is studied in detail. Since the solvent plays an central role in mediating the process, studying electron-transfer reactions requires us to first understand the structure and dynamics of the solvent molecules (water molecules in our case). Therefore, we have also studied the static and dynamical properties of (heavy) water at ambient conditions with extensive first-principles molecular-dynamics simulations in the canonical ensemble, with temperatures ranging between 325 K and 400 K.(cont.) Density-functional theory, paired with a modern exchange-correlation functional (PBE), provides an excellent agreement for the structural properties and binding energy of the water monomer and dimer. On the other hand, contrary to a long-standing belief, the structural and dynamical properties of the bulk liquid show a clear enhancement of the local structure compared to experimental results; a distinctive transition to liquid-like diffusion occurs in the simulations only at the elevated temperature of 400 K. The local coordination and structure of water is still a very debated matter and in collaboration with experimentalists at the European Synchrotron Radiation Facility in Grenoble, we have characterized the structure and the local environment in water with a combination of inelastic X-ray scattering and first-principles calculations, under conditions ranging from the normal state to the supercritical regime. The same temperature dependence of the Compton profile is observed in experiment and simulation. A well-defined linear correlation is identified between Compton profile differences and changes in the number of hydrogen bonds per molecule, that is consistent with well-established structural models, and that confirms the prevailing picture of hydrogen bonding under normal conditions.(cont.) While close to the critical point we observe a clear signature of density fluctuations, supercritical water is characterized by a sharp increase in under-coordinated clusters, with a significant number of dimers and trimers. Last, we implemented a Hubbard U correction in our first-principles molecular dynamics to improve the hybridization between a transition metal ion and its surroundings. The implementation has been tested for ferrous and ferric ions solvation in water. The effects of the Hubbard U correction on the electron-transfer reaction is also studied.by Patrick Hoi Land Sit.Ph.D

Rationalizing the Band Gap Tunability of Semiconductors via Electronic Structure Calculations

The polymorphs of titanium dioxide and various diamond-like semiconductor materials are promising candidates in photovoltaic solar cell applications. Several of these polymorphs have been studied with experimental and computational methods, which often aim at tuning the electronic structure, particularly the band gap value of the crystalline solid. Prior studies report that the addition of a substituent into the structure of titanium dioxide decreases its band gap value, but the reasons for this are unknown. Possible explanations for the change in band gap involve the substituent atom\u27s crystal radius, electronegativity, and ionization energy. Understanding the cause of these changes will provide great insight in designing new materials. The hypothesis of this work is that a substituent atom\u27s crystal radius has a greater impact on the change in band gap of titanium dioxide than the substituent atom\u27s electronegativity. In an aim to test this hypothesis, atoms of differing chemical properties were selected for substitution into the rutile and anatase polymorphs of titanium dioxide. The electronic structure (band structure and density of states) of the substituted systems was calculated using the full-potential linearized-augmented plane wave approach of density functional theory in the WIEN2k software. Upon calculating the electronic structure, the relationships between the band gap value and various chemical properties were evaluated

Measurements of electric field noise and light-induced charging in Al and Cu surface electrode ion traps at cryogenic temperatures

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 85-89).Ion traps provide an excellent tool for controlling and observing the state of a single trapped ion. For this reason, ion traps have been proposed as a possible system for large-scale quantum computation. However, many obstacles must be overcome before quantum computing can become a reality. In particular, perturbations in the electric field due to noise and electrode charging must be reduced to increase coherence of the motional quantum state. Gold has been a popular choice in the past due to its inert properties; however, it is undesirable due its incompatibility with CMOS technology. This has led to increased research into alternative CMOS-compatible materials, such as aluminum and copper. This thesis presents measurements of electric field noise and light-induced charging in aluminum, copper, and gold surface electrode traps. In addition, the effect of oxide growth on field noise and electrode charging is explored by controlling the thickness of aluminum oxide on several aluminum traps. The measurements show that electric field noise can be suppressed in aluminum traps to approximately 10-18 V2 cm-2 Hz-1, matching the noise exhibited in gold traps, and that copper traps exhibit noise within an order of magnitude of that in aluminum and gold. However, the natural oxide of aluminum poses many problems towards high-performance aluminum ion traps. The electric field noise is shown to be strongly dependent on the oxide thickness, increasing the noise by a factor of about 10 until saturation at a thickness of 13 nm. Charging of surface electrodes is shown to be highly dependent upon the material, but the model presented does not match the experimental data and is found to be incomplete. These results indicate that ion traps made out of CMOS-compatible materials can perform as well as more traditional traps fabricated from gold with respect to heating and charging as long as methods are developed for controlling oxide growth.by Nathan S. Lachenmyer.S.B

Large-Scale Atomistic Simulations of Complex and Functional Properties of Ferroic Materials

Ferroelectric (FE) nanostructures have attracted considerable attention as our abilities improve to synthesize them and to predict their properties by theoretical means. Depolarizing field effects at interfaces of FE heterostructures are particularly notable for causing topological defects such as FE vortices and negative dielectric responses in superlattices. In this thesis, I employ two large-scale atomistic techniques, the first-principles-based effective Hamiltonian (HEff) method and the linear-scaling three-dimensional fragment (LS3DF) method. I use these methods to explore optical rotation in FE vortices, electro-optic effects in FE vortices and skyrmions, and voltage amplification via negative capacitance in ferroelectric-paraelectric superlattices. We employ HEff in Monte Carlo and molecular dynamics schemes to maximize spontaneous optical rotation in a BaTiO$_3$/SrTiO$_3$ nanocomposite. For a small bias field, maximal optical rotation was realized at room temperature. The result has acquired greater relevance since Ramesh and coworkers observed ``emergent chirality in FE vortex arrays in PbTiO$_3$/SrTiO$_3$ superlattices. In a similar nanocomposite as above, we use the combined HEff and LS3DF method to study how band gap and band alignment evolves along the path from a polar-toroidal to an electrical skyrmion state. Temperature control of the vortex provides substantially larger range of control of bandgap and band alignment than field control of the skyrmion. Using temperature and electric fields to manipulate polarization and bond angle distortion in both constituent materials provides an additional handle for bandgap engineering in such nanostructures. We then use HEff to study BaTiO$_3$/SrTiO$_3$ superlattices as a platform for negative differential capacitance. We implement an atomistic framework amenable to simulation of negative capacitance in strained superlattices. In these systems, we predict misfit epitaxial strain control allows for broadly extending the operable temperature range for negative. By manipulating this strain, we observed switching of negative capacitance behavior between both constituent materials of the superlattice at low temperature