acase@tulane.edu

Transition metal oxides have attracted widespread attention due to their broad range of fascinating exotic phenomena such as multiferroicity, superconductivity, colossal magnetoresistance and metal-to-insulator transition. Due to the interplay between spin, charge, lattice and orbital degrees of freedom of strongly correlated d electrons, these physical properties are extremely sensitive to the external perturbations such as magnetic field, charge carrier doping and pressure, which provide a unique chance in search for novel exotic quantum states. Ruthenate systems are a typical strongly correlated system, with rich ordered states and their properties are extremely sensitive to external stimuli. Recently, the experimental observation of spin-orbit coupling induced Mott insulator in Sr2IrO4 as well as the theoretical prediction of topological insulating state in other iridates, have attracted tremendous interest in the physics of strong correlation and spin-orbit coupling in 4d/5d compounds. We observe an itinerant ferromagnetic ground state of Ca2RuO4 film in stark contrast to the Mott-insulating state in bulk Ca2RuO4. We have also established the epitaxial strain effect on the transport and magnetic properties for the (Ca,Sr)2RuO4 thin films. For Sr2IrO4 thin films, we will show that the Jeff = 1/2 moment orientation can be modulated by epitaxial strain. In addition, we discovered novel Ba7Ir3O13+x thin films which exhibit colossal permittivity. / acase@tulane.edu

Weak bonds such as hydrogen bond, pi-pi stacking and van der Waals interaction are much weaker in the strength but play a more important role for the existence of various lives. For example, they are the major intermolecular interactions in the liquid and solid structure of water and determine the 3 dimensional structure of protein and DNA, which are the crucial organic molecules in lives. As a result, studying these weak bonds can lead to the better understanding of fundamental knowledge of lives. Kohn-Sham (K-S) Density Functional Theory (DFT) is an accurate and effect way to investigate the fundamental properties for many-body systems, in which, only the exchange-correlation energy as a functional of electron density need to be approximated. However, weak interaction system is still a challenge problem for KS-DFT. In this dissertation work, several standard density functionals are used to study these weak interactions in the solid state structure ice as long as nucleic bases molecules in the biologic system. It is found that the hydrogen bond can be well described by most semilocal functionals: the mismatch problem of ice Ih and AgI for GGA functional can be solved by using the higher level meta-GGA functionals and the binding length and energy between nucleic bases in DNA can be well described. However, the more accurate dispersion correction is strongly needed for van der Waals interactions and pi stacking for super-high pressure ice phases and large size biologic molecules, where van der Waals interaction takes major role. Finally, the basic structural properties of various phases of ice and DNA can be understood based on the investigation with appropriate functionals. / acase@tulane.edu

acase@tulane.edu

acase@tulane.edu

Density functional theory (DFT) is a widely used quantum mechanical method for the simulation of the electronic structure of atoms, molecules, and solids. The only part that needs to be approximated is the exchange-correlation energy as a functional of the electron density. After many-year development, there is a huge variety of exchange-correlation functionals. According to the ingredients, an exchange-correlation functional can be classified as a semi-local functional or beyond. A semi-local functional can be nonempirical or empirical and only uses locality information, such as electron density, gradient of the density, Laplacian of the density, and kinetic energy density. Unlike a non-local functional that uses non-locality information, a semi-local functional is computationally efficient and can be applied to large systems. The meta-generalized gradient approximation (meta-GGA), which is the highest-level semi-local functional, has the potential to give a good description for condensed matter physics and quantum chemistry. We built the self-consistent revised Tao-Perdew-Staroverov-Scuseria (revTPSS) meta-GGA into the band-structure program BAND to test the performances of some self-consistent semi-local functionals on lattice constant with a 58-solid test set. The self-consistent effect of revTPSS was also discussed. The vibration of a crystal has a contribution to the ground state energy of a system, which is the zero-point energy at zero temperature. It has anharmonicity at the equilibrium geometry. The standard DFT doesn’t consider the zero-point energy of a crystal. We used density functional perturbation theory (DFPT), which is a powerful and flexible theoretical technique within the density functional framework, to study the zero-point energy and make a correction to the lattice constant. The method was compared to a traditional zero-point anharmonic expansion method that is based on the Debye and Dugdale-MacDonald approximations. We also tested some new meta-GGA functionals (revTPSS, regularized revTPSS, and meta-GGA made simple) on a big molecular test set - GMTKN30 - that is composed of 30 smaller test sets and covers a large cross section of chemically relevant properties. The performances of these new meta-GGAs were compared with some other popular functionals or meta-GGAs. / acase@tulane.edu