Theoretical Studies Of The Physical Properties Of Some Novel Materials

In the past decades, with the rapid progress in computational methods and theenhancements of computational ability, first principles calculations in the frameworkof density functional theory becomes a common tool in condensed matter physics,quantum chemistry and materials science. Unlike experimental studies, first principlescalculations are almost able to explore any system under any situation as long as themodel set up does not violate the physical rules. Theoretical calculations can not onlybe employed to explain the experimental results, to reveal the underlying physics, butalso to predict new materials, to design mew experimental routes and so on. This whyfirst-principles calculations play a more and more important role in the modern sciencesociety.The works in this thesis can be divided into following parts, the prediction of athree dimensional sp² boron allotrope, namely, K₄ boron; the electron-phonon cou-pling in gallium-doped germanium superconductor; the possibility of using a modelof doped FeAs single layer to study the doping effects in real iron-based supercondu-tors; surface properties of Ba(Fe_2-xCo_x)As₂ (0 0 1); a re-investigation of the crystalstructure of Ha¨ggite and the origin of the ferromagnetism in Ag_1.2V₃O₈.In the first chapter, a brief introduction of the framework of DFT, its recent progressand applications is given. Focusing on the efforts that have been made to find good ap-proximations for the exchange-correlation functionals, some modern functionals havebeen reviewed, including the local density approximation (LDA), generalized gradientapproximation (GGA), self-interaction correction (SIC), exact-exchange (EXX) andoptimized effective potential (OEP), etc. The extension of DFT to the time dependentregion and combination of DFT with the dynamic mean field theory are also intro-duced. In the end of this chapter, we present a short introduction to some popularsoftware packages.In the second chapter, based on first-principles equation-of-state and lattice dy-namics calculations, a new boron allotrope with K₄ structure is proposed, which gives the first example of a three-dimensional all-sp² network. K₄ boron has the lowest bulkmodule, cohesive energy and density amongα-B₁₂,β-B₁₀₅,γ-B₂₈ and K₄ boron, be-sides it is a semiconductor with an indirect gap about 1.5 eV. K₄ boron is not onlyconceptually attractive but also have great application potential.In the third chapter, use the state-of-art density functional perturbation theory, weinvestigate the electron-phonon coupling properties of the superconducting gallium-doped germanium. The doping of gallium introduces holes into germanium, resultedin a semicondutor-metal transition. The soften of optical phonon atΓhas been ob-served. In addition, we find the contribution of acoustic modes to the coupling con-stant is around 25%. The calculated superconducting transition temperature is 0.44 Kwhenμ?=0.1 is used, agreeing well with experimental results, indicating the origin ofsuperconductivity in this system can be well explained using a conventional phononmediated mechanism.In the fourth chapter, first-principles calculations have been performed on singlelayer FeAs. In this model where the inter layer interaction is ignored, we find thestructure of the FeAs layer in 1111 and 122 iron-supercondutors can not be reproducedaccurately in the framework of GGA and GGA+U. Besides, with both optimized andexperimental lattice parameters, the stripe AFM ground state of 1111 and 122 can notbe obtained in FeAs single layer. Since in the simple single layer model, the inter-layerinteractions between RO (R) layers and FeAs layers are excluded. Our results suggestthat this interactions may need to be considered to obtain correct geometry. This con-clusion is important for choosing a proper theoretical model in future investigation ofFe based superconductors. Surface properties of Ba(Fe_2-xCo_x)As₂ are also presentedin this chapter, combined with experimental results, we find the cleaved surface has a(?) structure where half of the Ba layer sit on the top of surface, the dI/dV curvesare also well explained.In the fifth chapter, two works on the electronic and magnetic structure of vana-dium oxides systems are presented. Firstly, we re-investigated the crystal structure ofHa¨ggite, we notice the chemical formula should be V₄O₆(OH)₄ rather than the longknowing V₄O₄(OH)₆. Based on this structure, the electronic structure shows a gap of1.4 eV, agreeing well with the one obtained from adsorption spectroscopy. Secondly, combined with EXANES results, we successfully explained the origin of the shouldpeak in the L edge of V in Ag_1.2V₃O₈, besides, we found the ferromagnetism in thissystem can be well interpreted using a standard bound magnetic polaron model.