First-principles Studies On Electronic Structures Of Compounds Containing Local D/f Electrons

The first principles method with the local (spin) density approximation isproved to be successful in describing some simple metals and semiconductors.But it fails to describe some systems containing partly filled d/f orbitals. We useBeyond L(S)DA (i. e., L(S)DA+U) method to study the electronic properties of3d transition metal oxides and 4f rare earth compounds. The theoretical resultsare compared with the experimental ones and some theoretical predictions aregiven. There have been a lot of experimental studies and first principles studiesfor the 3d transition metal oxides. Nevertheless, there is little theoretical studieson the effect of strain and defect induced from the experimental circumstances.For LSMO strained on the most commonly used substrates, we study the straineffect and effect of oxygen vacancy on the electronic structure of the material.L(S)DA method also fails to describe the rare earth compounds containing 4felectrons. These electrons are conventionally considered as being in the innershells and the interactions between them and the other electrons can be omitted.And therefore, they are often been treated as core states. We found that insome compounds, the interactions between the partly filled 4f electrons and theother electrons are strong, and they should not be considered as core states. Wechoose a rare earth compound BaTbO₃ and a silicide to study the interactionsbetween the 4f electrons and the other electrons, and the electronic structurecharacteristics of the compounds. It is found that the 4f electrons can be welltreated by means of L(S)DA+U method.In the first chapter, we introduce briefly the difficulties of L(S)DA method incalculating some systems containing partly filled d/f electrons and the researchbackground of 3d, 4d and 4f systems.In the second chapter, the density functional theory and L(S)DA+U methodare introduced firstly. Two first principles methods (the linear muffin-tin orbitalsmethod (LMTO)) and full potential linear augmented plane wave (FLAPW)method are followed. In the third chapter, we study the effects induced by strain and oxygenvacancies for La_2/3Sr_1/3MnO₃ (LSMO). In the first part, the effects of tetrago-nal strain on electronic and magnetic properties of LSMO are investigated withdensity-functional theory. As far as the structural properties are concerned, thecomparison between theory and experiments for LSMO strained on the most com-monly used substrates, shows an overall good agreement: the error is less than1.0～1.5%. The half-metallic behavior, particularly relevant for spin-injectionpurposes, is independent on the chosen substrate and is achieved for all the con-sidered in-plane lattice constants. The inclusion of a Hubbard-like contributionon the Mn d states, according to the so-called "LSDA+U" approach, is ratherineffective from the structural point of view, but much more important from theelectronic and magnetic point of view. As to strain effects, a smaller in-planelattice constant can stabilize the out-of-plane vs in-plane e_g orbital and signi-ficatively change their relative occupancy. Since e_g orbitals are key quantitiesfor the double-exchange mechanism, strain effects are confirmed to be crucial forthe resulting magnetic coupling. In the second part of the third chapter, theeffect of oxygen vacancies on the electronic and magnetic properties of LSMOhas been investigated by means of ab-initio calculations within the density func-tional formalism combined with photoemission. The simulations show that theintroduction of oxygen vacancies causes a shift, of the valence band features to-wards higher binding energies, and an increase of the degree of covalency of Mnbondings. There is an important vacancy-induced drawback: half-metallicity,typical of the perfectly stoichiometric system, is generally lost, due to defectivebands that cross the Fermi level. Photoemission experiments performed on epi-taxial thin films of LSMO with different content of oxygen vacancies grown byPulsed Laser Deposition, essentially confirm theoretical predictions. Our findingsclearly indicate that the control over oxygen deficiency should therefore be ex-perimentally achieved, to avoid unwanted consequences in terms of spin-injectionefficiency. When the LSDA method is used, we cannot obtain the results in agree-ment with the experimental ones.The fourth chapter studies the electronic structures of rare earth oxide andsilicide. The first principles calculations for rare earth compounds are very deli- cate. Contrary to previous theoretical studies on these compounds, 4f electronsare treated as valence state electrons, explicitly taking into account the on-siteCoulomb interactions. In the first part of the fourth chapter, the electronicstructures of BaTbO₃ have been studied. The interactions between Tb 4f and O2p are analyzed. Due to the good compatibility between YBa₂Cu₃O_7-δ(YBCO)and BaTbO₃, and even a large doping of YBCO with Ba or Tb does not cause anoticeable reduction of T_c, BaTbO₃ become the new candidate substrate for high-T_c superconductor devices. The ground magnetic phase is found to be G-typeantiferromagnetic structure (G-AFM) with lattice constant of 4.278 A and thelocal magnetic moment is found to be 5.83μB, consistent with the experimentalresults, showing the reliability of our method. The electronic structure analysisindicates the ionic character of Ba and strong hybridization between O 2p and Tb4f. This theoretical study is meaningful for one to explore new substrate mate-rial for high-T_c superconductor devices. In the second part of the fourth chapter,we investigated the electronic structures (especially 4f states) of hexagonal andtetragonal erbium silicides with density functional theory. The interactions be-tween Er 4f, 5d and Si electrons are analyzed. Total energies calculations fromFPLMTO show that the relaxed hexagonal ErSi_1.7 is more stable than the tetrag-onal structure, which can help to explain why the hexagonal structure appearsin Si (100) although tetragonal phase should form on Si (100) from the mismatchpoint of view. The calculated total density of states of the hexagonal ErSi_1.7agrees well with the experimental valence-band spectrum in a wide energy rangefrom 0 to 12 eV below the Fermi level, which show LSDA+U method can welldescribe the 4f electrons in rare earth compounds. By comparing the DOSs oferbium silicides in different vacancy configurations with the experimental photoe-mission spectrum, we found the period of vacancy layers along c_hex axis in ErSi_1.7is 1c rather than 2c, thus solving the long-term disputed problem. In addition,the calculated TDOS of tetragonal ErSi₂ could provide a theoretical prediction ofthe valence band spectrum for the future photoemission spectrum experimentalstudy on this rarely investigated structure. Our study also indicates that theoccupied 4f states in erbium silicides can locate in the energy range of 0～4.0eV below the Fermi energy, much different from the prediction of the previously adopted Er ion model. Studies on these two rare earth compounds show thatthere are strong interactions between the partly occupied 4f electrons and theother electrons, and therefore, the traditional 'ion model' can not describe thesesystems very well.The fifth chapter presents a summary of our studies.