Electronic Structure Of The Modulation Of The Transition Metal Compounds

Transitional Metal Elements includes ten groups of elements ranges fromâ…¢B toâ…¡B in the period table.This large group of material has unfilled d orbital and thus could form many novel compounds which has unique behavior in electronic,optic and magnetic properties.Moreover all these unique properties could be finely tuned by changing temperature,doping and pressure which in turn gives a very colorful phase diagram.For example,the coexistence of Superconductivity(SC) and Charge Ordering(CDW) in Transitional Metal Dichalcogenides(TMDs);the Metal-Insulator Transition (MIT) in Mangnite Compound;and the competition of Spin-Density-Wave (SDW) and Superconductivity(SC) in the newly discovered Iron Pnictides Superconductors.All these diagrams include rich physics information inside and could have potential in future applications and this make them hot topics in condensed matter physics.For the complexity of d-orbital,classical band structure theories are no longer valid in studying the transitional metal compound which makes experimental method the most direct and efficient way in understanding the unique properties of these novel materials.Advanced experimental techniques such as STM(Scanning Tunneling Microscopy) and ARPES (Angle Resolved Photoemission Spectroscopy) have been greatly developed during the last two decades.With modern ARPES we could not only obtain the band structure of solids but also the Fermi velocities and scattering properties et al.,which help to give a detail understanding of the orbital and band information.In the first half of this thesisâ… present the ARPES measurements on classical TMDs material 1T-Cu_xTiSe₂ and the new superconductor BaFe_2-xCo_xAs₂ with the following conclusions:1.The electronic structure of a new charge-density-wave system or superconductor,1T-CuxTiSe2,has been studied by photoemission spectroscopy.A correlated semiconductor band structure is revealed for the undoped case,which resolves a long-standing controversy in the system. With Cu doping,the charge density wave is suppressed by the raising of the chemical potential,while the superconductivity is enhanced by the enhancement of the density of states,and possibly suppressed at higher doping by the strong scattering.2.Systematic study of the electronic structure of new type electron dopped material BaFe_2-xCo_xAs₂ is finished and the evolution of the band structure with doping the observed.We note the different doping effect to the electronic structure at M and G.While being inspired by the powerful method,we feel somehow regret for the oversensitivity to surface which limits this method to only a few cleavable layered structure samples.Other interesting sample such as La1-xSrxMnO3 for the difficulty to obtaining a high quality surface is beyond our research area. To further develop the state of art photoemission technique,we make the following attempts to extend our research area:1.We make seamless combination of the ARPES and OxMBE(Oxide Molecular Beam Epitaxy) which could provide high quality single crystal for insitu measurements.I designed and built the MBE system and managed to construct our own evaporation source;I designed and finished the ozone distilling system.Now the Oxide MBE system in Fudan University is being debugged and would be ready to use in a few weeks.2.With the finical support,I participated in the epitaxial growth of La1-xSrxMnO3 films which has unique magnetic and electronic properties and is the key candidate for the next generation information storage material.With the ozone assistant MBE technique,we could grow better quality films than PLD ever did before and through the artificial engineering of the top-most layer we could increase the Curie temperature much higher than we have expected.