Electronic Structure and Photoemission Studies of Late Transition-metal Oxides PDF Download
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Author: Z.-X. Shen
Publisher:
ISBN:
Category : Electric conductivity
Languages : en
Pages : 162
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Author: Z.-X. Shen
Publisher:
ISBN:
Category : Electric conductivity
Languages : en
Pages : 162
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Author: Tobias Rödel
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Many transition metal oxides (TMOs) show complex physics, ranging from ferroelectricity to magnetism, high-Tc superconductivity and colossal magnetoresistance. The existence of a variety of ground states often occurs as different degrees of freedom (e.g. lattice, charge, spin, orbital) interact to form different competing phases which are quite similar in energy. The capability to epitaxially grow heterostructures of TMOs increased the complexity even more as new phenomena can emerge at the interface. One typical example is the two-dimensional electron system (2DES) at the interface of two insulating oxides, namely LaAlO3/SrTiO3, which shows metal-to-insulator transitions, magnetism or gate-tunable superconductivity. The origin of this thesis was the discovery of a similar 2DES at the bare surface of SrTiO3 fractured in vacuum, making it possible to study its electronic structure by angle-resolved photoemission spectroscopy (ARPES).In this thesis, the study of well-prepared surfaces, instead of small fractured facets, results in spectroscopic data showing line widths approaching the intrinsic value. This approach allows a detailed analysis of many-body phenomena like the renormalization of the self-energy due to electron-phonon interaction.Additionally, the understanding of the electronic structure of the 2DES at the surface of SrTiO3(001) was given an additional turn by the surprising discovery of a complex spin texture measured by spin-ARPES. In this thesis data is presented which contradicts these conclusions and discusses possible reasons for the discrepancy.One major motivation of this thesis was the question if and how the electronic structure and the properties of the 2DES can be changed or controlled. In this context, the study of 2DESs at (110) and (111) surface revealed that the electronic band structure of the 2DES (orbital ordering, symmetry of the Fermi surface, effective masses) can be tuned by confining the electrons at different surface orientations of the same material, namely SrTiO3.A major achievement of this thesis is the generalization of the existence of a 2DES in SrTiO3 to many other surfaces and interfaces of TMOs (TiO2 anatase, CaTiO3, BaTiO3) and even simpler oxides already used in modern applications (ZnO). In all these oxides, we identify oxygen vacancies as the origin for the creation of the 2DESs.In anatase and other doped d0 TMOs, both localized and itinerant electrons (2DES) can exist due to oxygen vacancies. Which of the two cases is energetically favorable depends on subtle differences as demonstrated by studying two polymorphs of the same material (anatase and rutile).In CaTiO3, the oxygen octahedron around the Ti ion is slightly tilted. This symmetry breaking results in the mixing of different d-orbitals demonstrating again why and how the electronic structure of the 2DES can be altered.In BaTiO3, the creation of a 2DES results in the coexistence of the two, usually mutual exclusive, phenomena of ferroelectricity and metallicity in the same material by spatially separating the two.Moreover, this work demonstrates that the 2DES also exists in ZnO which is - compared to the Ti-based oxides - rather a conventional semiconductor as the orbital character of the itinerant electrons is of s and not d-type.The main result of this thesis is the demonstration of a simple and versatile technique for the creation of 2DESs by evaporating Al on oxide surfaces. A redox reaction between metal and oxide results in a 2DES at the interface of the oxidized metal and the reduced oxide. In this thesis the study of such interfacial 2DESs was limited to photoemission studies in ultra high vacuum. However, this technique opens up the possibility to study 2DESs in functional oxides in ambient conditions by e.g. transport techniques, and might be an important step towards cost-efficient mass production of 2DESs in oxides for future applications.
Author: Dean E. Eastman
Publisher:
ISBN:
Category :
Languages : en
Pages : 21
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The author has developed new photoemission techniques and performed studies of several noble and transition metals (Cu, Ag, Au, Ni, Pd, Cr, etc.), semiconductors (Si, Ge, GaAs, GaP, InSb, ZnSe, CdTe, etc.), the Pd-H metal-gas system, magnetic rare earth, uranium, and transition metal compounds (EuO, EuS, GdS, US, SmS, TmTe, NiO, CoO, etc.), chemisorbed molecules and atoms on surfaces (CO on Ni) and the semiconductor-metal and semiconductor-vacuum interface (Ge, GaAs, GaSb, InSb, Ge with Sb overlayer, GaAs with Pd overlayer, etc.).
Author: R. J. Powell
Publisher:
ISBN:
Category :
Languages : en
Pages : 187
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The nature of the electronic states in the transition metal oxides has been the subject of controversy for some time. The difficulty arises because some of these oxides are insulators, and conventional band theory predicts that they should be metallic conductors because of their partially filled states. Models based on band theory, which have been proposed to explain this behavior, depend upon magnetic ordering. Localized models, on the other hand, explain the insulating nature as the result of weak interaction (small orbital overlap) of the metal ion d-electrons. Photoelectric emission and optical reflectance measurements have been performed on NiO, CoO, and V2O4 in order to determine details of their electronic structures and to examine the predictions of the theoretical models. Photoemission from these materials has been measured both above and below their magnetic transition temperatures. Details of experimental techniques are presented, including a new high-vacuum cleaving apparatus used for cleaving the NiO and CoO single crystals, and a method of preparing V2O4. From the results of these studies it is concluded that the highest lying filled states in the electronic structure of NiO and CoO are best represented by localized wave functions and that the various band models with magnetic ordering are not applicable to NiO or CoO. (Author).
Author: David James Payne
Publisher:
ISBN:
Category : Metallic oxides
Languages : en
Pages : 362
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Author: Martin Ellguth
Publisher: Logos Verlag Berlin GmbH
ISBN: 3832540024
Category : Science
Languages : en
Pages : 134
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Electron correlation is an important phenomenon of solid-state physics, which is actively studied both by experimentalists for the rich material properties which result from it and by theoreticians which face a lot of open questions on the way to a succesful many-body description of electron systems where the Coulomb interaction plays an important role. Ferromagnetic cobalt is an interesting candidate for the study of electron correlation, since the exchange interaction splits the band structure into majority-spin and minority-spin bands, which differ considerably in the strength of the electron-electron interaction.Using a revolutionary, parallelized approach to spin-resolved photoemission with an efficiency 3 to 4 orders of magnitude higher than previously possible, the spin-dependent manifestations of the electron correlation are revealed in unprecedented detail, allowing for a characterization of the self energy. As an additional phenomenon of the electron correlation, unusual waterfall features, previously only observed in superconductors, occur in the photoemission spectra of cobalt. Further subjects include a comprehensive mapping of the fcc cobalt Fermi surface and an investigation of unoccupied quantum well states in ultrathin cobalt films on copper accessed by spin-resolved, non-linear photoemission. The principle of the imaging spin filter and the data analysis routine are discussed in-depth in a dedicated chapter.
Author: Peter David Alexander Mann
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Electronic structure of transition metal oxides has been a subject of intense research since decades due to the wide spectrum of properties that they exhibit, like high temperature superconductivity, metal-insulator transitions (MIT), phase separation etc. Among these, colossal magnetoresistance (CMR), i.e. a sharp drop in the electrical resistance by the application of an external magnetic field, is a property of fundamental and technological importance. In the present study we investigate several of these interesting properties ranging from colossal magnetoresistance, metal-insulator transitions and phase separation phenomena on a wide range of magnetoresistive systems. All these properties originate in transition metal oxides due to a competition between the strong inter-atomic Coulomb interaction strength within the transition metal d electrons and a large hopping interaction strength between the metal d and oxygen 2p states. In this thesis we report the investigation of the electronic and magnetic structures of some magnetoresistive oxides, including various double perovskites and manganites, using various high energy spectroscopies in conjunction with various theoretical approaches. The samples for the present experimental investigation were prepared by different synthetic routes, such as solid state reaction, nitrate method, d.c arc melting and float zone method, and were characterized by x-ray diffraction, four probe resistivity, magnetic susceptibility, optical absorption and energy dispersive analysis of x-rays while some of the samples were supplied by our collaborators. Various spectroscopic techniques like x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), bremsstrahlung isochromat spectroscopy (BIS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism spectroscopy (XMCD), electron energy loss spectroscopy (EELS), spatially resolved photoelectron spectroscopy and M· ossbauer spectroscopy were used.
Author: Stanford University. Stanford Electronics Laboratories
Publisher:
ISBN:
Category :
Languages : en
Pages : 224
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Author: Zhihai Zhu
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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