Magnetic metal-organic / topological insulator heterostructures / Marc González Cuxart ; Directors: Prof. Aitor Mugarza, Prof. Sergio O. Valenzuela, Dr. César Moreno ; Tutor: Prof. Jordi Sort
González Cuxart, Marc
Mugarza, Aitor, dir.
Valenzuela, Sergio O., dir.
Moreno, Cesar, dir.
Sort Viñas, Jordi, dir.
Institut Català de Nanociència i Nanotecnologia

Publicació: : Universitat Autònoma de Barcelona, 2019.
Descripció: 1 recurs en línia (146 pàgines)
Resum: Topological Insulators (TIs) have become one of the wonder materials of condensed matter physics over the last decade due to their novel properties, possessing an insulating bulk in coexistence with metallic boundaries. They present an inverted band gap consequence of strong spin orbit coupling, which gives rise to the conductive boundary states with linear dispersion, characteristic of Dirac fermions, and helical spin polarization. Numerous materials have been predicted and observed to have TI signatures, holding great perspective for the realization of novel applications in spintronics, quantum computing and metrology. The experimental realization of three-dimensional TIs with the the Fermi Level located well in the bulk band gap is a challenging task because of their relatively small gap of hundreds of meV, and their high sensibility to crystal defects and impurities. These can induce electron doping that activates bulk conduction channels, thus burying the contribution of the surface states to the transport. Molecular Beam Epitaxy (MBE) has been reported to be the most suitable growth method to overcome this hindrance, due to its capability to grow single crystals with fine control over the crystal defects and impurity level. The first part of this thesis deals with the growth of high-quality TIs that maintain their pristine insulating bulk behaviour. By using MBE, we studied the impact of different substrates and growth parameters to the synthesis of Bismuth Telluride (Bi2Te3) thin films, and the growth of the ternary compound Bismuth-Antimony Telluride. We were able to grow insulating Bi2Te3 thin films with complete suppression of the \twin" domains, mirror-symmetric domains that contribute to the self-doping of the crystal. By a combination of the initial interaction with the lattice-matched Barium Fluoride substrate and the high working temperatures, the growth of Bi2Te3 single-crystalline films is achieved already from the first layer. More importantly, the films present low-doping level with the the Fermi Level kept in the bulk band gap. The correlation between the lack of \twin" domains (measured by Re ection High-Energy Electron Diffraction, X-ray Diffraction and Atomic Force Microscopy) and the low-doping level measured by Angle- Resolved Photoemission Spectroscopy (ARPES), indicates the relation between the crystal quality and the capability to preserve the bulk insulating character. This result contrasts to other TIs grown on more conventional substrates, typically presenting large lattice misfits that lead to the formation of an initial polycrystalilne or amorphous seed layer. In parallel, we explored a complemeniii tary approach to the growth of insulating Bi2Te3, based on the addition of Sb at the expense of Te atoms. A sequence of Bismuth-Antimony Telluride films with different x content were measured by X-ray Photoemission Spectroscopy (XPS) and ARPES, showing that the the Fermi Level can be gradually brought to the bulk valence band. The realization of such TIs, with a controlled level of the the Fermi Level position is of special interest for counteracting the n-doping effects typically induced by the addition of magnetic materials. The second and more extended part of this thesis is devoted to the study of interfaces formed by magnetic Metal-organic molecules deposited on the TI thin films. Interfacing TI surfaces with magnetic materials can give rise to novel magnetoelectronic phenomena, involving the manipulation of spin-torques (Inverse Edelstein Effect), or the realization of spin polarized edge states (Quantum Anomalous Hall Effect). The realization of such spin-related effects rely on the capability to control the interfacial magnetic and electronic interactions. The use of organic molecules to cage magnetic ions has been proved to be a versatile approach to engineer inter-ions and ions-surface interactions, due to the exible design that molecules offer and to their ability to form structurally perfect selfassembled structures. Moreover, they can also act as building blocks for covalent or coordination structures via on-surface reactions. As a first approach to tune the interfacial properties with Metal-organic molecules, we showed how the ligand chemistry allows a progressive control over the magnetic interactions between a hosted Co ion and a prototypical Au surface. The spin states and magnetic moments are comprehensively studied thanks to the complementary use of local spectroscopic Scanning Tunneling Spectroscopy and non-local magnetic sensitive X-ray Magnetic Circular Dichroism (XMCD) techniques, which are supported theoretically by Density Functional Theory (DFT). We were able to continuously cover the range of magnetic Co ion-substrate interactions, from a strong interacting scenario where the magnetic moment is quenched, to a gradual decrease of the interaction revealed by a lower Kondo screening of the spin. In addition, by changing the Au surface for a TI surface, the interfacial interactions reach the weakest limit in which the molecular magnetic structure is completely decoupled from the substrate electrons. Thereafter, we explored the electronic and magnetic interactions between the Topological Surface State of the Bi2Te3 thin film and Co ions caged in two different planar molecules such as Cobalt - Tetrakis (4-Promophenyl) Porphyrin (CoTBrPP) and Cobalt - Phthalocyanine (CoPc). We found a Metal-organic / TI interface with unperturbed electronic and magnetic properties. This is assessed by a coverage dependent ARPES study in which the Topological Surface State persists upon the deposition of one (CoTBrPP or CoPc) molecular layer. On the other hand, XMCD and Scanning Tunneling Spectroscopy measurements reveal the preservation of the pristine CoTBrPP magnetic moment and electronic structure respectively. Furthermore, a comprehensive Scanning Tunneling Microscopy (STM) and DFT study of the CoTBrPP adsorption geometry describes weak molecule-surface interactions, and corroborates the electronic decoupling of the Metal-organic layer from the TI surface. In an analogue study with CoPc we find slightly stronger interactions yet within the non-perturbative regime, that suggesting ligand chemistry can be used to tune magnetic interactions without affecting the overall properties of each component of the heterostructure. Subsequently, the Br-functionalized CoTBrPP on Bi2Te3 system was used to induce on-surface synthesis of Metal-organic coordination networks on TI. These more entangled structures are of great interest as a framework in which magnetic ions could arrange in ordered and mechanically stable arrays. Two different coordination phases are selectively created after CoTBrPP dehalogenation upon thermal activation. We track the chemical reaction by XPS, and investigate the morphological and electronic properties of the final products by combining Scanning Tunneling Spectroscopy (STS) and DFT calculations. We conclude that the resulting structures consists of CoTPP coordinated with Te atoms incorporated from the substrate, and thanks to the supporting DFT calculations, we are able to explain the presence of linear chains and irregular coordinated networks. In parallel, the presence of unperturbed Topological Surface State upon the formation of the Metal-organic structures is confirmed by a coverage-dependent ARPES study. Overall, the first part of the thesis constitutes an extensive study of MBE grown of Bi2Te3 thin films, in which different substrates and growth conditions are discussed. Furthermore, the results provide a route for the enhancement of the crystal quality of simple diatomic TIs, crucial for the preservation of their bulk insulating behaviour. The results presented in the second part conceive the capabilities of organic molecules to tune magnetic interactions between Co atoms and Bi2Te3 films, and pave the way for the on-TI surface synthesis of magnetic supramolecular structures.
Drets: L'accés als continguts d'aquesta tesi queda condicionat a l'acceptació de les condicions d'ús establertes per la següent llicència Creative Commons: Creative Commons
Llengua: Anglès
Document: Tesis i dissertacions electròniques ; Tesi doctoral ; Versió publicada
Matèria: Aïlladors ; Materials magnètics ; Microscòpia d'efecte túnel
ISBN: 9788449086519

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Documents de recerca > Documents dels grups de recerca de la UAB > Centres i grups de recerca (producció científica) > Ciències > Institut Català de Nanociència i Nanotecnologia (ICN2)
Documents de recerca > Tesis doctorals

 Registre creat el 2019-10-14, darrera modificació el 2021-08-07

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