Electronic and spin transport in Bismuthene with magnetic impurities

Topological insulators have remained as candidates for future electronic devices since their first experimental realization in the past decade. The existence of topologically protected edge states could be exploited to generate a robust platform and develop quantum computers. In this work we explore the role of magnetic impurities in the transport properties of topological insulators, in particular, we study the effect on the edge states conductivity. By means of realistic $\it{ab}$ $\it{initio}$ calculations we simulate the interaction between magnetic adatoms and topological insulators, furthermore, our main goal is to obtain the transport properties for large samples as it would be possible to localize edge states at large scales

Simulação computacional de materiais bidimensionais: funcionalização de defeitos extensos no grafeno e retenção de contaminantes por argilominerais

In this work are considered two bidimensional systems, with distints chacacteristcs and applicabilitys. Is studied the adsorption of transition metals (MT) Fe, Co, Mn and Ru in extended defects, formed by graphene grain boundaries. First in pristine graphene The hollow site of carbon hexagon, in pristine graphene, are the most stable for MT adsorption. The Dirac cone in eletronic structure of graphene was manteined with the presence of MT. For the considered grain boundaries the MT has a greater stability for absorption in the grain boundaries sites in comparison with pristine graphene. Through the energy barrier values, are observed diffusion chanels for MT localized on the grain boundaries. This diffusion chanels indicate a possible formation of nanolines of MT in graphene. For the first stage of the nanolines, ate observed a better stability for the system with greater MT concentration, due to MT-MT interactions. Also, due to the magnetic moment of the MT, the nanolines show a magnetization. For the most stable configurations the system are metallics, particularly for Fe the band structure indicates an anisotropic spin current. In a second study, are considereted the retention capacity for metallic contaminants Cd and Hg in clayminerals, kaolinite (KAO) and montmorillonite (MMT). Through the adsorption energies of contaminantes in the clayminerals, was observed a increase in stability with the increase of contaminants concentration, due to the interaction Cd-Cd and Hg-Hg. Also, was observed that KAO has a strong interaction beteween monolayers than MMT. In this sence, for the adsoption process of contaminantes in the natural form of KAO and MMT, the latter has a better retention capacity, due to the small net work for contaminant intercalation. However, when the modification of clayminerals, with molecules that increase the spacing between monolayers, exist a optimal condition, which the contaminant absorption are more stable in KAO system than in MMT. In the Langmuir adsorption model for the clayminerals in the optimal monolayer spacing, the retention capacity for Cd and Hg in KAO system are 21% greater than in MMT system. Also, for the X-ray Absorption Near Edge Spectroscopy (XANES) for the K edge of Cd and Hg, are found a positive shift of absorption edge with the decreasing of monolayer spacing. This result indicates a possible way to determine the concentration of adsorbed contaminats in relation to unabsorbed ones, from the decomposition of experimental XANES in the obteined spectras.Dissertação (Mestrado)Neste trabalho foram estudados dois sistemas bidimensionais, com diferentes características e aplicabilidade. Estudou-se a adsorção¸ dos metais de transição (MT) Fe, Co, Mn, e Ru sobre defeitos extensos, formados por fronteiras de grãos no grafeno. Primeiramente para o grafeno pristino, a adsorção¸ dos MT é mais estável no sítio central ao hexágono formado pelos carbonos. Na estrutura eletrônica o cone de Dirac do grafeno é preservado na presença dos MT. Para as fronteiras de grãos consideradas os MT possuem uma maior estabilidade, perante a adsorção¸, sobre os sítios da fronteira em relação ao do grafeno pristino. Através das barreiras de energia, observou-se canais de difusão para os MT sobre o defeito extenso. Estes canais de difusão indicam uma possível formação de nanolinhas de MT sobre o grafeno. Estudando um primeiro estágio das nanolinhas, observa-se a maior estabilidade do sistema com uma maior concentração dos MT sobre o defeito extenso, devido à interação MT-MT. Devido a magnetização dos MT de transição considerados, as nanolinhas possuem uma magnetização o não nula. Para as configuração mais estáveis os sistemas são metálicos, em particular para o Fe as estrutura de bandas indicam a existência de uma corrente anisotrópica por polarização de spin. No segundo estudo, foi considerada a capacidade de retenção de contaminantes metálicos Cd e Hg, nos argilominerais kaolinita (KAO) e montmorilonita (MMT). Através das energias de adsorção¸ dos contaminantes nos argilominerais, foi observada o aumento da estabilidade do sistema com a concentração, devido `a maior interaçãoo Cd-Cd e Hg-Hg. Foi observado ainda, que a KAO possui ligação mais forte entre monocamadas comparado com a MMT. Desta forma no processo de adsorção¸ ˜ao dos contaminantes nas formas naturais da KAO e MMT, esta ´ultima possui uma melhor capacidade de retenção de contaminantes, uma vez que o gasto energético necessário na intercalação dos contaminantes ´e menor. No entanto, quando se considera a modificação dos argilominerais, com moléculas que aumentam o espaçamento de suas monocamadas, existe uma condição ótima em que a KAO se torna mais favorável no processo de intercalação dos contaminantes. Através do modelo de adsorção de Langmuir para os argilominerais no espaçamento ótimo, obteve-se uma capacidade de retenção para a KAO 21% maior que a capacidade da MMT. A determinação do Espectro de Absorção de raios-X (XANES) próximos `a borda K para o Cd e Hg, permitiu a verificação de um deslocamento positivo da borda de adsorção¸ com a diminuição do espaçamento entre monocamadas. Este resultado indica a possibilidade da determinação da porcentagem de metais adsorvidos entre as monocamadas em relação aos não adsorvidos, a partir da decomposição do XANES experimental nos espectros encontrados

Computational simulations applied to materials physics: topological phases in 2D system

Throughout this work we explore new two-dimensional lattices constructed from the Archimedean tiling of the plane, characterizing its band structure and topological quantum spin Hall phases. As a proof of principle, within a combination of density functional theory and effective Hamiltonians, we have explored the material realization of Archimedean lattices. Fist, focusing in the kagome lattice, we predict within magnetic metal-organic frameworks the arising of quantum anomalous Hall effects. Additionally, from the time reversal symmetric counterpart, i.e. quantum spin Hall effect, we include a new multilayer degree of freedom into the control of metal-organic frameworks electronic properties. Within this multilayer system a layer localization effect of the topological states can be controlled through a external electric field. Focusing in a different Archimedean lattice, we shown its material realization in a two-dimensional boron allotrope. This material presents linear dispersive Dirac fermions in a px/py orbital pseudospin subspace. Here we shown a helical behavior of these orbital pseudospinors, i.e., its direction locked with the momentum direction, in a analogous way as the three dimensional topological insulator surface states. Furthermore, by adding a layer degree of freedom in addition to the orbital one, i.e., stacking of borophene bilayers, allows a control of the orbital texture by engineering the stacking order and external electric field. Lastly, we explored the construction of high-degeneracy points, which are not predicted by space-group symmetries. Here we show that pair permutation symmetries stabilizes such points, which are presents in a two Archimedean lattices, but also in other 3D lattices. We have found the recipe for constructing such lattices, where its experimental realization was proposed in metal lattices designed over silicon carbide surface oxidized by a ordered silica phase. In such systems, the post-transition metals s orbitals give rise to surface states localized neatly within the substrate’s semiconducting energy gap.CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorCENAPAD - Centro Nacional de Processamento de Alto Desempenho em São PauloFAPEMIG - Fundação de Amparo a Pesquisa do Estado de Minas GeraisTese (Doutorado)Ao longo deste trabalho serão exploradas novas redes bidimensionais construídas a partir da ladrilhagem Arquimediana do plano, sendo caracterizadas suas estruturas de bandas e fases Hall quânticas de spin. Utilizando uma combinação de teoria do funcional da densidade e Hamiltonianos efetivos, explorou-se realizações materiais das redes de Arquimedes. Primeiramente, focando na rede de kagome, foi predito a existência de fase Hall quântica anômala em materiais organometálicos magnéticos. Ainda mais, tomando o seu análogo com simetria de reversão temporal, i.e., efeito Hall quântico de spin, foi incluído um grau de liberdade adicional para o controle das propriedades eletrônicas em redes organometálicas, a saber, o seu empilhamento em multicamadas. Neste sistema empilhado a localização dos estados topológicos de borda é controlado através de um campo elétrico externo. Ainda dentro do escopo das redes de Arquimedes, mostramos sua realização material em um alótropo de boro. Este material apresenta estados de Dirac linearmente dispersivos em um subespaço formado por um pseudospin dos orbitais px/py . Neste caso, mostramos o comportamento hélico deste pseudospinor orbital, i.e., sua direção vinculada à direção do momento; um quadro análogo aos estados de superfı́cie de isolantes topológicos tridimensionais. Na sequência, considerando-se sistemas de bicamadas, estudamos as formas das texturas orbitais em função da quiralidades das camadas empilhadas e o efeito de um elétrico externo. Por fim explorou-se a construção de redes com pontos de alta-degenerescência, as quais não são previstas pelos grupos de simetria espacial. Aqui mostrou-se que simetrias de permutação de pares estabilizam tais pontos, sendo os mesmo observados em algumas redes de Arquimedes e outras redes 3D. Encontramos os ingredientes mı́nimos para a construção de tais redes, onde a sua realização experimental foi explorada no design de redes metálicas sobre a superfı́cie do carbeto de silício oxidado, onde consideramos uma fase ordenada de sı́lica que é observada experimentalmente. Neste sistema os orbitais s dos metais de pós-transição dão origem a estados de superfı́cie localizados exatamente no gap semicondutor do substrato

Weak antilocalization in quasi-two-dimensional electronic states of epitaxial LuSb thin films

Observation of large non-saturating magnetoresistance in rare-earth monopnictides has raised enormous interest in understanding the role of its electronic structure. Here, by a combination of molecular-beam epitaxy, low-temperature transport, angle-resolved photoemssion spectroscopy, and hybrid density functional theory we have unveiled the bandstructure of LuSb, where electron-hole compensation is identified as a mechanism responsible for large magnetoresistance in this topologically trivial compound. In contrast to bulk single crystal analogues, quasi-two-dimensional behavior is observed in our thin films for both electron and holelike carriers, indicative of dimensional confinement of the electronic states. Introduction of defects through growth parameter tuning results in the appearance of quantum interference effects at low temperatures, which has allowed us to identify the dominant inelastic scattering processes and elucidate the role of spin-orbit coupling. Our findings open up new possibilities of band structure engineering and control of transport properties in rare-earth monopnictides via epitaxial synthesis.Comment: 20 pages, 12 figures; includes supplementary informatio

Revealing quantum Hall states in epitaxial topological half-Heusler semimetal

Prediction of topological surface states (TSS) in half-Heusler compounds raises exciting possibilities to realize exotic electronic states and novel devices by exploiting their multifunctional nature. However, an important prerequisite is identification of macroscopic physical observables of the TSS, which has been difficult in these semi-metallic systems due to prohibitively large number of bulk carriers. Here, we introduce compensation alloying in epitaxial thin films as an effective route to tune the chemical potential and simultaneously reduce the bulk carrier concentration by more than two orders of magnitude compared to the parent compound. Linear magnetoresistance is shown to appear as a precursor phase that transmutes into a TSS induced quantum Hall phase on further reduction of the coupling between the surface states and the bulk carriers. Our approach paves the way to reveal and manipulate exotic properties of topological phases in Heusler compounds.Comment: 8 pages, 4 figures. Supplementary Infromation contains 7 sections and 17 figure

Vacancy localization effects on MX2 transition metal dichalcogenides: a systematic ab-initio study

Two-dimensional transition metal dichalcogenides (MX$_2$) vacancy formation energetics is extensively investigated. Within an ab-initio approach we study the MX$_2$ systems, with M=Mo, W, Ni, Pd and Pt, and X=S, Se, and Te. Here we classify that chalcogen vacancies are always energetic favorable over the transition metal ones. However, for late transition metals Pd $4d$, and Pt $5d$ the metal vacancy are experimentally achievable, bringing up localized magnetic moments within the semiconducting matrix. By quantifying the localization of the chalcogen vacancy states we evidentiate that it rules the intra- and inter-vacancy interactions that establish both the number of vacancy states neatly lying within the semiconducting gap, as well as its electronic dispersion and SOC splitting. Combining different vacancies and phase variability 1T and 1H of the explored systems allow us to construct a guiding picture for the vacancy states localization

The role of functional thiolated molecules on the enhanced electronic transport of interconnected MoS2_2 nanostructures

Molecular linkers have emerged as an effective strategy to improve electronic transport properties on solution-processed layered materials via defect functionalization. However, a detailed discussion on the microscopic mechanisms behind the beneficial effects of functionalization is still missing. Here, by first-principles calculations based on density functional theory, we investigate the effects on the electronic properties of interconnected MoS$_2$ model flakes systems upon functionalization with different thiol molecule linkers, namely thiophenol, 1,4-benzenedithiol, 1,2-ethanedithiol, and 1,3-propanedithiol. The bonding of benzene- and ethanedithiol bridging adjacent armchair MoS$_2$ nanoflakes leads to electronic states just above or at the Fermi level, thus forming a molecular channel for electronic transport between flakes. In addition, the molecular linker reduces the potential barrier for thermally activated hopping between neighboring flakes, improving the conductivity as verified in experiments. The comprehension of such mechanisms helps in future developments of solution-processed layered materials for use on 2D electronic devices