Orientador: Antônio Carlos Ferreira Seridonio / Resumo: Investigamos teoricamente um sistema composto por duas impurezas afastadas e enterradas em um semimetal de Weyl. Analisamos a densidade de estados local para duas situações: com simetrias de reversão temporal e inversão preservadas e; simetria de reversão temporal quebrada e inversão preservada. Na situação em que as duas simetrias são preservadas, o Hamiltoniano descreve um semimetal de Dirac. Sendo assim, verificamos a densidade de estados local em dois pontos diferentes do semimetal de Dirac e os orbitais moleculares formado pelas impurezas. É possível observar que em alguns pontos, a densidade de estados total, que pode ser obtido experimentalmente via espectroscopia de varredura por tunelamento, há a presença de apenas dois picos, enquanto que em outro ponto há a presença de quatro picos. Sendo assim, a presença de dois picos nos leva a crer que não há interação entre as impurezas, entretanto, em outro ponto que contém quatro picos em sua densidade de estados, mostra que há interação entre as impurezas. Analisamos os orbitais moleculares realizando uma topografa espacial da densidade de estados e é possível observar estados ligante e antiligante entre as impurezas com orbitais s. A segunda situação, com a quebra de simetria de reversão temporal e simetria de inversão preservada, temos um semimetal de Weyl com as bandas de energias separadas no espaço dos momentos e com energias degeneradas. Verificamos na densidade de estados local, uma magnetização das impurezas devido ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: We investigate theoretically the setup composed by two distant impurities and burried in Weyl semimetal. We analyze a local density of states for two situations: with time reversal symmetry and inversion symmetry preserved and; time reversal symmetry broken and inversion symmetry preserved. In the situation that the two symmetries are preserved, the Hamiltonian describes a Dirac semimetal. Therefore, we verified the local density of states in two different points of Dirac semimetal and the molecular orbital formed by the impurities. It is possible to observe that in some points, the total density of states, which can be obtained experimentally via scanning tunneling microscope, there is the presence of just two peaks, while that in another point there is the presence of four peaks. Therefore, the presence of two peaks leads us to belive that there is no interaction between impurities, however, in the another point that contain four peaks in the density of states, show that there is interaction between the impurities. We analyze the molecular orbital realizing a spacial topography of density of states and it is possible to observe bonding and antibonding states between impurities with s orbital. The second situation, with the time reversal symmetry broken and inversion symmetry preserved, we have a Weyl semimetal with separated energy bands in momentum space and degenerate energy. We verified in the local density of states, a magnetization of the impurities due to the time rev... (Complete abstract click electronic access below) / Mestre

Orientador: Antônio Carlos Ferreira Seridônio / Resumo: Por meio da equação de Weyl que descreve o bulk de um semimetal de Weyl, inserimos duas impurezas no interior deste semimetal, com o intuito de medir a condutância desse sistema por meio de uma ponta de um microscópio de corrente de tunelamento (STM: scanning tunneling microscope), assim como exploramos teoricamente a influência que o semimetal de Dirac-Weyl causa na estrutura dos orbitais das impurezas. Verificamos que nessas condições apresentadas: (i) é possível resgatar e obter os mesmos resultados publicados por Phys. Rev. B 96, 041112(R) (2017) para um semimetal de Dirac em três dimensões; (ii) ao quebrarmos a simetria de inversão do sistema, ocorre um alargamento dos picos de ressonância, até o ponto que uma impureza deixa de sentir a presença da outra, o que caracteriza um sistema que segue o modelo de Anderson de uma impureza (SIAM: single impurity Anderson model), ao passo que, caso continuemos aumentando o valor deste parâmetro, a estrutura de bandas do material torna-se um metal; (iii) ao quebrarmos a simetria de reversão temporal nas direções dos eixos x e z, ocorre formação de um par de orbitais de ligações π para os estados ligantes e antiligantes, o que significa que os elétrons são excitados de ondas s para ondas p. / Abstract: Through the Weyl equation that describes the bulk of a Weyl semimetal, we inserted two impurities inside this semimetal, in order to measure the conductance of this system by means of a scanning tunneling microscope (STM), as well as we theoretically explore the influence that Dirac-Weyl semimetal causes on the structure of the impurities orbitals. We verified that under these conditions presented: (i) it is possible to retrieve and obtain the same results published by Phys. Rev. B 96, 041112 (R) (2017) for a three-dimensional Dirac Semimetal; (ii) when we break the inversion symmetry of the system, there is a widening of the resonance peaks, to the point that one impurity cannot feel the presence of the other one, which characterizes a single impurity Anderson model (SIAM) system, whereas, if we continue increasing the value of this parameter, the bands structure of the material becomes a metal; (iii) when we break the time reversal symmetry in the x and z-axis directions, a pair of π-bond orbitals forms for the bonding and antibonding states, which means that electrons are excited from s-waves to p-waves. / Mestre

Orientador: Antonio Carlos Ferreira Seridonio / Resumo: Embora o estado fundamental de moléculas covalentes diatômicas na natureza seja inevitavelmente ligante com primeiro estado excitado antiligante, foi demonstrado teoricamente que um par de impurezas, colocadas dentro de um semimetal de Dirac tridimensional, pode exibir um estado fundamental antiligante. Esse contraste com a natureza de moléculas isoladas surge devido a emergência de uma inesperada interação de longo alcance mediada pelos elétrons de condução com comportamento relativístico inerente ao semimetal de Dirac. Os perfis dos orbitais moleculares ligante e antiligante desse estado molecular são obtidos por meio da determinação teórica da densidade local de estados na superfície, cuja medida experimental pode ser realizada com auxílio da microscopia de corrente de tunelamento. Para o semimetal de Weyl, foi evidenciado que a quebra de simetria de reversão temporal é responsável por uma transição energética de s- para p-wave nos orbitais individuais das impurezas. Como consequência dessa transição e da característica direcional dos orbitais p-wave, a interferência entre as impurezas produz orbitais do tipo sigma quando frontais e do tipo \pi quando paralelas. Além disso, foi verificado que o surgimento do efeito magneto quiral, devido a separação dos nós de Weyl com quiralidades opostas, produz polarização nos orbitais moleculares via oscilações de Friedel. Por fim, foi analisado o efeito dos graus de liberdade de vibração da rede, presentes em qualquer sistema realísti... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Although the ground state of the diatomic molecules in nature is inevitably bonding with its first excited state is antibonding, it was demonstrate theoretically that a pair of impurities, placed buried in three-dimensional Dirac semimetals, may exhibit an antibonding ground state. This contrast with the nature of isolated molecules emerges due to an unexpected long-range interaction mediated by the conduction electrons with relativist behavior inherent to Dirac semimetal. The bonding and antibonding molecular profiles were obtained by theoretical determination of the local density states on the system surface, whose experimental measurement can be performed with the help of tunneling current microscopy. For theWeyl semimetal, it was evidenced that the time reversal symmetry break is responsible for an energy transition from s- to p-wave in the individual orbitals of the impurities. As a consequence of this transition and the directional characteristic of the p-wave orbitals, the interference between the impurities produces p-type orbitals when frontal and -type orbitals when parallel. In addition, it was found that the appearance of the chiral magneto effect, due to the separation of the Weyl nodes with opposite chiralities, produces polarization in the molecular orbitals via Friedel oscillations. Lastly, it was addressed the effect of vibrational degrees of freedom, which are present in any realistic system, in the formation and features of (anti)bonding molecular state, ... (Complete abstract click electronic access below) / Doutor

acase@tulane.edu / 1 / Jinyu Liu

Nodal semimetals host either degenerate points (Dirac/Weyl points) or lines whose band topology in Brillouin zone can be classified either as trivial (normal nodal semimetals) or non trivial (topological nodal semimetals). This thesis investigates the electronic structure of two different categories of topological nodal semimetals probed by angleresolved photoemission spectroscopy (ARPES): The first material is Indium Bismuth (InBi). InBi is a semimetal with simple tetragonal structure with P4/nmm space group. This space group is predicted to host protected nodal lines along the perpendicular momentum direction at the high symmetry lines of the Brillouin zone boundary even under strong spin-orbit coupling (SOC) situation. As a semimetal with two heavy elements, InBi is a suitable candidate to test the prediction. The investigation by ARPES demonstrates not only that InBi hosts the nodal line in the presence of strong SOC, it also shows the signature of type-II Dirac crossing along the perpendicular momentum direction from the center of Brillouin zone. However, as the nodal line observed is trivial in nature, there is no exotic drumhead surface states observed in this material. This finding demonstrates that Dirac crossings can be protected in a non-symmorphic space group. The second material is NbIrTe₄ which is a semimetal that breaks inversion symmetry predicted to host only four Weyl points. This simplest configuration is confirmed by the measurement from the top and bottom surface of NbIrTe₄ showing only a pair of Fermi arcs each. Furthermore, it is found that the Fermi arc connectivity on the bottom surface experiences re-wiring as it evolves from Weyl points energy to the ARPES Fermi energy level. This change is attributed to the hybridisation between the surface and the bulk states as their projection lie within the vicinity of each other. The finding in this work demonstrates that although Fermi arcs are guaranteed in Weyl semimetals, their shape and connectivity are not protected and may be altered accordingly.

No description available.

We present an effective field theory for interacting electrons in clean semimetals (both three dimensional Dirac semimetals and graphene) in terms of their soft or massless bosonic degrees of freedom. We show, by means of a Ward identity, that the intrinsic semimetal ground state breaks the Sp(4M) symmetry of the theory. In Fermi liquids this enables one to identify the massive, non-Goldstone modes of the theory and integrate them out. Due to the vanishing density of states in semimetals, unlike in Fermi liquids, both Goldstone and non-Goldstone modes are equally soft, and so all two-particle correlations need to be kept. The resulting theory is not perturbative with respect to the electron-electron interaction; rather, it is controlled by means of a systematic loop expansion and allows one to determine the exact asymptotic form of observables in the limits of small frequencies and/or wave vectors. Equivalently, it provides a mechanism of determining the long time-tail and long wavelength behavior of observables and excitations. As a representative application, we use the theory to compute the zero-bias anomaly for the density of states for both short and long-range interactions in two and three dimensions. We find that the leading nonanalyticity in semimetals with a long-ranged interaction appears at the same order in frequency as the one in Fermi liquids, since the effects of the vanishing density of states at the Fermi level are offset by the breakdown of screening. Consequently, we are able to provide a logical scheme to determine the leading non-analytical behavior of observables in semimetals using knowledge of the corresponding non-analyticities in a Fermi liquid. / 2020-01-11

No description available.

This thesis aims to contribute to the understanding of quantum materials by employing a combination of experimental techniques, such as angle-resolved photoemission spectroscopy and magnetic and transport measurements. Further collaborative support in the form of first-principles calculations is included and discussed in tandem. In this thesis, a lanthanide-based semimetal of the ZrSiS type, is investigated. Multiple nodal lines which remain gapless are observed along the X-R direction of the Brillouin zone. We also present a nodal line that is observed further below the Fermi level and aligned in the G-M direction; this nodal line appears to be sensitive to light source polarization. A surface state at the X point is also observed. Finally, this thesis includes some discussion on the development of a sample growth laboratory, along with the presentation and characterization of grown Bi2Se3 samples. With potential applications in the fields of spintronics and novel microelectronic devices, the experimental realization and understanding of quantum materials is key to a deeper understanding of materials physics.

The topological aspects of superconductivity on doped Weyl semimetals are investigated. / Topological phases of matter have sparked significant experimental and theoretical interest due to the topologically robust edge modes they host, as well as their classification through rich mathematics. An interesting example of a topological phase in three dimensions, the Weyl semimetal, can exhibit topological ordering through the existence of Fermi arcs on the surfaces of the material. For the doped Weyl semimetal, we investigate possible resulting Weyl superconducting states --- both the inter-Fermi surface pairing state following Li and Haldane, and the intra-Fermi surface pairing state following Burkov --- in this thesis, and study their topological properties by computing the gapless Weyl-Majorana edge modes they host. The results obtained in Ref. \cite{LH} for the inter-Fermi surface superconducting state are reproduced, and the bulk and edge properties of the intra-Fermi surface pairing superconducting state are studied in detail. / Thesis / Master of Science (MSc) / In this thesis, we study an interesting class of topological materials called the Weyl semimetal as well as its associated superconducting phases. A description of the Fermi arcs on Weyl semimetals are given, and the topological properties of the inter-Fermi surface and intra-Fermi surface pairing states are studied in detail.