Thesis advisor: Kenneth S. Burch / Since their first experimental realizations in the 2000s, bulk electronic topological materials have been one of the most actively studied areas of condensed matter physics. Among the more recently discovered classes of topological materials are the Weyl semimetals whose low energy excitations behave like massless, relativistic particles with well-defined chirality. These material systems display exotic behavior such as surface Fermi arc states, and the chiral anomaly in which parallel magnetic and electric fields lead to an imbalance of left- and right-handed particles. Much of the research into these materials has focused on the electronic properties, but relatively little has been directed towards understanding the vibrational properties of these systems, or of the interplay between the electronic and vibrational degrees of freedom. Further, the technological potential of these materials is still underdeveloped, with the search for physical properties enhanced by the topological nature of these materials being sought after. In this dissertation we address both of these issues. In Chapters III and IV we present temperature dependent Raman investigations of the the Weyl semimetals WP2, NbAs, and TaAs. Measurements of the optical phonon linewidths are used to identify the available phonon decay paths, with ab-initio calculations and group theory used to aid the interpretation of these results. We find that some phonons display linewidths indicative of dominant decay into electron-hole pairs near the Fermi surface, rather than decay into acoustic phonons. In light of these results we discuss the role of phonon-electron coupling in the transport properties of these Weyl semimetals. In Chapter V, we discuss the construction of our "PVIC" setup for the measurement of nonlinear photocurrents. We discuss the experimental capabilities that the system was designed to possess, the operating principles behind key components of the system, and give examples of the operating procedures for using the setup. The penultimate chapter, Chapter VI, presents the results of photocurrent measurements using this setup on the Weyl semimetal TaAs. Through careful analysis of the photocurrent polarization dependence, we identify a colossal bulk photovoltaic effect in this material which exceeds the response displayed by previously studied materials by an order of magnitude. Calculations of the second-order optical conductivity tensor show that this result is consistent with the divergent Berry connection of the Weyl nodes in TaAs. In addition to these topics, Chapter II addresses the results of Raman measurements on thin film heterostructures of the topological insulator Bi2Se3 and the magnetic semiconductor EuS. By investigating the paramagnetic Raman signal in films with different compositions of EuS and Bi2Se3 we provide indirect evidence of charge transfer between the two layers. We also track the evolution of phonon energies with varying film thicknesses on multiple substrates which provides insight into the interfacial strain between layers. We conclude the dissertation in Chapter VII with a summary of the main results from each preceding chapter, and give suggestions for future experiments that further investigate these topics. / Thesis (PhD) — Boston College, 2020. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

No description available.

Thesis advisor: Fazel Tafti / Since the idea of topology was realized in real materials, the hunt is on for new candidates of topological semimetals with novel electromagnetic responses. For example, topological states can be highly conductive due to a topological protection, which can be destroyed in a magnetic field and lead to an extremely high magnetoresistance. In Weyl semimetals, a transverse current that would usually require a magnetic field to emerge, can be generated by intrinsic Berry curvature without a magnetic field -- the celebrated anomalous Hall effect. In this dissertation, both phenomena mentioned above are studied in rare-earth monopnictides and RAlX material family (R=rare-earths, X=Ge/Si), respectively. The monopnictides are ideal for the study of extreme magnetoresistance because of their topological transitions and abundant magnetic phases. In LaAs, we untied the connection between topological states and the extreme magnetoresistance, the origin of which is clarified. In HoBi, we found an unusual onset of extreme magnetoresistance controlled by a magnetic phase dome. On the other hand, RAlX material family is a new class of Weyl semimetals breaking both inversion and time-reversal symmetries. In particular, in PrAlGeₓSi₁₋ₓ (x=0-1), we unveiled the first transition from intrinsic to extrinsic anomalous Hall effect in ferromagnetic Weyl semimetals, and the role of topology is discussed. In CeAlSi, we found that the Fermi level can be tuned as close as 1 meV away from the Weyl nodes; moreover, a novel anomalous Hall response appears only when the Fermi level is tuned to be near the Weyl nodes. Thus, we established a new transport response solely induced by Weyl nodes. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Weyl-Semimetalle haben bemerkenswerte Eigenschaften. Ihr elektrischer Widerstand steigt linear und unsaturiert mit einem angelegten Magnetfeld, diverse Ergebnisse deuten darauf hin, dass sie einen unordnungsinduzierten Metall-Isolator-Phasenübergang aufweisen und ihre Ladungsträger zeigen die chirale Anomalie, d.h., die Nichtkonservierung der chiralen Ladung. Diese Eigenschaften haben ihren Ursprung in der Niedrigenergiephysik der Weyl- Semimetalle, die von Weyl-Punkten, Berührungspunkten zwischen Leitungs- und Valenzband an der Fermi-Energie mit einer linearen Dispersionsrelation, dominiert wird. Diese Berührungspunkte sind topologisch geschützt, d.h., kleine Störung können ihnen nichts anhaben. Weyl-Semimetalle sind daher Beispiele für topologische Semimetalle, Materialien mit geschützten niedrigdimensionalen Berührungenspunkten, -linien, oder -oberflächen an der Fermi-Energie. In dieser Arbeit zeigen wir, wie die Eigenschaften von Weyl-Semimetallen durch Unordnung, Magnetfelder und Deformationen beeinflusst werden. Wir zeigen außerdem eine Querverbindung zwischen Weyl-Semimetallen und nodal line-Semimetallen, topologisch geschützten Semimetallen mit einer eindimensionalen Fermi-Fläche. Durch die Nutzung von Gitter- und Niedrigenergiekontinuumsmodellen können wir Wege aufzeigen, wie man unsere Ergebnisse sowohl aus einer Festkörperphysik- als auch aus einer Hochenergiephysikperspektive verstehen kann. Insbesondere identifizieren wir eine experimentelle Signatur der chiralen Anomalie: die blaue Note, ein charakteristisches Muster in Form einer Note, das mit Hilfe von winkelaufgelöster Photoelektronenspektroskopie gemessen werden kann. Ein weiteres wichtiges Charakteristikum ist der Magnetwiderstand, der in Weyl-Semimetallen vom Winkel zwischen einem angelegten Magnetfeld und der Transportrichtung abhängt. Durch den Einfluss der chiralen Anomalie ist der longitudinale Magnetwiderstand negativ, der transversale Widerstand hingegen wächst linear und grenzenlos mit dem angelegten Magnetfeld. In dieser Dissertation untersuchen wir beide Charakteristiken analytisch und numerisch. Inspiriert durch Experimente, in denen ein scharfes Leitfähigkeitsmaximum für parallele elektrische und Magnetfelder observiert wurde, zeigen wir, dass die Leitfähigkeit vom Winkel zwischen den angelegten Feldern und dem Abstandsvektor der Weyl-Punkte abhängt und dass sie insbesondere für Felder parallel zum Abstandsvektor ein scharfes Maximum aufweist. Dieser Effekt ist besonders ausgeprägt, wenn nur das niedrigste Landau-Niveau zur Leitfähigkeit beiträgt, er bleibt aber auch bei höheren Energien beobachtbar. Für parallelen Magnettransport untersuchen wir starke Unordnung, die außerhalb des von der Störungstheorie abgedeckten Bereichs liegt, numerisch und beobachten einen positiven Magnetwiderstand, qualitativ ähnlich zu experimentellen Daten. Aus Deformationen in Weyl-Semimetallen entstehen sogenannte chirale oder auch axiale Felder, die ähnliche Konsequenzen wie externe elektromagnetische Felder haben, wobei noch viele Details im Verborgenen liegen. Wir untersuchen Deformationen aus zwei verschiedenen Perspektiven: zunächst zeigen wir, wie zwei widersprüchliche Vorhersagen aus der Quantenfeldtheorie, die konsistenten und kovarianten Anomalien, in einem Gittermodell beobachtbar sind. Dann untersuchen wir elektrischen Transport unter Einfluss von axialen Magnetfeldern und zeigen, dass Moden, die sich in unterschiedliche Richtungen bewegen, räumlich getrennt sind. Diese räumliche Trennung hat eine unübliches Wachstums des elektrischen Leitwerts mit der transversalen Systembreite zur Folge. Des weiteren zeigen wir, wie ein nodal line-Semimetall aus einem Weyl-Semimetall entstehen kann, das einer Supergitterstruktur ausgesetzt ist. Wir interpretieren die Oberflächenzustände mit Hilfe der interzellulären Zak-Phase und zeigen zwei verschiedene Mechanismen, die die Bandstruktur vor der Öffnung einer Bandlücke schützen, auf. Um unsere Diskussion abzuschließen, untersuchen wir Transport in nodal line-Semimetallen in Kürze und stellen ihre Quantenfeldtheorie vor. Schließlich wenden wir uns wechselwirkenden Phasen zu und zeigen, welche Konsequenzen die Symmetrieklassifizierung des Sachdev- Ye-Kitaev-Modells hat – ein Modell von Teilchen mit zufälligen Wechselwirkungsstärken, dessen Topologie von der Anzahl der enthaltenen Teilchen bestimmt wird.:1 Introduction 2 Topological Band Theory 2.1 Geometric Phase and Berry Phase 2.1.1 The Adiabatic Theorem 2.1.2 The Zak Phase 2.2 Tenfold Classification of Topological Insulators and Superconductors 2.3 Topological Semimetals 2.3.1 Weyl Semimetals 2.3.2 Nodal Line Semimetals 2.4 Bulk-boundary Correspondence from the Intercellular Zak Phase 2.4.1 Intra- and Intercellular Zak Phase 2.4.2 Bulk-boundary Correspondence 2.4.3 Conclusion 3 Field Theory Perspective on Topological Phases 3.1 Topological Insulators 3.2 Weyl Fermions and the Chiral Anomaly 3.3 Visualizing the Chiral Anomaly with Photoemission Spectroscopy 3.3.1 The Chiral Anomaly in Condensed Matter Systems 3.3.2 Model and Methods 3.3.3 ARPES Spectra for Weyl and Dirac Semimetals 3.3.4 Experimental Details 3.3.5 Summary and Conclusion 3.4 The Consistent and Covariant Anomalies 3.5 Consistent and Covariant Anomalies on a Lattice 3.5.1 Model and Methods 3.5.2 Lattice Results for Consistent and Covariant Anomalies 3.5.3 Influence of the Mass Term 3.5.4 The Quest for One Third 3.6 The Action of Nodal Line Semimetals 4 Transport in Topological Semimetals 4.1 Longitudinal Magnetoresistance in Weyl Semimetals 4.2 Transversal Magnetoresistance in Weyl Semimetals 4.2.1 Model 4.2.2 Mesoscopic Transport in Clean Samples 4.2.3 Numerical Magnetotransport in the Presence of Disorder 4.2.4 Born-Kubo Analytical Bulk Conductivity 4.2.5 Numerical Results in Disordered Samples 4.2.6 Conclusion 4.3 Transport in the Presence of Axial Magnetic Fields 4.3.1 Model and Methods 4.3.2 Longitudinal Magnetotransport for Axial Fields 4.3.3 Conclusion 4.4 Transport in Nodal Line Semimetals 5 Nodal Line Semimetals from Weyl Superlattices 5.1 Weyl Semimetal on a Superlattice 5.2 Emergent Nodal Phases 5.3 Symmetry Classification of the Nodal Line 5.4 Surface States 5.5 Stability against Wave Vector Mismatch 5.6 Time-reversal Symmetric Weyl Semimetal 5.7 Conclusion 6 Symmetry Classification of the SYK Model 6.1 Model and Topological Classification 6.2 Overlap of Time-reversed Partners 6.2.1 Even Number of Majoranas 6.2.2 Odd Number of Majoranas 6.3 Spectral Function 6.3.1 Zero Temperature 6.3.2 Infinite Temperature 6.4 Symmetry-breaking Terms 6.5 Lattice Model 6.6 Conclusion 7 Conclusion and Outlook Appendix A Zak Phase and Extra Charge Accumulation Appendix B Material-specific Details for ARPES B.1 Relaxation Rates B.2 ARPES in Finite Magnetic Fields B.3 Estimates of the Chiral Chemical Potential Difference Appendix C Weyl Nodes in a Magnetic Field C.1 Scattering between Different Landau Levels C.2 Analytical Born-Kubo Calculation of Transversal Magnetoconductivity C.2.1 Disorder Scattering in Born Approximation C.2.2 Transversal Magnetoconductivity from Kubo Formula Appendix D Transfer Matrix Method D.1 Longitudinal Magnetic Field D.2 Transversal Magnetic Field Bibliography Acknowledgments List of Publications Versicherung / Weyl semimetals have remarkable properties. Their resistance grows linearly and unsaturated with an applied transversal magnetic field, and they are expected to show a disorder-induced metal-insulator transition. Their charge carriers exhibit the chiral anomaly, i.e., the nonconservation of chiral charge. These properties emerge from their low-energy physics, which are dominated by Weyl nodes: zero-dimensional band crossings at the Fermi energy with a linear dispersion. The band crossings are topologically protected, i.e., they cannot be lifted by small perturbations. Thus, Weyl semimetals are examples of topological semimetals, materials with protected lower-dimensional band crossing close to the Fermi surface. In this work, we show how the properties of Weyl semimetals are affected by disorder, magnetic fields, and strain. We further provide a link between Weyl semimetals and nodal line semimetals, topological semimetals with a one-dimensional Fermi surface. By using both lattice and low-energy continuum models, we present ways to understand the results from a condensed-matter and a quantum-field-theory perspective. In particular, we identify an experimental signature of the chiral anomaly: the blue note, a characteristic note-shaped pattern that can be measured in photoemission spectroscopy. Another important signature is the magnetoresistance. In Weyl semimetals, its behavior depends on the angle between the magnetic field and the transport direction. For parallel transport, a negative longitudinal magnetoresistance as a manifestation of the chiral anomaly is observed; for orthogonal transport, the transversal magnetoresistance shows a linear and unsaturated growth. In this thesis, we investigate both regimes analytically and numerically. Inspired by experiments that show a sharply peaked magnetoresistance for parallel fields, we show that the longitudinal magnetoresistance depends on the angle between applied fields and the Weyl node separation, and that it is sharply peaked for fields parallel to the node separation. This effect is especially strong in the limit where only the lowest Landau level contributes to the magnetoresistance, but it survives at higher chemical potentials. For transversal magnetotransport, we numerically investigate the strong-disorder regime that is beyond the reach of perturbation theory and observe a positive magnetoresistance, qualitatively similar to recent experiments. Strain in Weyl semimetals creates so-called axial fields that result in phenomena similar to the ones driven by electric and magnetic fields, but with some yet unknown consequences. We investigate strain from two perspectives: first, we show how two different predictions from quantum field theory, the consistent and covariant anomalies, manifest on a lattice. Second, we investigate transport in the presence of axial magnetic fields and show that counterpropagating modes are spatially separated, resulting in an unusual scaling of the conductance with the system’s width. We further show how a nodal line semimetal can emerge from a Weyl semimetal on a superlattice. We interpret the presence of surface states in terms of the intercellular Zak phase and show two distinct mechanisms that protect the spectrum from opening a gap. To complete our discussion, transport in nodal line semimetals is briefly discussed, as well as the quantum field theory that describes the low-energy features of these materials. Finally, we conclude this work by showing manifestations of the different symmetry classes that can be realized in the Sachdev-Ye-Kitaev model—a model of randomly interacting particles whose topology is deeply connected to the number of particles.:1 Introduction 2 Topological Band Theory 2.1 Geometric Phase and Berry Phase 2.1.1 The Adiabatic Theorem 2.1.2 The Zak Phase 2.2 Tenfold Classification of Topological Insulators and Superconductors 2.3 Topological Semimetals 2.3.1 Weyl Semimetals 2.3.2 Nodal Line Semimetals 2.4 Bulk-boundary Correspondence from the Intercellular Zak Phase 2.4.1 Intra- and Intercellular Zak Phase 2.4.2 Bulk-boundary Correspondence 2.4.3 Conclusion 3 Field Theory Perspective on Topological Phases 3.1 Topological Insulators 3.2 Weyl Fermions and the Chiral Anomaly 3.3 Visualizing the Chiral Anomaly with Photoemission Spectroscopy 3.3.1 The Chiral Anomaly in Condensed Matter Systems 3.3.2 Model and Methods 3.3.3 ARPES Spectra for Weyl and Dirac Semimetals 3.3.4 Experimental Details 3.3.5 Summary and Conclusion 3.4 The Consistent and Covariant Anomalies 3.5 Consistent and Covariant Anomalies on a Lattice 3.5.1 Model and Methods 3.5.2 Lattice Results for Consistent and Covariant Anomalies 3.5.3 Influence of the Mass Term 3.5.4 The Quest for One Third 3.6 The Action of Nodal Line Semimetals 4 Transport in Topological Semimetals 4.1 Longitudinal Magnetoresistance in Weyl Semimetals 4.2 Transversal Magnetoresistance in Weyl Semimetals 4.2.1 Model 4.2.2 Mesoscopic Transport in Clean Samples 4.2.3 Numerical Magnetotransport in the Presence of Disorder 4.2.4 Born-Kubo Analytical Bulk Conductivity 4.2.5 Numerical Results in Disordered Samples 4.2.6 Conclusion 4.3 Transport in the Presence of Axial Magnetic Fields 4.3.1 Model and Methods 4.3.2 Longitudinal Magnetotransport for Axial Fields 4.3.3 Conclusion 4.4 Transport in Nodal Line Semimetals 5 Nodal Line Semimetals from Weyl Superlattices 5.1 Weyl Semimetal on a Superlattice 5.2 Emergent Nodal Phases 5.3 Symmetry Classification of the Nodal Line 5.4 Surface States 5.5 Stability against Wave Vector Mismatch 5.6 Time-reversal Symmetric Weyl Semimetal 5.7 Conclusion 6 Symmetry Classification of the SYK Model 6.1 Model and Topological Classification 6.2 Overlap of Time-reversed Partners 6.2.1 Even Number of Majoranas 6.2.2 Odd Number of Majoranas 6.3 Spectral Function 6.3.1 Zero Temperature 6.3.2 Infinite Temperature 6.4 Symmetry-breaking Terms 6.5 Lattice Model 6.6 Conclusion 7 Conclusion and Outlook Appendix A Zak Phase and Extra Charge Accumulation Appendix B Material-specific Details for ARPES B.1 Relaxation Rates B.2 ARPES in Finite Magnetic Fields B.3 Estimates of the Chiral Chemical Potential Difference Appendix C Weyl Nodes in a Magnetic Field C.1 Scattering between Different Landau Levels C.2 Analytical Born-Kubo Calculation of Transversal Magnetoconductivity C.2.1 Disorder Scattering in Born Approximation C.2.2 Transversal Magnetoconductivity from Kubo Formula Appendix D Transfer Matrix Method D.1 Longitudinal Magnetic Field D.2 Transversal Magnetic Field Bibliography Acknowledgments List of Publications Versicherung

Topological materials have discovered ultrahigh magnetoresistance, chiral anomalies, the inherent anomalous Hall effect, and unique Fermi arc surface states. Topological materials now include insulators, metals, and semimetals. Weyl semimetals (WSM) are topological materials that show linear dispersion with crossings in their band structure which creates the pair of Weyl nodes of opposite chirality. WSMs have topological Fermi arc surface states connecting opposing chirality Weyl nodes. Spin-orbit coupling can result in the band opening in Dirac nodal rings, and creating the pair of Weyl nodes either by breaking the time-reversal or spatial inversion symmetry (but not both) 1-3. The chirality of a Weyl node is set by the Berry flux through a closed surface in reciprocal space around it. The purpose of this thesis was to characterize and investigate the thermodynamic stability of WSM. To accomplish these goals, quantum mechanical modeling at the level of density functional theory (DFT) was used. WloopPHI, a Python module, integrates the characterization of WSMs into WIEN2k, a full-potential all-electron density functional theory package. It calculates the chirality of the Weyl node (monopole charge) with an enhanced Wilson loop method and Berry phase approach. First, TaAs, a well-characterized Weyl semimetal, validates the code theoretically. We then used the approach to characterize the newly discovered WSM YRh6Ge4, and we found a set of Weyl points into it. Further, we study the stability of the kagome-based materials A3Sn2S2, where A is Co, Rh, or Ru, in the context of the ternary phase diagrams and competing binary compounds using DFT. We demonstrated that Co3Sn2S2 and Rh3Sn2S2 are stable compounds by examining the convex hull and ternary phase diagrams. It is feasible to synthesize Co3Sn2S2 by a chemical reaction between SnS, CoSn and Co9S8. Moreover, Rh3Sn2S2 can be produced by SnS, RhSn and Rh3S4. On the other hand, we found that Ru3Sn2S2 is a thermodynamically unstable material with respect to RuS2, Ru3S7 and Ru. Our work provides some insights for confirming materials using the DFT approach. 1. S. M. Young et al. Dirac Semimetal in Three Dimensions. Physical Review Letters108(14) (2012), 140405. 2. J. Liu and D. Vanderbilt. Weyl semimetals from noncentrosymmetric topological insulators. Physical Review B 90(15) (2014), 155316. 3. H. Weng et al. Weyl Semimetal Phase in Noncentrosymmetric Transition-Metal Monophosphides. Physical Review X 5(1) (2015), 011029. / Thesis / Master of Applied Science (MASc)

No description available.

The importance and promise that topological materials hold has been recently underscored by the award of the Nobel Prize in Physics in 2016 ``for theoretical discoveries of topological phase transitions and topological phases of matter." This dissertation explores the novel qualities and useful topologically protected surface states of topological insulators and semimetals. Topological materials have protected qualities which are not removed by weak perturbations. The manifestations of these qualities in topological insulators are spin-momentum-locked surface states, and in Weyl and Dirac semimetals they are unconventional open surface states (Fermi arcs) with anomalous electrical transport properties. There is great promise in utilizing the topologically protected surface states in electronics of the future, including spintronics, quantum computers, and highly sensitive devices. Physicists and chemists are also interested in the fundamental physics and exotic fermions exhibited in topological materials and in heterostructures including them. Chapter 1 provides an introduction to the concepts and methods of topological band theory. Chapter 2 investigates the spin and spin-orbital texture and electronic structures of the surface states at side surfaces of a topological insulator, Bi2Se3, by using slab models within density functional theory. Two representative, experimentally achieved surfaces are examined, and it is shown that careful consideration of the crystal symmetry is necessary to understand the physics of the surface state Dirac cones at these surfaces. This advances the existing literature by properly taking into account surface relaxation and symmetry beyond what is contained in effective bulk model Hamiltonians. Chapter 3 examines the Fermi arcs of a topological Dirac semimetal (DSM) in the presence of asymmetric charge transfer, of the kind which would be present in heterostructures. Asymmetric charge transfer allows one to accurately identify the projections of Dirac nodes despite the existence of a band gap and to engineer the properties of the Fermi arcs, including spin texture. Chapter 4 investigates the effect of an external magnetic field applied to a DSM. The breaking of time reversal symmetry splits the Dirac nodes into topologically charged Weyl nodes which exhibit Fermi arcs as well as conventionally-closed surface states as one varies the chemical potential. / Ph. D. / The importance and promise that topological materials hold has been recently underscored by the award of the Nobel Prize in Physics in 2016 “for theoretical discoveries of topological phase transitions and topological phases of matter.” This dissertation explores the novel qualities and useful topologically protected surface states of topological insulators and semimetals. Topological materials have protected qualities which are not removed by weak perturbations to the system. The manifestations of these qualities in topological insulators are spin-momentum-locked surface states which can be used to develop spin-polarized currents in electronics. Further, these states have linear dispersion at a special momentum point, called the Dirac cone. Conventionally these surface states form closed loops in momentum space. But in two other species of topological materials, Weyl and Dirac semimetals, the surface states form open arcs (called Fermi arcs) and these cause anomalous electrical transport properties including Hall conductivity and Nernst effect. Weyl and Dirac semimetals also have special momentum points (nodes) at which the bulk conduction and valence bands touch with linear dispersion. There is great promise in utilizing the topologically protected surface states in the electronics of the future, including spintronics, quantum computers, and highly sensitive devices. Physicists and chemists are also interested in the fundamental physics and exotic fermions exhibited in topological materials and in heterostructures including them. Chapter 1 provides an introduction to the concepts and methods of topological band theory. Chapter 2 investigates the spin and spin-orbital texture and electronic structures of the surface states of a topological insulator, Bi₂Se₃, at its side surfaces (beyond the familiar cleaving surface). We use slab models within density functional theory (DFT) to investigate two representative, experimentally achieved surfaces, and it is shown that careful consideration of the threefold rotational crystal symmetry is necessary to understand the physics of the surface state Dirac cones at these surfaces. The differing atomic orbital and cationic/anionic characters of the topological states are examined. This advances the existing literature by properly taking into account how the atoms at the surface relax at the interface with the vacuum and the full symmetry beyond what is contained in effective bulk model Hamiltonians. Chapter 3 examines the Fermi arcs of a topological Dirac semimetal (DSM) in the presence of asymmetric charge transfer at only one surface, of the kind which would be present in heterostructures comprised of DSMs and topologically-trivial materials. We use a thin slab model within DFT to calculate the electronic structure of the DSM. Asymmetric charge transfer allows one to accurately identify the projections of the linearly dispersing Dirac nodes despite the existence of a bulk band gap and to engineer the properties of the surface Fermi arcs, including their spin texture. Chapter 4 investigates the effect of an external magnetic field applied to a DSM. The breaking of time reversal symmetry splits the Dirac nodes into topologically charged Weyl nodes which exhibit Fermi arcs as well as conventionally-closed surface states as one varies the chemical potential. The topological charge of the Weyl nodes is what makes them, and their Fermi arcs, robust against weak perturbations such as strain. Meticulously determining the topological index, or Chern number, of Fermi surface sheets demonstrates the bulk-boundary correspondence between the Weyl nodes and their Fermi arcs, and provides evidence for the existence of multiple-charge double Weyl nodes which, until now, have only been discussed sparingly in the literature on topological DSMs.

In this work we exploit the unique characteristics of a Dirac semimetal material to be symmetry-protected, to investigate dierent topological phase transitions provided by chemical dopings, focusing in particular on the electronic, magnetic and topological properties of the doped systems, studied by the mean of rst-principles methods based on density functional theory (DFT) approach. In particular these doped systems, besides being of interest for investigating the role of topology in solid state physics, could have a great potential for practical application since the dierent topological phases that come along with the chemical dopings allow one to exploit the unique properties of topological materials. The starting point for our study will be the material called cadmium-arsenide (Cd3As2), an example of a topological Dirac semimetal, which is chemically stable at ambient conditions. Chapter I presents a general introduction to topology, especially in condensed matter physics, and to the main physical properties of the topological materials we mentioned. Then, in chapter II, we briey present the methods and the computational tools that we used for our study. In chapter III a more detailed introduction to our work is given, along with a schemetic view of the path we followed, together with the results that we obtained for pristine Cd3As2, which we use as bench mark for our computational methods. Finally, in chapter IV and V, the results for the doped systems are presented and discussed, respectevely for the non-magnetic (IV) and magnetic (V) dopings. Our study has enabled us to discern how doping can give rise to see dierent topological phase transitions. Specically our work shows that dierent realizations of non-magnetic doping gives rise to dierent topological phases: the topological Weyl semimetal phase, which is of great interest since it can support a robust quantum spin Hall eect, and a very special mixed Dirac + Weyl phase, where surprisingly both a Dirac and a Weyl phase can coexist in the same system. Furthermore, magnetically doped systems show the emergence of a magnetic Weyl phase, which can support a quantum anomalous Hall eect. Our work can be the starting point for future studies, both theoretical and experimental, in which the unique physical properties we found in the doped Cd3As2 systems can be further investigated, in order to exploit them for practical applications.

This thesis focuses on the development of nonlinear optical techniques and the measurement of topological properties of the Weyl semimetals. The first portion of this thesis describes technical developments of the nonlinear optical spectroscopic probes rotational anisotropy second harmonic generation (RA-SHG) and transient grating. In our work on SHG, we describe a fast-reflective optic-based rotational anisotropy nonlinear harmonic generation spectrometer built upon synchronization of stepper motors and a voice-coil fast turning motor with data recorded by a data acquisition card. This device enables fast accumulation of significantly more data points than traditional SHG spectrometers and further allows spectral measurement over a broad wavelength range to be performed without optical realignment. We then describe the Fourier domain RA-SHG, allows direct measurements of the RA-SHG signal components of Cn symmetry. This method is based on the fast scanning RA-SHG device described above and operates by recording the nth harmonics of the fast scanning signal using a lock-in amplifier. Finally, we describe a novel method of performing transient grating measurements based on low power laser diodes, a laser diode pulser, a digital delay generator, and a data acquisition card. The RA-SHG technique was applied to the chiral Weyl semimetal RhSi, where a spectrum of the sole SHG tensor element χ(2) i jk was measured over the unprecedented 0.275-1.5 eV incoming photon energy range. Our data shows evidence of a strong surface state response and are detailed enough to reveal the second order corrections to the linear band structure as well as the Pauli blocking condition which was observed to occur at ∼630 meV. We also describe measurements of the linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE) in RhSi deriving from topological Fermi arc states. While the magnitude of the CPGE response broadly matched theoretical predictions, the data also exhibit an inexplicably high degree of symmetry in the response as a function of incoming polarization in both CPGE and LPGE channels. Collaborative work on the SHG spectrum from TaAs is also described, from which we attribute the origin of the SHG response peak to the third cumulant of the Bloch wavefunction. Further collaborative studies of the CPGE in RhSi (111) revealed a response that was likely due to the topological band structure, but that also shows that the theoretically predicted quantized CPGE was not observed due to impurities and from contributions from sources other than the Weyl nodes. Finally, we briefly summarized how the crystal structure of PrAlGe1-xSix was revealed to be non-centrosymmetric using the RA-SHG technique. Transition from intrinsic to extrinsic anomalous Hall effect by tuning the dopant concentration x was studied in this ferromagnetic Weyl semimetal. / Physics

<p>In this thesis, we present the results for the finite frequency response of a variety of materials. These materials all share the common theme that their low energy excitations are Dirac-like. This coincidence was not by design, and highlights the now-ubiquitous nature of Dirac-quasiparticles in condensed matter physics. We present results for graphene, the high temperature superconducting cuprates, and Weyl semi metals. For graphene, our calculations revolve around a new experimental technique: Near field infrared spectroscopy. Conventionally it is ok to use the $\vec{q}\rightarrow 0$ limit when calculating the low energy optical response. This new technique is able to directly probe the finite $\vec{q}$ response by using an atomic force microscope tip as an antenna. We computed the optical conductivity of graphene at finite wavevector and studied how the quasiparticle peak is altered by disorder and the electron-phonon interaction. The calculations on the high $T_c$ cuprates use a model of the pseudogap phase known as the Yang, Rice and Zhang (YRZ) model. We employed the model to study the resistivity in the pseudogap regime, both in-plane and along the c-axis. We used a coherent tunneling matrix element to describe transport along the c-axis. We found that the model was able to reproduce the metaliclike behavior in the plane while being resistive out of plane. We then extended the model to the finite frequency response, as well as the superconducting phase. We found a pseduogap feature at finite frequency that was previously explained through an interlayer collective mode. We also found that microwave spectroscopy puts strong limits on the form of the scattering rate. Finally, we computed the optical response of Weyl semimetals subjected to an applied magnetic field. Weyl semimetals are a topological phase of matter that have yet to be observed. The form of the conductivity contains a series of asymmetric peaks, whose spacing is a signature of the underlying relativistic dispersion. These peaks remain robust, even with moderate disorder.</p> / Doctor of Philosophy (PhD)