@phdthesis{Northe2019,
  author    = {Northe, Christian},
  title     = {Interfaces and Information in Gauge/Gravity Duality},
  doi       = {10.25972/OPUS-19159},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-191594},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {This dissertation employs gauge/gravity duality to investigate features of ( 2 + 1 ) -dimensional quantum gravity in Anti-de Sitter space (AdS) and its relation to conformal field theory (CFT) in 1 + 1 dimensions. Concretely, we contribute to research on the frontier of gauge/gravity with condensed matter as well as the frontier with quantum informa- tion. The first research topic of this thesis is motivated by the Kondo model, which describes the screening of magnetic impurities in metals by conduction electrons at low temperatures. This process has a de- scription in the language of string theory via fluctuating surfaces in spacetime, called branes. At high temperatures the unscreened Kondo impurity is modelled by a stack of pointlike branes. At low tempera- tures this stack condenses into a single spherical, two-dimensional brane which embodies the screened impurity. This thesis demonstrates how this condensation process is naturally reinvoked in the holographic D1/D5 system. We find brane configu- rations mimicking the Kondo impurities at high and low energies and establish the corresponding brane condensation, where the brane grows two additional dimensions. We construct supergravity solutions, which fully take into account the effect of the brane on its surrounding space- time before and after the condensation takes place. This enables us to compute the full impurity entropies through which we confirm the validity of the g-theorem. The second research topic is rooted in the connection of geometry with quantum information. The motivation stems from the "complexity equals volume" proposal, which relates the volume of wormholes to the cicruit complexity of a thermal quantum state. We approach this proposal from a pragmatic point of view by studying the properties of certain volumes in gravity and their description in the CFT. We study subregion complexities, which are the volumes of the re- gions subtended by Ryu-Takayanagi (RT) geodesics. On the gravity side we reveal their topological properties in the vacuum and in ther- mal states, where they turn out to be temperature independent. On the field theory side we develop and proof a formula using kinematic space which computes subregion complexities without referencing the bulk. We apply our formula to global AdS 3 , the conical defect and a black hole. While entanglement, i.e. minimal boundary anchored geodesics, suffices to produce vacuum geometries, for the conical defect we also need geodesics windings non-trivially around the singularity. The black hole geometry requires additional thermal contributions.},
  subject      = {Information},
  language  = {en}
}
@phdthesis{Laubach2014,
  author    = {Laubach, Manuel},
  title     = {Nichtmagnetische Isolatoren in Hexagonalen Gittermodellen},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-106987},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {Wir untersuchen zunächst das Hubbard-Modell des anisotropen Dreiecksgitters als effektive Beschreibung der Mott-Phase in verschiedenen organischen Verbindungen mit dreieckiger Gitterstruktur. Um die Eigenschaften am absoluten Nullpunkt zu bestimmen benutzen wir die variationelle Cluster Näherung (engl. variational cluster approximation VCA) und erhalten das Phasendiagramm als Funktion der Anisotropie und der Wechselwirkungsstärke. Wir finden f{\"u}r schwache Wechselwirkung ein Metall. F{\"u}r starke Wechselwirkung finden wir je nach Stärke der Anisotropie eine Néel oder eine 120◦-Néel antiferromagnetische Ordnung. In einem Bereich mittlerer Wechselwirkung entsteht in der Nähe des isotropen Dreiecksgitters ein nichtmagnetischer Isolator. Der Metall-Isolator-Übergang hängt maßgeblich von der Anisotropie ab, genauso wie die Art der magnetischen Ordnung und das Erscheinen und die Ausdehnung der nichtmagnetischen Isolatorphase. Spin-Bahn Kopplung ist der ausschlaggebende Parameter, der elektronische Bandmodelle in topologische Isolatoren wandelt. Spin-Bahn Kopplung im Allgemeinen beinhaltet auch den Rashba Term, der die SU(2) Symmetrie vollständig bricht. Sobald man auch Wechselwirkungen ber{\"u}cksichtigt, m{\"u}ssen sich viele theoretische Methoden auf die Analyse vereinfachter Modelle beschränken, die nur Spin-Bahn Kopplungen enthalten, welche die U(1) Symmetrie erhalten und damit eine Rashba Kopplung ausschließen. Wir versuchen diese bisher bestehende L{\"u}cke zu schließen und untersuchen das Kane-Mele Hubbard (KMH) Modell mit Rashba Spin-Bahn Kopplung und präsentieren eine systematische Analyse des Effekts der Rashba Spin-Bahn Kopplung in einem korrelierten zweidimensionalen topologischen Isolator. Wir wenden die VCA auf dieses Problem an und bestimmen das Phasendiagramm mit Wechselwirkung durch die Berechnung der lokalen Zustandsdichte, der Magnetisierung, der Einteilchenspektralfunktion und der Randzustände. Nach einer ausf{\"u}hrlichen Auswertung des KMH-Modells, bei erhaltener U(1) Symmetrie, finden wir auch f{\"u}r endliche Wechselwirkung, dass eine zusätzliche Rashba Kopplung zu neuen elektronischen Phasen f{\"u}hrt, wie eine metallische Phase und eine topologische Isolatorphase ohne Bandl{\"u}cke in der lokalen Zustandsdichte, die aber eine direkte Bandl{\"u}cke f{\"u}r jeden Wellenvektor besitzt. F{\"u}r eine Klasse von 5d Übergangsmetallen untersuchen wir ein KMH ähnliches Modell mit multidirektionaler Spin-Bahn Kopplung, das wegen seiner Relevanz f{\"u}r die Natrium-Iridate (engl. sodium iridate) als SI Modell bezeichnet wird. Diese intrinsische Kopplung bricht die SU(2) Symmetrie bereits vollständig und dennoch erhält man wegen der speziellen Form f{\"u}r starke Wechselwirkung wieder einen rotationssymmetrischen Néel-AFM Isolator. Der topologische Isolator des SIH-Modells ist adiabatisch mit dem des KMH-Modells verbunden, jedoch sind die Randströme hier nicht mehr spinpolarisiert. Wir verallgemeinern das Konzept der Klein-Transformation, das bereits erfolgreich auf Spin-Hamiltonians angewandt wurde, und wenden es auf ein Hubbard-Modell mit rein imaginären spinabhängigen H{\"u}pfen an, das im Grenzfall unendlicher Wechselwirkung in das Kitaev-Heisenberg Modell {\"u}bergeht. Dadurch erhält man ein Modell des Dreiecksgitters mit reellen spinunabhängigen H{\"u}pfen, das aber eine mehratomige Einheitszelle besitzt. F{\"u}r schwache Wechselwirkung ist das System ein Dirac Halbmetall und f{\"u}r starke Wechselwirkung erhält man eine 120◦-Néel antiferromagnetische Ordnung. F{\"u}r mittlere Wechselwirkung findet man aber einen relativ großen Bereich in dem eine nichtmagnetische Isolatorphase stabil ist. Unsere Ergebnisse deuten auf die mögliche Existenz einer Quanten Spinfl{\"u}ssigkeit hin.},
  subject      = {Hexagonaler Kristall},
  language  = {de}
}
@phdthesis{Rothe2015,
  author    = {Rothe, Dietrich Gernot},
  title     = {Spin Transport in Topological Insulators and Geometrical Spin Control},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-125628},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {In the field of spintronics, spin manipulation and spin transport are the main principles that need to be implemented. The main focus of this thesis is to analyse semiconductor systems where high fidelity in these principles can be achieved. To this end, we use numerical methods for precise results, supplemented by simpler analytical models for interpretation. The material system of 2D topological insulators, HgTe/CdTe quantum wells, is interesting not only because it provides a topologically distinct phase of matter, physically manifested in its protected transport properties, but also since within this system, ballistic transport of high quality can be realized, with Rashba spin-orbit coupling and electron densities that are tunable by electrical gating. Extending the Bernvevig-Hughes-Zhang model for 2D topological insulators, we derive an effective four-band model including Rashba spin-orbit terms due to an applied potential that breaks the spatial inversion symmetry of the quantum well. Spin transport in this system shows interesting physics because the effects of Rashba spin-orbit terms and the intrinsic Dirac-like spin-orbit terms compete. We show that the resulting spin Hall signal can be dominated by the effect of Rashba spin-orbit coupling. Based on spin splitting due to the latter, we propose a beam splitter setup for all-electrical generation and detection of spin currents. Its working principle is similar to optical birefringence. In this setup, we analyse spin current and spin polarization signals of different spin vector components and show that large in-plane spin polarization of the current can be obtained. Since spin is not a conserved quantity of the model, we first analyse the transport of helicity, a conserved quantity even in presence of Rashba spin-orbit terms. The polarization defined in terms of helicity is related to in-plane polarization of the physical spin. Further, we analyse thermoelectric transport in a setup showing the spin Hall effect. Due to spin-orbit coupling, an applied temperature gradient generates a transverse spin current, i.e. a spin Nernst effect, which is related to the spin Hall effect by a Mott-like relation. In the metallic energy regimes, the signals are qualitatively explained by simple analytic models. In the insulating regime, we observe a spin Nernst signal that originates from the finite-size induced overlap of edge states. In the part on methods, we discuss two complementary methods for construction of effective semiconductor models, the envelope function theory and the method of invariants. Further, we present elements of transport theory, with some emphasis on spin-dependent signals. We show the connections of the adiabatic theorem of quantum mechanics to the semiclassical theory of electronic transport and to the characterization of topological phases. Further, as application of the adiabatic theorem to a control problem, we show that universal control of a single spin in a heavy-hole quantum dot is experimentally realizable without breaking time reversal invariance, but using a quadrupole field which is adiabatically changed as control knob. For experimental realization, we propose a GaAs/GaAlAs quantum well system.},
  subject      = {Elektronischer Transport},
  language  = {en}
}
@phdthesis{Boettcher2021,
  author    = {B{\"o}ttcher, Jan Frederic},
  title     = {Fate of Topological States of Matter in the Presence of External Magnetic Fields},
  doi       = {10.25972/OPUS-22045},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-220451},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {The quantum Hall (QH) effect, which can be induced in a two-dimensional (2D) electron gas by an external magnetic field, paved the way for topological concepts in condensed matter physics. While the QH effect can for that reason not exist without Landau levels, there is a plethora of topological phases of matter that can exist even in the absence of a magnetic field. For instance, the quantum spin Hall (QSH), the quantum anomalous Hall (QAH), and the three-dimensional (3D) topological insulator (TI) phase are insulating phases of matter that owe their nontrivial topology to an inverted band structure. The latter results from a strong spin-orbit interaction or, generally, from strong relativistic corrections. The main objective of this thesis is to explore the fate of these preexisting topological states of matter, when they are subjected to an external magnetic field, and analyze their connection to quantum anomalies. In particular, the realization of the parity anomaly in solid state systems is discussed. Furthermore, band structure engineering, i.e., changing the quantum well thickness, the strain, and the material composition, is employed to manipulate and investigate various topological properties of the prototype TI HgTe. Like the QH phase, the QAH phase exhibits unidirectionally propagating metallic edge channels. But in contrast to the QH phase, it can exist without Landau levels. As such, the QAH phase is a condensed matter analog of the parity anomaly. We demonstrate that this connection facilitates a distinction between QH and QAH states in the presence of a magnetic field. We debunk therefore the widespread belief that these two topological phases of matter cannot be distinguished, since they are both described by a \$\mathbb{Z}\$ topological invariant. To be more precise, we demonstrate that the QAH topology remains encoded in a peculiar topological quantity, the spectral asymmetry, which quantifies the differences in the number of states between the conduction and valence band. Deriving the effective action of QAH insulators in magnetic fields, we show that the spectral asymmetry is thereby linked to a unique Chern-Simons term which contains the information about the QAH edge states. As a consequence, we reveal that counterpropagating QH and QAH edge states can emerge when a QAH insulator is subjected to an external magnetic field. These helical-like states exhibit exotic properties which make it possible to disentangle QH and QAH phases. Our findings are of particular importance for paramagnetic TIs in which an external magnetic field is required to induce the QAH phase. A byproduct of the band inversion is the formation of additional extrema in the valence band dispersion at large momenta (the `camelback'). We develop a numerical implementation of the \$8 \times 8\$ Kane model to investigate signatures of the camelback in (Hg,Mn)Te quantum wells. Varying the quantum well thickness, as well as the Mn-concentration, we show that the class of topologically nontrivial quantum wells can be subdivided into direct gap and indirect gap TIs. In direct gap TIs, we show that, in the bulk \$p\$-regime, pinning of the chemical potential to the camelback can cause an onset to QH plateaus at exceptionally low magnetic fields (tens of mT). In contrast, in indirect gap TIs, the camelback prevents the observation of QH plateaus in the bulk \$p\$-regime up to large magnetic fields (a few tesla). These findings allowed us to attribute recent experimental observations in (Hg,Mn)Te quantum wells to the camelback. Although our discussion focuses on (Hg,Mn)Te, our model should likewise apply to other topological materials which exhibit a camelback feature in their valence band dispersion. Furthermore, we employ the numerical implementation of the \$8\times 8\$ Kane model to explore the crossover from a 2D QSH to a 3D TI phase in strained HgTe quantum wells. The latter exhibit 2D topological surface states at their interfaces which, as we demonstrate, are very sensitive to the local symmetry of the crystal lattice and electrostatic gating. We determine the classical cyclotron frequency of surface electrons and compare our findings with experiments on strained HgTe.},
  subject      = {Topologie},
  language  = {en}
}
@phdthesis{Reinthaler2015,
  author    = {Reinthaler, Rolf Walter},
  title     = {Charge and Spin Transport in Topological Insulator Heterojunctions},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-135611},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Over the last decade, the field of topological insulators has become one of the most vivid areas in solid state physics. This novel class of materials is characterized by an insulating bulk gap, which, in two-dimensional, time-reversal symmetric systems, is closed by helical edge states. The latter make topological insulators promising candidates for applications in high fidelity spintronics and topological quantum computing. This thesis contributes to bringing these fascinating concepts to life by analyzing transport through heterostructures formed by two-dimensional topological insulators in contact with metals or superconductors. To this end, analytical and numerical calculations are employed. Especially, a generalized wave matching approach is used to describe the edge and bulk states in finite size tunneling junctions on the same footing. The numerical study of non-superconducting systems focuses on two-terminal metal/topological insulator/metal junctions. Unexpectedly, the conductance signals originating from the bulk and the edge contributions are not additive. While for a long junction, the transport is determined purely by edge states, for a short junction, the conductance signal is built from both bulk and edge states in a ratio, which depends on the width of the sample. Further, short junctions show a non-monotonic conductance as a function of the sample length, which distinguishes the topologically non-trivial regime from the trivial one. Surprisingly, the non-monotonic conductance of the topological insulator can be traced to the formation of an effectively propagating solution, which is robust against scalar disorder. The analysis of the competition of edge and bulk contributions in nanostructures is extended to transport through topological insulator/superconductor/topological insulator tunneling junctions. If the dimensions of the superconductor are small enough, its evanescent bulk modes can couple edge states at opposite sample borders, generating significant and tunable crossed Andreev reflection. In experiments, the latter process is normally disguised by simultaneous electron transmission. However, the helical edge states enforce a spatial separation of both competing processes for each Kramers' partner, allowing to propose an all-electrical measurement of crossed Andreev reflection. Further, an analytical study of the hybrid system of helical edge states and conventional superconductors in finite magnetic fields leads to the novel superconducting quantum spin Hall effect. It is characterized by edge states. Both the helicity and the protection against scalar disorder of these edge states are unaffected by an in-plane magnetic field. At the same time its superconducting gap and its magnetotransport signals can be tuned in weak magnetic fields, because the combination of helical edge states and superconductivity results in a giant g-factor. This is manifested in a non-monotonic excess current and peak splitting of the dI/dV characteristics as a function of the magnetic field. In consequence, the superconducting quantum spin Hall effect is an effective generator and detector for spin currents. The research presented here deepens the understanding of the competition of bulk and edge transport in heterostructures based on topological insulators. Moreover it proposes feasible experiments to all-electrically measure crossed Andreev reflection and to test the spin polarization of helical edge states.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@phdthesis{Tutschku2021,
  author    = {Tutschku, Christian Klaus},
  title     = {Anomaly Induced Transport And Hall Viscous Effects In 2+1 Space-Time Dimensions},
  doi       = {10.25972/OPUS-23913},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-239131},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {The main goal of this thesis is to elucidate the sense in which recent experimental progress in condensed matter physics, namely the verification of two-dimensional Dirac-like materials and their control in ballistic- as well as hydrodynamic transport experiments enables the observation of a well-known 'high-energy' phenomenon: The parity anomaly of planar quantum electrodynamics (QED\(_{2+1}\)). In a nutshell, the low-energy physics of two-dimensional Quantum Anomalous Hall (QAH) insulators like (Hg,Mn)Te quantum wells or magnetically doped (Bi,Sb)Te thin films can be described by the combined response of two 2+1 space-time dimensional Chern insulators with a linear dispersion in momentum. Due to their Dirac-like spectra, each of those Chern insulators is directly related to the parity anomaly of planar quantum electrodynamics. However, in contrast to a pure QED\(_{2+1}\) system, the Lagrangian of each Chern insulator is described by two different mass terms: A conventional momentum-independent Dirac mass \(m\), as well as a momentum-dependent so-called Newtonian mass term \(B \vert \mathbf{k} \vert^2\). According to the parity anomaly it is not possible to well-define a parity- and U(1) gauge invariant quantum system in 2+1 space-time dimensions. More precisely, starting with a parity symmetric theory at the classical level, insisting on gauge-invariance at the quantum level necessarily induces parity-odd terms in the calculation of the quantum effective action. The role of the Dirac mass term in the calculation of the effective QED\(_{2+1}\) action has been initially studied in Phys. Rev. Lett. 51, 2077 (1983). Even in the presence of a Dirac mass, the associated fermion determinant diverges and lacks gauge invariance. This requires a proper regularization/renormalizaiton scheme and, as such, transfers the peculiarities of the parity anomaly to the massive case. In the scope of this thesis, we connect the momentum-dependent Newtonian mass term of a Chern insulator to the parity anomaly. In particular, we reveal, that in the calculation of the effective action, before renormalization, the Newtonian mass term acts similarly to a parity-breaking element of a high-energy regularization scheme. This calculation allows us to derive the finite frequency correction to the DC Hall conductivity of a QAH insulator. We derive that the leading order AC correction contains a term proportional to the Chern number. This term originates from the Newtonian mass and can be measured via electrical or via magneto-optical experiments. The Newtonian mass, in particular, significantly changes the resonance structure of the AC Hall conductivity in comparison to pure Dirac systems like graphene. In addition, we study the effective action of the aforementioned Chern insulators in external out-of-plane magnetic fields. We show that as a consequence of the parity anomaly the QAH phase in (Hg,Mn)Te quantum wells or in magnetically doped (Bi,Sb)Te thin films survives in out-of-plane magnetic fields, violates the Onsager relation, and can therefore be distinguished from a conventional quantum Hall (QH) response. As a smoking-gun of the QAH phase in increasing magnetic fields, we predict a transition from a quantized Hall plateau with \(\sigma_\mathrm{xy}= -\mathrm{e}^2/\mathrm{h}\) to a not perfectly quantized plateau which is caused by scattering processes between counter-propagating QH and QAH edge states. This transition is expected to be of significant relevance in paramagnetic QAH insulators like (Hg,Mn)Te/CdTe quantum wells, in which the exchange interaction competes against the out-of-plane magnetic field. All of the aforementioned results do not incorporate finite temperature effects. In order to shed light on such phenomena, we further analyze the finite temperature Hall response of 2+1 dimensional Chern insulators under the combined influence of a chemical potential and an out-of-plane magnetic field. As we have mentioned above, this non-dissipative transport coefficient is directly related to the parity anomaly of planar quantum electrodynamics. Within the scope of our analysis we show that the parity anomaly itself is not renormalized by finite temperature effects. However, the parity anomaly induces two terms of different physical origin in the effective Chern-Simons action of a QAH insulator, which are directly proportional to its Hall conductivity. The first term is temperature and chemical potential independent and solely encodes the intrinsic topological response. The second term specifies the non-topological thermal response of conduction- and valence band modes, respectively. We show that the relativistic mass \(m\) of a Chern insulator counteracts finite temperature effects, whereas its non-relativistic Newtonian mass \(B \vert \mathbf{k} \vert^2 \) enhances these corrections. In addition, we are extending our associated analysis to finite out-of-plane magnetic fields, and relate the thermal response of a Chern insulator therein to the spectral asymmetry, which is a measure of the parity anomaly in out-of-plane magnetic fields. In the second part of this thesis, we study the hydrodynamic properties of two-dimensional electron systems with a broken time-reversal and parity symmetry. Within this analysis we are mainly focusing on the non-dissipative transport features originating from a peculiar hydrodynamic transport coefficient: The Hall viscosity \(\eta_\mathrm{H}\). In out-of-plane magnetic fields, the Hall viscous force directly competes with the Lorentz force, as both mechanisms contribute to the overall Hall voltage. In our theoretical considerations, we present a way of uniquely distinguishing these two contributions in a two-dimensional channel geometry by calculating their functional dependencies on all external parameters. We are in particular deriving that the ratio of the Hall viscous contribution to the Lorentz force contribution is negative and that its absolute value decreases with an increasing width, slip-length and carrier density. Instead, it increases with the electron-electron mean free path in the channel geometry considered. We show that in typical materials such as GaAs the Hall viscous contribution can dominate the Lorentz signal up to a few tens of millitesla until the total Hall voltage vanishes and eventually is exceeded by the Lorentz contribution. Last but not least, we derive that the total Hall electric field has a parabolic form originating from Lorentz effects. Most remarkably, the offset of this parabola is directly characterized by the Hall viscosity. Therefore, in summary, our results pave the way to measure and to identify the Hall viscosity via both global and local measurements of the entire Hall voltage.},
  subject      = {Anomalie},
  language  = {en}
}
@phdthesis{Mayer2023,
  author    = {Mayer, Julian Benedikt},
  title     = {Topological phases in Luttinger materials},
  doi       = {10.25972/OPUS-32736},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-327368},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {The hunt for topological materials is one of the main topics of recent research in condensed matter physics. We analyze the 4-band Luttinger model, which considers the total angular momentum \(j = 3/2\) hole states of many semiconductors. Our analysis shows that this model hosts a wide array of topological phases and allows analytical calculations of the related topological surface states. The existence of these surface states is highly desired due to their strong protection against perturbations. In the first part of the thesis, we predict the existence of either one or two two-dimensional (2D) surface states of topological origin in the three-dimensional (3D) quadratic-node semimetal phase of the Luttinger model, called the Luttinger semimetal phase. We associate the origin of these states with the inverted order of s and p-orbital states in the band structure and approximate chiral symmetry around the node. Hence, our findings are essential for many materials, including HgTe, α-Sn, and iridate compounds. Such materials are often modified with strain engineering by growing the crystal on a substrate with a different lattice constant, which adds a deformation potential to the electrons. While tensile strain is often used to drive such materials into a gapped topological insulator regime, we apply compressive strain to induce a topological semimetal regime. Here, we differentiate between Dirac and Weyl semimetals based on inversion and time-reversal symmetry being simultaneously present or not. One major part of this thesis is the theoretical study of the evolution of the Luttinger semimetal surface states in these topological semimetal phases. The relative strength of the compressive strain and typical bulk inversion asymmetry (BIA) terms allow the definition of a symmetry hierarchy in the system. The cubic symmetric \(O_h\) Luttinger model is the highest symmetry low-energy parent model. Since the BIA terms in the Weyl semimetal phase are small in most materials, we find a narrow energy and momentum range around the Weyl points where the surface states form Fermi arcs between two Weyl nodes with opposite chirality. Consequently, we see 2D momentum planes between the Weyl points, which can be considered as effective 2D Chern insulators with chiral edge states connecting the valence and conduction band in the bulk gap. Exceeding the range of the BIA terms, the compressive strain becomes dominating, and the system behaves like a Dirac semimetal with two doubly degenerate linear Dirac nodes in the band structure. For energies larger than the compressive strain strength, the quadratic terms in the Luttinger model dominate and surface band structure is indistinguishable from an unperturbed Luttinger semimetal. To conclude this symmetry hierarchy, we analyze the limit of the Luttinger model when the remote \(j = 1/2\) electron states show a considerable hybridization with the \(j = 3/2\) hole states around the Fermi level. Here, the Luttinger model is not valid anymore and one needs to consider more complicated models, like the 6-band Kane Hamiltonian. In the second part of this thesis, we analyze theoretically two different setups for s-wave superconductivity proximitized \(j = 3/2\) particles in Luttinger materials under a magnetic field. First, we explore a one-dimensional wire setup, where the intrinsic BIA of inversion asymmetric crystals opens a topological gap in the bulk states. In contrast to wires, modeled by a quadratic dispersion with Rashba or Dresselhaus spin-orbit coupling, we find two topological phase transitions due to the different effects of magnetic fields to \(|j_z| = 3/2\) heavy-hole (HH) and \(|j_z| = 1/2\) light-hole (LH) states. Second, we discuss a two-dimensional Josephson junction setup, where we find Andreev-bound states inside the superconducting gap. Here, the intrinsic spin-orbit coupling of the Luttinger model is sufficient to open a topological gap even in the presence of inversion symmetry. This originates from the hybridization of the light and heavy-hole bands in combination with the superconducting pairing. Consequently, both setups can form Majorana-bound states at the boundaries of the system. The existence of these states are highly relevant in the scientific community due to their nonabelian braiding statistics and stability against decoherence, making them a prime candidate for the realization of topological quantum computation. Majorana-bound states form at zero energy and are protected by the topological gap. We predict that our findings of the topological superconductor phase of the Luttinger model are valid for both semimetal and metal phases. Hence, our study is additionally relevant for metallic systems, like p-doped GaAs. This opens a new avenue for the search for topological superconductivity.},
  language  = {en}
}
@phdthesis{Zhao2024,
  author    = {Zhao, Suting},
  title     = {Symmetry Resolution of Entanglement in Holography},
  doi       = {10.25972/OPUS-36385},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-363854},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2024},
  abstract  = {This thesis investigates the charged moments and the symmetry-resolved entanglement entropy in the context of the AdS3/CFT2 duality. In the first part, I focus on the holographic U(1) Chern-Simons-Einstein gravity, a toy model of AdS3/CFT2 with U(1) Kac-Moody symmetry. I start with the vacuum background with a single entangling interval. I show that, apart from a partition function in the grand canonical ensemble, the charged moments can also be interpreted as the two-point function of vertex operators on the replica surface. For the holographic description, I propose a duality between the bulk U(1) Wilson line and the boundary vertex operators. I verify this duality by deriving the effective action for the Chern-Simons fields and comparing the result with the vertex correlator. In the twist field approach, I show that the charged moments are given by the correlation function of the charged twist operators and the additional background operators. To solve the correlation functions involved, I prove the factorization of the U(1) extended conformal block into a U(1) block and a Virasoro block. The general expression for the U(1) block is derived by directly summing over the current descendant states, and the result shows that it takes an identical form as the vertex correlators. This leads to the conclusion that the disjoint Wilson lines compute the neutral U(1) block. The final result for the symmetry-resolved entanglement entropy shows that it is always charge-independent in this model. In the second part, I study charged moments in higher spin holography, where the boundary theory is a CFT with W3 symmetry. I define the notion of the higher spin charged moments by introducing a spin-3 modular charge operator. Restricting to the vacuum background with a single entangling interval, I employ the grand canonical ensemble interpretation and calculate the charged moments via the known higher spin black hole solution. On the CFT side, I perform a perturbative expansion for the higher spin charged moments in terms of the connected correlation functions of the spin-3 modular charge operators. Using the recursion relation for the correlation functions of the W3 currents, I evaluate the charged moments up to the quartic order of the chemical potential. The final expression matches with the holographic result. My results both for U(1) Chern-Simons Einstein gravity and W3 higher spin gravity constitute novel checks of the AdS3/CFT2 correspondence.},
  subject      = {AdS-CFT-Korrespondenz},
  language  = {en}
}