Topological materials from a symmetry perspective

149 p.Esta tesis doctoral se ubica dentro del reciente campo de física topológica, un área que ha cobrado interés dentro de la física de la materia condensada. Se hace especial hincapié en la conexión entre simetría y topología. Primero, describimos el formalismo de Química Cuántica Topológica, que es capaz de predecir y diagnosticar propiedades topológicas con base en las simetrías del sistema y en cómo transforman las funciones de Bloch en la Zona de Brillouin. Después, aplicamos el formalismo a un material aislante concreto, PbTe, del que se predijo que podía tener estado anómalos en la superficie en 1986. Se encuentra que el modelo es topológico, pero que la topología del material es fuertemente dependiente del parámetro de red del material real. Se hace también un análisis detallado de la topología de CoS2, una pirita ferromagnética que ha sido estudiada durante décadas. Se encuentran nodos de Weyl y líneas nodales, con sus respectivos estados de superficie, `drumhead¿ en la proyección de las líneas nodales, y arcos de Fermi que conectan la proyección de los nodos de Weyl. Completamos el estudio con una comparativa entre nuestros cálculos y medidas experimentales. Por último, encontramos un término de viscosidad nuevo, puramente tridimensional, y lo calculamos en un modelo magnético quiral, con posibles aplicaciones en materiales reales

Large anomalous Hall, Nernst effect and topological phases in the 3d-4d/5d based oxide double perovskites

Magnetism and spin-orbit coupling are two fundamental and interconnected properties of oxide materials, that can give rise to various topological transport phenomena, including anomalous Hall and anomalous Nernst effects. These transport responses can be significantly enhanced by designing an electronic structure with a large Berry curvature. In this context, rocksalt-ordered double perovskites (DP), denoted as A2BB’O6, with two distinct transition metal sites are very powerful platforms for exploration and research. In this work, we present a comprehensive study based on the intrinsic anomalous transport in cubic and tetragonal stable DP compounds with 3d-4d/5d elements. Our findings reveal that certain DP compounds show a large anomalous Hall effect, displaying topological band crossings in the proximity of the Fermi energy

Monopole-like orbital-momentum locking and the induced orbital transport in topological chiral semimetals

The interplay between chirality and topology nurtures many exotic electronic properties. For instance, topological chiral semimetals display multifold chiral fermions that manifest nontrivial topological charge and spin texture. They are an ideal playground for exploring chirality-driven exotic physical phenomena. In this work, we reveal a monopole-like orbital-momentum locking texture on the three-dimensional Fermi surfaces of topological chiral semimetals with B20 structures (e.g., RhSi and PdGa). This orbital texture enables a large orbital Hall effect (OHE) and a giant orbital magnetoelectric (OME) effect in the presence of current flow. Different enantiomers exhibit the same OHE which can be converted to the spin Hall effect by spin-orbit coupling in materials. In contrast, the OME effect is chirality-dependent and much larger than its spin counterpart. Our work reveals the crucial role of orbital texture for understanding OHE and OME effects in topological chiral semimetals and paves the path for applications in orbitronics, spintronics, and enantiomer recognition.Comment: 23 pages, 5 figure

Spin-momentum locking from topological quantum chemistry: applications to multifold fermions

In spin-orbit coupled crystals, symmetries can protect multifold degeneracies with large Chern numbers and Brillouin zone spanning topological surface states. In this work, we explore the extent to which the nontrivial topology of chiral multifold fermions impacts the spin texture of bulk states. To do so, we formulate a definition of spin-momentum locking in terms of reduced density matrices. Using tools from the theory of topological quantum chemistry, we show how the reduced density matrix can be determined from the knowledge of the basis orbitals and band representation forming the multifold fermion. We show how on-site spin orbit coupling, crystal field splitting, and Wyckoff position multiplicity compete to determine the spin texture of states near chiral fermions. We compute the spin texture of multifold fermions in several representative examples from space groups $P432$ (207) and $P2_13$ (198). We show that the winding number of the spin around the Fermi surface can take many different integer values, from zero all the way to $\pm 7$. Finally, we conclude by showing how to apply our theory to real materials using the example of PtGa in space group $P2_13$.Comment: 28 pages, 6 figure

Atomically Sharp Internal Interface in a Chiral Weyl Semimetal Nanowire

Internal interfaces in Weyl semimetals (WSMs) are predicted to host distinct topological features that are different from the commonly studied external interfaces (crystal-to-vacuum boundaries). However, the lack of atomically sharp and crystallographically oriented internal interfaces in WSMs makes it difficult to experimentally investigate hidden topological states buried inside the material. Here, we study a unique internal interface known as merohedral twin boundary in chemically synthesized single-crystal nanowires (NWs) of CoSi, a chiral WSM of space group P213 (No. 198). High resolution scanning transmission electron microscopy reveals that this internal interface is (001) twin plane and connects two enantiomeric counterparts at an atomically sharp interface with inversion twinning. Ab-initio calculations show localized internal Fermi arcs at the (001) twin boundary that can be clearly distinguished from both external Fermi arcs and bulk states. These merohedrally twinned CoSi NWs provide an ideal material system to probe unexplored topological properties associated with internal interfaces in WSMs.Comment: 19 pages, 4 figure

Spectral and optical properties of Ag3Au(Se2,Te2) and dark matterdetection

Paper • The following article is Open access Spectral and optical properties of Ag3Au(Se2,Te2) and dark matter detection M-Á Sánchez-Martínez6,1 , I Robredo6,2,3, A Bidaurrazaga3, A Bergara2,3,4, F de Juan2,5, A G Grushin1 and M G Vergniory7,2,5 Published 29 October 2019 • © 2019 The Author(s). Published by IOP Publishing Ltd Journal of Physics: Materials, Volume 3, Number 1 Focus on Topological Matter Citation M-Á Sánchez-Martínez et al 2020 J. Phys. Mater. 3 014001 Download Article PDF Figures References 692 Total downloads 4 4 total citations on Dimensions. Turn on MathJax Share this article Share this content via email Share on Facebook Share on Twitter Share on Google+ Share on Mendeley Article information Abstract In this work we study the electronic structure of ${\mathrm{Ag}}_{3}{\mathrm{AuSe}}_{2}$ and ${\mathrm{Ag}}_{3}{\mathrm{AuTe}}_{2}$, two chiral insulators whose gap can be tuned through small changes in the lattice parameter by applying hydrostatic pressure or choosing different growth protocols. Based on first principles calculations we compute their band structure for different values of the lattice parameters and show that while ${\mathrm{Ag}}_{3}{\mathrm{AuSe}}_{2}$ retains its direct narrow gap at the Γ point, ${\mathrm{Ag}}_{3}{\mathrm{AuTe}}_{2}$ can turn into a metal. Focusing on ${\mathrm{Ag}}_{3}{\mathrm{AuSe}}_{2}$ we derive a low energy model around Γ using group theory, which we use to calculate the optical conductivity for different values of the lattice constant. We discuss our results in the context of detection of light dark matter particles, which have masses of the order of a keV, and conclude that ${\mathrm{Ag}}_{3}{\mathrm{AuSe}}_{2}$ satisfies three important requirements for a suitable detector: small Fermi velocities, meV band gap, and low photon screening. Our work motivates the growth of high-quality and large samples of ${\mathrm{Ag}}_{3}{\mathrm{AuSe}}_{2}$ to be used as target materials in dark matter detectors.We acknowledge support from the European Union's Horizon 2020 research and innovation programme under the Marie-Sklodowska-Curie grant agreement No. 754303 and the GreQuE Cofund programme (MASM). AGG is also supported by the ANR under the grant ANR-18-CE30-0001-01 and the European FET-OPEN SCHINES project No. 829044. MGV acknowledges the IS2016-75862-P national project of the Spanish MINECO. AB acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13)

Unconventional Charge-to-Spin Conversion in Graphene/MoTe2 van der Waals Heterostructures

Spin-charge interconversion (SCI) is a central phenomenon to the development of spintronic devices from materials with strong spin-orbit coupling (SOC). In the case of materials with high crystal symmetry, the only allowed SCI processes are those where the spin-current, charge-current, and spin-polarization directions are orthogonal to each other. Consequently, standard SCI experiments are designed to maximize the signals arising from the SCI processes with conventional mutually orthogonal geometry. However, in low-symmetry materials, certain nonorthogonal SCI processes are also allowed. Since the standard SCI experiment is limited to charge current flowing only in one direction in the SOC material, certain allowed SCI configurations remain unexplored. Here, we perform a thorough SCI study in a graphene-based lateral spin valve combined with low-symmetry MoTe2. Due to a very low contact resistance between the two materials, we can detect SCI signals using both a standard configuration, where the charge current is applied along MoTe2, and a recently introduced [three-dimensional- (3D) current] configuration, where the charge-current flow can be controlled in three directions within the heterostructure. As a result, we observe three different SCI components, one orthogonal and two nonorthogonal, adding valuable insight into the SCI processes in low-symmetry materials. The large SCI signals obtained at room temperature, along with the versatility of the 3D-current configuration, provide feasibility and flexibility to the design of the next generation of spin-based devices.This work is supported by the Spanish MICINN under Projects No. RTI2018-094861-B-I00, No. PGC2018-101988-B-C21, No. PID2019-109905GB-C21, No. MAT2017-88377-C2-2-R, and the Maria de Maeztu Units of Excellence Programme (Grants No. MDM-2016-0618 and No. CEX2020-001038-M); the “Valleytronics” Intel Science Technology Center; the Gipuzkoa Regional Council under Projects No. 2021-CIEN-000037-01 and No. 2021-CIEN-000070-01; and the European Union H2020 under the Marie Sklodowska-Curie Actions (Grants No. 0766025-QuESTech and No. 794982-2DSTOP). N.O. thanks the Spanish MICINN for support from a Ph.D. fellowship (Grant No. BES-2017-07963). J.I.-A. acknowledges support from the “Juan de la Cierva-Formación” program by the Spanish MICINN (Grant No. FJC2018-038688-I) for a postdoctoral fellowship. R.C. acknowledges funding from Generalitat Valenciana through Grant No. CIDEGENT/2018/004 M.G.V. and I.R. thanks support from the Spanish MICINN (grant PID2019-109905GBC21), the German Research Foundation DFG (grant nr. GA3314/1-1-FOR 5249 QUAST) and the European Research Council ERC (Grant No. 101020833)

Weyl-fermions, Fermi-arcs, and minority-spin carriers in ferromagnetic CoS2

The pyrite compound CoS2 has been intensively studied in the past due to its itinerant ferromagnetism and potential for half-metallicity, which make it a promising material for spintronic applications. However, its electronic structure remains only poorly understood. Here we use complementary bulk- and surface-sensitive angle-resolved photoelectron spectroscopy and ab-initio calculations to provide a complete picture of its band structure. We discover Weyl-cones at the Fermi-level, which presents CoS2 in a new light as a rare member of the recently discovered class of magnetic topological metals. We directly observe the topological Fermi-arc surface states that link the Weyl-nodes, which will influence the performance of CoS2 as a spin-injector by modifying its spin-polarization at interfaces. Additionally, we are for the first time able to directly observe a minority-spin bulk electron pocket in the corner of the Brillouin zone, which proves that CoS2 cannot be a true half-metal. Beyond settling the longstanding debate about half-metallicity in CoS2, our results provide a prime example of how the topology of magnetic materials can affect their use in spintronic applications

Controllable orbital angular momentum monopoles in chiral topological semimetals

The emerging field of orbitronics aims at generating and controlling currents of electronic orbital angular momentum (OAM) for information processing. Structurally chiral topological crystals could be particularly suitable orbitronic materials because they have been predicted to host topological band degeneracies in reciprocal space that are monopoles of OAM. Around such a monopole, the OAM is locked isotopically parallel or antiparallel to the direction of the electron's momentum, which could be used to generate large and controllable OAM currents. However, OAM monopoles have not yet been directly observed in chiral crystals, and no handle to control their polarity has been discovered. Here, we use circular dichroism in angle-resolved photoelectron spectroscopy (CD-ARPES) to image OAM monopoles in the chiral topological semimetals PtGa and PdGa. Moreover, we also demonstrate that the polarity of the monopole can be controlled via the structural handedness of the host crystal by imaging OAM monopoles and anti-monopoles in the two enantiomers of PdGa, respectively. For most photon energies used in our study, we observe a sign change in the CD-ARPES spectrum when comparing positive and negative momenta along the light direction near the topological degeneracy. This is consistent with the conventional view that CD-ARPES measures the projection of the OAM monopole along the photon momentum. For some photon energies, however, this sign change disappears, which can be understood from our numerical simulations as the interference of polar atomic OAM contributions, consistent with the presence of OAM monopoles. Our results highlight the potential of chiral crystals for orbitronic device applications, and our methodology could enable the discovery of even more complicated nodal OAM textures that could be exploited for orbitronics.Comment: 16 pages, 8 figure