Geometry-induced spin-filtering in photoemission maps from WTe2_2 surface states

We demonstrate that an important quantum material WTe$_2$ exhibits a new type of geometry-induced spin-filtering effect in photoemission, stemming from low symmetry that is responsible for its exotic transport properties. Through the laser-driven spin-polarized angle-resolved photoemission Fermi surface mapping, we showcase highly asymmetric spin textures of electrons photoemitted from the surface states of WTe$_2$. Such asymmetries are not present in the initial state spin textures, which are bound by the time-reversal and crystal lattice mirror plane symmetries. The findings are reproduced qualitatively by theoretical modeling within the one-step model photoemission formalism. The effect could be understood within the free-electron final state model as an interference due to emission from different atomic sites. The observed effect is a manifestation of time-reversal symmetry breaking of the initial state in the photoemission process, and as such it cannot be eliminated, but only its magnitude influenced, by special experimental geometries.Comment: 5 pages, 3 figure

Ramseyova teorie aneb příklady, které jsou pro počítač příliš složité

summary:Ramsey theory deals with problems asking for the minimum number of elements which guarantees a particular property. These include e.g. Party Problem, Happy End Problem or Van der Waerden Problem. Solving the problems of this kind is rather time-consuming for a computer, but the result can often be obtained by mathematical reasoning with pencil and paper. Ramsey theory was deeply developed by a famous Hungarian mathematician Pál Erdös

Physical and chemical properties of CeOx/SrTiO3(100) films

A study of epitaxial growth of CeOx on SrTiO3(100) surface in CeO2(100)/SrTiO3(100) orientation by reactive vapor deposition in oxygen atmosphere is desirable for widening the current knowledge in the field of ceria-based model catalysts. This work was aimed at determining physical parameters necessary for the growth of CeO2(100) films (substrate temperature, oxygen pressure and deposition rate), atomic structure of the CeO2(100) surface and structure-stoichiometry relation during reversible CeO2↔Ce2O3 transition. Surface science methods were used in order to follow the structural and chemical characteristics of prepared films, such as XPS, XPD, ISS and LEED. The morphology of the prepared films was examined in detail using scanning tunnelling microscope (STM). We also examined the interaction with platinum, which represents the next step towards designing a ceria based catalysts with low noble metal content. Also the redox interaction properties of CeOx and hydrogen were studied

Physical and chemical properties of CeOx/SrTiO3(100) films

Práce se zabývá růstem epitaxních vrstev CeOx na povrchu SrTiO3(100) v orientaci CeO2(100)/SrTiO3(100) připravených pomocí reaktivního napařování v kyslíkové atmosféře a je velmi žádoucí pro doplnění stávajících poznatků v oblasti studia modelových katalyzátorů na bázi oxidu ceru. Byly studovány fyzikální podmínky potřebné k růstu orientovaných vrstev CeO2(100), zejména vliv teploty substrátu při depozici, tlak kyslíku a rychlost depozice, dále atomární struktura povrchu CeOx(100) a vztah mezi strukturou a stechiometrií při reverzibilním přechodu mezi CeO2↔Ce2O3. Ke sledování strukturních a chemických vlastností připravených vrstev bylo využito metod povrchové analýzy XPS, ISS, LEED, XPD a UPS. Morfologie vrstev byla podrobněji zkoumána rastrovací tunelovou mikroskopií (STM). Dále byla zkoumána interakce těchto vrstev s platinou, což je zajímavé zejména z pohledu katalyzátorů s nízkým obsahem drahých kovů. Současně byly zkoumány oxidační a redukční vlastnosti vrstev CeOx při interakci s vodíkem.A study of epitaxial growth of CeOx on SrTiO3(100) surface in CeO2(100)/SrTiO3(100) orientation by reactive vapor deposition in oxygen atmosphere is desirable for widening the current knowledge in the field of ceria-based model catalysts. This work was aimed at determining physical parameters necessary for the growth of CeO2(100) films (substrate temperature, oxygen pressure and deposition rate), atomic structure of the CeO2(100) surface and structure-stoichiometry relation during reversible CeO2↔Ce2O3 transition. Surface science methods were used in order to follow the structural and chemical characteristics of prepared films, such as XPS, XPD, ISS and LEED. The morphology of the prepared films was examined in detail using scanning tunnelling microscope (STM). We also examined the interaction with platinum, which represents the next step towards designing a ceria based catalysts with low noble metal content. Also the redox interaction properties of CeOx and hydrogen were studied.Department of Surface and Plasma ScienceKatedra fyziky povrchů a plazmatuMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Spin and orbital nature of electronic structure in transition metal dichalcogenides

Chapitre 1: Développement actuelCe chapitre établit le concept général des TMDCs avec des potentielles applications et introduit toutes les connaissances contemporaines nécessaires sur WTe2, WSe2 et HfTe2 qui serviront de base à nos efforts supplémentaires pour élargir la compréhension des processus et propriétés physiques fondamentaux. Nous introduisons les informations structurelles pour chaque matériau. En particulier pour WTe2, le concept de semi-métal de Weyl de type II est décrit ainsi que les propriétés globales de la zone de Brillouin. Pour WSe2 nous introduisons le modèle minimaliste des points K / K´ proches du maximum de la bande de valence qui sera utilisé plus tard pour introduire une nouvelle observable physique - le «dichroïsme à inversion du temps». Enfin, le concept de signal de polarisation en spin dans les systèmes à symétrie d'inversion spacialle et d'inversion du temps sera développé dans le contexte de la structure de bande HfTe2.Chapitre 2: Outils théoriques pour sonder/analyser la structure de bandeDans ce chapitre, nous décrivons le concept de la méthode KKR de la fonction de Green à diffusion multiple. Nous commençons par l'équation de Dirac comme outil principal pour complètement décrire le système quantique. Son implémentation dans le code SPR-KKR sera détaillés par étapes individuelles. Graduellement nous parlerons de la théorie de la fonctionnelle de la densité (DFT), des équations de Kohn-Sham, des termes de corrélation-échange du potentiel, méthode des fonctions de Green et le modèle en une étape.Chapitre 3: Méthode expérimentaleCe chapitre commence par une description du processus de photoémission et des différences conceptuelles entre le modèle en trois étapes et le modèle en une étape. Nous introduisons ensuite les méthodes expérimentales que nous avons utilisé lors de l'acquisition des données - la spectroscopie de photoémission et son angle, spin et angle, modifications du temps et de l'angle avec introduction éventuelle du spin, du temps et de l'angle résolus spectroscopie de photoémission (STAR-PES).Chapitre 4: Résultats expérimentaux et théoriques - WTe2Dans le chapitre consacré à l'échantillon WTe2, nous nous sommes concentrés sur la mise en service de la technologie de pointe installation laser de génération qui, en combinaison avec la chambre expérimentale de photo-émission à résolution en spin et en angle, nous a permis non seulement à sonder/analyser l'évolution de la texture en spin du semi-métal au voisinage des supposés points de Weyl mais a également servi de preuve du principe de la méthode comme tel. Nous avons nommé cette nouvelle méthode de spectroscopie de photoémission résolues en spin, en temps et en angle (STAR-PES).Chapitre 5: Résultats expérimentaux et théoriques - WSe2Le but de notre effort était de trouver une méthode qui démêlerait la texture orbitale de la texture de rotation. Comme l’analyse de la texture orbitale avec une lumière polarisée linéairement est de trouver un échantillon hébergeant une opération de symétrie qui équivaudrait à l'opération d'inversion du temps, nous avons défini un nouveau observable "le Dichroïsme d'inversion de temps".Chapitre 6: Résultats expérimentaux et théoriques - HfTe2Et enfin, une fois que nous avons trouvé un outil pour analyser à la fois les textures orbitale et de spin du matériau nous avons tenté de décrire l'origine de la polarisation en spin dans un système où il n'y a pas d'autre origine du couplage spin-orbite que la cassure de la symétrie d'inversion spatiale induite par la surface comme le système que nous avons choisi a une polarisation en spin nulle des états initiaux en volume en raison à la fois de l'inversion et de la symétrie d'inversion temporelle de le groupe spatial pour lequel nous avons utilisé un autre TMDC approprié – HfTe2.• Chapter 1: Current developmentThis chapter establishes the general concept of TMDCs with possible applications and introduces all the necessities from the contemporary knowledge on WTe2, WSe2 and HfTe2 which will be used as a basis for our further efforts to expand the understanding of the fundamental physical processes and properties. We introduce structural information for each material. In particular, for WTe2 the concept of Weyl type II semimetal is described as well as the overall properties of the Brillouin zone. For WSe2 we introduce the minimalistic model of the K/K´points close to the valence band maximum which will later on be used in order to introduce a novel physical observable - "time-reversal dichroism". Lastly, the concept of spin polarization signal in systems with time-reversal and spatial inversion symmetry will be elaborated on in the context of HfTe2 bandstructure.• Chapter 2: Theoretical tools to probe the band structureIn this chapter we describe the concept of the multiple scattering Green´s function KKR method. We start with the Dirac equation as the main tool to fully describe the quantum system following with the individual steps which will speak about its implementation into the SPR-KKR code. Step by step we will speak about the density functional theory (DFT), Kohn Sham equations, exchange correlation terms of the potential, Green´s functions method, and one step model.• Chapter 3: Experimental MethodThis chapter starts with description of the photoemission process and the conceptual differences between the three-step and one-step models. We then introduce the experimental methods which we used for the data acquisition - photoemission spectroscopy and its angle, spin and angle, time andangle modifications with eventual introduction of the spin, time and angle- resolved photoemission spectroscopy (STAR-PES).• Chapter 4: Experimental and Theoretical Results - WTe2In the chapter related to WTe2 sample we focused on commissioning of the state of the art high harmonic generation laser facility which in combination with the spin and angle-resolved photoesmission experimental chamber allowed us not only to probe the evolution of the spin texture of the semimetal in the vicinity of supposed Weyl points but also served as a proof of principleof the method as such. We named this new method spin- time- and angle-resolved photoemission spectroscopy (STAR-PES).• Chapter 5: Experimental and Theoretical Results - WSe2Once we showed that this experimental configuration was capable of reliable probing of the spin texture of the material on the femtosecond scale we adjusted our focus on another sample - WSe2 with the largest known spin-splitting in the valence band among all TMDCs. The goal of our effortwas to find a method that would disentangle the orbital texture from the spin texture. Since a necessary requirement to probe the orbital texture with linearly polarized light is to find a sample symmetry operation which would be equivalent to the time-reversal operation, we defined a new physical observable "Time-Reversal Dichroism".• Chapter 6: Experimental and theoretical results - HfTe2And lastly, once we have found a tool to probe both orbital and spin texture of the material we have attempted to describe the origin of spin polarization in a system where there is no other origin of the spin-orbit coupling than breaking of the spatial inversion symmetry induced by the surface asthe system we chose has zero spin polarization of initial states in bulk due to both spatial inversion and time-reversal symmetries of the space group for which we used yet another suitable TMDC - HfTe2.The interpretation in the last three chapters would not have been possible without the theoreticalcalculations which we performed and which were necessary in all cases to explain the underlying physics

Processing of two-step γ cascade data from 151Eu(n,γ)152Eu reaction

1 Fotonové silové funkce, které v rámci statistického přístupu popisují rozdělení pravděpodobnosti vyzáření fotonu daným jádrem při deexcitaci ze vzbuzeného stavu, lze zkoumat například pomocí dvoukrokových gamma kaskád. Metoda dvoukrokových gamma kaskád byla poprvé představena v padesátých letech minulého století. Předmětem našeho zkoumání je studium dvoukrokových gamma kaskád následujících záchyt tepelného neutronu v reakci 151 Eu(n,γ)152 Eu. Experimentální data byla měřena na výzkumném reaktoru v Řeži u Prahy. Získaná emisní spektra z tepelného záchytu neutronu nejprve energeticky okalibrujeme, poté provedeme účinnostní kalibraci a po dalším zpracování porovnáme naměřená data s výsledky teoretických simulací.1 Photon strength functions in terms of statistical approach to a nuclear decay describe the probability distribution of photon emission by given nucleus. They can be studied using two step gamma cascades. The two step gamma cascade method was firstly introduced in fifties of the 20th century. Our aim is to study two step gamma cascades following thermal neutron capture in 151 Eu. The experiment was performed at the Nuclear Physics Institute ASCR in Řež near Prague. The acquired emission spectra of photons from thermal neutron capture will be processed by energetic and efficiency calibration and after further processing will be compared to the output of theoretical models.Institute of Particle and Nuclear PhysicsÚstav částicové a jaderné fyzikyMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Geometry-Induced Spin Filtering in Photoemission Maps from WTe 2 Surface States

We demonstrate that an important quantum material WTe2 exhibits a new type of geometry-induced spin filtering effect in photoemission, stemming from low symmetry that is responsible for its exotic transport properties. Through the laser-driven spin-polarized angle-resolved photoemission Fermi surface mapping, we showcase highly asymmetric spin textures of electrons photoemitted from the surface states of WTe2. Such asymmetries are not present in the initial state spin textures, which are bound by the time-reversal and crystal lattice mirror plane symmetries. The findings are reproduced qualitatively by theoretical modeling within the one-step model photoemission formalism. The effect could be understood within the free-electron final state model as an interference due to emission from different atomic sites. The observed effect is a manifestation of time-reversal symmetry breaking of the initial state in the photoemission process, and as such it cannot be eliminated, but only its magnitude influenced, by special experimental geometries

Assessing Nontrivial Topology in Weyl Semimetals by Dichroic Photoemission

The electronic structure of Weyl semimetals features Berry flux monopoles in the bulk and Fermi arcs at the surface. While angle-resolved photoelectron spectroscopy (ARPES) is successfully used to map the bulk and surface bands, it remains a challenge to explicitly resolve and pinpoint these topological features. Here we combine state-of-the-art photoemission theory and experiments over a wide range of excitation energies for the Weyl semimetals TaAs and TaP. Our results show that simple surface-band-counting schemes, proposed previously to identify nonzero Chern numbers, are ambiguous due to pronounced momentum-dependent spectral weight variations and the pronounced surface-bulk hybridization. Instead, our findings indicate that dichroic ARPES provides an improved approach to identify Fermi arcs but requires an accurate description of the photoelectron final state

Angle-resolved photoemission calculations of WTe2 compared to experiment

Molybdenum dichalcogenides are probably the most studied single layer TMDCs by virtue of being appealing for sundry possible applications suchlike transistors, diodes, solar cells or more fundamental studies of spin or valley pseudospin and their interactions. Tungsten-based counterparts are on the other hand evincing much stronger spin-orbit coupling due to which all the spin-related effects are more stable at room temperature and thus more feasible for application. WTe2, a type-II Weyl semimetal is in particular interesting due to having two pairs of spin-differentiated Weyl points above Fermi energy. We have conducted several experiments following the evolution of the band dispersion in the vicinity of X and Y points of the Brillouin zone of WTe2 which is substantial for understanding the fundamental properties of the structure-property relation of the system. Ab-initio set of photoemission calculations was performed using SPR-KKR package and compared to experimental results