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We combined a spin-resolved photoemission spectrometer with a high-harmonic generation (HHG) laser source in order to perform spin-, time- and angle-resolved photoemission spectroscopy (STARPES) experiments on the transition metal dichalcogenide bulk WTe$_2$, a possible Weyl type-II semimetal. Measurements at different femtosecond pump-probe delays and comparison with spin-resolved one-step photoemission calculations provide insight into the spin polarization of electrons above the Fermi level in the region where Weyl points of WTe$_2$ are expected. We observe a spin accumulation above the Weyl points region, that is consistent with a spin-selective bottleneck effect due to the presence of spin polarized cone-like electronic structure. Our results support the feasibility of STARPES with HHG, which despite being experimentally challenging provides a unique way to study spin dynamics in photoemission., 6 pages, 4 figures

The growth of Bi on both the In-terminated (A) face and the As-terminated (B) face of InAs(1 1 1) has been investigated by low-energy electron diffraction, scanning tunnelling microscopy, and photoelectron spectroscopy using synchrotron radiation. The changes upon Bi deposition of the In 4d and Bi 5d 5/2 photoelectron signals allow to get a comprehensive picture of the Bi/InAs(1 1 1) interface. From the early stage the Bi growth on the A face is epitaxial, contrary to that on the B face that proceeds via the formation of islands. Angle-resolved photoelectron spectra show that the electronic structure of a Bi deposit of ≈10 bi-layers on the A face is identical to that of bulk Bi, while more than ≈30 bi-layers are needed for the B face. Both bulk and surface electronic states observed are well accounted for by fully relativistic ab initio calculations performed using the one-step model of photoemission. These calculations are used to analyse the dichroic photoemission data recorded in the vicinity of the Fermi level around the Γ ¯ point of the Brillouin zone.

APCOM2018 - 24th International conference Applied Physics of Condensed Matter, Štrbské Pleso, Slovaquie, 20 to 22/06/2018"Solid State Physics and Radioactive Irradiation"; International audience; Thin Bi films are interesting candidates for spintronic applications due to a large spin-orbit splitting that, combined with the loss of inversion symmetry at the surface, results in a band structure that is not spin-degenerate. In recent years, applications for topological insulators based on Bi and Bi alloys have as well attracted much attention. Here we present angle-resolved photoemission spectroscopy studies of the Bi/InAs(100) interface. Bismuth deposition followed by annealing of the surface results in the formation of one full Bi monolayer decorated by Bi nanolines. We found that the building up of the interface does not affect the electronic structure of the substrate. As a consequence of weak interaction, Bi states are placed in the gaps of the electronic structure of InAs(100). We observe a strong resonance of the Bi electronic states close to the Fermi level; its intensity depends on the photon energy and the photon polarization. These states show nearly no dispersion when measured perpendicular to the nanolines, confirming their onedimensionality.

The various physical properties of transition metal oxides make them the good candidates toward possible new industrial applications, and more precisely in the novel photovoltaic productions. In fact, the rich fundamental electronic properties of transition metal oxides essentially arise from strong Coulomb interactions within the electrons of the system. Understanding of such correlation effects continues to be challenging for modern solid state physics, which make the combination of experimental and theoretical data essential for further progress in the field. Spectroscopic investigations of the prototypical correlated SrTiO3 thin films doped with several transition metal (TM) ions (TM=Ni, Fe and Cr) are the main settings of this thesis. With the combination of core-level photoelectron spectroscopy (XPS), angle-resolved photoelectron spectrocopy (ARPES) and photoelectron diffraction (XPD) on one side; and first-principle calculations by means of multiple scattering theory on the other side, a good understanding of the doping on the structural and electronic properties has been here achieved. In SrTiO3 polycrystalline thin films, grown by magnetron sputtering method, the mean size of the crystallites increases with TM-doping content. This finding was the motivation to start further studies and get a better conception of the fundamental electronic band structure of transition metal doped SrTiO3 films. For this reason, high quality of ordered pristine andNi/Fe/Cr-doped SrTiO3 films were epitaxially prepared by pulsed laser deposition (PLD). The electronic band structure calculations for the ground state, as well as one-step model of photoemission calculations, which were obtained by means of Korringa-Khon-Rostoker (KKR) Greens’s function method, predicted the formation of localised 3d-impurity bands in the band gap of SrTiO3 close to the valence band maxima. The measured band dispersion and valence bands at the resonance confirmed these findings. The two-dimensional electron gas (2DEG), observed at the surface of SrTiO3 doped with Ni, by means of resonant-ARPES, can be indirectly controlled by the Ni content in the films. Ni doping seems to be affecting the tetragonal distortion of the TiO6 octahedron and results in changes of the structure symmetry and the energy splitting of the Ti 3d surface localized bands. The XPD measurements confirmed the 4-fold symmetry of Ni-doped SrTiO3 film’s structure, and a change in the lattice parameter compared to pure SrTiO3 films was detected. Ni impurity atoms incorporate the hosting system by substituting the Ti sites., Les divers propriétés physiques des oxydes de métaux de transitions font d’eux de bons candidats pour de potentiels nouvelles applications industrielles, et plus précisement dans de nouvelles productions photovoltaiques. Concrètement, les riches propriétés électroniques fondamentales proviennent de fortes interactions de Coulomb au sein des électrons du système. La compréhension de tels effets corrélés continue un défit pour la physique du solide moderne, ce qui signifie qu’une combinaison des faces expérimentale et théorique de la recherche est essentielle pour progrésser dans ce domaine. Les études spéctroscopiques des prototypes des couches minces corrélées de SrTiO3dopées avec divers ions de métaux de transitions (MT) (e.g. Ni, Fe ou Cr) sont la colonne vértébrale de cette thèse. Avec la combinaison de la photoémission spectroscopique des niveaux de coeurs, la photoémission résolue en angle (ARPES) et la photoémission de diffraction (XPD) d’une part; et les calculs de principe-premier par le biais de la théorie des diffusions multiples d’une autre part, une bonne compréhension du dopage de la structue et des propiétés électroniques a été obtenu ici. Dans les couches minces polycristallines de SrTiO3, crû par pulvérisation magnétron, la taille moyenne des cristaux augmentent avec le dopage aux MT. Cette découverte était une motivation pour étudier et approfondir la connaissance des structures de bandes électroniques fondamentales du SrTiO3 dopé aux MTs. Pour cette raison, haute qualité des couches minces monocrystallines de pure SrTiO3 et de SrTiO3 dopé avec Ni, Fe et Cr ont été préparer en epitaxie par ablation laser pulsé (PLD). Les calculs des structures de bandes électroniques des états fondamentaux, aussi que les calculs de principe-premier de photoémission obtenu par le biais de la méthode Korringa- Khon-Rostoker (KKR), ont prédit la formation des états localisés 3d liés aux impuretés dans la bande de gap du SrTiO3 et proche du maximum de la bande de valence. Les mesures de la dispersion des bandes électroniques et les bandes de valences à la résonance confirment nos résultats. Le gaz bidimensionnelle (2DEG), vu à la surface du SrTiO3 dopé au Ni, par la résonance-ARPES, peut être indirectement contrôlé par le dopage et sa concentration dans les couches.Le dopage au Ni semble modifier la distortion de l’octaèdre TiO6, ce qui abouti à des changements dans la symmetrie de la structure et la partition des sous-bandes de Ti 3d localisées à la surface. Les mesures XPD confirment la symmetrie d’ordre quatre de la structure des couches minces SrTiO3 dopé au Ni, et un changement du paramètre de maille par raport a celui du pure SrTiO3 a été détecté. Les atomes Ni intègrent le système de base en se substituant aux sites des Ti.