Fermiologie, réduction de dimensionalité et couplage électron-phonon dans les métaux de transition dichalcogénides
Auteur / Autrice : | Zakariae El Youbi |
Direction : | Karol Hricovini, Christine Richter, Cephise Cacho |
Type : | Thèse de doctorat |
Discipline(s) : | Physique - Cergy |
Date : | Soutenance le 12/03/2021 |
Etablissement(s) : | CY Cergy Paris Université |
Ecole(s) doctorale(s) : | École doctorale Sciences et ingénierie (Cergy-Pontoise, Val d'Oise) |
Partenaire(s) de recherche : | Laboratoire : Laboratoire de Physique des Matériaux et des Surfaces (Cergy-Pontoise, Val d'Oise) - Laboratoire de Physique des Matériaux et des Surfaces / LPMS |
Jury : | Examinateurs / Examinatrices : Karol Hricovini, Christine Richter, Cephise Cacho, Pavel Dudin, Neil Wilson, Christine Boeglin, Véronique Brouet, Matthew D. Watson |
Rapporteurs / Rapporteuses : Pavel Dudin, Neil Wilson |
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Résumé
The dosing of layered materials with alkali metals has become a commonly usedstrategy in ARPES experiments. However, precisely what occurs under such conditions, both structurally and electronically, has remained a matter of debate. Here we performed a systematic study of 1T-HfTe2, a prototypical semimetal of the transition metal dichalcogenide family. By utilizing photon energy-dependent angle-resolved photoemission spectroscopy (ARPES), we have investigated the electronic structure of this material as a function of Potassium (K) deposition. From the kz maps, we observed the appearance of 2D dispersive bands after electron dosing, with an increasing sharpness of the bands, consistent with the wavefunction confinement at the topmost layer. In our highest-dosing cases, a monolayer-like electronic structure emerges, presumably as a result of intercalation of the alkali metal. Here, by bringing the topmost valence band below EF, we could directly measure a band overlap of ∼ 0.2 eV. However, 3D bulk-like states still contribute to the spectra even after considerable dosing. Our work on HfTe2 provides a reference point for the increasingly popular studies of the alkali metal dosing of semimetals using ARPES. In our second work, we investigated the electronic band structure of the two polytypes of NbS2, namely 2H-NbS2 and 3R-Nb1+xS2 combining ARPES and DFT calculations again. The measured Fermi surfaces show a remarkable difference in size, reflecting a significantly increased band filling in 3R-Nb1+xS2 due to the presence of additional Nb interstitials. Thus we found that the stoichiometry, rather than the stacking arrangement, is the most important factor in the difference in electronic and physical properties of the two phases. Our high resolution data on the 2H phase shows strong kinks in the spectral function that are fingerprints of the electron-phonon coupling. However, the strength of the coupling is found to be much larger for the the sections of bands withNb 4dx2-y2,xy character than for the Nb 4d3z2-r2. Our results provide an experimental framework for interpreting the two-gap superconductivity and latent CDW in 2H-NbS2