Marco Grioni

Biographie

Marco Grioni est né à Milan (Italie). Après une thèse au Politecnico di Milano en 1982, il occupe des positions de recherche à Minneapolis (USA), Nijmegen (Pays Bas), Orsay (France) et Neuchâtel, où il devient Privat Docent en Physique du Solide en 1994. A l'EPFL depuis 1996, il est nommé professeur titulaire en 2005. Il enseigne la physique générale et dirige actuellement le Laboratoire de Spectroscopie Electronique de l'Institut de Physique.

Ses recherches portent sur les propriétés électroniques de nouveaux matériaux tels que les supraconducteurs à haute température, les métaux à basse dimension et les fermions lourds, qu'il étudie par la spectroscopie des photoélectrons (ARPES) à très haute résolution, par ultrafast time-resolved ARPES, et par d'autres techniques utilisant le rayonnement synchrotron. Auteur d'env. 220 publications, il participe activement au niveau suisse et européen au développement de nouveaux instruments. Entre 2003 et 2006 il a été chairman de l'organisation des utilisateurs du synchrotron européen de Grenoble (ESRF). De 2010 à 2017 il a été chairman du Scientific Advisory Committee du synchrotron national français SOLEIL.

Official source

https://people.epfl.ch/marco.grioni?lang=fr

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Hidden bulk and surface effects in the spin polarization of the nodal-line semimetal ZrSiTe

Helmuth Berger, Alberto Crepaldi, Mauro Fanciulli, Gianmarco Gatti, Daniel Gosalbez Martinez, Marco Grioni, Arnaud Magrez, Oleg Yazyev

Local inversion symmetry breaking in centrosymmetric materials can lead to large spin polarization of the electronic band structure in separate sectors of the unit cell. Here, the authors reveal such hidden spin polarisation in ZrSiTe using spin and angle resolved photoemission spectroscopy in combination with ab initio band structure calculations and investigate the resultant spin polarised bulk and surface properties In non-magnetic materials the combination of inversion symmetry breaking (ISB) and spin-orbit coupling (SOC) determines the spin polarization of the band structure. However, a local spin polarization can also arise in centrosymmetric crystals containing ISB subunits. This is namely the case for the nodal-line semimetal ZrSiTe where, by combining spin- and angle-resolved photoelectron spectroscopy with ab initio band structure calculations, we reveal a complex spin polarization. In the bulk, the valence and conduction bands exhibit opposite spin orientations in two spatially separated two-dimensional ZrTe sectors within the unit cell, yielding no net polarization. We also observe spin-polarized surface states that are well separated in energy and momentum from the bulk bands. A layer-by-layer analysis of the spin polarization allows us to unveil the complex evolution of the signal in the bulk states near the surface, thus bringing the intertwined nature of surface and bulk effects to the fore.

NATURE RESEARCH2021

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Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr3 by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass of 0.26 +/- 0.02 m(e), 50% heavier than the bare mass m(0) = 0.17 m(e) predicted by density functional theory. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Frohlich coupling parameter alpha = 1.81. A good agreement with our experimental data is obtained within the Feynman polaron model, validating a viable theoretical method to predict the carrier effective mass of LHPs ab initio.

AMER PHYSICAL SOC2020

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Metal monochalcogenides (MX) have recently been rediscovered as two-dimensional materials with electronic properties highly dependent on the number of layers. Although some intriguing properties appear in the few-layer regime, the carrier mobility of MX compounds increases with the number of layers, motivating the interest in multilayered heterostructures or bulk materials. By means of angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory calculations, we compare the electronic band structure of bulk epsilon-GaSe and epsilon-InSe semiconductors. We focus our attention on the top valence band of the two compounds along main symmetry directions, discussing the effect of spin-orbit coupling and contributions from post-transition-metal (Ga or In) and Se atoms. Our results show that the top valence band at Gamma point is dominated by Se p(z) states, while the main effect of Ga or In appears more deeply in binding energy, at the Brillouin zone corners, and in the conduction band. These findings explain also the experimental observation of a hole effective mass rather insensitive to the post-transition metal. Finally, by means of spin-resolved ARPES and surface band structure calculations we describe Rashba-Bychkov spin splitting of surface states in epsilon-InSe.

AMER PHYSICAL SOC2020

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