Non-trivial Surface-band Dispersion on Bi(111)

We performed angle-resolved photoelectron spectroscopy of the Bi(111) surface to demonstrate that this surface support edge states of non-trivial topology. Along the $\bar{\Gamma}\bar{M}$-direction of the surface Brillouin zone, a surface-state band disperses from the projected bulk valence bands at $\bar{\Gamma}$ to the conduction bands at $\bar{M}$ continuously, indicating the non-trivial topological order of three-dimensional Bi bands. We ascribe this finding to the absence of band inversion at the $L$ point of the bulk Bi Brillouin zone. According to our analysis, a modification of tight-binding parameters can account for the non-trivial band structure of Bi without any other significant change on other physical properties.Comment: 13 pages, 4 figures. This manuscript has been accepted in New Journal of Physic

Origin of the different electronic structure of Rh- and Ru-doped Sr2IrO4

One way to induce insulator to metal transitions in the spin-orbit Mott insulator Sr2IrO4 is to substitute iridium with transition metals (Ru, Rh). However, this creates intriguing inhomogeneous metallic states, which cannot be described by a simple doping effect. We detail the electronic structure of the Ru-doped case with angle-resolved photoemission and show that, contrary to Rh, it cannot be connected to the undoped case by a rigid shift. We further identify bands below $E_F$ coexisting with the metallic ones that we assign to non-bonding Ir sites. We rationalize the differences between Rh and Ru by a different hybridization with oxygen, which mediates the coupling to Ir and sensitively affects the effective doping. We argue that the spin-orbit coupling does not control neither the charge transfer nor the transition threshold

Symmetry of the Fermi surface and evolution of the electronic structure across the paramagnetic-helimagnetic transition in MnSi/Si(111)

MnSi has been extensively studied for five decades, nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevented from a comprehensive understanding the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS symmetry and the evolution of the electronic structure across the paramagnetic-helimagnetic transition at T$_C$ $\sim$ 40 K, along with the appearance of sharp quasiparticle emission below T$_C$. The shape of the resulting FS is found to fulfill robust nesting effects. These effects can be at the origin of strong magnetic fluctuations not accounted for by state-of-art quasiparticle self-consistent GW approximation. From this perspective, the unforeseen quasiparticle damping detected in the paramagnetic phase and relaxing only below T$_C$, along with the persistence of the d-bands splitting well above T$_C$, at odds with a simple Stoner model for itinerant magnetism, open the search for exotic magnetic interactions favored by FS nesting and affecting the quasiparticles lifetime

First direct observation of a nearly ideal graphene band structure

Angle-resolved photoemission and X-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(000-1) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films cause adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K-point. Each cone corresponds to an individual macro-scale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.Comment: 5 pages, 4 figure

Le site aurignacien de plein-air de Combemenue à Brignac-la-Plaine (Corrèze) : apport de la géochéologie et de l’étude de l’industrie lithique à la compréhension des processus taphonomiques

Le site paléolithique de Combemenue en Corrèze a livré un niveau d’occupation d’Aurignacien récent enfoui à faible profondeur, sur un replat près du sommet d’un versant. Le contexte de faible enfouissement laissant suspecter des perturbations significatives du niveau archéologique dues à une longue exposition aux agents naturels en surface ou en subsurface du sol, une étude taphonomique détaillée a été entreprise. Différents points ont été examinés : la distribution spatiale du matériel, la disposition des objets (fabrique), leur granulométrie, leur état de surface ainsi que les remontages. Les résultats obtenus ont été confrontés de manière à proposer un scénario qui rende compte au mieux de l’ensemble des observations faites sur le site. Cette étude indique que l’assemblage lithique originel a subi un appauvrissement sélectif en petits éléments sous l’action du ruissellement. Simultanément, il est possible qu’une partie du matériel de plus grande taille initialement présent ait été soustrait du site par les mécanismes sédimentaires. Ces modifications ont eu des répercussions sur la distribution spatiale des vestiges. Celle-ci se caractérise par une absence de concentration claire, tant lorsqu’on considère la répartition de l’ensemble du matériel que celle de catégories particulières d’objets. Une diffusion progressive des vestiges par le ruissellement ou les phénomènes périglaciaires rend bien compte des transformations observées. En revanche, les déplacements n’ont pas entraîné d’altération physique importante des pièces, dont la majorité ne porte pas de stigmate postérieur à sa production ou son utilisation par les Aurignaciens. Les transformations décrites ici pour le site de Combemenue sont probablement représentatives de celles subies par un grand nombre de sites paléolithiques localisés sur une pente négligeable dans un contexte géomorphologique peu favorable à un enfouissement rapide.An Upper Aurignacian level located on a hilltop flat surface has been found at Combemenue (Brignac-la-Plaine, Corrèze, France) during field surveys along the A89 highway. Geomorphological evidence, and particularly the shallow depth of the level, strongly suggests that it has undergone significant perturbation due to a long-lasting exposure to sedimentary and pedological processes at the soil surface or subsurface. Therefore, a detailed taphonomic study has been made to evaluate the impact of natural processes in site patterning. It involves examination of the spatial distribution of the artefacts, their orientation and dip (fabric), their size distribution, their alteration and refits. The results have been compared to the data derived from the techno-economical analysis. This study indicates that the lithic assemblage has undergone a selective impoverishment in small-sized artefacts as the result of surface wash. A part of the coarser artefacts may have been also removed from the site by the same processes. The lack of any clear artefact concentration indicates that the spatial distribution has been modified significantly. This can be adequately described by a model of progressive diffusion of the artefacts by surface wash or periglacial processes. By contrast, the movements did not provoke strong physical alteration of the pieces. The Combemenue site is thought to be representative of a number of palaeolithic sites located in similar geomorphological contexts, which are characterized by low sedimentation rates

Effect of the valence state on the band magnetocrystalline anisotropy in two-dimensional rare-earth/noble-metal compounds

[EN] In intermetallic compounds with zero orbital momentum (L = 0) the magnetic anisotropy and the electronic band structure are interconnected. Here, we investigate this connection in divalent Eu and trivalent Gd intermetallic compounds. We find by x-ray magnetic circular dichroism an out-of-plane easy magnetization axis in two-dimensional atom-thick EuAu2. Angle-resolved photoemission spectroscopy and density-functional theory prove that this is due to strong f-d band hybridization and Eu2+ valence. In contrast, the easy in-plane magnetization of the structurally equivalent GdAu2 is ruled by spin-orbit-split d bands, notably Weyl nodal lines, occupied in the Gd3+ state. Regardless of the L value, we predict a similar itinerant electron contribution to the anisotropy of analogous compounds.Discussions with the late J. I. Cerda are warmly thanked. Financial support from Spanish Ministerio deCiencia e Innovacion (projects MAT-2017-88374-P, PID2020-116093RB-C44 and PID2019-103910GB-I00 funded by MCIN/AEI/10.13039/501100011033/) , the Basque Govern-ment (Grants No. IT-1255-19 and No. IT1260-19) , and the University of the Basque Country UPV/EHU (Grant No. GIU18/138) is acknowledged. L.F. acknowledges funding from the European Union's Horizon 2020 research and in-novation programme through the Marie Skodowska-Curie Grant Agreement MagicFACE No. 797109. We acknowl-edge SOLEIL for provision of synchrotron radiation facilities at CASSIOPEE beamline under proposal 20181362. The XMCD experiments were performed at BOREAS beamline at ALBA Synchrotron with the collaboration of ALBA staff. Computational resources were provided by DIPC