INSP - UPMC - 4 place Jussieu - 75005 Paris - Barre 22-23, 3e étage, salle 317
Matteo Calandra, Directeur de recherche au CNRS, IMPMC, Université Pierre et Marie Curie - Jussieu Paris
During decades, the exploration of physics in reduced dimension has been mostly speculative as it was difficult to achieve experimentally the 2D limit. However, the scenario completely changed in 2004 with the discovery of graphene. In the last twelve years, the interest in two-dimensional crystals  composed of few atomic layers (nanolayers) has grown enormously mainly because large size 2D samples can be obtained by mechanical and liquid exfoliation or by epitaxial and CVD growth.
2D films open new perspectives as they allow for a different and more efficient control of charge carriers, either by electrochemical doping and intercalation via field effect or by deposition of foreign atoms (such as alkali atoms or alkaline earths). Control of charge carriers means a fine tuning of the interplay between the electron-electron and the electron-phonon interactions, leading to stabilization of unexpected long-range ordered phases.
However, despite the fact that 2D materials are very versatile, sample characterization of these crystals is often difficult and requires a different approach from conventional X-ray and Neutron diffraction, routinely performed in bulk samples but hardly feasible in 2D crystal. For this reason, a synergy between high-resolution Raman and optical spectroscopy, STM and transport measurements and advanced theoretical modeling is crucial to characterize and understand structural, electronic and vibrational properties of 2D materials.
In this talk, I will demonstrate via practical examples from my work, the key role of first-principles electronic structure calculations in (i) characterizing samples, (ii) understanding the physics of nanolayers and, finally, (iii) in devising new 2D materials with tailored physical properties. I will first consider rhombohedral stacked multilayer graphene [2,3]. I will show what are the Raman fingerprints of the rhombohedral stacking  and why a magnetic state can be stabilized despite the lack of d-states . I will then present a first-principle approach to describe the electronic structure and transport properties in field-effect configuration and its application in few-layer transition metal dichalcogenides [6,7]. Finally, I will show experimental data confirming the theoretical prediction of superconductivity in alkali and alkaline-earth decorated graphene .
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 D. Pierucci et al., Evidence for Flat Bands near the Fermi Level in Epitaxial Rhombohedral Multilayer Graphene, ACS Nano, 9, 5432 (2015).
 Y. Henni, et al., Rhombohedral Multilayer Graphene : A Magneto-Raman Scattering Study, Nano Lett., 16, 3710 (2016).
 Torche et al., The Raman fingerprint of Rombohedral graphite, in preparation.
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