Sommaire:

After the isolation of graphene in the mid 2000s, the focus of interest in nanotechnology applications has largely shifted towards two-dimensional (2D) materials [1].The technological challenge now resides in the ability to stack together atomically different thin layers in order to build new kinds of “van der Waals heterostructures” with the goal of realising devices with customized functionalities [2]. In order to do that a detailed knowledge of the change in the material fundamental properties going from the 3D to the 2D case is compulsory.
In this talk I will show how in both thermal and optical responses lowering dimensionality leads to the manifestation of interesting emergent phenomena.
Concerning thermal properties I will present a fully first principle characterization [3] of the thermal conductivity of graphite, graphene and related compound, such as hexagonal boron nitride (hBN) and molybdenum disulfide (MoS2). I will explain how, at variance with typical three-dimensional solids, collective phonon excitations, and not single phonons, are the main heat carriers. These excitations, characterized by mean free paths of the order of hundreds of micrometers, lead to the observation of Poiseuille and Ziman hydrodynamics regimes, hitherto typically confined to ultra-low temperatures in the corrispondent 3D case. Most remarkably, several of these two-dimensional materials admit wave-like heat diffusion, with the possibility to observe second sound even at room temperature for graphene, graphane and hBN. [4,5]
For the optical part I will show how the transition from 3D to 2D leads to a reduced effect of the screening and to an optical response dominated by strong electron-hole (e-h) interaction with the formation of bound e-h pairs, i.e. excitons [6]. I will show how the common description of weak (Wannier) and tightly (Frenkel) bound excitons, generally used for describing standard 3D materials, has to be modified for the 2D case and how, in the framework of ab initio many body GW and Bethe Salpether calculations, it is possible to derive a general formalism for describing excitons in layered crystals starting from the knowledge of the excitations of a single layer [7].

[1] K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, PNAS 102, 10451 (2005).
[2] A. K. Geim and I. V. Grigorieva, Nature 499, 419 (2013).
[3] G. Fugallo, L. Paulatto, M. Lazzeri and F. Mauri, Phys. Rev. B, 88, 045430 (2013)
[4] G. Fugallo ​, A Cepellotti, L Paulatto, M Lazzeri, N Marzari, F Mauri, Nano letters 14 (11), 6109­6114 (2014).
[5] A Cepellotti, G. Fugallo​, L Paulatto, M Lazzeri, F Mauri, N Marzari, Nature Comm. 6, 6400 (2015)
[6] G. Fugallo​, M. Aramini, J. Koskelo, K. Watanabe, T. Taniguchi, M. Hakala, S. Huotari, M. Gatti, F. Sottile, Phys. Rev. B 92 (16), 165122 (2015)
[7] J. Koskelo, G. Fugallo​, M. Hakala, M. Gatti, F.Sottile, P. Cudazzo Phys. Rev. B 95, 035125 (2017)

Pour plus d'informations, merci de contacter Cassabois G.