Francesco Mauri - Measuring charge carrier excitations (phonons, plasmons and excitons) in suspended 2D materials, metals and superconductors by electron energy loss spectroscopy and by optical IR spectroscopy

Abstract Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) makes finally possible to measure the dispersion of charge excitations of mono-layers suspended in vacuum [1,2]. EELS of freestanding neutral graphene demonstrates the importance of many-body effects (e-e repulsion and excitonic attraction) in the description of the prominent electronic excitations, the onset and of the π-plasmon [2]. TEM-EELS is capable to measure the phonon dispersions of mono and few-layer flakes of graphene and h-BN [1]. In particular, the h-BN data demonstrate the peculiar LO-TO phonon splitting, linear in the phonon momentum, predicted to occur in 2D membranes [3]. The coupling between TEM electron-beam and the phonons can be described, from first principles, defining, for each atom, a momentum-dependent effective charge vector Z(q) [1]. In insulators, in the limit of small momentum (q->0), such charges are closely related to the better-known Born effective atomic tensors, that describe the coupling between IR light and optical phonons [1,4,5]. In the same regimes, Z(q) can also be used to efficiently compute, within DFT, the quadrupole and octupole atomic tensors in systems containing hundreds of atoms [4,5]. Finally, the momentum dependent charges can also be used to extend the definition of Born effective charges to metals, and thus to describe, from first principles, the optical vibrational signature in the reflectivity of metals and superconductors [6]. In contrast to the insulating cases, Born effective charges of metals crucially depend on the collision regime of conducting electrons (from the undamped, collisionless regime to the overdamped, collision-dominated limit). Such an approach enables the interpretation of vibrational reflectance measurements of both superconducting and bad metals, as I illustrate for the case of strongly electron–phonon-coupled superhydride H3S [6]. I acknowledge support from the MORE-TEM ERC-SYN project, grant agreement No 951215. References [1]R. Senga, K. Suenaga, P. Barone, S. Morishita, F. Mauri, T. Pichler, Nature, 573, 247-250 (2019) [2]A. Guandalini, R. Senga, Y. Lin, K. Suenaga, A. Ferretti, D. Varsano, A. Recchia, P. Barone, F. Mauri, T. Pichler, C. Kramberger, Nano Lett., 23, 11835-11841 (2023) [3]T. Sohier, M. Gibertini, M. Calandra, F. Mauri, N. Marzari, Nano Lett., 17, 3758-3763 (2017) [4]F. Macheda, P. Barone, F. Mauri, Phys. Rev. Lett., 129, 185902 (2022) [5]F. Macheda, P. Barone, F. Mauri, to be published [6]G. Marchese, F. Macheda, L. Binci, M. Calandra, P. Barone, F. Mauri, Nat. Phys., 20, 88-94 (2023)