DAY 1

Abstract
Recent progress in electron and photon spectroscopy now provides resolution down to the milli-eV, both in large scale infrastructures and tabletop experiments. Phonon, magnon, and other low energy excitations have become accessible to measurements, and even essential to the interpretation of full spectra. This is an especially exciting era given the central role of electron-boson coupling in solid state physics, whether in superconductivity, thermoelectricity, or photovoltaics. Further, the coupling quantifies thermal shifts and broadenings in the scattering energies of electrons, neutrons, muons or photons. In this talk I review our first principles calculations of the spectroscopic signatures and the consequences of phonon coupling, focusing on Angle-Resolved Photo-Emission Spectra (exemplified in LaB6 and diamond), photoluminescence and absorption (at defects in 2D materials). Employing the tools of many bodies perturbation theory as well as model Hamiltonians we can now predict finite temperature and polaronic properties, satellites and sideband features, both in pristine bulk systems and defected or nanostructured ones.


References
[1]A. Rattanachata, L. Nicolaï, H. Martins, G. Conti, M. Verstraete, M. Gehlmann, S. Ueda, K. Kobayashi, I. Vishik, C. Schneider, C. Fadley, A. Gray, J. Minár, S. Nemšák, Phys. Rev. Materials, 5, 055002 (2021)
[2]J. de Abreu, J. Nery, M. Giantomassi, X. Gonze, M. Verstraete, Phys. Chem. Chem. Phys., 24, 12580-12591 (2022)
[3]P. de Melo, J. de Abreu, B. Guster, M. Giantomassi, Z. Zanolli, X. Gonze, M. Verstraete, npj. Comput. Mater., 9, 147 (2023)
[4]P. M. C. de Melo, Z. Zanolli, M. Verstraete, Adv. Quantum. Tech., 4, (2021)
[5]F. Libbi, P. de Melo, Z. Zanolli, M. Verstraete, N. Marzari, Phys. Rev. Lett., 128, 167401 (2022)

Abstract
The analysis of lattice vibrations provides vital information on a variety of material properties and is of great experimental interest. Most of the available techniques and software tools to compute phonons rely on harmonic approximations of the classical ionic motion. When the material contains light ions or is subjected to temperatures and/or pressures deviating significantly from ambient conditions, anharmonic and quantum effects may alter its phonon characteristics. Simulating these effects, however, comes at a numerical cost still too high to model large systems within a first principle model of the interactions. Here, we present an approach that facilitates anharmonic quantum phonon calculations via accurate and relatively low cost ab initio molecular dynamics. We leverage the power of the recently introduced mass zero constrained dynamics in the orbital free DFT framework [1], together with a computational framework that relates anharmonic phonon spectra to time correlation functions [2]. Path integral and quantum thermal bath dynamics are employed and compared to incorporate quantum nuclear effects.


References
[1]A. Coretti, T. Baird, R. Vuilleumier, S. Bonella, The Journal of Chemical Physics, 157, (2022)
[2]T. Morresi, L. Paulatto, R. Vuilleumier, M. Casula, J. Chem. Phys., 154, 224108 (2021)
[3]M. Martinez, M. Gaigeot, D. Borgis, R. Vuilleumier, The Journal of Chemical Physics, 125, (2006)

Abstract
Combining density functional theory with many-body techniques is enabling rapid advances in first-principles calculations of nonequilibrium dynamics in materials.

In this talk, I will present new quantitative methods to model coupled electrons and phonons, focusing on the challenging regime where interactions are strong and correlation effects are dominant. We will discuss three exciting frontiers for these studies: 1) transport in quantum materials with strong electron-phonon (e-ph) coupling and electron correlations; 2) ultrafast electron and lattice dynamics, including quantitative predictions of time-domain spectroscopies and extensions to excitons and magnons; 3) leveraging data-driven methods to compress the electronic interactions and dynamics and significantly speed-up their computation.

The talk will conclude with a discussion of PERTURBO, an open-source code developed in my group which provides quantitative tools to study electron interactions and dynamics from first principles in both conventional and quantum materials.

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)