Coherences and Populations in Mixed Classical–Quantistic Bosonic Systems—A Many–Body Perspective

Excitons are routinely used to interpret a wealth of physical concepts [3], both at equilibrium and out–of–equilibrium. The commonly accepted picture is simple: a photo–excitation creates a bound electron–hole pair that evolves as a population in time inducing an observable induced field and governing the carriers distribution. Phonons [1, 7] are equally ubiquitous in condensed matter physics. The electron–phonon interaction is at the basis of the description of such disparate phenomena. There are few key differences between excitons and phonons. Excitons are not exact bosons while phonons are. On the other hand one of the most successful approach to calculate phonons is based on Density–Functional Perturbation Theory, a semi–classical approach where atoms are treated within the classical Born–Oppenheimer approximation. On the contrary excitons are obtained from a fully quantistic theory.<br/>In this talk I will discuss how the external field (in the case of excitons) and trajectories (in the case of phonons) are just different manifestations of coherence. I will, then, discuss their link with the Many–Body Green’s function concept. I will also investigate the processes that lead to a coherence transfer to the quantistic dynamics leading to the formation of real populations and, eventually, macroscopic phenomena. I will first describe three different aspects of the excitonic physics: the photo– induced band insulator to excitonic insulator (EI) transition [5], the ultrafast rise time of the transient absorption in an MoS2 monolayer [6] and the discovery of the exciton internal structure by means of exciton–phonon coupling [4]. In the case of phonons I will discuss the clear and sharp distinction between the coherent, classical, trajectory concept and the quantistic fluctuations [2]. I will then conclude my talk by describing the rich physics of a paradigmatic<br/>mixed classical–quantistic system, the Semi–classical Swing where classical fields oscillate immersed in a quantistic liquid. The different aspects discussed will contribute to give an unconventional description of the excitonic an phononic states opening the path to further theoretical and experimental investigations.<br/>References<br/>[1] Giustino, F. Electron-phonon interactions from first principles. Rev. Mod. Phys. 89 (Feb 2017), 015003.<br/>[2] Marini, A. Equilibrium and out-of-equilibrium over-screening free phonon self-energy in realistic materials, 2022.<br/>[3] Onida, G., Reining, L., and Rubio, A. Electronic excitations: densityfunctional versus many-body green’s-function approaches. Rev. Mod. Phys. 74, 2 (Jun 2002), 601–659.<br/>[4] Paleari, F., and Marini, A. Exciton-phonon interaction calls for a revision of the “exciton” concept. Phys. Rev. B 106 (Sep 2022), 125403.<br/>[5] Perfetto, E., Sangalli, D., Marini, A., and Stefanucci, G. Pumpdriven normal-to-excitonic insulator transition: Josephson oscillations and signatures of bec-bcs crossover in time-resolved arpes. Phys. Rev. Materials 3 (2019), 2791.<br/>[6] Smejkal, V., Libisch, F., Molina-Sanchez, A., Trovatello, C., Wirtz, L., and Marini, A. Time-dependent screening explains the ultrafast excitonic signal rise in 2d semiconductors. ACS Nano 15 (2021), 1179–1185.<br/>[7] Stefano Baroni, Stefano de Gironcoli, A. D. C. P. G. Phonons and related crystal properties from density-functional perturbation theory. REVIEWS OF MODERN PHYSICS 73 (2001), 515.