The study of the suppression of an order parameter by an external perturbation and the following recovery of the broken-symmetry phase is a problem relevant to systems even beyond condensed matter physics. In the context of pump-probe experiments, it has been tackled considering the suppression of the charge density wave order parameter in several compounds, and it was found a relevant role played by the fluctuations [1,2]. In particular, two alternative scenarios have emerged: One in which spatial fluctuations remain relatively small during the whole process [1] and the other in which they become large [2,3]. The theoretical description of this situation finds its strongest limitation in the different time scales at play in this context, which are of the order of a few tens of femtoseconds for the electrons and several picoseconds for the phonons and the other relevant order parameters. For this reason, in most cases, one relies on a phenomenological description, even if recent experiments have challenged the accuracy of Ginzburg-Landau (GL) theories in this context [3,4].
In this presentation, we challenge the assumptions of the usually employed GL theories. In the first part, we consider the “small fluctuations” scenario, showing that other degrees of freedom besides the order parameter might provide a relevant contribution to the dynamics, and they should thus be explicitly included in the description of the process [5]. In the second part, we analyze the “large fluctuations” limit, the description of which requires the development of a new theoretical framework that goes beyond phenomenological theories [6]. With this tool, we study the dynamics of photoexcitation in the Holstein model, showing the emergence of ultrafast disordering already in this minimal setup [7].
[1] Zong et al., Nat. Phys. 15, 27-31 (2019)
[2] Wall et al., Science 362, 572 (2018)
[3] Perez-Salinas et al., Nat. Comm. 13, 238 (2022)
[4] Maklar et al., Nat. Comm. 12, 2499 (2021)
[5] Grandi, Eckstein, PRB 103, 245117 (2021)
[6] Picano, Grandi, Werner, Eckstein, PRB 108, 035115 (2023)
[7] Picano, Grandi, Eckstein, PRB 107, 245112 (2023)