Computational and Applied Mathematics seminar

Abstract: Materials are composed of a vast number of ions and electrons, arranging themselves in regular patterns. Due to immense progress in ultrafast spectroscopy, ranging from the low-frequency infrared to high-frequency X-ray regimes, collective excitations of charge, spin, lattice, and orbital degrees of freedom can now be triggered and probed on their characteristic time and length scales. Probing such excitations far from equilibrium motivates the field of ultrafast thermodynamics, which translates well-understood concepts, such as entropy and heat, to picosecond dynamics. I will introduce the concept of ultrafast thermodynamics using the example of entropy production due to laser-driven phonons [1] and magnons [2] using stochastic thermodynamics. I will show that these processes are well described by Langevin equations and introduce their derivation on a few examples. Furthermore, I will show extensions of the formalism to non-Markovian behavior and quantum mechanics [3].

[1] L. Caprini, H. Löwen & R. M. Geilhufe, Nature Communications, 15, 94 (2024)

[2] F. Tietjen, R. M. Geilhufe, PNAS Nexus, 4, 3 (2025)

[3] Y. Qiao, R. M. Geilhufe, arXiv:2512.17669