The excitation of matter with intense, ultrashort pulses of light induces a non-equilibrium state that can give rise to electronic, structural or magnetic phase transitions with exciting prospects for novel device functionalities. The optically excited non-equilibrium state relaxes through interactions between charge carriers, phonons or other quasiparticles that occur over a large time and energy scale. The study of such many-body dynamics, therefore, requires techniques which are capable of following the flow of energy across all pertinent scales.

The emission of light through the process of high-harmonic generation meets these challenges as pulse lengths can be in the attosecond domain with photon energies reaching the soft X-ray regime [1,2]. The absorption of X-rays is particularly intriguing as it allows for the probing of changes in the electronic density of states above and below the Fermi energy. Consequently, the broadband spectrum of a soft X-ray high-harmonic pulse is ideal for investigating the many-body interactions of a non-equilibrium state.

In this talk I will present results demonstrating the application of attosecond soft x-ray spectroscopy for the investigation of many-body dynamics over a vast temporal range. In particular, I will focus on strongly coupled electron and lattice dynamics in highly photo-excited graphite [3,4], and the laser-induced spin dynamics in GdFe [5].

[1] Summers et al., J. Ultrafast Sci. 3, 4 (2023).
[2] Biegert, Europhysics News 55 (1), 12-15 (2024).
[3] Sidiropoulos et al., Phys. Rev. X 11, 041060 (2021).
[4] Sidiropoulos et al., Nat. Commun 14, 7407 (2023).
[5] Hennecke et al., Phys. Rev. Res. 4 (2), L022062 (2022).