Picosecond ultrasonics with x-rays - applications to energy transport and magnetisation dynamics

Picosecond ultrasonics with x-ray probe pulses (PUX) provide unique access to coherent longitudinal acoustic phonons (coherent strain wave packets) and heat transport at the nanoscale (flow of incoherent excitations)[1]. Bragg-peak shifts are especially useful experimental observables in nano-layered systems, where all layers can be simultaneously probed and identified by their Bragg angle. Contemporary laser-based sources of hard x-rays with femtosecond pulse duration have sufficient x-ray flux and stability to analyze the dynamics of films with single-digit nanometer thickness, and large-scale facilities even yield access to nanoparticle dynamics. This presentation will show that Ultrafast X-ray diffraction (UXRD) and extensions to Ultrafast Reciprocal Space Mapping (URSM) are broadly applicable to fundamental physics question and materials science.
We will highlight fascinating phenomena such as the counterintuitive localization of heat via dissipation and heat transport without heating [2]. We shall discuss coupling of strain and heat to magnetization dynamics [4,5] and magnetic phase transitions [6] by simultaneous magneto-optic and structural probing. Ultrathin metal superlattices are introduced as a metamaterial and means to transduce large amplitude terahertz strain waves via electronic pressure on metallic interfaces.[7]

[1] M. Mattern, et al.,, Photoacoustics 31, 100503 (2023)
[2] F. Stete, et al.,, Nat Commun 16, 8168 (2025).
[3] J.E. Pudell, et al.,, Advanced Functional Materials 30, 2004555 (2020).
[4] J. Jarecki, et al., Communications Physics 7, 12 (2024).
[5] C. Walz, et al., Appl. Phys. Lett. 127, 052406 (2025).
[6] M. Mattern, et al.,, APL Materials 12, 051124 (2024).
[7] Bargheer et al., https://doi.org/10.21203/rs.3.rs-6597328/v1

If you would like to meet with Matias during his visit, please contact Michael Fechner.