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 [2] and heat transport at metallic interfaces dominated by phonons [3]. We shall discuss coupling of strain and heat to magnetization dynamics [4,5] and magnetic phase transitions [6]. Ultrathin metal superlattices are introduced as a metamaterial and means to transduce large amplitude terahertz strain waves via electronic pressure on metallic interfaces.[7]
References
[1] Mattern, A. von Reppert, S.P. Zeuschner, M. Herzog, J.-E. Pudell, and M. Bargheer, Concepts and use cases for picosecond ultrasonics with x-rays, Photoacoustics 31, 100503 (2023)
[2] Stete, S. Kesarwani, C. Ruhmlieb, F. Schulz, M. Bargheer, H. Lange, Inverted Temperature Gradients in Gold-Palladium Antenna-Reactor Nanoparticles, Nat Commun 16, 8168 (2025).
[3] Herzog, A. von Reppert, J.-E. Pudell, C. Henkel, M. Kronseder, C.H. Back, A. Maznev, and M. Bargheer, Phonon-dominated energy transport in purely metallic heterostructures, Adv. Funct. Mater. 32, 2206179 (2022).
[4] Jarecki, M. Mattern, F.-C. Weber, J.-E. Pudell, X.-G. Wang, J.-C. Rojas-Sánchez, M. Hehn, A. von Reppert, and M. Bargheer, Controlling effective field contributions to laser-induced magnetization precession by heterostructure design, Communications Physics 7, 12 (2024).
[5] Walz, F.-C. Weber, S.-P. Zeuschner, K. Dumesnil, A. von Reppert, M. Bargheer, Large strain contribution to the laser-driven magnetization response of magnetostrictive TbFe2, Appl. Phys. Lett. 127, 052406 (2025).
[6] Mattern, J. Jarecki, J. A. Arregi, V. Uhlíř, M. Rössle, and M. Bargheer, Speed limits of the laser-induced phase transition in FeRh, APL Materials 12, 051124 (2024).
[7] Bargheer et al., Electron pressure drives THz phonons in metal-metal superlattices, https://doi.org/10.21203/rs.3.rs-6597328/v1