Non-equilibrium Dynamics of the Electron, Spin, and Structural Degrees of Freedom in Condensed Matter

Matter can be excited by means of external stimuli into non-equilibrium states. Analysis of the real time dynamics evolving under these conditions provides unique insights into elementary processes and fundamental interactions that govern the dynamics. Considering the electronic, spin, and structural degrees of freedom in condensed matter it is an essential question on which time scales these subsystems remain isolated / are interacting. These studies profit from a multi-modal experimental approach that addresses these subsystems specifically. We performed x-ray absorption spectroscopy at the synchrotron light source BESSY II [1], European X-ray Free Electron Laser (XFEL) [2,3,4], and table-top in the laboratory [5]. Ultrafast electron diffraction studies were performed at the MeV electron source at Stanford [6]. In this talk work that combines ultrafast x-ray absorption and electron diffraction experiments will be presented. In Fe/MgO heterostructures we identify electron-phonon coupling by a hybrid mode directly at the interface. This mode mediates energy transfer from electrons to phonons very efficiently on sub-picosecond timescales [7]. The ultrafast magnetization dynamics of these heterostructures exhibits a pronounced dependence on the Fe film thickness which is explained by the reduced spin coordination at the Fe-MgO interface [8]. Using the excellent spectral resolution of the SCS instrument at XFEL we analyzed the temporal evolution of the S=0 to S=2 transition in Fe-based spin crossover molecules that is mediated by an 11% bond length and the corresponding ligand field variations [9]. Most recently, we focus on the extended spectral range using EXAFS at the Ba L3 and the Ti K absorption edges of BaTiO3 to establish a structural probe with local sensitivity. Such methodology is desired to distinguish displacive and order-disorder types of structural transitions. The impact on the electric dipole dynamics will be discussed based on first results.
Funding by the Deutsche Forschungsgemeinschaft through SFB 1242 is gratefully acknowledged.

[1] K. Holldack et al., J. Synch. Rad. 21, 1090 (2014).
[2] L. LeGuyader et al., J. Synchrotron Rad. 30, 284 (2023).
[3] T. Lojewski et al., Mat. Research Lett. 11, 655 (2023).
[4] T. Lojewski et al., Phys. Rev. B 110, 245120 (2024).
[5] O. A. Naranjo-Montoya et al., Rev. Sci. Instrument. 95, 103001 (2024).
[6] S. Weathersby et al., Rev. Sci. Instr. 86, 073702 (2015)
[7] N. Rothenbach et al., Phys. Rev. B 100, 174301 (2019).
[8] W. Windsor et al., under review.
[9] L. Kämmerer et al., ACS Nano 18, 34596 (2024).

Uwe will be at MPSD on Thursday and Friday; to schedule a meeting, please contact Michael Fechner.