The electronic band structure and crystal structure are the two complementary identifiers of solid-state materials. Although convenient instruments and reconstruction algorithms have made large, empirical, crystal structure databases possible, extracting the quasiparticle dispersion (closely related to band structure) from photoemission band mapping data is currently limited by the available computational methods. To cope with the growing size and scale of photoemission data, here we develop a pipeline including probabilistic machine learning and the associated data processing, optimization, and evaluation methods for band-structure reconstruction, leveraging theoretical calculations. The pipeline reconstructs all 14 valence bands of a semiconductor and shows excellent performance on benchmarks and other materials datasets. The reconstruction uncovers previously inaccessible momentum-space structural information on both global and local [more...]

Sang successfully defended his PhD thesis in Dec. 2022. Congratulations and best wishes for your future!

Understanding the microscopic mechanisms behind ultrafast magnetization dynamics remains an important issue in solid state physics. Numerous studies have successfully applied variations of three-temperature models to describe experimental ultrafast magnetization dynamics. By introducing effective temperatures for the transient electronic, lattice and spin degrees of freedom (see figure), the 3TM provides an intuitive, phenomenological approach for the quantitative analysis of ultrafast magnetization dynamics using three coupled differential equations to describe the mutual energy transfer between the subsystems. However, comparison of model predictions are often only made with one of the subsystems.
In our study (Adv. Mater. Interfaces 2201340 (2022)), we investigate the femtosecond dynamics of electronic temperature, sub-surface ferromagnetic ordering [more...]

In the quest to continue Moore´s law, utilizing the electron´s spin degree of freedom poses a promising approach. A material class that intrinsically enables efficient spintronic applications are quantum spin Hall (QSH) insulators, also termed 2D topological insulators, as they allow for dissipationless spin-currents in their edges. However, exploiting the transport properties of such edge states is so far restricted to cryogenic temperatures, as only a few QSH materials with small bulk band gaps are available. Here, graphene-like 2D structures of heavy atoms, most notably bismuthene, i.e., a honeycomb lattice of Bi atoms on a semiconductor substrate, offer a route to QSH conductivity far beyond room-temperature due to their large spin-orbit coupling.
In our recent study [Maklar et al., Nano Letters (2022)], we present a detailed investigation of the ultrafast [more...]

In our recent combined experimental and theoretical study [Maklar et al., Phys. Rev. Lett. 128, 026406 (2022)], we investigate how a dynamical insulator-to-metal transition affects fundamental interactions, such as electron-electron and electron-phonon scattering. We utilize optical excitation to transiently alter the energy gap of a charge-density-wave compound and observe a concurrent, highly unusual modulation of the relaxation rate of hot quasiparticles. State-of-the-art calculations based on non-equilibrium Green’s functions provide a microscopic view onto the interplay of quasiparticle scattering and the transiently modified electronic band structure, highlighting the critical role of the phase space of electron-electron interaction. Our results vividly demonstrate the possibility of controlling quasiparticle relaxation rates [more...]