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...]

Active control over macroscopic properties of solids is highly desirable for a broad range of applications. A promising pathway to on-demand material properties are metastable hidden states, which are nonequilibrium phases that can only be reached after a quench by ultrashort optical or electrical pulses. As hidden states often host new emergent properties and can be switched on ultrafast timescales through non-thermal reaction pathways, they offer exciting novel functionalities for solid-state quantum devices. Yet, the fundamental processes that govern the dynamical pathway to hidden phases remain a largely open subject. Thus, switching is mostly based on empirical protocols, resulting in low efficiencies and limited control over stability.
In our recent study (arXiv link), we investigate the dynamical pathway to the metastable hidden quantum [more...]