Ultrafast excitonic relaxation cascade in magnetic 2D semiconductors

• Applicants: Professor Dr. Ermin Malic; Professor Dr. Marcel Reutzel
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 571553489

The proposed project is designed to provide so far inaccessible microscopic insights into the coupling of excitonic and magnetic degrees of freedom in two-dimensional (2D) semiconducting magnets. Building on our joint experiment-theory preliminary work on exciton optics and dynamics in transition metal dichacogenides (TMDs), we will focus on the 2D semiconductor magnet CrSBr as well as CrSBr/TMD heterostructures. We will characterize the nature of excitonic properties on the level of the quantum mechanical wavefunction and identify the microscopic scattering channels that mediate the ultrafast formation, thermalization, and decay of Coulomb-bound quasiparticles. To achieve this goal, we will combine femtosecond momentum microscopy experiments with fully microscopic many particle calculations. This will allow the reconstruction of the exciton wavefunction following the concept of orbital imaging. Moreover, momentum microscopy will provide a detailed view on the energy- and center-of-mass momentum-resolved excitonic occupation dynamics on the timescale of 50 fs and the length scale of 100 nm. In direct comparison to microscopic many-particle calculations based on the density-matrix formalism, we will then characterize the impact of exciton-phonon, exciton-exciton, and exciton-magnon interactions on the ultrafast exciton relaxation cascade. We will focus on key open questions in the field that can only be accessed by combining our joint momentum-resolved view on excitons: (i) Characterize how the quasi-1D nature of the exciton wavefunction and its degree of interlayer hybridization depend on the quantities of magnetic order (controlled by the sample temperature) and the strength of Coulomb correlations (controlled by the layer-thickness). (ii) Reveal the impact of magnetic order and the strength of electron-hole Coulomb correlations on the ultrafast exciton relaxation cascade, in particular resolving the interplay of exciton-phonon and exciton-magnon interactions. (iii) Explore CrSBr/TMD heterostructures to control the ultrafast valley exciton physics in the TMD layer via magnetic order in proximity. Overall, this joint experiment-theory project combining time- and momentum-resolved microscopy experiments with a material-specific and predictive many-particle theory will provide valuable new microscopic insights into the ultrafast exciton dynamics in magnetic 2D materials.

• DFG Programme: Research Grants