Project B11 is a new project, which entered the TRR 227 “Ultrafast Spin Dynamics” via a successful supplementary proposal in 2024. B11 focuses on the ultrafast structure and dynamics of excitons in two-dimensional (2D) materials and heterostructures. The exciton is a central quasiparticle in transition metal dichalcogenides (TMDCs), but also in 2D semiconductor magnets of interest to the second funding period of the TRR. A detailed understanding of excitons and how they are influenced by magnetic order is essential to assess the potential of TMDC/2D magnets as a platform for optical control of valley polarization and spintronic devices. We will employ polarization-resolved broadband coherent two-dimensional electronic spectroscopy (2DES) to probe how excitons interact with spins, phonons and electronic excitations, see the figure to this end. We will investigate the excitonic structure (manifolds of excitonic states, fine structure) and dynamics in 2D semiconductor magnets and TMDC/2D magnet heterostructures with 10 fs and 10 meV resolution as a function of light helicity. The temporal resolution will be essential to resolve processes such as charge-transfer or polaron formation, which can be expected to occur in < 100 fs. We will exploit the ability of the 2DES method to distinguish static from dynamic sources of disorder to resolve the broad optical lineshapes of 2D magnets. We aim to reveal exciton dephasing mechanisms and deliver quantities such as spin-dependent charge-transfer time.

Publications (2^nd Funding Period 2024 - 2025)

• Momentum-resolved signatures of carrier screening effects on electron–phonon coupling in MoS2
  Y. Pan, P. -N. Hildebrandt, D. Zahn, M. Zacharias, Y. W. Windsor, R. Ernstofer, F. Caruso, and H. Seiler
  ACS Nano 19, 11, 11381-11389 (2025) - DOI: 10.1021/acsnano.5c00744

• Element-specific ultrafast lattice dynamics in monolayer WSe2
  H. Jung, S. Dong, D. Zahn, T. Vasileiadis, H. Seiler, R. Schneider, S. Michaelis de Vasconcellos, V.C.A. Taylor, R. Bratschitsch, R. Ernstorfer, and Y.W. Windsor
  Nano Lett. 2024, 24, 43, 13671–13677 (2024)  - DOI: 10.1021/acs.nanolett.4c03611

• Observation of multi-directional energy transfer in a hybrid plasmonic–excitonic nanostructure 
  T. Pincelli, T. Vasileiadis, S. Dong, S. Beaulieu, M. Dendzik, D. Zahn, S.-E. Lee, H. Seiler, Y. Qi, R. P. Xian, J. Maklar, E. Coy, N. S. Mueller, Y. Okamura, S. Reich, M. Wolf, L. Rettig, and R. Ernstorfer 
  Adv. Mater. 35, 2209100 (2023)  - DOI: 10.1002/adma.202209100

• Direct observation of ultrafast lattice distortions during exciton–polaron formation in lead halide perovskite nanocrystals 
  H. Seiler, D. Zahn, V. C. A. Taylor, M. I. Bodnarchuk, Y. W. Windsor, M. V. Kovalenko, and R. Ernstorfer  
  ACS Nano 17, 1979 (2023)  - DOI: 10.1021/acsnano.2c06727

• Intrinsic energy flow in laser-excited 3d ferromagnets 
  D. Zahn, F. Jakobs, H. Seiler, T. A. Butcher, D. Engel, J. Vorberger, U. Atxitia, Y. W. Windsor, and R. Ernstorfer 
  Phys. Rev. Res. 4, 013104 (2022)  - DOI: 10.1103/PhysRevResearch.4.013104