Ultrafast Formation and Annihilation of Strongly Bound, Anisotropic Excitons

Van der Waals (vdW) layered materials with long-range magnetic order have the potential to enable novel optoelectronic and spintronic applications. Among these, CrSBr is an air-stable, direct band gap semiconductor that hosts interlayer antiferromagnetic order, a highly anisotropic electronic structure, and strongly bound excitons. In particular, excitons in CrSBr have been shown to inherit the quasi-one-dimensional nature of the material and also couple to the underlying spinorder. However, mechanisms of exciton formation, dissociation, and interaction with free carriers remain largely unexplored, despite being crucial for spintronic and optoelectronic applications. Here, we employ time- and angle-resolved photoemission spectroscopy to map the electronic structure and excited state dynamics in CrSBr. We directly resolve an exceptionally large exciton binding energy (~800 meV) and a highly anisotropic momentum space distribution of the exciton, revealing its quasi-1D real-space character. We observe an excitation-density-dependent interconversion between bound excitons and quasi-free carriers on sub- to few-picosecond timescales, indicating that many-body effects govern the excited-state dynamics and optical properties during the initial stages of relaxation. Our work highlights the strongly bound, anisotropic character of excitons in CrSBr, as well as the microscopic interactions steering relaxation pathways after photoexcitation in elevated density regimes relevant for future device applications.

💡 Research Summary

This study provides a comprehensive investigation into the fundamental properties and ultrafast dynamics of excitons in the air-stable, magnetic van der Waals semiconductor chromium sulfur bromide (CrSBr). Utilizing time- and angle-resolved photoemission spectroscopy (trARPES) combined with momentum microscopy, the researchers directly probe the momentum-resolved electronic structure and excited-state dynamics with femtosecond temporal resolution.

The key findings are threefold. First, the authors directly resolve two distinct, quasi-dispersionless features in the photoemission spectra following photoexcitation. By assigning these to the lowest-energy bound exciton state and the single-particle conduction band minimum (CBM), they measure an exceptionally large exciton binding energy of approximately 800 meV at 120 K (below the Néel temperature, T_N ~132 K), which slightly reduces to about 785 meV at room temperature. This giant binding energy, nearly an order of magnitude larger than in typical 2D transition metal dichalcogenides, indicates strongly localized, Frenkel-like excitonic character.

Second, by Fourier-transforming the two-dimensional momentum-space distribution of the exciton signal, the team reconstructs its real-space wavefunction envelope. This reveals a highly anisotropic, elongated shape with Bohr radii of approximately 0.35 nm along the crystal a-axis and 0.80 nm along the b-axis. This quantitative visualization directly confirms the predicted quasi-one-dimensional character of the exciton, inherited from the anisotropic crystal structure of CrSBr consisting of weakly coupled Cr-S chains.

Third, and most dynamically, the research uncovers a complex, excitation-condition-dependent interconversion between bound excitons and quasi-free carriers. When exciting near the exciton resonance (1.36 eV), the exciton population rises instantly, while the CBM population exhibits a delayed rise on a few-hundred-femtosecond timescale. This delay shortens, and the relative CBM signal intensity increases, as the excitation fluence is raised. This behavior is attributed to exciton-exciton annihilation at elevated densities, a many-body scattering process that dissociates bound excitons into free carriers. Conversely, for above-band-gap excitation (1.94 eV, 3.10 eV), the dynamics reverse: hot electrons populate the CBM first via cooling, followed by a rise of the exciton signal on a sub-picosecond timescale, indicative of efficient exciton formation from thermalized carriers.

The study concludes that the early-stage photoresponse of CrSBr is governed by a delicate balance between bound excitons and free carriers, steered by many-body interactions whose dominance depends critically on the photoexcitation density and energy. These insights are crucial for future opto-spintronic device applications that seek to harness the strong magneto-excitonic coupling in CrSBr, as they highlight the importance of controlling excitation parameters to manipulate the non-equilibrium state of the system for desired functionality. The work establishes trARPES as a powerful tool for directly visualizing anisotropic exciton wavefunctions and disentangling complex many-body dynamics in quantum materials.