Electronic Structures and Orbital-Selective Coherence-Incoherence Crossover in CsFe₂As₂

Understanding the role of electron correlations in quasiparticle coherence is central to multiorbital iron-based superconductors. Here, we investigate the electronic structure and temperature-dependent quasiparticle dynamics of CsFe_2As_2 using polarization-resolved laser- and synchrotron-based angle-resolved photoemission spectroscopy combined with first-principles calculations. We resolve multiple hole-like Fermi surface sheets around the \mathbf\Gamma and M points, with the \alpha(\alpha') and \beta bands predominantly derived from Fe 3d_xz/yz orbitals and the \gamma band mainly from d_xy. The calculations reveal a strongly three-dimensional and multiorbital Fermi surface topology, while discrepancies in the detailed orbital character indicate significant correlation effects. Temperature-dependent ARPES further reveals an orbital-selective coherence-incoherence crossover. The \alpha band exhibits a gradual loss of coherence over approximately 50-85\,K, accompanied by suppressed spectral weight, increasing linewidth, and evolving dispersion, whereas the \beta band shows an abrupt collapse of coherent spectral weight near 50-60\,K with little change in its dispersion. Together with the resistivity anomaly near 80\,K, these results establish a microscopic connection between orbital-selective quasiparticle coherence and macroscopic transport behavior. Our findings provide direct momentum-resolved evidence for band-selective correlation effects in the strongly correlated multiorbital state of CsFe_2As_2.