Electronic Structures and Orbital-Selective Coherence-Incoherence Crossover in CsFe 2 As 2

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Abstract 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$_2$As$_2$ 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$_2$As$_2$.

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