Revealing the Heavy Quasiparticles in the Heavy-Fermion Superconductor CeCu2Si2.

The superconducting order parameter of the first heavy-fermion superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here, we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce $4f$ electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-$f$ electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk $X$ point and moderately heavy three-dimensional hole pockets near the $Z$ point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy-fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.