| Abstract: | The cooper pairs in the heavy-fermion superconductor CeCu_2Si_2 are formed of heavy fermions. Therefore, the heavy fermions are fundamental to the emergence of unconventional superconductivity and associated non-Fermi-liquid behavior in the normal state. The interplay between localization and itinerancy manifested on the electronic structure is key for understanding the heavyfermion behavior. Here, via the first-principle density functional theory(DFT) combined with single-site dynamical mean-field theory(DMFT), we investigate the temperature(T) evolution of the electronic structure of CeCu_2Si_2 in the normal state, focusing on the role of the 4f states in the low energy regime. Two characteristic temperature scales of this evolution, which accompanied the heavy-fermion formation, are established. The coherence onset temperature is around 130K, whereas the heavy-fermion band formation temperature is between 40 and 80K; both characteristic temperature scales are higher than the transport coherence temperature. Furthermore, the heavy-fermion formation is confirmed by calculating its effective mass variation with the temperature. Based on the calculated T-dependent evolution of the 4 f orbital occupancy and electronic structure, an explanation on the behavior of the temperature evolution of the correlation strength of CeCu_2Si_2 is provided. Our results offer a comprehensive microscopic picture of the heavy-fermion formation in CeCu_2Si_2, which is essential for further understanding the emergent superconducting pairing mechanism. |
