Hybridization and correlation between f- and d-orbital electrons in a valence fluctuating compound EuNi2P2

The interaction between localized $f$ and itinerant conduction electrons is crucial in the electronic properties of heavy fermion and valence fluctuating compounds. Using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structure of the archetypical valence fluctuating compound ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$ that hosts multiple $f$ electrons. At low temperatures, we reveal the hybridization between Eu $4f$ and Ni $3d$ states, which contributes to the electron mass enhancement, consistent with the periodic Anderson model. With increasing temperature, interestingly, we observe opposite temperature evolution of electron spectral function above and below the Kondo coherence temperature near 110 K, which is in contrast to the monotonic valence change and beyond the expectation of the periodic Anderson model. We argue that both f-d hybridization and correlation are imperative in the electronic properties of ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$. Our results shed light on the understanding of novel properties, such as heavy fermion behaviors and valence fluctuation of rare-earth transition-metal intermetallic compounds with multiple $f$ electrons.