The weak electronic correlations and absence of heavy Fermion state in KNi$_2$Se$_2$
We have studied the low-lying electronic structure of a new ThCr$_2$Si$_2$-type superconductor KNi$_2$Se$_2$ with angle-resolved photoemission spectroscopy. Three bands intersect the Fermi level, forming complicated Fermi surface topology, which is sharply different from its isostructural superconductor K$_x$Fe$_{2-y}$Se$_2$. The Fermi surface shows weak variation along the $k_z$ direction, indicating its quasi-two-dimensional nature. Further comparison with the density functional theory calculations demonstrates that there exist relatively weak correlations and substantial hybridization of the Ni 3$d$ and the Se 4$p$ orbitals in the low-lying electronic structure. Our results indicate that the large density of states at the Fermi energy leads to the reported mass enhancement based on the specific heat measurements. Moreover, no anomaly is observed in the spectra when entering the fluctuating charge density wave state reported earlier.