Camelback-shaped band reconciles heavy-electron behavior with weak electronic Coulomb correlations in superconducting TlNi$_{2}$Se$_{2}$

Combining photoemission spectroscopy, Raman spectroscopy, and first-principles calculations, we characterize superconducting TlNi$_{2}$Se$_{2}$ as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides, in TlNi$_2$Se$_2$ it lies in the immediate vicinity of the Fermi level, giving rise to a pronounced van Hove singularity (VHS). The resulting heavy band mass resolves the apparent puzzle of a large normal-state Sommerfeld coefficient in this weakly correlated compound. The correlation effect evolution in pnictides upon d-shell filling in the presence of significant Hund's exchange coupling will also be discussed.