Unusual correlation physics in a BCS superconductor, ZrB12

• Despite being a 4d transition metal based material and conventional superconductor, the electron correlation strength is unusually large.
• The charge transfer energy is smaller than the correlation strength albeit being an early transition metal based systems.
• Emergence of charge transfer physics down the group in the periodic table.

We investigate the electronic structure of a conventional superconductor, ZrB12, employing multiband Hubbard model and configuration interaction calculations. The calculated spectral functions within the cluster approximation provide a remarkable representation of both 3d and 3p spectra of Zr2+ species for the same set of electron–electron interaction parameters. Despite ZrB12 being a 4d transition metal based conventional superconductor, the electron correlation strength is found to be large. Ironically, the charge transfer energy is found to be smaller than the correlation strength placing this material in the charge transfer regime in the Zaanen–Swatzky–Allen phase diagram, which is unusual for early transition metal based systems.