Valence Band Character of NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$ using 3p-3d Resonant ARPES

Understanding the strong correlated system is one of the most challenging tasks in condensed matter physics. Especially, the metal insulator transition (MIT) has been one of the major topics recent few decades. NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$ is known as one of famous material which has MIT. The cubic pyrite NiS$_{2}$ is a charge-transfer (CT) insulator. NiS$_{2}$ attracts particular interest as it easily forms a solid solution with NiSe$_{2}$ (NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}})$ which, while being isoelectronic and isostructural to NiS$_{2}$, is nevertheless a good metal. MIT, induced by Se alloying, is observed at low temperature (T) for x$=$0.45. Perucchi and his collaborators revealed closed relation between MIT and band width through comparison of infrared spectroscopy result and LDA calculation. However, it was only an indirect observation, and is inconsistent with recent proposal that NiS$_{2}$ is not a CT insulator but an insulator due to the bonding-antibonding splitting in the S -- S (Se -- Se) dimers. To reveal the true mechanism in the MIT in NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$, resonant photoemission experiment is essential. According to competing theories (CT insulator and insulator due to bonding-antibonding splitting), it is expected that the character of the main band that is responsible for the MIT should be different. Therefore, we performed 3p-\textgreater 3d resonant ARPES for various Se dopings (x$=$0.43; insulator, x$=$0.5, 0.7, 2.0; metal) and observed a significant change between on- and off-resonances near the MIT. Our experimental result supports that the origin of MIT in NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$ is the CT theory rather than the dimer theory.