Strength of correlations in a silver-based cuprate analog

Abstract

${\mathrm{AgF}}_{2}$ has been proposed as a cuprate analog, which requires strong correlation and marked covalence. On the other hand, fluorides are usually quite ionic, and $4d$ transition metals tend to be less correlated than their $3d$ counterparts, which calls for further scrutiny. We combine valence band photoemission and Auger-Meitner spectroscopy of AgF and ${\mathrm{AgF}}_{2}$ together with computations in small clusters to estimate values of the Ag $4d$ Coulomb interaction ${U}_{4d}$ and charge-transfer energy ${\mathrm{\ensuremath{\Delta}}}_{pd}$. Based on these values, ${\mathrm{AgF}}_{2}$ can be classified as a charge-transfer correlated insulator according to the Zaanen-Sawatzky-Allen classification scheme. Thus, we confirm that the material is a cuprate analog from the point of view of correlations, suggesting that it should become a high-temperature superconductor if metallization is achieved by doping. We present also a computation of the Hubbard $U$ in density functional ``$+U$'' methods and discuss its relation to the Hubbard $U$ in spectroscopies.