$\mathrm{CuAg}{\left({\mathrm{SO}}_4\right)}_2$: A doubly strongly correlated altermagnetic three-dimensional analog of the parent compounds of high-$T_c$ cuprates

The discovery of high-temperature superconductivity (HTSC) in strongly correlated cuprates opened a new chapter in condensed matter physics, breaking existing stereotypes of what is a material base for a good superconductor ("Matthias rules"), at the same time emphasizing the richness and challenge of strongly correlated physics, personified by the most strongly correlated $3d$ ion, ${\mathrm{Cu}}^{2+}$. A recently reported new compound, $\mathrm{CuAg}{\left({\mathrm{SO}}_4\right)}_2$, combines in a fascinating way the same ion with the most strongly correlated $4d$ one, ${\mathrm{Ag}}^{2+}$. In this Letter, we present a detailed analysis of electronic and magnetic properties of this material, and show that it is very different from the HTSC cuprates in several different ways, and opens a door into further research of superconductivity and magnetism, in particular altermagnetism, in strongly correlated materials.