Can dd excitations mediate pairing?

The Cu-3d states in the high-T-c cuprates are often described as a single band of 3d(x2-y2) states, with the other four 3d states having about 2 to 3 eV higher energy due to the lower-than-octahedral crystal field at the copper sites. However, excitations to these higher energy states observed with RIXS show indications of strong coupling to doped holes in the 3d(x2-y2) band. This relaunches a decades-old question of the possible role of the orbital degrees of freedom that once motivated Bednorz and Muller to search for superconductivity in these systems. Here we explore a direction different from the Jahn-Teller electron-phonon coupling considered by Bednorz and Muller, namely the interaction between holes mediated by dd excitations.