Ion-size effects in HTS cuprates -- dielectric versus magnetic pairing

We have been exploring the systematic effects of changing ion size on superconducting and normal-state properties of the HTS cuprates. In the model system RA$_{2}$Cu$_{3}$O$_{\mathrm{y}}$ (where R $=$ La,\textellipsis , Lu and A$=$Ba$_{\mathrm{2-x}}$Sr$_{\mathrm{x}})$ the maximum T$_{\mathrm{c}}$ in the dome-shaped phase curve can be shifted from 70 to 110 K in the model system. Surprisingly T$_{\mathrm{c}}$ correlates with the dielectric properties and not the magnetic properties. This highlights the fundamental importance of charge fluctuation and dielectric screening in the cuprates and may signal a novel pairing mechanism having its origin with quantized waves of electronic polarization.