Ion-size effects in cuprate superconductors - implications for pairing

The standard separation in metals between valence and core states leads in cuprate superconductors to constituent ions which may sustain both charge and spin fluctuations. We change ion size in RA$_2$Cu$_3$O$_y$ (A=(Ba,Sr) and R=(La, ... Lu,Y)) in order to systematically alter both dielectric and magnetic properties. Dielectric fluctuation is characterized by ionic polarizability while magnetic fluctuation is characterized by exchange interactions measurable by Raman scattering. The range of transition temperatures is 70 to 107 K and we find that these correlate only with the dielectric properties, a behavior which persists with external pressure. 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.