Optic breathing mode, resonant charge fluctuations, and high-Tcsuperconductivity in the layered perovskites

Frozen-phonon optical breathing mode calclations on ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{M}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ (M=Ba,Sr,. . .) demonstrate the following: (i) a large (≤3 eV/Å) electron-phonon interaction matrix element for the &—O $p_x$,y orbitals, (ii) a high frequency (∼110 meV), and (iii) large charge fluctuations (≃0.3 electrons). Especially significant is the possible resonant coupling to, and enhancement of, the natural ground-state ${\mathrm{Cu}}^{2+}$-${\mathrm{Cu}}^{3+}$ charge fluctuations between in-plane Cu atoms. The relation of these results to the high $T_c$ (and previous theoretical descriptions of superconductivity) is indicated.