Virtual electric quadrupole fluctuations: A mechanism for highTc

We present the first detailed study of a mechanism for high-$T_c$ superconductivity where oxygen-hole pairing is mediated via the exchange of virtual fluctuations of copper electric quadrupole moments. The excitation scale Δ≃0.1–0.5 eV is large enough for high $T_c$, but the interaction is still significantly retarded. This mechanism explains the absence of superconductivity in doped ${\mathrm{La}}_2$ ${\mathrm{NiO}}_4$, the pressure dependence of $T_c$, and the different character of 60-K and 90-K phases of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (through filling-dependent gap nodes). Experimental tests of the model are proposed.