Virtual electric quadrupole fluctuations: a mechanism for high T<SUB>c</SUB>

We present the first detailed study of a mechanism for high-Tc superconductivity where oxygen-hole pairing is mediated via the exchange of virtual fluctuations of copper electric quadrupole moments. The excitation scale &#916;0.1-0.5 eV is large enough for high Tc, but the interaction is still significantly retarded. This mechanism explains the absence of superconductivity in doped La2NiO4, the pressure dependence of Tc, and the different character of 60-K and 90-K phases of YBa2Cu3O7-&#948; (through filling-dependent gap nodes). Experimental tests of the model are proposed