Pairing in cuprates from high-energy electronic states

The in-plane optical conductivity of Bi2Sr2CaCu2O 8 + δ films with small carrier density (underdoped) up to large carrier density (overdoped) is derived from accurate reflectivity data. Integrating the conductivity up to increasingly higher frequencies yields the energy scale involved in the formation of the condensate. At least in the underdoped sample, states extending up to 2 eV contribute to the superfluid. This anomalously large energy scale may be assigned to a change of in-plane kinetic energy at the superconducting transition, and is compatible with an electronic pairing mechanism.