Energy Scales Of The Excitations Associated With Superconductivity: An Analysis Of The Infrared And Visible In-Plane Response Of Bi2sr2cacu2o8+Delta

The ab-plane infrared and visible (3 meV-3 eV) response of Bi2Sr2CaCU2O8+delta (Bi-2212) thin films (prepared by r.f. sputtering on SrTiO3) has been measured between 300 K and 10 K for different doping levels. In the superconducting state, dramatic differences appear between the underdoped and overdoped regimes regarding the electrodynamics of the formation of the superfluid condensate. In the over-doped regime, the superfluid grows up by removing states from energies below 60 meV. This energy is of the order of a few times the superconducting gap. In this respect, overdoped Bi-2212 exhibits a conventional behavior. In the underdoped regime, states extending up to 2 eV contribute to the superfluid. This anomalously large energy scale may be assigned to a change of electronic kinetic energy at the superconducting transition, and is compatible with an electronic pairing mechanism.