The in-plane infrared and visible (3 meV-3 eV) reflectivity of Bi2Sr2CaCu2O 8+δ (Bi-2212) thin films is measured between 300 K and 10 K for different doping levels with unprecedented accuracy. The optical conductivity is derived through an accurate fitting procedure. We study the transfer of spectral weight from finite energy into the superfluid as the system becomes superconducting. In the over-doped regime, the superfluid develops at the expense of states lying below 60 meV, a conventional energy of the order of a few times the superconducting gap. In the underdoped regime, spectral weight is removed from up to 2 eV, far beyond any conventional scale. The intraband spectral weight change between the normal and superconducting state, if analyzed in terms of a change of kinetic energy is ∼ 1 meV. Compared to the condensation energy, this figure addresses the issue of a kinetic energy driven mechanism.

We have deposited Bi2Sr2CaCu2O8+δ/Bi2Sr2YCu2O8+δ/Bi2Sr2CaCu2 O+δ (2212/22Y2/2212) heterostructures by an in situ dc sputtering technique at high oxygen pressures on (001) SrTiO3 substrates. The formation of highly c-axis oriented trilayers with sharp interfaces is demonstrated by x-ray diffraction and transmission electron microscope (TEM) analysis. Both the top and the bottom 2212 layers are superconducting below 87 K. Tunneling phenomena on junctions fabricated from these trilayers are observed. The conductance versus voltage curves at low temperatures exhibit a change of slope indicative of a gap structure at about 30 mV, a zero-bias peak, as well as linear background at high voltages.

No abstract

No abstract

No abstract