Tunneling density of states in a-axis YBCO films

The authors report a novel and simple method for the fabrication of high quality a-axis YBCO/Insulator/Normal metal junctions. The insulating layer is formed by in-situ oxidation of a metallic layer having a strong electronegativity (in the present case Al) deposited on the YBCO surface. The junction resistance shows an exponential dependence on the Al thickness over several decades of resistance with values consistent with the known resistance of high quality tunnel junctions, when the Al film is fully oxidized. The thermal and mechanical stability of the junctions, the high yield of the method and its simplicity are very promising for future applications and for the study of the underlying physics. Because some of the oxygen content of the YBCO film is used to oxidize the Al layer, the YBCO film underneath the junction tends to be in the underdoped regime. The authors discuss this result in terms of strong coupling effects and in light of the theory of Emery and Kivelson, who have predicted that in the underdoped regime there exists a mean field Tc much larger than the measured one, which determines the value of the superconducting gap. They believe that the proposed method is general enough to be used for the preparation of junctions on all high Tc oxides