Chemical nature of the barrier in Pb/YBa2Cu3O7−x tunneling structures

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Abstract
Several reports of reproducible tunneling measurements on YBa2Cu3O7−x thin films or single crystals with a Pb counterelectrode have recently appeared. The nature of the tunnel barrier, formed by air exposure, in these structures has been unknown. In the present work, the chemical nature of the tunnel barrier is studied with x‐ray photoelectron spectroscopy(XPS). Laser‐ablated filmsgrown on LaAlO3 which have been chemically etched and heated in air are found to form nonsuperconducting surface Ba species, evident in an increase of the high binding energy Ba 3d and O 1s signals. A deposited Pbfilm ∼10 Å thick is found to be oxidized, and Cu+2 is partially reduced to Cu+1. The tunneling barrier thus appears to consist of species resulting from a combination of the air exposure and a reaction between the superconductor and the deposited Pb counterelectrode.

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