Planar Tunneling in YBa₂Cu₃O_7-d

Patrick J. Hentges and Professor Laura H. Greene have measured planar tunneling spectra of the high-temperature superconductor YBa2Cu3O7-d (YBCO) using multilayers of solution-deposited zirconia on the surface of the superconductor to form the insulating tunnel barrier.

The spectra display tunneling features that are as sharp, or sharper, than any planar tunneling spectra reported for YBCO to date (see IEEE Transactions on Applied Superconductivity, 13(2), 2003, 801-804).

The method for depositing these ultrathin zirconia tunneling barriers was developed in collaboration with Professor Walter G. Klemperer and Glenn Westwood (Dept. of Chemistry, UIUC). Multilayers of zirconia are deposited onto the YBCO surfaces by repeated reaction with tetra-n-propyl orthozirconate in hydrocarbon solution followed by reaction with water in n-propanol solution, yielding remarkably uniform tunneling barriers. The image shown here is a MultiMode AFM Contact Mode derivative image of YBa2Cu3O7-d (001) sputter-grown on a SrTiO3 substrate (higher resolution image).

Before deposition, the YBCO surface has a RMS roughness 0.19 nm, which is increased by 0.26 nm after 14 deposition cycles. The zirconia films also serve to passivate the YBCO surfaces. Zirconia planar tunnel junctions display sharp tunneling features after more than a year of storage under ambient conditions. The junctions studied are helping to elucidate the role of the tunneling cone, which, in planar tunneling spectroscopy, determines the extent of momentum-resolution of the density of states at the Fermi surface. There is a patent pending on this process.

The image below is a MultiMode AFM Contact Mode three-dimensional perspective projection image of YBa2Cu3O7-d (103) sputter-grown on a SrTiO3 substrate (higher resolution image).

The scale of both images is 1500 nm x 1500 nm, and both were obtained in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439.