Electric transport across Sr1−xLaxCuO2/Au/Nb planar

Abstract

Thin-film planar tunnel junctions with the electron-doped infinite-layer superconductor Sr 1−x La x CuO 2 (SLCO) with x ∼ 0.15 as bottom electrode, a thin Au interlayer, and Nb as top electrode were fabricated and characterized. Measurements of electric transport across these junctions provide information on the interface and surface properties of the SLCO thin films. No Cooper pair tunneling is observed; however, nonlinear current-voltage characteristics give evidence for quasiparticle (QP) tunneling across a thin insulating SLCO barrier at the SLCO/Au interface, with a single gap value ∼1.4 meV, originating from superconducting Nb. The absence of a superconducting SLCO gap in the QP conductance curves indicates a thin normal-conducting SLCO layer below the insulating SLCO barrier. To examine its origin, x-ray photoelectron spectroscopy (XPS) and x-ray Auger-electron spectroscopy (XAES) on SLCO thin films were performed. We observe a Cu valence of +1 in the SLCO surface layer (within ∼3 nm thickness) and of +2 in deeper regions, as expected for fully oxidized CuO 2 planes in the bulk. Hence, the XPS and XAES results for the SLCO films are consistent with the QP tunneling spectra observed for our planar SLCO/Au/Nb junctions.

Properties of the electron-doped infinite-layer superconductor Sr1−xLaxCuO2Tomaschko1, Leca2, Selistrovski3 et al. 2012Phys. Rev. BSelf Cite182100View full textAdd to dashboardCite

“…This implies that both phases coexist side by side and not on top of each other. We reported on the same observation in a recent paper 44 . In conclusion, the formation of the IL-r phase is a process that develops laterally on distinct nuclei and not along the film normal.…”

“…Indeed, in a recent paper, we have proven the existence of a thin (∼ 3 nm), oxygen deficient SLCO surface layer, which most probably forms during vacuum annealing. 44 Such a reduced SLCO surface layer explains well the small inplane parameter a SLCO observed by RHEED. Additionally to RHEED, we checked the morphology of the SLCO films with AFM.…”

Importance of grain-boundary Josephson junctions in the electron-doped infinite-layer cuprate superconductor Sr1−xLaxCuOTomaschko1, Leca2, Selistrovski3 et al. 2011Phys. Rev. BSelf Cite

“…28 We note that our values of I 0 R n and j 0 fall right onto the scaling line I 0 R n ∝ 1/ρ 1.5 n shown by Gross and Mayer 36 for YBCO GBJs, which was suggested to be related to the oxygen stoichiometry at the barrier. Thus, we speculate that our small I 0 R n product is a fingerprint of oxygen vacancies at the barrier due to vacuum annealing 26,27 of the as-deposited SLCO films, which would also be in line with the conclusions of Hilgenkamp and Mannhart. 14 The intrinsically shunted junction (ISJ) model 14,36 is based upon such barrier defects, which are supposed to form localized states.…”

“…5,11 This can be attributed to nonoptimum doping of our SLCO films (x = 0.15) and/or to strain effects due to the lattice mismatch of the BaTiO 3 buffer layer and SLCO (a BTO = 3.997Å, a bulk SLCO = 3.949Å). 26,37 For T < T c down to ∼15 K, we observe a "foot structure" in R(T ). Such behavior has been observed, e.g., in YBCO and NCCO GBJs before and was attributed to thermally activated phase slippage (TAPS), 29,38 as described by Ambegaokar and Halperin.…”

“…Very recently, we reported on thin-film planar Sr 1−x La x CuO 2 (SLCO)/Au/Nb junctions, 26 which showed quasiparticle (QP) tunneling, but no Cooper pair (CP) tunneling. In this work, we report on the fabrication and characterization of SLCO bicrystal GBJs.…”