Electric transport across Sr${}_{1-x}$La${}_x$CuO${}_2$/Au/Nb planar tunnel junctions and x-ray photoelectron and Auger-electron spectroscopy on Sr${}_{1-x}$La${}_x$CuO${}_2$ thin films

Thin-film planar tunnel junctions with the electron-doped infinite-layer superconductor Sr${}_{1-x}$La${}_x$CuO${}_2$ (SLCO) with $x\sim 0.15$ 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 $\sim$$1.4$meV, originating from superconducting Nb. The absence of a superconducting SLCO gap in the QP conductance curves indicates a thin normal-conducting SLCO layer below the insulating SLCO barrier. To examine its origin, x-ray photoelectron spectroscopy (XPS) and x-ray Auger-electron spectroscopy (XAES) on SLCO thin films were performed. We observe a Cu valence of +1 in the SLCO surface layer (within $\sim$$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.

Figure 1

Schematic layout (a) and micrograph (b) of planar SLCO/Au/Nb junction. The stacking sequence from bottom to top is SLCO (26 nm)/Au (5 nm)/Nb (21 nm)/Au${}^{*}$ (5 nm)/Nb${}^{*}$ (75 nm). The first three layers forming the junction were deposited in situ. The uppermost two layers (labeled with ${}^{*}$) are required for the patterning process (Ref. 24).

Figure 2

Resistance $R$ vs temperature $T$ measured across the SLCO/Au/Nb junction ($R_{1\text{–}3}$) and along the SLCO bottom layer ($R_{1\text{–}2}$). Inset: $R\left(T\right)$ of the unpatterned SLCO/Au/Nb trilayer.

Figure 3

Transport characteristics of a planar SLCO/Au/Nb junction for different temperatures: (a) $I\left(V\right)$ curves over large voltage range $\left|V\right|⩽300$ mV and (b) zoom-in of low-voltage regime $\left|V\right|⩽3$ mV. (c) Differential conductance curves $dI/dV\left(V\right)$. Inset shows $dI/dV\left(V\right)$ at $T=8.9$ K on a larger voltage scale.

Figure 4

XPS data from SLCO-1 (with IL crystal structure) showing relevant photoemission signals for O 1$s$ (a), Sr 3$d$ (b), and Cu 2$p$ (c) lines at normal incidence 0${}^{\circ }$ and tilt angle ${60}^{\circ }$, corresponding to information depths of 6 and 3 nm, respectively.

Figure 5

Comparison of XP spectra of the superconducting IL (full symbols) and the insulating IL-r sample (open symbols). Upper graphs show normal ($0^{\circ }$) and lower graphs grazing incidence (${60}^{\circ }$) for O 1$s$ (a), Sr 3$d$ (b), and Cu 2$p$ (c). The clear differences between the spectra allow us to exactly identify the superconducting phase as well as surface contaminations.

Figure 6

X-ray excited Cu $L_3{M}_{45}{M}_{45}$ Auger spectra of superconducting IL, insulating IL-r, and Cu reference sample, for normal ($0^{\circ }$) and grazing incidence (${60}^{\circ }$). Curves are shifted equidistantly along the vertical axis by 1. The dashed lines indicate the peak positions of Cu${}^0$, Cu${}^{1+}$, and Cu${}^{2+}$ as expected for cuprate superconductors according to the literature (see text).

Figure 7

XRD data of (single-phase) IL thin-film SLCO-1a and of the strongly vacuum-annealed thin-film SLCO-2a with dominant IL-r phase. Main graph shows $\Theta \text{–}2\Theta$ scans. Inset shows rocking curves around the (002) peaks.