Importance of grain boundary Josephson junctions in the electron-doped infinite-layer cuprate superconductor Sr$_{1-x}$La$_x$CuO$_2$

TL;DR

This study demonstrates high critical current densities and detailed tunneling characteristics in grain boundary Josephson junctions of electron-doped Sr$_{1-x}$La$_x$CuO$_2$, revealing key superconducting properties and resonances.

Contribution

First demonstration of high critical current density and detailed tunneling spectroscopy in grain boundary Josephson junctions of electron-doped Sr$_{1-x}$La$_x$CuO$_2$ superconductor.

Findings

High critical current density > 10^3 A/cm^2 at 4.2 K.

Observation of Fraunhofer-like $j_c(H)$ pattern and flux resonances.

V-shaped superconducting gap with $ ext{Δ}_0 \\approx 2.4$ meV.

Abstract

Grain boundary bicrystal Josephson junctions of the electron-doped infinite-layer superconductor Sr$_{1-x}$La$_x$CuO$_2$ ($x = 0.15$) were grown by pulsed laser deposition. BaTiO$_3$-buffered $24\,^\circ$ [001]-tilt symmetric SrTiO$_3$ bicrystals were used as substrates. We examined both Cooper pair (CP) and quasiparticle (QP) tunneling by electric transport measurements at temperatures down to 4.2\,K. CP tunneling revealed an extraordinary high critical current density for electron-doped cuprates of $j_c > 10^3\,$A/cm$^2$ at 4.2\,K. Thermally activated phase slippage was observed as dissipative mechanism close to the transition temperature. Out-of-plane magnetic fields $H$ revealed a remarkably regular Fraunhofer-like $j_c(H)$ pattern as well as Fiske and flux flow resonances, both yielding a Swihart velocity of $3.1\cdot10^6\,$m/s. Furthermore, we examined the superconducting gap by means of QP tunneling spectroscopy. The gap was found to be V-shaped with an extrapolated zero temperature energy gap $\Delta_0 \approx 2.4\,$meV. No zero bias conductance peak was observed.