Granular Superconductivity in La$_{2}$PrNi$_{2}$O$_{7-\delta}$ Thin Films

TL;DR

This study uncovers that the two-step resistive transition in La$_{2}$PrNi$_{2}$O$_{7- ext{delta}}$ thin films is due to their granular nature, involving distinct superconducting phases coupled by Josephson junctions, which hampers achieving higher zero-resistance temperatures.

Contribution

It identifies the microscopic origin of the two-step transition as granular superconductivity and highlights the importance of oxygen homogeneity for better superconducting properties.

Findings

Two-step transition caused by coexistence of two superconducting grain phases.

Lower transition temperature around 10 K limits zero-resistance achievement.

Granular nature impedes spectroscopic studies and bulk superconductivity.

Abstract

Superconductivity realized in bilayer nickelate thin films enables direct spectroscopic and transport studies at ambient pressure. However, a persistent two-step resistive transition remains a major barrier to achieving optimal superconducting properties. Here, we show that the two-step transition in La$_2$PrNi$_2$O$_{7-\delta}$ thin films originates from the granular nature of superconductivity, specifically, the coexistence of two distinct superconducting grain phases coupled by a Josephson junction network. A secondary, lower-temperature transition appears in the $R(T)$ curve, even when residual resistance becomes vanishingly small near 30 K. This two-step behavior significantly lowers the zero-resistance transition temperature, $T_{c, zero}$$\approx$ 10 K, and limits advanced spectroscopic studies. Our findings reveal the microscopic mechanism underlying the two-step transition in thin films and underscore the need for improved oxygen homogeneity to achieve bulk superconductivity in this system.