Strong-coupling superconductivity in layered nickel-based LaO$_{1-x}$F$_x$NiAs

TL;DR

This study reports the synthesis and characterization of layered nickel-based LaO$_{1-x}$F$_x$NiAs superconductors, revealing strong-coupling behavior and multi-gap features with transition temperatures up to 3.8 K.

Contribution

It provides new experimental evidence of strong-coupling and multi-gap superconductivity in nickel-based layered compounds, expanding understanding of their pairing mechanisms.

Findings

Superconductivity observed with Tc up to 3.8 K.

Specific heat jump exceeds weak-coupling BCS predictions.

Deviations from single-band s- or d-wave models suggest multi-gap superconductivity.

Abstract

A series of layered nickel-based LaO$_{1-x}$F$_x$NiAs compounds with x=0 to 0.15 are synthesized by solid state reactions. The pure LaONiAs exhibits bulk superconductivity with Tc $\sim$ 2.75 K. Partial substitution of oxygen by fluorine increases the transition temperature to $\sim 3.8$ K. The LaO$_{0.9}$F$_{0.1}$NiAs sample shows a sharp superconducting transition and a sharp specific heat jump at the critical temperature. The magnitude of the specific heat jump is much larger than that expected from the weak-coupling BCS theory, indicating that this superconductor is in the strong coupling regime. Furthermore, the temperature dependence of the specific heat deviates strongly from the theoretical result for the single-band s- or d-wave superconductor, but shows some character of a multi-gap system.