Evidence for a full energy gap for nickel-pnictide LaNiAsO_{1-x}F_x superconductors by ^{75}As nuclear quadrupole resonance

TL;DR

This study uses NQR and NMR techniques to demonstrate that nickel-pnictide LaNiAsO_{1-x}F_x superconductors have a fully open energy gap, contrasting with iron-pnictides, and highlights the role of Fermi-surface topology in their electronic properties.

Contribution

First systematic NQR and NMR investigation revealing a full superconducting gap in nickel-pnictides and contrasting their electronic behavior with iron-pnictides.

Findings

Superconducting gap is fully opened in LaNiAsO_{1-x}F_x.

Weak electron correlations in the normal state.

Differences attributed to electronic band structure and Fermi-surface topology.

Abstract

We report systematic ^{75}As-NQR and ^{139}La-NMR studies on nickel-pnictide superconductors LaNiAsO_{1-x}F_x (x=0, 0.06, 0.10 and 0.12). The spin lattice relaxation rate 1/T_1 decreases below T_c with a well-defined coherence peak and follows an exponential decay at low temperatures. This result indicates that the superconducting gap is fully opened, and is strikingly different from that observed in iron-pnictide analogs. In the normal state, 1/T_1T is constant in the temperature range T_c \sim 4 K < T <10 K for all compounds and up to T=250 K for x=0 and 0.06, which indicates weak electron correlations and is also different from the iron analog. We argue that the differences between the iron and nickel pnictides arise from the different electronic band structure. Our results highlight the importance of the peculiar Fermi-surface topology in iron-pnictides.