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

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.

4 pages, 5 figures

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• The Puzzle of High Temperature Superconductivity in Layered Iron Pnictides and Chalcogenides
• Quantum Criticality in Electron-doped BaFe_{2-x}Ni_xAs_2
• ${}^{75}$As NQR and NMR studies of superconductivity and electron correlations in iron arsenide Li${}_x$FeAs
• Antiferromagnetic Spin Fluctuation above the Superconducting Dome and the Full-Gaps Superconducting State in LaFeAsO1-xFx Revealed by 75As-Nuclear Quadrupole Resonance
• Coexistence of multiple charge-density waves and superconductivity in SrPt2As2 revealed by 75As-NMR/NQR and 195Pt-NMR
• Fully gapped superconductivity in SrNi${}_2$P${}_2$
• Multiple Superconducting Gaps, Anisotropic Spin Fluctuations and Spin-Orbit Coupling in Iron-Pnictides
• Magnetic and structural properties of the iron silicide superconductor LaFeSiH
• Multiple superconducting gap and anisotropic spin fluctuations in iron arsenides: Comparison with nickel analog