Evidence for a full energy gap in the nickel pnictide superconductorLaNiAsO1−xFxfromA

Abstract

We report systematic 75 As-NQR and 139 La-NMR studies on nickel-pnictide superconductors LaNiAsO1−xFx (x=0, 0.06, 0.10 and 0.12). The spin lattice relaxation rate 1/T1 decreases below Tc 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/T1T is constant in the temperature range Tc ∼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. 74.25.Nf, 74.90.+n

Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAsT1, Li2, Kawasaki3 et al. 2012Phys. Rev. Lett.Self Cite7612795View full textAdd to dashboardCite

“…The low-frequency NQR peak for x = 0.04 gives a smaller θ ∼ 5 K. The increase of θ with increasing x means that the system moves away from the magnetic instability (MI) where θ = 0 K. With further doping, for x = 0.10 and 0.15, no enhancement of 1/T 1 T is seen. Instead, 1/T 1 T decreases with decreasing T , which was recently explained by the loss of the DOS due to a topological change of the Fermi surface [9,25]. The results in previous reports of the lack of the AFSF for x ≥ 0.10 [12,13,15] are consistent with our results for x = 0.10 and 0.15.…”

“…Previous nuclear-magnetic resonance (NMR) and nuclear-quadrupole resonance (NQR) measurements have found that the superconductivity is in the spin-singlet state with multiple gaps [4][5][6]. Recent systematic measurements on Ba(Fe 1−x Co x ) 2 As 2 [7], CaFe 2 As 2 under pressure [8], LaNiAsO 1−x F x [9], and BaFe 2 (As 1−x P x ) 2 [10] have suggested that the antiferromagnetic spin fluctuation (AFSF) originated from their multiple electronic bands correlates with the appearance of the pertinent superconducting properties. On the other hand, there are also reports suggesting that AFSF is not important to realize high T c .…”

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

“…86. 443 The data show a peak at T c for each sample, called the Hebel-Slichter peak, which is a signature of a conventional s-wave superconducting state. The data follow exponential temperature dependences below T c , indicated as solid curves in the figure, with superconducting energy gaps 2∆ that are close to the BCS value of 3.53 k B T c .…”

Quantum criticality in electron-doped BaFe2−xNixAs2

“…5b, where K 0 is T-independent, while the second is due to the band that sinks below the Fermi level 21,22 . Correspondingly, we can write…”