NMR evidence for an intimate relationship between antiferromagnetic spin fluctuations and extended s-wave superconductivity in monocrystalline SrFe2(As1−xPx)2

Abstract

We report on a systematic P31-NMR study on an iron- (Fe-) based superconductor SrFe2(As1−xPx)2 (Sr122AsP) in which a superconducting (SC) transition temperature Tc at x=0.35 increases from Tc=26K up to 33 K by annealing an as-grown monocrystalline sample. The present NMR study has unraveled that Tc reaches the highest value of 33 K at x=0.35 around a quantum critical point at which antiferromagnetic (AFM) order disappears. When noting that the SC transition disappears at x=0.6 where the AFM spin fluctuations (SFs) are no longer present, we remark that the onset and increase in Tc are apparently associated with the emergence and enhancement, respectively, of the AFM-SFs. In the SC state, the residual density of state (RDOS) at the Fermi energy EF in the SC state becomes much smaller for the annealed sample than for the as-grown one, suggesting that some inhomogeneity and/or imperfection for the latter increases the RDOS as expected for the unconventional SC state with a nodal gap. These findings in Sr122AsP are consistent with the unconventional s±-wave Cooper pairing state that is mediated by the AFM-SFs. We also discuss other key ingredients besides the AFM-SFs to increase Tc further.

Source-reported materials — not catalogue approval

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