NMR evidence for an intimate relationship between antiferromagnetic spin fluctuations and extended s-wave superconductivity in mono-crystalline SrFe2(As_{1-x}P_{x})2

Abstract

We report systematic 31P-NMR study on iron (Fe)-based superconductors SrFe2(As_{1-x}P_{x})2 (Sr122AsP), in which a superconducting (SC) transition temperature Tc at x=0.35 increases from Tc=26 K up to 33 K by annealing an as-grown mono-crystalline sample. The present NMR study has unraveled that Tc reaches a 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 of Tc are apparently associated with the emergence and enhancement of AFM-SFs, respectively. 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 RDOS as expected for unconventional SC state with nodal gap. These findings in Sr122AsP are consistent with the unconventional s(+-)-wave Cooper pairing state that is mediated by AFM-SFs. We also discuss other key-ingredients besides the AFM-SFs to increase Tc further.

6 pages, 4 figures, Accepted for publication in Phys. Rev. B

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• Multiple Antiferromagnetic Spin Fluctuations and Novel Evolution of Tc in Iron-based superconductors LaFe(As1-xPx)(O1-yFy) revealed by 31P-NMR Studies
• Universal relationship between low-energy antiferromagnetic fluctuations and superconductivity in BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$
• High-$T_c$ Iron-phosphide Superconductivity Enhanced by Reemergent Antiferromagnetic Spin Fluctuations in (Sr${}_4$Sc${}_2$O${}_6$)Fe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ probed by NMR
• Unconventional Multi-gap Superconductivity and Antiferromagnetic Spin Fluctuations in New Iron-arsenide LaFe2As2 in Heavily Electron-doped Regime
• ${}^{31}$P NMR studies of an iron-based superconductor Ba${}_{0.5}$Sr${}_{0.5}$Fe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ with $T_{\mathrm{c}}$ = 29 K
• Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor
• Electronic Structure of ${\mathrm{Sr}}_{1-y}{\mathrm{Ca}}_y\mathrm{F}{\mathrm{e}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ ($x$ = 0.25, $y$ = 0.08) Revealed by Angle-Resolved Photoemission Spectroscopy