Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor

Abstract

To elucidate the origin of the reenhanced high-Tc phase in the heavily electron-doped Fe pnictides, systematic As75 NMR studies are performed on heavily electron-doped LaFePnO0.75H0.25 by controlling the pnictogen height (hPn) from the Fe plane through the substitution at Pn(=As) site with Sb or P. The measurements of nuclear spin relaxation rate (1/T1) and Knight shift (K) reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gaplike feature at low energies are more enlarged in higher Tc compounds with higher hPn, while those are totally suppressed in nonsuperconducting compounds with lower hPn. This implies that the contribution of the finite-energy part in the spin-fluctuation spectrum is crucial for enhancing Tc in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from the dxy orbital that rises when hPn is high, together with the enhanced electron correlation effects.

Source-reported materials — not catalogue approval

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