In iron-based superconductors, the mechanism of superconductivity based on spin and/or orbital fluctuations has been discussed since its discovery, but has not yet been solved. In typical iron-based superconductors with an electronic structure consisting of hole and electron Fermi surfaces in similar sizes, spin fluctuations at low-energies increase toward Tc, and the relationship between spin fluctuations and Tc has been reported in many Fe-based superconductors. On the other hand, in the heavily electron-doped Fe-pnictides, the re-enhanced high-Tc phase has been observed without strong spin fluctuations critically enhanced toward Tc [1,2].
In order to elucidate the origin of re-enhanced high-Tc phase in the heavily electron-doped Fe-pnictides, 75As NMR studies are performed systematically on heavily electron-doped LaFePnO0.75H0.25 by controlling the pnictogen height (hPn) from Fe plane through the substitution at Pn(=As) site with Sb or P [3,4]. 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[5]. Such novel spin fluctuations with gap-like feature at low-energies was more enlarged in higher Tc compounds with higher hPn, while those are totally suppressed in non-superconducting compounds with lower hPn [5]. It suggests that the finite energy part in the spin fluctuation spectrum is rather important for enhancing Tc in heavily electron-doped regime, which is in contrast to many cases for the typical Fe-based compounds with hole and electron Fermi surfaces in similar sizes, where the spin fluctuations at low energies develops significantly toward low temperatures. The novel features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from dxy orbital that rises when hPn is high, together with the enhanced electron correlation effects.
The remaining problem is to reveal the possible roles of orbital fluctuations over wide doping region. Recently, we have investigated the Sb-doped LaFeAsO with both antiferromagnetic and orbital orders by means of the isotope 121,123Sb NMR method, and found that the electric relaxation mechanism is significantly enhanced toward structural transition temperature due to the orbital fluctuation. In this presentation, I would like to discuss the roles of spin and orbital fluctuations in LaFe(As1-xPnx)O1-y(F/H)y series.
[1] N. Fujiwara et al., Phys. Rev. Lett. 111, 097002 (2013).
[2] R. Sakurai et al., Phys. Rev. B 91, 064509 (2015).
[3] T. Kawashima et al., Scientific Report 11, 10006 (2021).
[4] K. Suzuki et al., Phys. Rev. Lett. 113, 027002 (2014).
[5] T. Kouchi et al., unpublished.
Keywords: Superconductivity, Iron-pnictide, NMR