Multiple Antiferromagnetic Spin Fluctuations and Novel Evolution of Tc in Iron-based superconductors LaFe(As1-xPx)(O1-yFy) revealed by 31P-NMR Studies

TL;DR

This study uses 31P-NMR to explore antiferromagnetic spin fluctuations in LaFe(As_{1-x}P_x)(O_{1-y}F_y), revealing their crucial role in enhancing superconducting transition temperature (Tc) through multiple orbital contributions.

Contribution

It uncovers two distinct types of AFMSFs with different energy characteristics and links their orbital origins to the nonmonotonic Tc variation in iron-based superconductors.

Findings

Two types of AFMSFs identified with different energy scales.

AFMSFs associated with specific Fe 3d orbitals (d_{xz/yz} and d_{xy}).

Nonmonotonic Tc variation explained by multiple AFMSFs from degenerated orbitals.

Abstract

We report on 31P-NMR studies of LaFe(As_{1-x}P_x)(O_{1-y}F_{y}) over wide compositions for 0<x<1 and 0<y<0.14, which provide clear evidence that antiferromagnetic spin fluctuations (AFMSFs) are one of the indispensable elements for enhancing Tc. Systematic 31P-NMR measurements revealed two types of AFMSFs in the temperature evolution, that is, one is the AFMSFs that develop rapidly down to Tc with low-energy characteristics, and the other, with relatively higher energy than the former, develops gradually upon cooling from high temperature. The low-energy AFMSFs in low y (electron doping) over a wide x (pnictogen height suppression) range are associated with the two orbitals of d_{xz/yz}, whereas the higher-energy ones for a wide y region around low x originate from the three orbitals of d_{xy} and d_{xz/yz}. We remark that the nonmonotonic variation of Tc as a function of x and y in LaFe(As_{1-x}P_x)(O_{1-y}F_y) is attributed to these multiple AFMSFs originating from degenerated multiple 3d orbitals inherent to Fe-pnictide superconductors.