NMR study of spin fluctuations and superconductivity in LaFeAsO$_{1-x}$H$_x$

We have performed NMR measurements in LaFeAsO$_{1-x}$H$_x$, an isomorphic compound of LaFeAsO$_{1-x}$F$_x$. LaFeAsO$_{1-x}$H$_x$ is most recently known for having double superconducting (SC) domes on H doping. LaFeAsO$_{1-x}$H$_x$ is an electron- doped system, and protons act as H$^{-1}$ as well as F$^{-1}$. The first SC dome is very similar between F and H doping, suggesting that H doping supplies the same amount of electrons as F doping. Interestingly, an excess amount of H up to x=0.5 can be replaced with O$^{2-}$. In the H-overdoped regime ($x > 0.2$), LaFeAsO$_{1-x}$H$_x$ undergoes the second superconducting state [1]. We measured the relaxation rate of LaFeAsO$_{1-x}$H$_x$ for x=0.2 and 0.4, and fond an anomalous electronic state; spin fluctuations measured from $1/T_1T$ is enhanced with increasing the doping level from $x=0.2$ to 0.4. The enhancement of spin fluctuations with increasing carrier doping is a new phenomenon that has not observed in LaFeAsO$_{1-x}$F$_x$ in which the upper limit of the doping level is at most $x=0.2$. We will discuss the phenomenon in relation to superconductivity.\\[4pt] [1] S. Iimura, $et. al.$, Nature Communications (2012)