Experimental evidence of T c enhancement without the influence of spin fluctuations: NMR study on LaFeAsO 1 − x H x under a pressure of 3.0 GPa

The electron-doped high-transition-temperature $({T}_{c})$ iron-based pnictide superconductor ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{H}}_{x}$ has a unique phase diagram: Superconducting double domes are sandwiched by antiferromagnetic phases at ambient pressure and they turn into a single dome with a maximum ${T}_{c}$ that exceeds 45 K at a pressure of 3.0 GPa. We studied whether spin fluctuations are involved in increasing ${T}_{c}$ under a pressure of 3.0 GPa by using the $^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) technique. The $^{75}\mathrm{As}$-NMR results for the powder samples show that ${T}_{c}$ increases up to 48 K without the influence of spin fluctuations. This fact indicates that spin fluctuations are not involved in raising ${T}_{c}$, which implies that other factors, such as orbital degrees of freedom, may be important for achieving a high ${T}_{c}$ of almost 50 K.