Experimental evidence of Tc enhancement without the influence of spin fluctuations: NMR study on LaFeAsO1-xHx under a pressure of 3.0 GPa

The electron-doped high-transition-temperature (T-c) iron-based pnictide superconductor LaFeAsO1-xHx 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 As-75 nuclear magnetic resonance (NMR) technique. The As-75-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.