Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and 77Se-NMR Study

We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature (T-c) increases sensitively under pressure, but shows a plateau between 0.5-1.5GPa. The maximum T-c, which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches similar to 37 K at 4.15 GPa. We also measure the nuclear spin-lattice relaxation rate 1/T-1 under pressure using Se-77-NMR measurement. 1/T-1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/T1T just above Tc shows a plateau as well as the pressure dependence of T-c, which gives clear evidence of the close relationship between 1/T1T and T-c. Spin fluctuations are suggested to contribute to the mechanism of superconductivity.