Pressure-Induced Antiferromagnetic Fluctuations in the Pnictide Superconductor FeSe0.5Te0.5:125Te NMR Study

To investigate the relationship between Superconductivity and low-energy spin fluctuations in the iron-based superconductor FeSe0.5Te0.5, we have conducted Te-125 NMR measurements at ambient pressure and 2GPa. As the superconducting transition temperature T-c is increased by applying pressure, the nuclear spin-lattice relaxation rate divided by temperature, 1/T1T, shows the development of antiferromagnetic fluctuations upon lowering temperature toward T-c. This supports the scenario that spill fluctuations promote superconducting pairing. The depressed Knight shift K-125 and the absence of a coherence peak in 1/T-1, below T-c are consistent with spin-singlet superconducting pairing with an anisotropic order parameter. In the normal metallic state, the comparison between the uniform and dynamic spin Susceptibilities Suggests the existence of a Fermi level located near the singularity of the band structure.