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 FeSe 0.5 Te 0.5 , we have conducted 125 Te NMR measurements at ambient pressure and 2 GPa. As the superconducting transition temperature T c is increased by applying pressure, the nuclear spin–lattice relaxation rate divided by temperature, 1/ T 1 T , shows the development of antiferromagnetic fluctuations upon lowering temperature toward T c . This supports the scenario that spin fluctuations promote superconducting pairing. The depressed Knight shift 125 K 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.