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

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To investigate the relationship between superconductivity and low-energy spin fluctuations in the iron-based superconductor FeSe0.5Te0.5, we have conducted 125Te NMR measurements at ambient pressure and 2 GPa. As the superconducting transition temperature $T_{\text{c}}$ 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 $T_{\text{c}}$. 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 $T_{\text{c}}$ 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.
2009-12-15