Why Does Undoped FeSe Become a High-Tc Superconductor under Pressure?

Unlike the parent phases of the iron-arsenide high-Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc approximately 9 K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to approximately 37 K. We investigate the electronic properties of FeSe using 77Se NMR to search for the key to the superconducting mechanism. We demonstrate that the electronic properties of FeSe are very similar to those of electron-doped FeAs superconductors, and that antiferromagnetic spin fluctuations are strongly enhanced near Tc. Furthermore, applied pressure enhances spin fluctuations. Our findings suggest a link between spin fluctuations and the superconducting mechanism in FeSe.