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

Unlike the parent phases of the iron-arsenide high-T-c superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with T-c similar to 9 K. Equally surprising is the fact that applied pressure dramatically enhances the modest T-c to similar to 37 K. We investigate the electronic properties of FeSe using Se-77 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 T-c. Furthermore, applied pressure enhances spin fluctuations. Our findings suggest a link between spin fluctuations and the superconducting mechanism in FeSe.