Why Does Undoped FeSe Become A High Tc Superconductor Under Pressure?

TL;DR

This study uses NMR to show that undoped FeSe's electronic properties resemble electron-doped FeAs superconductors and that pressure enhances spin fluctuations, which are linked to its high Tc superconductivity.

Contribution

It reveals the similarity of FeSe's electronic properties to doped FeAs superconductors and demonstrates pressure-induced enhancement of spin fluctuations.

Findings

Spin fluctuations are strongly enhanced near Tc.

Pressure increases spin fluctuations in FeSe.

Electronic properties of FeSe resemble electron-doped FeAs superconductors.

Abstract

Unlike the parent phases of the iron-arsenide high Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc~9K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to ~37K. 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.