Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe

Abstract

The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature ($T_{\mathrm{c}}$) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. More importantly, a pressure-induced fourfold increase of $T_{\mathrm{c}}$ has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to $\sim 9$ GPa, which uncover a hidden magnetic dome superseding the nematic order. Above $\sim 6$ GPa the sudden enhancement of superconductivity ($T_{\mathrm{c}}\le 38.3$ K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed above the high-$T_{\mathrm{c}}$ phase. The obtained phase diagram highlights unique features among iron-based superconductors, but bears some resemblance to that of high-$T_{\mathrm{c}}$ cuprates.

15 pages, 4 figures