Effect of magnetic frustration on nematicity and superconductivity in iron chalcogenides

Over the past few years iron chalcogenides have been intensively studied as part of the wider family of iron-based superconductors, with many intriguing results reported so far on intercalated and monolayer Fe Se. Nevertheless, bulk Fe Se itself remains an unusual case when compared with pnictogen-based iron superconductors, and may hold clues to understanding the more exotic derivatives of the Fe Se system. The Fe Se phase diagram is distinct from the pnictides: the orthorhombic distortion, which is likely to be of a 'spin-nematic' nature in numerous pnictides, is not accompanied by magnetic order in Fe Se, and the superconducting transition temperature T-c rises significantly with pressure before decreasing. Here we show that the magnetic interactions in Fe Se, as opposed to most pnictides, demonstrate an unusual and unanticipated frustration, which suppresses magnetic (but not nematic) order, triggers ferro-orbital order in the nematic phase and can naturally explain the non-monotonic pressure dependence of the superconducting critical temperature T-c(P).