The discovery of the iron-based superconductors a decade ago has put a new fire into the research of unconventional superconductivity. Even the simplest compound of the family – FeSe – has opened up a canvas of new scientific discussions.

Recently, we tackled a few problems that were open in this emblematic compound. There are different magnetic and superconducting phases present in the material and the questions on how they compete and transform from one phase into another have been puzzling the community for a long time. Muon spin rotation under high pressures turned out to be the key experimental technique to answer them – it offers the independent measurement of the volume fraction and magnetic moment as a function of a control parameter.

First, we could find a tricritical point in the pressure-temperature phase diagram, where the magnetic ordering phase transition at very high pressures switches from the second-order to the first order.
Find more at Phys. Rev. B 97, 224510.

Second, when looking at the version with the substitution of sulfur for selenium, we found an emergence of a magnetically ordered phase, similar to the case of the pure system but shifted to lower pressures. We could study the complex interplay between the superconductivity and the magnetism, but what we found to be the most surprising was the discovery of an extended dome of long-range magnetic order that spans a pressure range between previously reported separated magnetic phases.
Find more at Phys. Rev. Lett. 123, 147001