Recent experiments resolve an important open question concerning the interplay between magnetism and structure, which is ubiquitous in iron-based superconductors. Studies of the iron-selenium compound using x-ray diffraction and time-domain Mössbauer spectroscopy under applied pressure at the Department of Energy’s Advanced Photon Source confirm that structural nematicity—long-range, orientational order—and magnetic order in FeSe are indeed strongly and cooperatively coupled. At pressures between 1.0 GPa (about 10,000x that of ambient pressure) and 1.7 GPa, separate structural and magnetic transitions are observed that merge into a single first-order transition for pressures greater than 1.7 GPa. The results point to an underlying spin-driven nematic order as a basis for a possible universal description of the normal state properties of iron-based superconductors. Future work will look at the evolution of magnetism and structure as the pressure is increased to even higher values.
Evolution of the in-plane lattice parameters at various pressures determined from the splitting of the tetragonal (HH0) Bragg peaks.
Strong Cooperative Coupling of Pressure-induced Magnetic Order and Nematicity in FeSe