Distinct pressure evolution of coupled nematic and magnetic orders in FeSe

Abstract

We present a microscopic study of nematicity and magnetism in FeSe over a wide temperature and pressure range using high-energy x-ray diffraction and time-domain M\"ossbauer spectroscopy. The low-temperature magnetic hyperfine field increases monotonically up to $\ensuremath{\sim}6$ GPa. The orthorhombic distortion initially decreases under increasing pressure but is stabilized at intermediate pressures by cooperative coupling to the pressure-induced magnetic order. Close to the reported maximum of the superconducting critical temperature at $p=6.8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, the orthorhombic distortion suddenly disappears and a new tetragonal magnetic phase occurs. The pressure and temperature evolution of the structural and magnetic order parameters suggests that they have distinct origins.