Coupled magnetic and nematic fluctuations in S-doped FeSe

Iron selenide (FeSe) is the simplest of the iron-based superconductors. It has a nematic (tetragonal to orthorhombic) phase transition at T = 90K. Upon doping with sulphur, the nematic transition temperature is suppressed, until at 17% doping it disappears entirely at what has been called a nematic quantum critical point. The questions explored in this proposal are, to what extent are the spin and nematic fluctuations coupled, and what are the implications for superconductivity? We shall measure the spin fluctuation spectrum in the nematic quantum critical system, using a powder sample. This study was started on Mari, but due to an anomalous background signal and detector gaps, the signal could not be measured at low energies.