Orthorhombic distortion, superconductivity and magnetic fluctuations in FeSe

The recently observed scaling between magnetic and lattice fluctuations observed for Ba(Fe,Co)$_2$As$_2$ provides evidence that its tetragonal-to-orthorhombic structural transition is magnetically driven\footnote{Fernandes et al. PRL 111, 137001 (2013)}. Here, we study the interplay between structure, magnetism and superconductivity in FeSe, an iron-based superconductor which is particularly interesting because it orders magnetically only under high pressure, while a tetragonal-to-orthorhombic structural transition takes place at 90 K at ambient pressure. In contrast to the 122-systems, our high-resolution thermal-expansion data clearly demonstrate that orthorhombic distortion and superconductivity do not compete in FeSe\footnote{B{\"o}hmer et al. PRB 87, 180505 (2013)}, while the shear modulus softening is similar to the 122 systems. By studying magnetic fluctuations using nuclear magnetic resonance, we investigate whether FeSe is simply a case of extreme splitting of magnetic and structural phase transitions - and thus comparable to 122 systems - or whether its structural transition has a qualitatively different origin.