Origin of the tetragonal-to-orthorhombic (nematic) phase transition in   FeSe: a combined thermodynamic and NMR study
  The nature of the tetragonal-to-orthorhombic structural transition at $T_s\approx90$ K in single crystalline FeSe is studied using shear-modulus, heat-capacity, magnetization and NMR measurements. The transition is shown to be accompanied by a large shear-modulus softening, which is practically identical to that of underdoped Ba(Fe,Co)$_2$As$_2$, suggesting very similar strength of the electron-lattice coupling. On the other hand, a spin-fluctuation contribution to the spin-lattice relaxation rate is only observed below $T_s$. This indicates that the structural, or "nematic", phase transition in FeSe is not driven by magnetic fluctuations. 
A. E. B\"ohmer, T. Arai, F. Hardy, T. Hattori, T. Iye, T. Wolf, H. v.   L\"ohneysen, K. Ishida, and C. Meingast
cond-mat.supr-con