Origin of the tetragonal-to-orthorhombic phase transition in FeSe: a combined thermodynamic and NMR study of nematicity.

The nature of the tetragonal-to-orthorhombic structural transition at T_{s}≈90  K in single crystalline FeSe is studied using shear-modulus, heat-capacity, magnetization, and nuclear magnetic resonance 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 a 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.