Origin of the tetragonal-to-orthorhombic (nematic) phase transition in FeSe: a combined thermodynamic and NMR study

TL;DR

This study investigates the structural transition in FeSe at around 90 K, revealing it is driven by electron-lattice interactions rather than magnetic fluctuations, through combined thermodynamic and NMR measurements.

Contribution

It provides evidence that the nematic phase transition in FeSe is primarily due to electron-lattice coupling, not magnetic fluctuations, contrasting with other iron-based superconductors.

Findings

Large shear-modulus softening at transition

Spin fluctuations only below transition temperature

Transition driven by electron-lattice interactions

Abstract

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.