The link between superconductivity and nematicity revealed under pressure

The link between superconductivity and critical fluctuations emerging close to a distinct phase transition is one of the main unsolved problems in current research on quantum materials. It has been shown that the atomic vibrations of BCS (Bardeen, Cooper and Schrieffer) theory can be replaced by critical fluctuations of electronic origin. These fluctuations can mediate an effective attraction between electrons, giving rise to unconventional superconductivity. Initially envisioned for magnetic fluctuations, this scenario has been recently extended to the case of electron nematic fluctuations associated to an electronic order breaking the rotational symmetry of the crystal lattice. Nematic fluctuations could play a key role in the superconducting mechanism of several high-temperature superconductors like copper oxides and iron-based systems.

The article “Collapse of Critical Nematic Fluctuations in FeSe Under Pressure” explores experimentally this question on the iron-based superconductor FeSe. This remarkable compound shows a strong increase of its superconducting transition temperature (Tc), from 8K to almost 40K, upon application of hydrostatic pressure. The authors managed to track the evolution of nematic fluctuations up to 8 GPa via polarized Raman scattering measurements in a diamond anvil cell. Surprisingly, they have observed a quick collapse of these fluctuations upon increasing the pressure, demonstrating that they cannot be associated to the large Tc increase. However anomalies observed in the pressure dependence of lattice vibrations indicate an abrupt change of the Fermi surface topology which could be the main factor in explaining the Tc increase.

Contact:
Yann Gallais (yann.gallais@univ-paris-diderot.fr)

Reference:
Collapse of Critical Nematic Fluctuations in FeSe under Pressure, Pierre Massat, Yundi Quan, Romain Grasset, Marie-Aude Méasson, Maximilien Cazayous, Alain Sacuto, Sandra Karlsson, Pierre Strobel, Pierre Toulemonde, Zhiping Yin, and Yann Gallais, Phys. Rev. Lett. 121, 077001 (2018)