Pure nematic state in iron-based superconductor

Lattice and electronic states of thin FeSe films on LaAlO${}_3$ substrates are investigated in the vicinity of the nematic phase transition. No evidence of structural phase transition is found by x-ray diffraction below $T^{\ast }\sim 90$ K, while results obtained from resistivity measurement and angle-resolved photoemission spectroscopy clearly show the appearance of a nematic state. These results indicate formation of a pure nematic state in the iron-based superconductor and provide conclusive evidence that the nematic state originates from the electronic degrees of freedom. This pure nematicity in the thin film implies difference in the electron-lattice interaction from bulk FeSe crystals. FeSe films provide valuable playgrounds for observing the pure response of “bare” electron systems free from the electron-lattice interaction, and should make important contribution to investigate nematicity and its relationship with superconductivity.