Revealing the A1g-type strain effect on superconductivity and nematicity in FeSe thin flake* *Project supported by the National Key R&D Program of China (Grant Nos. 2017YFA0303000 and 2016YFA0300201), the National Natural Science Foundation of China (Grant No. 11888101), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB25000000), and the Anhui Initiative in Quantum Information Technologies (Grant

Abstract

The driving mechanism of nematicity and its twist with superconductivity in iron-based superconductors are still under debate. Recently, a dominant B1g-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe1–x Co x )2As2, suggesting a strong interplay between nematicity and superconductivity. Since the long-range spin order is absent in FeSe superconductor, whether a similar strain effect could be also observed or not is an interesting question. Here, by utilizing a flexible film as substrate, we successfully achieve a wide-range-strain tuning of FeSe thin flake, in which both the tensile and compressive strain could reach up to ∼0.7%, and systematically study the strain effect on both superconducting and nematic transition (T c and T s) in the FeSe thin flake. Our results reveal a predominant A1g-type strain effect on T c. Meanwhile, T s exhibits a monotonic anti-correlation with T c and the maximum T c reaches to 12 K when T s is strongly suppressed under the maximum compressive strain. Finally, in comparison with the results in the underdoped Ba(Fe1–x Co x )2As2, the absence of B1g-type strain effect in FeSe further supports the role of stripe-type spin fluctuations on superconductivity. In addition, our work also supports that the orbital degree of freedom plays a key role to drive the nematic transition in FeSe.