Revealing the A_1g-type strain effect on superconductivity and nematicity in FeSe thin flake

The driving mechanism of nematicity and its twist with superconductivity in iron-based superconductors are still under debate. Recently, a dominant B_1g-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe_1-xCo_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 A_1g-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(Fe_1-xCo_x)₂As₂, the absence of B_1g-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.