Revealing the A1g-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 B1g-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe_(1-x)Co_(x))_(2)As_(2),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 Ts)in the FeSe thin flake.Our results reveal a predominant A1g-type strain effect on T_(c).Meanwhile,Ts exhibits a monotonic anti-correlation with T_(c)and the maximum T_(c)reaches to 12 K when Ts is strongly suppressed under the maximum compressive strain.Finally,in comparison with the results in the underdoped Ba(Fe_(1-x)Co_(x))_(2)As_(2),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.