Transport properties of FeSe thin films with various degrees of strain

Iron chalcogenide superconductor, FeSe, is a suitable material to investigate the role of the nematic order for the superconductivity in iron pnictides and chalcogenides because it does not show a magnetic order, different from other iron based materials. In order to clarify the relation between the superconductivity and the nematicity in iron chalcogenides, we investigated effects of in-plane strain in FeSe films. This method is more favorable than studies on chemical pressure effects in that there is no need for substitution of impurity.
We have successfully grown FeSe thin films with systematically varied degrees of strain, from tensile to compressive strain, via pulsed laser deposition. Magneto-transport measurements of the grown films suggested that the carrier density increased as the a-axis length shrinks, consistent with an ARPES observation[1]. The superconducting transition temperature also varied depending on the strain from below 2 K up to 12 K, which is higher than that of bulk crystals, while only a slight change was observed in the nematic transition temperature. These results imply that the nematic order is not important for superconductivity in this material.
[1] G. N. Phan et al., PRB 95, 224507 (2017).