Enhancement of superconductivity in FeSe thin crystals induced by biaxial compressive strain

We report on the enhancement of superconductivity in FeSe thin crystals induced by in-plane biaxial compressive strain, with an underlying scotch tape as an in-situ strain generator. It is found that, due to the compressive strain, the superconducting transition temperature Tc ≈ 9 K of FeSe is increased by 30%-40% and the upper critical field Hc2(0) ≈ 14.8 T is increased by ∼ 20%. In parallel, the T*, which characterizes an onset of enhanced spin fluctuations, is raised up from 69 K to 87 K. On the other hand, the structural transition temperature Ts ≈ 94 K, below which an orthorhombic structure and an electronic nematic phase settle in, is suppressed down by ∼ 5 K. These findings reveal clear evolutions of the orders/fluctuations under strain effect in FeSe, the structurally simplest iron-based superconductor where the lattice/spin/charge degrees of freedom are closely coupled to one another. Moreover, the presented research provides a simple and clean way to manipulate the superconductivity in the layered iron compounds and may promote applications in related materials.