Anisotropy of the upper critical field and its thickness dependence in superconducting FeSe electric-double-layer transistors

High-temperature superconductivity in FeSe thin films has attracted keen interest owing to the counterintuitive enhancement of the superconducting critical parameters in the monolayer-thin limit. Here, the authors apply an electrochemical etching technique to the FeSe-based electric-double-layer transistor (FeSe-EDLT) to investigate systematically the thickness and angular dependence of the upper critical magnetic field (H${}_c2$) below 10 nm. Compared with the isotropic H${}_c2$ in low-T${}_c$ bulk FeSe, the high-${T}_c$ FeSe-EDLT exhibits highly anisotropic and three-dimensional behavior, with a much smaller out-of-plane coherence length than the interlayer distance between FeSe sheets. The authors consider this to be the reason for the robust high-${T}_c$ superconductivity of monolayer FeSe.