FeSe is a unique superconductor that can be manipulated to enhance its superconductivity using different routes, while its monolayer form grown on different substrates reaches a record high temperature for a two-dimensional system. In order to understand the role played by the substrate and the reduced dimensionality on superconductivity, we examine the superconducting properties of exfoliated FeSe thin flakes by reducing the thickness from bulk down towards 9 nm. Magnetotransport measurements performed in magnetic fields up to 16 T and temperatures down to 2 K help to build up complete superconducting phase diagrams of different thickness flakes. While the thick flakes resemble the bulk behaviour, by reducing the thickness the superconductivity of FeSe flakes is suppressed. The observation of the vortex-antivortex unbinding transition in different flakes provide a direct signature of a dominant two-dimensional pairing channel. However, the upper critical field reflects the evolution of the multi-band nature of superconductivity in FeSe becoming highly two-dimensional and strongly anisotropic only in the thin limit. Our study provides detailed insights into the evolution of the superconducting properties of a multi-band superconductor FeSe in the thin limit in the absence of a dopant substrate.

FeSe是一种独特的超导体，可以使用不同的途径对其进行操作以增强其超导性，而在不同基材上生长的单层形式则达到了二维系统的创纪录高温。为了了解基材的作用和尺寸减小对超导性的影响，我们通过将厚度从体积减小到9 nm来检查剥落的FeSe薄片的超导性能。在最高16 T的磁场和最低2 K的温度下进行的磁传输测量有助于建立不同厚度薄片的完整超导相图。尽管较厚的薄片类似于整体性能，但通过减小厚度，可以抑制FeSe薄片的超导性。在不同薄片中涡旋-反涡旋未结合转变的观察提供了显性二维配对通道的直接特征。然而，上临界场反映了FeSe中超导性的多频带性质的演变，仅在薄限内才变为高度二维且强各向异性。我们的研究提供了在没有掺杂剂衬底的情况下，在薄极限内多带超导体FeSe的超导性能演变的详细见解。