Layer thickness and substrate effects on superconductivity in epitaxial FeSe films on BLG/SiC(0001)

The layered nature and simple structure of FeSe reveal this iron-based superconductor as a unique building block for the design of artificial heterostructure materials. While superconductivity develops in ultrathin films of FeSe on <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>SrTi</a:mi><a:msub><a:mi mathvariant="normal">O</a:mi><a:mn>3</a:mn></a:msub></a:mrow></a:math> substrates, it remains unclear whether it can be developed on more chemically inert, layered materials such as graphene. Here, we report on the characterization of the structural, chemical, and electronic properties of few-layer and multilayer FeSe on bilayer graphene grown on SiC(0001) using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and x-ray photoelectron spectroscopy (XPS). STM imaging of our FeSe films with thicknesses up to three layers exhibits the tetragonal crystal structure of bulk FeSe, which is supported by XPS spectra consistent with the FeSe bulk counterpart. While our STS measurements at 340 mK reveal a metallic character for few-layer FeSe on BLG/SiC(0001), they show an absence of superconductivity, as the low-lying electronic structure exhibits a spatially anisotropic dip-like feature robust against magnetic fields. However, we find that superconductivity in FeSe/BLG/SiC(0001) starts to emerge from a thickness of five layers. Cooper pairing strength increases with film thickness, reaching critical temperatures up to 6 K above 20 layers. Our results underscore the significance of the substrate and thickness as key factors driving the suppression of superconductivity in FeSe in the 2D limit.