Anisotropic High Upper Critical Field in Ultrathin MgB₂ Film

Ultrathin superconducting films with high transition temperature and high critical magnetic field are desirable for many potential applications, such as single photon detector and hybrid superconductor-topological material structure for quantum computing. MgB ₂ is one of the highest ${T}_c$ s -wave superconductors and therefore is very promising for these applications. In this work, we studied the superconducting transition of ultrathin MgB ₂ film with film thickness in the nanometer scale. The ${T}_c$ of the nominal 3 nm- thick film is above 30 K. The upper critical field exhibits a good agreement with the phenomenological Ginzburg-Landau expression for a 2D superconducting film, i.e. , $B_{c2}^ \bot \propto 1 - T/{T}_c$ for an out-of-plane magnetic field and $B_{c2}^\parallel \propto {({1 - T/{T}_c})}^{0.5}$ for an in-plane magnetic field. The extrapolation of upper critical field yields $B_{c2}^ \bot ({0\ \mathrm{K}})\ = \ 7.5\ \mathrm{T}$ and $B_{c2}^\parallel \ ({0\ \mathrm{K}}) = \ 70\ \mathrm{T}$ in a 3 nm- thick film. The large anisotropy indicates that the superconductivity of ultrathin MgB ₂ film is strongly 2D in nature. The angular dependence of the upper critical field ${B}_{c2}(\theta)$ shows a sharp cusp feature when the magnetic field is parallel to the film. The 2D Tinkham model could describe the trend of angular dependent upper critical field, which once again indicates the 2D superconducting nature of the MgB ₂ films.