Two-dimensional superconductivity and violation of Pauli paramagnetic limit in MBE-grown Al nanofilms*

We have performed low-temperature transport measurements on Al nanofilms on GaAs substrates prepared by molecular beam epitaxy (MBE) with as-grown thicknesses of 3 nm, 3.5 nm, and 4 nm. Such MBE-grown nanofilms all show the Berezinskii-Kosterlitz-Thouless transition, which is compelling evidence for two-dimensional superconductivity, with enhanced transition temperatures (≈2.3 K) compared to that of bulk Al (1.2 K). When the magnetic field is applied parallel to the plane of the Al nanofilms, we measure the upper critical magnetic field H_c^∥ by the mid-points of resistive transitions at different temperatures T . In this way, we can estimate H_c^∥(T=0). It is found that H_c^∥(T=0) increases with decreasing film thickness. Interestingly, for the thinnest device, H_c^∥(T=0) exceeds the Pauli paramagnetic limit. Such experimental results can be ascribed to spin-orbit coupling effects on superconductivity, even though Al has a low atomic mass.