Evidence for the Berezinskii–Kosterlitz–Thouless transition in an Al nanofilm grown by molecular beam epitaxy*

In this work, we present experimental evidence for the Berezinskii–Kosterlitz–Thouless (BKT) transition in an atomic-scale aluminum nanofilm grown on a GaAs substrate by molecular beam epitaxy (MBE). Such a MBE-grown Al film has a higher superconducting critical temperature and a larger critical magnetic field compared to those of bulk Al. In a 4-nm-thick Al film, our results show that V~I³ can occur in both the low-voltage (the BKT transition temperature T_BKT =1.97 K) and high-voltage (T_BKT =2.17 K) regions where V and I correspond to the voltage across the device and the driving current, respectively. By fitting our data to the vortex/anti-vortex model and a model based on dynamical scaling, we unequivocally determine that T_BKT = 2.17 K in our nanofilm. Our new experimental results suggest that when one uses the temperature for which V~I³ to determine T_BKT, great care has to be taken as one also needs to fit one's data to both dynamical scaling and vortex/anti-vortex models.