Controlling dendritic flux avalanches by nanostructure of superconducting films

Niobium Nitrate (NbN) superconducting films are extensively used in superconducting electronics, for example as basic element of single-photon microwave resonators. Here we report on direct visualisation of magnetic flux penetration into a NbN thin film deposited by High-Temperature Chemical Vacuum Deposition (HTCVD). The film is of the thickness of 90.8 nm. It is deposited at temperature of 1200 C on a single-crystal α-Al2O3 (0001) c-axis substrate (sapphire). The visualisation is done by Magneto-Optical imaging allowing to see directly distribution of magnetic flux in the superconductor. It is found that at low temperatures magnetic flux penetrates into the film in the form of dendritic flux avalanches. Moreover, the shape of dendritic avalanches appeared to be very unusual, previously not reported in the literature. The branches of avalanches persistently follow one specific direction in the plane of the film. To clarify the origin of this effect, high-resolution Scanning Electron Microscopy and Atomic Force Microscopy have been used in combination with the Fast Fourier Transform of the obtained images. It was found that the origin of the selected direction in the dendritic flux penetration is deep on the nanometre scale, namely in nano-channels formed by the merging NbN crystallites during their growth. In this way, nanostructure of the film directly controls dendritic flux avalanches in the superconductor. Varying conditions of deposition would allow actively changing superconducting properties of the films.