Influence of hydrostatic pressure on superconducting properties of niobium thin film
We have studied superconducting properties of niobium thin films under hydrostatic pressures up to 3 GPa. The films with thickness of 100 nm were prepared in the high vacuum DC magnetron sputtering system (with critical temperature Tc= 8.95 K at ambient pressure). The produced high quality films have been characterized using electrical resistivity and magnetization measurements, X-ray diffraction, and atomic force microscope imaging. We have observed increase of Tc with increasing value of applied pressure (dTc/dp = 73 mK/GPa) up to 3 GPa. This observation is different to pressure effect observed on bulk sample of Nb (see Fig. 1). After the releasing the pressure, we observed decrease of Tc value to 8.86 K (denoted by star in Fig. 1), which is lower than before applying of pressure. We suppose that the application of pressure can release the strains induced in thin film during sputtering. The change of microstructure by application of pressure was studied by XRD and by AFM imaging. Fig. 2 shows XRD pattern of  reflection of bcc structure of Nb thin films as prepared (full circles) and thin film after pressure treatment (open squares). The observed pattern shows that in as prepared thin film we have expansion of the Nb lattice parameter. After pressure treatment we have two peaks which correspond to grains with expanded lattice parameter (small peak at around 37 degrees) and grains with lattice parameter value close to bulk Nb. The increased background point to increase of amorphous phase. This picture of changes in grain structure is in agreement with AFM imaging results (see Fig. 3).
Figure 1: Pressure dependence of TC for niobium thin film (circles) and for bulk sample of Nb (squares) from Ref. . Star represents Tc after releasing of pressure measured at ambient pressure. Inset shows the temperature dependence of resistivity of Nb thin film for different values of applied pressure (r.p. means released pressure).
Figure 2: XRD pattern showing  reflection of bcc structure of Nb thin films for as prepared thin film (full circles) and thin film after pressure treatment (open squares). The  peak of the bulk Nb at 2Θ≈ 38.54 ° is shown by the black vertical line.
Figure 3: AFM picture of thin films: a) as prepared, and b) after pressure treatment. On the right side are shown pictures with highlighted grains.