We study superconducting properties of NbN thin film samples with different thicknesses and an ultra-thin NbTiN meander nanowire sample. For the ultra-thin samples, we found that the temperature dependence of upper critical field (H c2) in parallel to surface orientation shows bending curvature close to critical temperature T c, suggesting a two-dimensional (2D) nature of the samples. The 2D behavior is further supported by the angular dependence measurements of H c2 for the thinnest samples. The temperature dependence of parallel upper critical field for the thick films could be described by a model based on the anisotropic Ginzburg–Landau theory. Interestingly, the results measured in the field perpendicular to the film surface orientation show a similar bending curvature but in a much narrow temperature region close to T c for the ultra-thin samples. We suggest that this feature could be due to suppression of pair-breaking caused by local in-homogeneity. We further propose the temperature dependence of perpendicular H c2 as a measure of uniformity of superconducting ultra-thin films. For the thick samples, we find that H c2 shows maxima for both parallel and perpendicular orientations. The H c2 peak for the perpendicular orientation is believed to be due to the columnar structure formed during the growth of the thick films. The presence of columnar structure is confirmed by transmission electron microscopy (TEM). In addition, we have measured the angular dependence of magneto-resistance, and the results are consistent with the H c2 data.

The effect of 600 keV He + ion irradiation on the temperature and magnetic field dependence of the critical current density J C in high quality BaFe 1.84 Co 0.16 As 2 (Co-doped Ba122 type) thin films is investigated. The films are prepared by pulsed-laser-deposition (PLD) on CaF 2 (00 ) substrates. The irradiation dosages are varied in between 110 13 to 110 16 cm -2 . Upon irradiation, the superconducting transition temperature T C drops slightly from 23 K for the unirradiated sample to about 20 K for the sample with the highest irradiation level. The J C values of the thin film samples are calculated by using the Bean critical state model. The results showed that J C could be enhanced substantially. The maximum J C value at 4.5 K temperature is enhanced up to 2.4 MAcm -2 under 1 T field. The analysis of pinning force dependence on magnetic field shows that the pinning behavior is not changed in the irradiated samples, suggesting more pinning centers of similar nature to those of presented in the unirradiated samples are introduced by the irradiation process. The results indicate that the irradiation of light element ions He + with relatively low energy could increase the critical current density in iron based superconductors.