2. Pinning Efficiency of Artificial Pinning Centers in Superconductor Nanocomposite Films

Nanoscale c-axis aligned one-dimensional artificial pinning centers (1D APCs) can provide strong correlated pinning and therefore reduce the magnetic field (H) orientation-dependence of the critical current density, J _c, an issue stemming from the layered structure of REBa₂Cu₃O_7-δ (RE-123, RE = rare earth including Y, Gd, Sm, etc.). The 1D APCs are self-assembled in RE-123 matrix driven by the strain field initiated from the 1D APC/RE-123 interface strained due to the lattice mismatch. A fundamental question arises as to how the microstructure and pinning efficiency of a 1D APC are affected by the 1D APC/YBCO interface? In order to shed light on this question, two model systems of 1D APCs of BaZrO₃ (BZO) and BaHfO₃ (BHO) of comparable lateral dimensions (5–6 nm) are selected in our recent studies. However, the interface lattice mismatch with RE-123 such as YBCO and the elastic properties of BZO and BHO differ subtly. The purpose of these studies is to quantify the impact of such differences on the microstructure (especially the areal concentration) and pinning efficiency of the 1D APCs in the BZO (BHO)/YBCO nanocomposite films with APC doping levels varied in the range of 2–6 vol.%. Intriguingly, we have found the impact is substantial. Specifically, the BZO/RE-123 interface is semi-coherent with a large number of dislocations consistent with prior reports, while that of the BHO/RE-123 remains coherent even at high BHO doping levels. This affects the microstructure and pining efficiency of BZO and BHO 1D APCs significantly. The BZO 1D APC concentration and hence matching field B∗ estimated from the TEM characterization increases linearly with the BZO doping, in contrast to a nonlinear trend peaked at 4 vol.% in the BHO case. In addition, a maximum pinning force density (F _pmax) at B//c ~182.0 GNm⁻³ at B _max > 9.0 T (instrument limit) and 65 K is obtained in BHO/YBCO nanocomposites, which is significantly higher than the F _pmax~73.0 GNm⁻³ at B _max = 5.0 T in its BZO/YBCO counterpart. Moreover, the B _max/B∗ ratio in both cases decreases monotonically with APC doping. However, it is up to 2.5–3.5 in the BHO/YBCO case in contrast to the maximum of 0.6–0.7 in the BZO/YBCO case. This result reveals the critical effect of APC/RE-123 interface on the microstructure and pinning efficiency of 1D APCs.