Improvement of high Tc superconductor by near-optimum pinning centers created by high Z, high-energy ions

Damage tracks left by high Z, high-energy ions in high temperature superconductor (HTS) can serve the need to pin in place the magnetic field quanta. Such pinning centers (PCs) can serve to dramatically increase the critical current density, Jc. Specific energy loss, Se, which is effective in the YBCO superconductor (YBa2Cu3O7−δ) lies in the range 0.7 ⩽ Se ⩽ 3.5 keV/Å. At 77 K, a sharp peak in Jc occurs at Se ∼ 2.1 keV/Å, for fluences of 1012 ions/cm2, where record in-field Jc is achieved in large-grain YBCO. For example, Jc = 340 kA/cm2 at 77 K, applied field of 1 T. At closely similar conditions record in-field Jc is achieved in thick coated conductor, e.g., Jc = 543 kA/cm2 at 77 K, 1 T. When corrected to the optimum Se, this Jc increases to 770 kA/cm2. These near-optimum PCs have very small diameter of damage (dd ∼ 6.6 nm) close to the value, predicted theoretically. However, they are very discontinuous, whereas theory predicts that continuous columnar PCs are the best to obtain high Jc. We find that the advantages of discontinuous PCs at Se = 2.1 keV/Å, are (a) a factor of 12 less damage to the HTS (resulting in improved percolation and Tc) and (b) entanglement of fluxoids (even for parallel PCs). This results in five times higher Jc than can be achieved with continuous columnar PCs. In large-grain HTS, these pinning centers increase Jc by a factor of 17, indicating that the dominant reduction of Jc in large-grain HTS is due not to weak links, voids, non-optimum oxygenation, etc., but to a dearth of pinning centers.