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

Abstract

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, J c. Specific energy loss, S e, which is effective in the YBCO superconductor (YBa 2Cu 3O 7−δ) lies in the range 0.7 ⩽ S e ⩽ 3.5 keV/Å. At 77 K, a sharp peak in J c occurs at S e ∼ 2.1 keV/Å, for fluences of 10 12 ions/cm 2, where record in-field J c is achieved in large-grain YBCO. For example, J c = 340 kA/cm 2 at 77 K, applied field of 1 T. At closely similar conditions record in-field J c is achieved in thick coated conductor, e.g., J c = 543 kA/cm 2 at 77 K, 1 T. When corrected to the optimum S e, this J c increases to 770 kA/cm 2. These near-optimum PCs have very small diameter of damage ( d d ∼ 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 J c. We find that the advantages of discontinuous PCs at S e = 2.1 keV/Å, are (a) a factor of 12 less damage to the HTS (resulting in improved percolation and T c) and (b) entanglement of fluxoids (even for parallel PCs). This results in five times higher J c than can be achieved with continuous columnar PCs. In large-grain HTS, these pinning centers increase J c by a factor of 17, indicating that the dominant reduction of J c in large-grain HTS is due not to weak links, voids, non-optimum oxygenation, etc., but to a dearth of pinning centers.