Point defects in YBa₂Cu₃O_7−xstudied using positron annihilation

Abstract

Fast neutron irradiation is a powerful technique for introducing additional pinning centers into high temperature superconductors. The spherical defects with sizes of a few nanometers are considered to be effective pinning centers, enhancing J c . Their morphology is well-known and has already been investigated by several authors in great detail. However, only very little is known about the nature and density of smaller and point defects, which are invisible in transmission electron microscopy. Positron annihilation lifetime spectroscopy was applied to investigate the nature and the concentration of small point-like defects. In this work, the influence of small point defects, such as vacancies and vacancy clusters, on the superconducting properties of YBa 2 Cu 3 O 7−x bulks was studied; these were introduced by irradiation in the TRIGA Mark II reactor in Vienna. J c and T c measurements were performed prior to and after each irradiation step. The samples were irradiated up to a fast neutron (>0.1 MeV) fluence of 6 × 10 21 m −2 . The two kinds of defects-the large collision cascades and the small point-like defects-contribute to the decrease of T c as well as to the J c enhancement in astonishingly similar ways.

Analysing neutron radiation damage in YBa2Cu3O7–x high‐temperature superconductor tapes

“…The sample we have studied here has been subjected to the highest dose. The critical temperature ( T c ) decreased linearly with dose from 90 to 81 K and it was speculated that this was a result of the formation of Cu‐O di‐vacancies along the b ‐axis 30 . We have shown here that it is possible to extract from the same samples a measure of damage of a different kind based on quantifying the randomness in HRTEM images, although it does not at this stage give information on the crystallographic nature of this damage.…”

“…So when REBCO samples are irradiated by species that create amorphous cascades, these defects can be detected both by diffraction contrast and by high‐resolution TEM techniques, but there is no agreed methodology to define the more subtle lattice damage caused by smaller defects that must exist between these cascades to explain the systematic degradation in T c values found after irradiation by both neutrons and ions. Chudy et al 30 . used positron annihilation lifetime spectroscopy (PALS) to quantify the density of small negatively charged defects after doses of 1.2 × 10 21 and 6 × 10 21 n/m 2 in bulk REBCO, and found an increasing concentration with irradiation time of both ‘small defects’ and cascades.…”

Critical current anisotropy in Nd-1111 single crystals and the influence of neutron irradiation

“…Fast neutron irradiation is known to result in a variety of defects, ranging from single displaced atoms to spherical defect cascades of about 5 nm in diameter. [3,[29][30][31] Angle-resolved magnetization measurements were performed on crystal #2 in a 5 T vector Vibrating Sample Magnetometer (VSM). Previous studies described similar measurements on superconducting thin films, [16,32,33] where the currents flow parallel to the lateral surface at all orientations.…”

Effects of 250 MeV Au-ion Irradiation on the Superconducting Properties of Ba1-xKxFe2As2 Single Crystals

“…Defects produced by irradiation can be controlled by choosing the particle and its energy [1]. Indeed, depending on the kind of ions, randomly distributed point defects [2,3], small cascades [4,5], correlated defects or columnar defects with nanometric section [6,7] can be created. Moreover, the same particle can produce completely different defects depending on its energy: a few MeV energy protons generate point defects in cuprate superconductors whereas nearly GeV energy protons create splayed columnar defects, which result from induced fission of the target atoms [8].…”