High-Resolution Spectrochemical Analysis of Columnar Defects Formed in Bi_2Sr_2CaCu_2O_x by Swift Heavy Ion Irradiation

摘要

Single-crystal specimen of high-temperature superconductor Bi_2Sr_2CaCu_2O_x was irradiated with 3.5 GeV Xe ions at room temperature up to 1.0 * 10~(11) ion/cm~2. Significant enhancement of magnetization by the irradiation was confirmed. The irradiated specimens were studied by using a high-resolution transmission electron microscope with field emission electron gun (FE-TEM), and, also, with an energy dispersive x-ray analyzer (EDS). Columnar defects with diameter of about 6 nm were observed along the incident ion tracks. By nanoscale high-resolution x-ray spectrochemical analysis across the columnar defect, enrichment of Cu and depletion of Bi, Sr and Ca in the columnar defect center, and vice versa in the outskirts of the defect, was found for the first time. Oxygen depletion in the defect, and increased d istribution outside of the delect were also found by electron energy loss spectroscopy. This experimental evidence suggests that the columnar defects are formed as a consequence of Coulomb explosion induced by the electronic excitation of the high-energy heavy ions.