Fig. 9-7(a)

A schematic diagram for the formation of columnar defects in high Tc materials by high energy heavy ion irradiation.

Fig. 9-7(b)

Columnar defects observed by an electron microscope. Columnar defects are seen as circular spots in this photo.

Fig. 9-7(c)

Expanded part of a columnar defect. The center of the defect is changed to be amorphous.

Fig. 9-8 The change of critical current density with irradiation. Improvement of the critical current density by the introduction of columnar defects.

In the superconducting state the dc resistivity is exactly zero or at least so close to zero that electrical currents flow persistently without any attenuation of electric power. For the practical use of ordinary superconductors, cooling to a liquid helium temperature (4.2 K) was indispensable until the discovery of high Tc superconductors High Tc superconductors open the way to use superconducting materials at a liquid nitrogen temperature (77 K). These high Tc materials, however, have too small a critical current density in a magnetic field for practical use. The study for the improvement of the critical current density is now the subject of keen, world-wide competition. One of the improvements will be obtained by trapping magnetic flux in defects produced by ion irradiation. Significant improvement is found in high Tc materials irradiated with high energy heavy ions. In Fig. 9-7 (a), a schematic diagram of the formation of columnar defects is shown. Figures (b) and (c) represent the columnar defects and the expanded part of a columnar defect with diameter of 7 nm, as observed by an electron microscope. The change of the critical current density prior to and after irradiation is shown in Fig. 9-8. The enhancement in the critical current density is about two orders of magnitude.

Reference Xinghua Gao et al., Effects of 230MeV Au14+ irradiation on Bi2Sr2CaCu2Ox, Physica C., 250(3-4), 325 (1995).

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