The variation of pinning efficiency in YBCO films containing columnar defects

YBa2Cu3O7-x (YBCO) films were irradiated by ions with different electronic energy loss Se (17.7 keV/nm Kr ions, 35.8 keV/nm Ta ions and 43.7 keV/nm Bi ions) at identical fluence of 5 x 1010 ions/cm2. Existences of columnar defects, i.e. latent tracks, in Ta ions and Bi ions irradiated YBCO films have been confirmed by TEM (transmission electron microscope) analysis, respectively. Variations of pinning efficiency eta, which can be represented as ratio between pinning force density Fp,i of irradiated sample and Fp,p of pristine sample at the same temperature, with temperature and magnetic field in YBCO films irradiated with different ion species were systematically studied. eta in all the irradiated samples show monotonically increasing trend with increasing temperature. Nevertheless, as the number of vortices, i.e. external magnetic field B, increases, eta in YBCO film containing latent tracks gradually reaches its maximum value at low magnetic field and then decreases slowly, totally different from YBCO film containing no latent tracks. This phenomenon may provide a fresh idea to judging the existence of columnar defects in YBCO films. Peak position Bp is far below matching field (B phi approximate to 1.0 T) and peak value increased with increasing temperature. Moreover, sample containing larger-size columnar defects (which can be produced by ions with higher Se) exhibits higher but narrower peak in eta-B curve, which is also important for future application.