SIMPLE-MODEL OF VORTEX PINNING AT ROUGH SUPERCONDUCTOR SURFACES IN YBA2CU3O7-DELTA THIN-FILMS

All epitactic YBa2Cu3O7-delta thin films show an island mediated growt h mode, which results in a characteristic surface roughness, The thick ness modulations influence the self energy of a vortex line and theref ore can act as effective pinning centers and contribute to the high cr itical current densities. An exact calculation of the pinning energy m ust take into account the structural changes in the structure of a vor tex line due to the complex surface morphology and accordingly represe nts a rather difficult problem. For a first approach, we have approxim ately calculated the surface pinning energy of vortex lines in thickne ss-modulated YBaCuO thin films within the framework of Ginzburg-Landau theory. The dependence of pinning energy, single vortex pinning force and critical current density j(c) on the roughness parameters of the surface are determined quantitatively assuming that the vortex lines r emain rigid. In this model the surface roughness is approximated with a periodic thickness modulation of the superconducting film with wavel ength l(r) and amplitude Delta d of thickness modulation. The average film thickness is (d) over bar. Additionally the effect of vortex-line widening, which increases the magnetic extension of a vortex line in thin films with (d) over bar less than or equal to 2 lambda(ab) is con sidered. The calculated j(c) values with realistic surface roughnesses , observed in YBa2Cu3O7-delta thin films, have values of some 10(10) A /m(2), which is between 10% and 30% of the total j(c) in high quality epitactic thin films. The highest j(c) from surface pinning is yielded at film thickness (d) over bar approximate to 2 lambda(ab), large thi ckness variations Delta d and roughness wavelengths of l(r) approximat e to lambda(ab).