Determination of pinning forces on vortex lines in YBa2Cu3O7-δ

The structure adopted by the flux line lattice (FLL) in YBa2Cu3O7-delta (YBCO) depends strongly on the angle (theta) between the applied field and the superconducting cuprate planes. If this angle is held constant and the field rotated about an axis normal to the cuprate planes, the structure of the FLL remains constant, while the direction of the Lorentz force on the vortex lines changes. Measurements of the critical current density (j(c)) with respect to this rotation angle (phi) thus probe pinning behaviour while excluding effects due to changes in the structure of the FLL. If the value of theta is small, such that the FLL adapts the kinked (string-pancake) state, the variation in j(c) seen with phi must be due to the behaviour of the string vortex segments. High-resolution measurements of the variation of j(c) with this second angle phi have been made on thin (120 nm) YBCO films. A simple model is proposed to explain this variation in terms of dragging of vortex strings by the vortex pancake elements. From this model and experimental data an estimate has been made for the value of the pinning force density on vortex strings for motion in the: a-b planes. This estimate is in agreement with values found for the string pinning force density found in earlier measurements on YBCO films grown on vicinal-cut substrates.