Anisotropy induced crossover from fractal to non-fractal flux penetration in high-T-c thin films

We present a magneto-optical study on (i) the origin of the fractal behavior and (ii) means to decrease the irregularity of the magnetic flux front in high-T-c thin films. In particular, we will refer to the crossover from fractal to non-fractal flux penetration induced by anisotropy in the critical current density. We study Tl2Ba2CuO6+x thin films sputtered on vicinal SrTiO3 substrates with vicinal angles of 0(0), 0.5(0), 2.5(0) and 4(0). The films were patterned in shapes of disks, squares and tilted squares. Circular samples are used to determine the current anisotropy axis, whereas square samples allow to evaluate the amplitude of this anisotropy. We observe that: (1) for low values of the anisotropy the nux front has a pronounced fractal character; (2) the fractal behavior of the flux penetration is destroyed by increasing the anisotropy in the critical current. The fractal dimension decreases from 1.21 to 1.09 for an increase in anisotropy from 1 to 8; (3) the disappearance of fractality is also find in experiments on YBa2Cu3O7-x epitaxial thin films deposited on well-oriented (vicinal angle 00) NdGaO3 substrates. Due to the so-called Cu-O chains of YBa2Cu3O7-x which run along the b-axis, the electronic structure is anisotropic, leading to e.g. anisotropy in the normal state resistivity. Experimentally, we find the relation: j(c)(a)/j(c)(b) = (rho(b)/rho(a))(1/2) between the anisotropy in critical current density j(c) and the normal state resistivity along a and b axes. This is in agreement with a simple Drude model. Numerical simulations of anisotropic and fractal flux penetration for thin film samples are compared with experiment.