Figure 6
Illustrating the method to measure the anisotropy parameter γ = λ c /λ ab for YBa2Cu3O7 (YBCO) films and single crystals. Normalized values (black symbols) of the critical current density, \({J}_{c}^{n}\), are plotted versus normalized conductor dimension b/λ ab . The solid black curve is the scaling curve for isotropic superconductors and is identical to the black curve in Fig. 5. The black dashed curve is the solid curve displaced along the x-axis by the factor γ. By extrapolating the data on the sloping (1/b) arm back to \({J}_{c}^{n}=1\) the value of γ may be read off as the abscissa. Here we obtain γ = 7. Parenthesised data at small b is for a single 4.5 nm thick film of YBCO at different doping levels achieved by ionic-liquid gating27. Values of λ(0) are obtained from the doping-dependent superfluid density28. The two data points at the large b are for two 30 μm thick single crystals30 and the next highest b value is for a 4.5 μm thick STI tape measured in our laboratory. The remainder are films reported in Table 1. The blue data points are the scaled surface field, \({B}_{s}^{n}\) and blue dashed curve is the scaling curve given in the SI by Eq. S7, \({B}_{s}^{n}=\gamma \,\tanh ({\gamma }^{-1}b/{\lambda }_{ab})\). The red data points are the scaled surface current density, \({J}_{s}^{n}\) and the red dashed line is the scaling curve \({J}_{s}^{n}=1\). The red data points are made smaller so as to make the black data points under them visible.
