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Optimal channel networks are fractal structures that bear a striking resemblance to real rivers. They are obtained by minimizing an energy functional associated with spanning trees. We show that large network development effectively occurs al zero temperature since the entropy scales subdominantly with system size compared to the energy. Thus these networks develop under generic conditions and freeze into a static scale-free structure. We suggest a link of optimal channel networks with self-organized critical systems and critical phenomena which exhibit spatial and temporal fractality, the former under generic conditions and the latter on fine tuning.
c' axis oriented YBa2Cu3O7-δ (YBCO) films grown on (100) SrTiO3 substrates by both laser ablation and ion beam sputtering. The effect of the magnetic field on the resistivity is investigated with the field parallel to the c' axis. The zero field transition widths vary from 1 K to 4 K in different films. However the data exhibits 3d X-Y critical scaling having introduced an `effective' magnetic field characteristic of the length scale of the disorder in the f