The complex conductance $G\left(\omega \right)$ of site-diluted Josephson junction arrays close to the percolation threshold was measured over a wide range of frequencies ω. Well below $T_c$ both the superfluid $\left[\omega \mathrm{Im}G\right]$ and dissipative $\left[\mathrm{Re}G\right]$ components are independent of ω below a critical frequency ${\omega }_c$, whereas $G\left(\omega \right)\propto {\omega }^{-u}$ with $u\approx \frac{1}{2}$ for $\omega >{\omega }_c$. This is shown to reflect the crossover from a Euclidean regime $(\omega <{\omega }_c)$ dominated by phononlike modes of the phase system to a fractal regime $(\omega >{\omega }_c)$, where the relevant excitations are localized fractons. Percolative critical exponents extracted from the data are consistent with theoretical predictions.

• Received 25 June 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.3905