A thin circular disk of radius $$R$$ has a surface charge density given by $$\sigma(r) = \sigma_0 \left(\frac{r}{R}\right)^3$$, where $$r$$ is the distance from the center and $$\sigma_0 = 2.0 \times 10^{-6}$$ C/m$$^2$$. Determine the magnitude of the electric field at a point on the disk’s axis, a distance $$z = R/2$$ from its center. Use the value $$\varepsilon_0 = 8.85 \times 10^{-12}$$ C$$^2$$/(N·m$$^2$$).