Fractional decay in the spontaneous emission of a two-level system

We find that when the environment of a two-level system has an energy spectrum with a lower bound but without an upper one, the survival probability of the spontaneous emission of the two-level system scales with the spatial dimension $D$ and the exponent $n$ of the energy dispersion $|\vec{k}|^n$ of the environment in the form $1-\alpha t^{2-D/n}$ in the short-time and in the form $\alpha t^{D/n-2}$ in the long-time regime. The former fractional scaling of the survival probability leads to a quantum Zeno effect with a different scaling of the Zeno time.