We have
$$\frac{n^b}{a^n}=e^{b\log n-n\log a}=e^{-n\left(\log a-b\frac{\log n}{n}\right)}\to 0$$
indeed
- $\log a-b\cdot \frac{\log n}{n} \to \log a-b\cdot 0=\log a>0$
and therefore
- ${n\cdot \left(\log a-b\cdot \frac{\log n}{n}\right)}\to \infty$
For $\frac{\log n}{n}\to 0$ refer to Prove $\log(x) \lt x^n$ for all $n \gt 0$.