We have
[tex]e^{-iy \log(n)} = e^{\log(n^{-iy})} = n^{-iy}[/tex]
and
[tex]\dfrac1{n^x} \cdot e^{-iy \log(n)} = \dfrac{n^{-iy}}{n^x} = \dfrac1{n^{x+iy}} = \dfrac1{n^z}[/tex]
The sum
[tex]\displaystyle \sum_{n=1}^\infty \frac1{n^z}[/tex]
is exactly the definition of the Riemann zeta function [tex]\zeta(z)[/tex].