  Topological superconductors supporting Majorana Fermions with non-abelian
statistics are presently a subject of intense theoretical and experimental
effort. It has been proposed that the observation of a half-frequency or a
fractional Josephson effect is a more reliable test for topological
superconductivity than the search for end zero modes. Low-energy end modes can
occur accidentally due to impurities. In fact, the fractional Josephson effect
has been observed for the semiconductor nanowire system. Here we consider the
ac Josephson effect in a conventional s-wave superconductor-normal
metal-superconductor junction at a finite voltage. Using a Floquet-Keldysh
treatment of the finite voltage junction, we show that the power dissipated
from the junction, which measures the ac Josephson effect, can show a peak at
half (or even incommensurate fractions) of the Josephson frequency. A similar
conclusion is shown to hold for the Shapiro step measurement. The ac fractional
Josephson peak can also be understood simply in terms of Landau-Zener processes
associated with the Andreev bound state spectrum of the junction.
