  The classical problem of whether $m$th-powers with or without zero in a
finite field $\mathbb{F}_q$ form a difference set has been extensively studied,
and is related to many topics, such as flag transitive finite projective
planes. In this paper new necessary and sufficient conditions are established
including those via a system of polynomial equations on Gauss sums. The author
thereby solves the problem for even $q$ which is neglected in the literature,
and extends the nonexistence list for even $m$ up to $22$. Moreover,
conjectures toward the complete classification are posed.
