  Here, we show that partial transposition, which is initially introduced to
study entanglement, can also inspire many results on quantum discord including:
(I) a discord criterion of spectrum invariant under partial transposition,
stating that one state must contain discord if its spectrum is changed by the
action of partial transposition, (II) an approach to estimate the geometric
quantum discord and the one-way deficit based on the change of spectrum. To
compare with entanglement theory, we also lower-bound the geometric quantum
entanglement and the entanglement of relative entropy. Thus, on one hand, we
illustrate an approach to specify and estimate discord based on partial
transposition. On the other hand, we show that, entanglement and discord, two
basic notions of nonclassical correlations, can be placed on the same ground
such that their interplay and distinction can be illustrated in within a
universal framework.
