  A search is presented for direct top squark pair production using events with
at least two leptons including a same-flavour opposite-sign pair with invariant
mass consistent with the $Z$ boson mass, jets tagged as originating from
$b$-quarks and missing transverse momentum. The analysis is performed with
proton-proton collision data at $\sqrt{s}=8$ TeV collected with the ATLAS
detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3
fb$^{-1}$. No excess beyond the Standard Model expectation is observed.
Interpretations of the results are provided in models based on the direct pair
production of the heavier top squark state ($\tilde{t}_2$) followed by the
decay to the lighter top squark state ($\tilde{t}_1$) via $\tilde{t}_2 \to
Z\tilde{t}_1$, and for $\tilde{t}_1$ pair production in natural gauge-mediated
supersymmetry breaking scenarios where the neutralino ($\tilde{\chi}^0_1$) is
the next-to-lightest supersymmetric particle and decays producing a $Z$ boson
and a gravitino ($\tilde{G}$) via the $\tilde{\chi}^0_1 \to Z\tilde{G}$
process.
