  Results of a search for supersymmetry via direct production of
third-generation squarks are reported, using $20.3$ fb$^{-1}$ of proton-proton
collision data at $\sqrt{s} = 8$ TeV recorded by the ATLAS experiment at the
LHC in 2012. Two different analysis strategies based on monojet-like and
$c$-tagged event selections are carried out to optimize the sensitivity for
direct top squark pair production in the decay channel to a charm quark and the
lightest neutralino ($\tilde{t}_1 \to c + \tilde{\chi}_{1}^{0}$) across the top
squark--neutralino mass parameter space. No excess above the Standard Model
background expectation is observed. The results are interpreted in the context
of direct pair production of top squarks and presented in terms of exclusion
limits in the ($m_{\tilde{t}_1}$, $m_{\tilde{\chi}_{1}^{0}}$) parameter space.
A top squark of mass up to about 240 GeV is excluded at 95$\%$ confidence level
for arbitrary neutralino masses, within the kinematic boundaries. Top squark
masses up to 270 GeV are excluded for a neutralino mass of 200 GeV. In a
scenario where the top squark and the lightest neutralino are nearly degenerate
in mass, top squark masses up to 260 GeV are excluded. The results from the
monojet-like analysis are also interpreted in terms of compressed scenarios for
top squark pair production in the decay channel $\tilde{t}_1 \to b + ff^{'} +
\tilde{\chi}^{0}_{1}$ and sbottom pair production with $\tilde{b}_1 \to b +
\tilde{\chi}^{0}_{1}$, leading to a similar exclusion for nearly
mass-degenerate third-generation squarks and the lightest neutralino. The
results in this paper significantly extend previous results at colliders.
