  A search for electroweak production of supersymmetric particles in scenarios
with compressed mass spectra in final states with two low-momentum leptons and
missing transverse momentum is presented. This search uses proton-proton
collision data recorded by the ATLAS detector at the Large Hadron Collider in
2015-2016, corresponding to 36.1 fb$^{-1}$ of integrated luminosity at
$\sqrt{s}=13$ TeV. Events with same-flavor pairs of electrons or muons with
opposite electric charge are selected. The data are found to be consistent with
the Standard Model prediction. Results are interpreted using simplified models
of R-parity-conserving supersymmetry in which there is a small mass difference
between the masses of the produced supersymmetric particles and the lightest
neutralino. Exclusion limits at 95% confidence level are set on
next-to-lightest neutralino masses of up to 145 GeV for Higgsino production and
175 GeV for wino production, and slepton masses of up to 190 GeV for pair
production of sleptons. In the compressed mass regime, the exclusion limits
extend down to mass splittings of 2.5 GeV for Higgsino production, 2 GeV for
wino production, and 1 GeV for slepton production. The results are also
interpreted in the context of a radiatively-driven natural supersymmetry model
with non-universal Higgs boson masses.
