  The surface terminations of 122-type alkaline earth metal iron pnictides
AEFe2As2 (AE = Ca, Ba) are investigated with scanning tunneling
microscopy/spectroscopy (STM/STS). Cleaving these crystals at a cryogenic
temperature yields a large majority of terminations with atomically resolved
square-root-two (rt2) or 1*2 lattice, as well as the very rare terminations
with 1*1 symmetry. By means of lattice alignment and chemical marking, we
identify these terminations as rt2-AE, 1*2-As, and rt2-Fe surfaces,
respectively. Layer-resolved spectroscopy on these terminating surfaces reveals
a well-defined superconducting gap on the As terminations, while the gap
features become weaker and absent on AE and Fe terminations respectively. The
local gap features are hardly affected by the surface reconstruction on As or
AE surface, whereas a suppression of them along with the in-gap states can be
induced by As vacancies. The emergence of two impurity resonance peaks at +-2
meV is consistent with the sign-reversal pairing symmetry. The definite
identification of surface terminations and their spectroscopic signatures shall
provide a more comprehensive understanding of the high-temperature
superconductivity in multilayered iron pnictides.
