Observation of unreconstructed square atomic square lattice in Ca(Fe_0.965Co_0.035)₂As₂ cleaved at very low temperatures

Scanning tunneling microscopy (STM) has been by now made in many pnictide layered superconductors. In AFe₂As₂ (A=Ca, Sr, Ba), surface reconstructions dominate The STM topographic images and no surface termination with The bulk atomic lattice has been reported. Here, we present STM measurements on The surface of Ca(Fe_0.965Co_0.035)₂As₂ cleaved in-situ below liquid helium temperatures. We observe The ubiquitous surface reconstruction consisting of 1D stripes that can adopt two degenerate orientations with respect to The crystal lattice. By searching over many different scanning windows, we find that low temperature cleaving also exposes surfaces showing The As unreconstructed lattice of The bulk tetragonal crystal, over areas of some tens of nm lateral size.