Interrogating the superconductor Ca(PtAs)(FePtAs) Layer-by-layer.

Ever since the discovery of high-T superconductivity in layered cuprates, the roles that individual layers play have been debated, due to difficulty in layer-by-layer characterization. While there is similar challenge in many Fe-based layered superconductors, the newly-discovered Ca(PtAs)(FeAs) provides opportunities to explore superconductivity layer by layer, because it contains both superconducting building blocks (FeAs layers) and intermediate PtAs layers. Cleaving a single crystal under ultra-high vacuum results in multiple terminations: an ordered PtAs layer, two reconstructed Ca layers on the top of a PtAs layer, and disordered Ca layer on the top of FeAs layer. The electronic properties of individual layers are studied using scanning tunneling microscopy/spectroscopy (STM/S), which reveals different spectra for each surface. Remarkably superconducting coherence peaks are seen only on the ordered Ca/PtAs layer. Our results indicate that an ordered structure with proper charge balance is required in order to preserve superconductivity.