Surface terminations and layer-resolved spectroscopy in 122 iron pnictide superconductors

TL;DR

This study uses STM/STS to identify surface terminations in 122 iron pnictide superconductors and links these to their superconducting properties, revealing how surface structure affects superconductivity and impurity effects.

Contribution

It provides the first detailed layer-resolved spectroscopy of different surface terminations in 122 iron pnictides, clarifying their roles in superconductivity.

Findings

Superconducting gap observed on As terminations

Weak or absent gaps on AE and Fe terminations

Impurity resonances support sign-reversal pairing symmetry

Abstract

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.