Surface reconstruction modulated superconductivity on quasi-2D iron pnictide superconductor KCa₂Fe₄As₄F₂

Abstract Iron-based superconductors (FeSCs) feature a complex phase diagram, and their diverse cleavage terminations offer a versatile platform for modulating surface electronic states and investigating the underlying superconducting mechanisms. In this study, we explore the surface modulation of KCa 2 Fe 4 As 4 F 2 using scanning tunneling microscopy/spectroscopy. Cryogenically cleaved surfaces reveal multiple configurations, including <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msqrt> <mml:mn>2</mml:mn> </mml:msqrt> <mml:mo>×</mml:mo> <mml:msqrt> <mml:mn>2</mml:mn> </mml:msqrt> </mml:mrow> </mml:math> reconstruction, 1 × 2 and 1 × 3 stripes, as well as nanoscale vacancies. Reducing potassium coverage induces hole doping, which shifts the density of states peak toward the Fermi level and suppresses the superconducting gap from 4.8 meV to 3.2 meV. This behavior is reminiscent of the Van Hove singularity observed in hole-doped 122-type FeSCs. The band structure does not undergo a simple rigid shift, and the evolution of superconductivity can be attributed to the interplay between surface carriers and electronic correlations. Additionally, a V-shaped gap is observed at a unique location preserving the FeAs bilayer structure, where interlayer coupling effects are likely involved. The diversity of surface structures and electronic states in K12442 enhances our understanding of FeSCs and facilitates the modulation and application of FeAs superconducting layers.