Atomic-scale electrostatic engineering of flat bands in a K3P Lieb lattice

Flat bands (FBs), characterized by dispersionless electronic states, strongly enhance electron correlations, making them promising platforms for exploring diverse strongly correlated phenomena. However, achieving FB materials and precise FB control remain significant challenges. Here, we report a material system: crystalline K3P thin films exhibiting a bilayer Lieb lattice structure, synthesized on Au(111) by thermal substitution of gold atoms in an intermediate Au9P18 phase with potassium. Combined atomically resolved scanning tunneling microscopy, scanning tunneling spectroscopy, and density functional theory calculations identify three highly anisotropic FBs. Crucially, native charged defects function as negative charge centres (CCs), generating radial band-bending variations at the atomic scale consistent with point-charge electrostatics. Spatial dI/dV mapping directly visualizes the equipotential contours surrounding multiple CCs. This work establishes K3P/Au(111) as a tunable FB platform and demonstrates an electrostatic engineering paradigm enabled by defect control, which enables the design of correlated quantum materials with atomic-scale precision. Flat bands provide ideal platforms for exploring strongly correlated electronic phenomena, but precise control of their energy levels remains challenging. This work demonstrates atomic-scale electrostatic engineering of flat bands in a K3P Lieb lattice, enabled by manipulating charged defects.