Surface Reconstruction and Orthogonal Decoupling in SrAl4 and EuAl4.

Surface-induced symmetry breaking in quantum materials can stabilize exotic electronic phases distinct from those in the bulk, yet its momentum-space manifestations remain elusive due to the domain-averaging effects. Here, by utilizing angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM), we present a microscopic investigation of the electronic structures of SrAl4 and EuAl4─layered tetragonal intermetallic compounds that exhibit well-characterized incommensurate charge density wave (CDW) transitions. Below the CDW transition temperatures, we uncover linearly dispersed electronic states and pronounced unidirectional replica bands orthogonal to the bulk CDW wave vector, evidencing the emergence of an in-plane C4 symmetry-breaking electronic order that is not dictated by the bulk incommensurate CDW. STM measurements further reveal a 1 × 2 surface reconstruction with quasi-one-dimensional modulations and half-unit-cell steps, traced to ordered 50% Sr/Eu vacancies, which vanish irreversibly upon thermal cycling, indicating decoupled surface and bulk orders. These findings establish SrAl4 and EuAl4 as model platforms for exploring surface-confined nematicity and emergent low-dimensional phases in quantum materials.