Surface polarity-driven valence-ordered aperiodic crystalline structure

Chemical and electronic properties of surfaces and interfaces are important for many technologically relevant processes, including information processing where properties of interfacial electronic states are crucial for device performance, and chemical reactions where the catalytic properties of surface depend on types and densities of active nucleation sites.  Aperiodic crystalline surfaces offer new opportunities due to their inherent inhomogeneity, resulting in electronic localization and properties vastly different from the surface of crystals with long-range order. Here, we demonstrate a new route to stabilization of an aperiodic crystalline structure at the surface of the single crystal palladium chromate.  Stabilized by a valence disproportionation driven by the surface polarity, the surface reconstruction does not exhibit any translational periodicity, but consists of a space-filling aperiodic tiling.  Measurement of the local density of states by tunneling spectroscopy reveals electronic localization in the surface layer.  Our results indicate that aperiodic orders can form on top of a periodic solid, creating a new class of emergent orders.