Two-Dimensional Pb Square Nets from Bulk (RO) n Pb (R = Rare Earth Metals, n=1,2)

Alltwo-dimensional (2D) materials of group IV elements from Sito Pb are stabilized by carrier doping and interface bonding fromsubstrates except graphene which can be free-standing. The involvementof strong hybrid of bonds, adsorption of exotic atomic species, andthe high concentration of crystalline defects are often unavoidable,complicating the measurement of the intrinsic properties. In thiswork, we report the discovery of seven kinds of hitherto unreportedbulk compounds (RO)( n )Pb(R = rare earth metals, n = 1,2),which consist of quasi-2D Pb square nets that are spatially and electronicallydetached from the [RO](& delta;+) blockinglayers. The band structures of these compounds near Fermi levels arerelatively clean and dominantly contributed by Pb, resembling theelectron-doped free-standing Pb monolayer. The R 2O2Pb compounds are metallic at ambient pressureand become superconductors under high pressures with much enhancedcritical fields. In particular, Gd2O2Pb (9.1 & mu;(B)/Gd) exhibits an interesting bulk response of latticedistortion in conjunction with the emergence of superconductivityand magnetic anomalies at a critical pressure of 10 GPa. Our findingsreveal the unexpected facets of 2D Pb sheets that are considerablydifferent from their bulk counterparts and provide an alternativeroute for exploring 2D properties in bulk materials.