Polar metals by geometric design

Ab initio calculations are used to identify the structural conditions under which a polar state in metals might be stabilized; this information is used to guide the experimental realization of new room-temperature polar metals. A new polar metal from theory to synthesis The ordered electric dipoles that characterize, for example, ferroelectric materials, are not something generally associated with a metal. Indeed, the free carriers responsible for metallic behaviour will typically eliminate polar ordering, to achieve an equilibrium state of zero net internal electric field (Gauss's law). But the possible existence of a polar state in metals is not fundamentally excluded, and some rare examples exist. Tae Heon Kim and colleagues now use ab initio calculations to identify the structural conditions under which such an exotic state might be stabilized, and then use this information to guide the experimental realization of new room-temperature polar metals. Gauss’s law dictates that the net electric field inside a conductor in electrostatic equilibrium is zero by effective charge screening; free carriers within a metal eliminate internal dipoles that may arise owing to asymmetric charge distributions 1 . Quantum physics supports this view 2 , demonstrating that delocalized electrons make a static macroscopic polarization, an ill-defined quantity in metals 3 —it is exceedingly unusual to find a polar metal that exhibits long-range ordered dipoles owing to cooperative atomic displacements aligned from dipolar interactions as in insulating phases 4 . Here we describe the quantum mechanical design and experimental realization of room-temperature polar metals in thin-film A NiO 3 perovskite nickelates using a strategy based on atomic-scale control of inversion-preserving (centric) displacements 5 . We predict with ab initio calculations that cooperative polar A cation displacements are geometrically stabilized with a non-equilibrium amplitude and tilt pattern of the corner-connected NiO 6 octahedra—the structural signatures of perovskites—owing to geometric constraints imposed by the underlying substrate. Heteroepitaxial thin-films grown on LaAlO 3 (111) substrates fulfil the design principles. We achieve both a conducting polar monoclinic oxide that is inaccessible in compositionally identical films grown on (001) substrates, and observe a hidden, previously unreported 6 , 7 , 8 , 9 , 10 , non-equilibrium structure in thin-film geometries. We expect that the geomet