Polar metals by geometric design.
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. Quantum physics supports this view, demonstrating that delocalized electrons make a static macroscopic polarization, an ill-defined quantity in metals--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. Here we describe the quantum mechanical design and experimental realization of room-temperature polar metals in thin-film ANiO3 perovskite nickelates using a strategy based on atomic-scale control of inversion-preserving (centric) displacements. 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 NiO6 octahedral--the structural signatures of perovskites--owing to geometric constraints imposed by the underlying substrate. Heteroepitaxial thin-films grown on LaAlO3 (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, non-equilibrium structure in thin-film geometries. We expect that the geometric stabilization approach will provide novel avenues for realizing new multifunctional materials with unusual coexisting properties.
Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
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Kim
Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.
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D
Puggioni
Department of Materials Science and Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA.
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Y
Yuan
Department of Chemical Engineering and Materials Science and Department of Physics and Astronomy, University of California-Irvine, Irvine, California 92697, USA.|National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences, Nanjing University, Nanjing, Jiangsu 210093, China.
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L
Xie
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
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H
Zhou
Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
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Campbell
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
P J
PJ
Ryan
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
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Y
Choi
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
J-W
JW
Kim
Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
J R
JR
Patzner
Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
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S
Ryu
Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
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JP
Podkaminer
Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
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J
Irwin
Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
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Y
Ma
School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA.
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CJ
Fennie
Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
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MS
Rzchowski
Department of Chemical Engineering and Materials Science and Department of Physics and Astronomy, University of California-Irvine, Irvine, California 92697, USA.
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XQ
Pan
Department of Materials Science and Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA.
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V
Gopalan
Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.
J M
JM
Rondinelli
Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
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CB
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