Abstract
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 distributions1. Quantum physics supports this view2, demonstrating that delocalized electrons make a static macroscopic polarization, an ill-defined quantity in metals3—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 phases4. 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) displacements5. 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 octahedra—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 unreported6,7,8,9,10, 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.
This is a preview of subscription content, access via your institution
Access options
Subscribe to this journal
Receive 51 print issues and online access
$199.00 per year
only $3.90 per issue
Buy this article
- Purchase on Springer Link
- Instant access to full article PDF
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
References
Jackson, J. Classical Electrodynamics 3rd edn (John Wiley & Sons, 1999)
Cohen-Tannoudji, C., Dupont-Roc, J. & Grynberg, G. Photons and Atoms: Introduction to Quantum Electrodynamics (John Wiley & Sons, 1989)
Resta, R. Why are insulators insulating and metals conducting? J. Phys. Condens. Matter 14, R625 (2002)
Cohen, R. E. Origin of ferroelectricity in perovskite oxides. Nature 358, 136–138 (1992)
Rondinelli, J. M., May, S. J. & Freeland, J. W. Control of octahedral connectivity in perovskite oxide heterostructures: an emerging route to multifunctional materials discovery. MRS Bull. 37, 261–270 (2012)
Breckenfeld, E., Chen, Z., Damodaran, A. R. & Martin, L. W. Effects of nonequilibrium growth, nonstoichiometry, and film orientation on the metal-to-insulator transition in NdNiO3 thin films. ACS Appl. Mater. Interfaces 6, 22436–22444 (2014)
Middey, S. et al. Polarity compensation in ultra-thin films of complex oxides: the case of a perovskite nickelate. Sci. Rep. 4, 6819 (2014)
Gibert, M., Zubko, P., Scherwitzl, R., Iniguez, J. & Triscone, J.-M. Exchange bias in LaNiO3/LaMnO3 superlattices. Nature Mater. 11, 195–198 (2012)
Hepting, M. et al. Tunable charge and spin order in PrNiO3 thin films and superlattices. Phys. Rev. Lett. 113, 227206 (2014)
Bousquet, E. et al. Improper ferroelectricity in perovskite oxide artificial superlattices. Nature 452, 732–736 (2008)
Anderson, P. W. & Blount, E. I. Symmetry considerations on martensitic transformations: “ferroelectric” metals? Phys. Rev. Lett. 14, 217–219 (1965)
Shi, Y. et al. A ferroelectric-like structural transition in a metal. Nature Mater. 12, 1024–1027 (2013)
Vaughey, J. T. et al. Synthesis and structure of a new family of cuprate superconductors: LnSr2Cu2GaO7 . Chem. Mater. 3, 935–940 (1991)
Puggioni, D. & Rondinelli, J. M. Designing a robustly metallic noncentrosymmetric ruthenate oxide with large thermopower anisotropy. Nature Commun. 5, 3432 (2013)
Catalan, G. Progress in perovskite nickelate research. Phase Transit. 81, 729–749 (2008)
Park, H., Millis, A. J. & Marianetti, C. A. Total energy calculations using DFT+DMFT: computing the pressure phase diagram of the rare earth nickelates. Phys. Rev. B 89, 245133 (2014)
Benedek, N. A. & Fennie, C. J. Why are there so few perovskite ferroelectrics? J. Phys. Chem. C 117, 13339–13349 (2013)
Bilc, D. I. & Singh, D. J. Frustration of tilts and A-site driven ferroelectricity in KNbO3–LiNbO3 alloys. Phys. Rev. Lett. 96, 147602 (2006)
Halilov, S. V., Fornari, M. & Singh, D. J. Lattice instabilities and ferroelectricity in AScO3 perovskite alloys. Phys. Rev. B 69, 174107 (2004)
Catalano, S. et al. Tailoring the electronic transitions of NdNiO3 films through (111)pc oriented interfaces. APL Mater. 3, 062506 (2015)
Rondinelli, J. M. & Spaldin, N. A. Substrate coherency driven octahedral rotations in perovskite oxide films. Phys. Rev. B 82, 113402 (2010)
Liu, J. et al. Strain-mediated metal-insulator transition in epitaxial ultrathin films of NdNiO3 . Appl. Phys. Lett. 96, 233110 (2010)
King, P. D. C. et al. Atomic-scale control of competing electronic phases in ultrathin LaNiO3 . Nature Nanotechnol. 9, 443–447 (2014)
Kumah, D. P. et al. Tuning the structure of nickelates to achieve two-dimensional electron conduction. Adv. Mater. 26, 1935–1940 (2014)
Pitcher, M. J. et al. Tilt engineering of spontaneous polarization and magnetization above 300 K in a bulk layered perovskite. Science 347, 420–424 (2015)
Oh, Y. S., Luo, X., Huang, F.-T., Wang, Y. & Cheong, S.-W. Experimental demonstration of hybrid improper ferroelectricity and the presence of abundant charged walls in (Ca,Sr)3Ti2O7 crystals. Nature Mater. 14, 407–413 (2015)
Jonietz, F. et al. Spin transfer torques in MnSi at ultralow current densities. Science 330, 1648–1651 (2010)
Seki, S., Yu, X. Z., Ishiwata, S. & Tokura, Y. Observation of skyrmions in multeferroic material. Science 336, 198–201 (2012)
Rüegg, A. & Fiete, G. A. Topological insulators from complex orbital order in transition-metal oxides heterostructures. Phys. Rev. B 84, 201103 (2011)
Okamoto, S. et al. Correlation effects in (111) bilayers of perovskite transition-metal oxides. Phys. Rev. B 89, 195121 (2014)
Perdew, J. P. et al. Restoring the density-gradient expansion for exchange in solids and surfaces. Phys. Rev. Lett. 100, 136406 (2008)
Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54, 11169–11186 (1996)
Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 59, 1758–1775 (1999)
Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 50, 17953–17979 (1994)
Monkhorst, H. J. & Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 13, 5188–5192 (1976)
Orobengoa, D., Capillas, C., Aroyo, M. I. & Perez-Mato, J. M. AMPLIMODES: symmetry-mode analysis on the Bilbao Crystallographic Server. J. Appl. Cryst. 42, 820–833 (2009)
Perez-Mato, J. M., Orobengoa, D. & Aroyo, M. I. Mode crystallography of distorted structures. Acta Crystallogr. A 66, 558–590 (2010)
Zhou, H. et al. Anomalous expansion of the copperapical oxygen distance in superconducting cuprate bilayers. Proc. Natl Acad. Sci. USA 107, 8103–8107 (2010)
Nelson-Cheeseman, B. B. et al. Polar cation ordering: a route to introducing >10% bond strain into layered oxide films. Adv. Funct. Mater. 24, 6884–6891 (2014)
Zhou, H., Pindak, R., Clarke, R., Steinberg, D. M. & Yacoby, Y. The limits of ultrahigh-resolution X-ray mapping: estimating uncertainties in thin-film and interface structures determined by phase retrieval methods. J. Phys. D Appl. Phys. 45, 195302 (2012)
Acknowledgements
This work was supported by the National Science Foundation (NSF) under Designing Materials to Revolutionize and Engineer our Future grant number DMR-1234096. Transport measurement at the University of Wisconsin–Madison was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), under award number DE-FG02-06ER46327. The work at Pennsylvania State University was supported by the DOE–BES, under award number DE-SC0012375 (Y.Y., V.G.). The work at Northwestern University was supported by the Army Research Office under award numbers W911NF-15-1-0017 (J.M.R.) and DOE–BES DE-SC0012375 (D.P.). The work at the Argonne National Laboratory is supported by the DOE–BES under contract number DE-AC-02-06CH11357 (H.Z., P.J.R., Y.C., J.W.K.). The computational work made use of the Haise and Kilrain clusters at the Navy DoD Supercomputing Resource Center under the High Performance Computing Modernization Program initiative of the US Department of Defense and NSF XSEDE (ACI-1053575). The work at Cornell University was funded by DMR-1056441 (C.J.F.).
Author information
Authors and Affiliations
Contributions
C.J.F. and C.B.E. conceived the project. T.H.K. and C.B.E. initiated the project, and fabricated and characterized the thin-film samples. C.B.E., M.S.R., V.G. and X.Q.P. supervised the experiments. D.P. and J.M.R. formulated the model and carried out the DFT study. L.X. and X.Q.P. carried out the transmission electron microscopy studies. Y.Y. and V.G. performed the SHG optical measurements. T.H.K., N.C., J.I., Y.M. and M.S.R. carried out the electrical transport measurements. T.H.K., J.P.P., J.R.P., S.R. and H.Z. performed the synchrotron CTR measurements. H.Z., J.R.P., S.R. and Y.Y. performed the COBRA analyses. Y.C., J.-W.K. and P.J.R. carried out the synchrotron diffraction and spectroscopy measurements. T.H.K., D.P., J.M.R., V.G., H.Z., M.S.R. and C.B.E. prepared the manuscript. C.B.E. directed the research.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Bond connectivity of heteroepitaxial perovskites along the (111) and (001) orientations.
a, b, Owing to the robust bond connectivity, the NiO6 octahedra is strongly coupled with the underlying AlO6 octahedra in NdNiO3/LaAlO3 (111) heterostructure through three bond contacts (indicated by the broken circle). The modification of oxygen octahedral structures can be better achieved in the perovskite (111) geometry (a) rather than the (001) geometry (b), which shows only one unique bond contact.
Extended Data Figure 2 Phonon frequencies and energy as a function of the tilting angle for C2/c (a−a−c−) NdNiO3 with the (111) geometry.
a, At small amplitude of the tilt Θ and the out-of-phase rotation angle ξ = 0° (as defined in the main text), we find two polar modes, Bu and Au, that lift spatial inversion symmetry resulting in the Cc and C2 space groups, respectively (a). The Bu and Au modes describe the polar displacements of Nd, Ni, and O atoms on the 
(Cc) and (001)pc (C2) planes. These two modes behave as in the a−a−c+ case (Fig. 1b), but become hard at smaller amplitude of tilting angle, 2.45° (Au ) and 6.38° (Bu ). Also, the Bu mode always has a lower frequency than the Au mode. b, For small amplitude of the tilt (Θ) and out-of-phase rotation (ξ) angles, we obtain an energy gain of ~2 meV per formular unit that is smaller than the a−a−c+ case (Fig. 1d).
Extended Data Figure 3 Epitaxial synthesis and structural characterization of NdNiO3/LaAlO3 (111) thin films.
a–c, An atomic force microscopy (AFM) topography image (a) of an as-grown 2.2-nm-thick NdNiO3 film, thickness-dependent evolution of in situ RHEED patterns (b), and intensity oscillation (c) during PLD deposition of a NdNiO3 film. In b, a yellow square shows that a (10) RHEED peak remains up to the 10th layer, but it disappears in thicker layers. This indicates that thin NdNiO3 samples with the film thickness below 10 layers (~2.2 nm) have a high crystalline quality compared with thicker NdNiO3 films. In c, black and red lines represent time-dependent evolution of (00) and 
peak intensity in b, respectively. d, A schematic diagram of three monoclinic domain variants, which are used for SHG polar fittings, in a NdNiO3 (111) thin film. The crossed circle represents the direction of out-of-plane components of A-site Nd displacements. Open arrows show three possible variants of in-plane components of the A-site Nd displacements. Owing to the three-fold symmetry of the LaAlO3 (111) substrate, three different domains exist with about equal amount. e, Observation of {integer integer half-integer} Bragg peaks in synchrotron X-ray diffraction. Note that the family of {integer integer half-integer} half-order Bragg peaks are directly related to the occurrence of off-symmetry A-site cation displacements in an orthorhombic system.
Extended Data Figure 4 X-ray spectroscopy and resonant X-ray diffraction measurements of epitaxial NdNiO3/LaAlO3 (111) and (001) films.
a, X-ray absorption spectroscopy (XAS) at the Ni K edge shows a clear pre-edge intensity, when the incident X-ray polarization (E) is along [111] of a NdNiO3 (111) film. The near-edge X-ray absorption fine structure (NEXAFS) indicates the Ni displacement is more pronounced along [111]pc of the NdNiO3 (111) film, while weaker along the other two in-plane directions. The response from a NdNiO3 (001) film is similar to the in-plane results from the NdNiO3 (111) film. The weak pre-edge intensity is due to transitions from the 1s to 3d levels. At the K edge, s–d electric dipole transition is forbidden for this octahedral case and weak quadrupole transition is allowed. However, as the central Ni atom is displaced from its cubic symmetric site, the displacement breaks the inversion symmetry, mixing p-state symmetry with unfilled d-states and allowing the dipole transition. The strength of the dipole transition is proportional to the square of the displacement along the incident X-ray polarization. b, Resonant X-ray diffraction (XRD) intensity at a 
film peak in pseudo-cubic notation (that is, equivalent to a (011) Bragg peak in orthorhombic notation) across the Ni K edge is shown. The Ni response at the 
arises from a combination of the finite size effect of the film (thickness fringes) and monoclinic distortion related to the Ni displacement. The error bars for both the XAS and resonant XRD results are calculated as the square root of the measured intensity.
Extended Data Figure 5 COBRA and STEM analyses of NdNiO3 thin films on LaAlO3 (001) substrates.
a–c, A schematic diagram (a), two-dimensional electron density maps sliced through the pseudocubic (110) plane (b), and STEM-ABF images (c) of NdNiO3/LaAlO3 (001) thin-film heterostructures along the pseudocubic [110] zone axis. The interface is marked by yellow dash–dotted lines. d, e, Magnified two-dimensional electron density maps of a NdNiO3 film (d) and a LaAlO3 substrate (e), which are indicated by (i) and (ii) in b, respectively. Red dashed lines represent the positions of oxygen atoms (marked with red colours), which are taken as references to measure the relative off-centre displacements of Nd (green) and La (black) atoms, respectively. No polar displacements of the Nd and La atoms are measured with respect to the oxygen atom positions. f, g, Magnified STEM-ABF images of a NdNiO3 film (f) and a LaAlO3 substrate (g), which are indicated by (iii) and (iv) in c, respectively. Red dotted lines are guidelines to show the tilting of NiO6 and AlO6 octahedra in the NdNiO3 and LaAlO3 layers, respectively. h, Layer-dependent evolution of the NiO6 and AlO6 octahedral tilt angles across the interface in the STEM-ABF image of c. The 0th layer represents the NdNiO3/LaAlO3 interfaces. We extract the error bars by calculating the standard deviation values from the measured tilting angles of about 10 unit cells in each layer. Blue and red dashed lines represent the octahedral tilting angles of bulk LaAlO3 and NdNiO3, respectively. Note that NiO6 octahedra in NdNiO3 (001) thin films exhibit bulk-like tilting angle magnitudes without suppression of octahedral tilt distortion.
Extended Data Figure 6 STEM analyses of octahedral tilt patterns in NdNiO3/LaAlO3 thin films.
a, A STEM image of a NdNiO3/LaAlO3 (111) heterostructure along the pseudocubic 
zone axis. Dotted red and solid yellow squares represent the NdNiO3 film and LaAlO3 substrate regions for fast Fourier transform (FFT) analyses, respectively. b, c, The corresponding FFT images of the NdNiO3 film (b) and the LaAlO3 substrate (c) regions in a. In c, yellow circles represent half-order spots due to the a−a−a− tilt pattern of the LaAlO3 substrate. In the FFT image of the NdNiO3 film, half-order spots do not appear, indicative of local suppression of in-phase c+ octahedral rotation. d, A STEM image of a NdNiO3/LaAlO3 (001) heterostructure along the pseudocubic [100] zone axis. e, f, The FFT images of the NdNiO3 film (e) and the LaAlO3 substrate (f) regions in d. In e, the dotted red circles represent half-order spots, which usually come from oxygen octahedral rotation. g, h, Simulated electron diffraction patterns of orthorhombic (Pbnm, a−a−c+) NdNiO3 (g) and rhombohedral (R
c, a−a−a−) LaAlO3 (h) along the pseudocubic [100] zone axis. In g, a dotted red circle represents a half-order peak induced by in-phase c+ octahedral rotation in orthorhombic NdNiO3. In rhombohedral LaAlO3, destructive interference occurs and then the half-order peak disappears in h.
Extended Data Figure 7 Thickness-dependent SHG and electrical transport experiments in NdNiO3 (111) thin films.
a–c, The temperature-dependent SHG experiments in 1.1- (a), 1.5- (b) and 2.2-nm-thick (c) NdNiO3 (111) thin films. d, The thickness dependence of the SHG responses. The SHG intensity is proportional to the square of the film thickness. e, The temperature-dependent resistance in 1.1-, 1.5- and 2.2-nm-thick NdNiO3 (111) films.
Extended Data Figure 8 SHG polarimetry of LaNiO3 (111) films and a capping-layer effect in NdNiO3 (111) films.
a, b, SHG polar plots of 2.23-nm-thick LaNiO3 thin films on LaAlO3 (111) substrates at a room temperature (RT; a) and 18 K (b). Two SHG components are measured, I2ω|| (blue circles) and I2ω⊥ (red circles) under the O1 and O2 sample orientation, depicted in Fig. 3a. The solid lines represent the theoretical fittings of monoclinic (m) point-group symmetry with equivalent three domain variants. c, The temperature-dependent resistivity of a 2.23-nm-thick LaNiO3 thin film on a LaAlO3 (111) substrate. d, Room-temperature SHG polar plots in a NdNiO3 (111) thin film with a LaAlO3 capping layer.
Extended Data Figure 9 Synchrotron CTR measurements in NdNiO3/LaAlO3 thin films.
a, b, CTR scans of processed raw (circles) and simulated (red curves) data along various non-specular CTRs in NdNiO3 (001) (a) and (111) (b) thin film, respectively. The data curves are offset for clarity of comparison. c, d, Schematics of symmetry inequivalent CTRs (HKL) measured in reciprocal lattices defined by the LaAlO3 substrate for both NdNiO3/LaAlO3 (001) (c) and (111) (d) thin films. The monoclinic structure of epitaxial NdNiO3 thin films breaks the high-order symmetry of the LaAlO3 substrate along both (001) and (111) orientations.
Rights and permissions
About this article
Cite this article
Kim, T., Puggioni, D., Yuan, Y. et al. Polar metals by geometric design. Nature 533, 68–72 (2016). https://doi.org/10.1038/nature17628
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/nature17628
This article is cited by
-
Nonzero spontaneous electric polarization in metals: novel predictive methods and applications
Scientific Reports (2024)
-
Designing spin and orbital sources of Berry curvature at oxide interfaces
Nature Materials (2023)
-
Extendable piezo/ferroelectricity in nonstoichiometric 2D transition metal dichalcogenides
Nature Communications (2023)
-
Magnetism and berry phase manipulation in an emergent structure of perovskite ruthenate by (111) strain engineering
npj Quantum Materials (2023)
-
Quantum fluctuation of ferroelectric order in polar metals
npj Quantum Materials (2023)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.
