Polar metal

A polar metal, metallic ferroelectric,[1] or ferroelectric metal[2] is a metal that contains an electric dipole moment. Its components have an ordered electric dipole. Such metals should be unexpected, because the charge should conduct by way of the free electrons in the metal and neutralize the polarized charge. However they do exist.[3] Probably the first report of a polar metal was in single crystals of the cuprate superconductors YBa2Cu3O7−δ.[4][5] A polarization was observed along one (001) axis by pyroelectric effect measurements, and the sign of the polarization was shown to be reversible, while its magnitude could be increased by poling with an electric field.[6] The polarization was found to disappear in the superconducting state.[7] The lattice distortions responsible were considered to be a result of oxygen ion displacements induced by doped charges that break inversion symmetry.[8][9] The effect was utilized for fabrication of pyroelectric detectors for space applications, having the advantage of large pyroelectric coefficient and low intrinsic resistance.[10] Another substance family that can produce a polar metal is the nickelate perovskites. One example interpreted to show polar metallic behavior is lanthanum nickelate, LaNiO3.[11][12] A thin film of LaNiO3 grown on the (111) crystal face of lanthanum aluminate, (LaAlO3) was interpreted to be both conductor and a polar material at room temperature.[11] The resistivity of this system, however, shows an upturn with decreasing temperature, hence does not strictly adhere to the definition of a metal. Also, when grown 3 or 4 unit cells thick (1-2 nm) on the (100) crystal face of LaAlO3, the LaNiO3 can be a polar insulator or polar metal depending on the atomic termination of the surface.[12] Lithium osmate,[13] LiOsO3 also undergoes a ferrorelectric transition when it is cooled below 140K. The point group changes from R3c to R3c losing its centrosymmetry.[14][15] At room temperature and below, lithium osmate is an electric conductor, in single crystal, polycrystalline or powder forms, and the ferroelectric form only appears below 140K. Above 140K the material behaves like a normal metal.[16] Artificial two-dimensional polar metal by charge transfer to a ferroelectric insulator has been realized in LaAlO3/Ba0.8Sr0.2TiO3/SrTiO3 complex oxide heterostructures.[17]

Native metallicity and ferroelectricity has been observed at room temperature in bulk single-crystalline tungsten ditelluride (WTe2); a transition metal dichalcogenide (TMDC). It has bistable and electrically switchable spontaneous polarization states indicating ferroelectricity.[18] Coexistence of metallic behavior and switchable electric polarization in WTe2, which is a layered material, has been observed in the low-thickness limit of two- and three-layers.[19] Calculations suggest this originates from vertical charge transfer between the layers, which is switched by interlayer sliding.[20] In April 2022 another polar metal at room temperature was reported which was also magnetic, skyrmions and the Rashba–Edelstein effect were observed.[21][22][23]

P. W. Anderson and E. I. Blount predicted that a ferroelectric metal could exist in 1965.[14] They were inspired to make this prediction based on superconducting transitions, and the ferroelectric transition in barium titanate. The prediction was that atoms do not move far and only a slight crystal non-symmetrical deformation occurs, say from cubic to tetragonal. This transition they called martensitic. They suggested looking at sodium tungsten bronze and InTl alloy. They realised that the free electrons in the metal would neutralise the effect of the polarization at a global level, but that the conduction electrons do not strongly affect transverse optical phonons, or the local electric field inherent in ferroelectricity.[24]

## References

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11. ^ a b Kim TH, Puggioni D, Yuan Y, Xie L, Zhou H, Campbell N, et al. (May 2016). "Polar metals by geometric design". Nature. 533 (7601): 68–72\. Bibcode:2016Natur.533...68K. doi:10.1038/nature17628. PMID 27096369. S2CID 518197.
12. ^ a b Kumah DP, Malashevich A, Disa AS, Arena DA, Walker FJ, Ismail-Beigi S, Ahn CH (6 November 2015). "Effect of Surface Termination on the Electronic Properties of LaNiO3 Films". Physical Review Applied. 2 (5): 054004. Bibcode:2014PhRvP...2e4004K. doi:10.1103/PhysRevApplied.2.054004.
13. ^ "When is a ferroelectric not a ferroelectric?". www.isis.stfc.ac.uk. 2013. Retrieved 21 April 2016.
14. ^ a b Shi Y, Guo Y, Wang X, Princep AJ, Khalyavin D, Manuel P, et al. (November 2013). "A ferroelectric-like structural transition in a metal". Nature Materials. 12 (11): 1024–1027\. arXiv:1509.01849. Bibcode:2013NatMa..12.1024S. doi:10.1038/nmat3754. PMID 24056805. S2CID 27226642.
15. ^ Paredes Aulestia EI, Cheung YW, Fang YW, He J, Yamaura K, Lai KT, Goh SK, Chen H (2018-07-02). "Pressure-induced enhancement of non-polar to polar transition temperature in metallic LiOsO3". Applied Physics Letters. 113 (1): 012902. arXiv:1806.00639. Bibcode:2018ApPhL.113a2902P. doi:10.1063/1.5035133. ISSN 0003-6951. S2CID 119058864.
16. ^ Shi Y, Guo Y, Wang X, Princep AJ, Khalyavin D, Manuel P, et al. (November 2013). "A ferroelectric-like structural transition in a metal". Nature Materials. 12 (11): 1024–1027\. arXiv:1509.01849. Bibcode:2013NatMa..12.1024S. doi:10.1038/nmat3754. PMID 24056805. S2CID 27226642.
17. ^ Zhou WX, Wu HJ, Zhou J, Zeng SW, Li CJ, Li MS, et al. (December 2019). "Artificial two-dimensional polar metal by charge transfer to a ferroelectric insulator". Communications Physics. 2 (1): 125. arXiv:2007.05903. Bibcode:2019CmPhy...2..125Z. doi:10.1038/s42005-019-0227-4. ISSN 2399-3650.
18. ^ Sharma P, Xiang FX, Shao DF, Zhang D, Tsymbal EY, Hamilton AR, Seidel J (July 2019). "A room-temperature ferroelectric semimetal". Science Advances. 5 (7): eaax5080. Bibcode:2019SciA....5.5080S. doi:10.1126/sciadv.aax5080. PMC 6611688. PMID 31281902.
19. ^ Fei Z, Zhao W, Palomaki TA, Sun B, Miller MK, Zhao Z, et al. (August 2018). "Ferroelectric switching of a two-dimensional metal". Nature. 560 (7718): 336–339\. arXiv:1809.04575. Bibcode:2018Natur.560..336F. doi:10.1038/s41586-018-0336-3. PMID 30038286. S2CID 49907122.
20. ^ Yang Q, Wu M, Li J (December 2018). "Origin of Two-Dimensional Vertical Ferroelectricity in WTe2 Bilayer and Multilayer". The Journal of Physical Chemistry Letters. 9 (24): 7160–7164\. doi:10.1021/acs.jpclett.8b03654. PMID 30540485. S2CID 56147713.
21. ^ Zhang H, Shao YT, Chen R, Chen X, Susarla S, Raftrey D, Reichanadter JT, Caretta L, Huang X, Settineri NS, Chen Z (2022-04-06). "A room temperature polar magnetic metal". Physical Review Materials. 6 (4): 044403. arXiv:2106.00833. Bibcode:2022PhRvM...6d4403Z. doi:10.1103/PhysRevMaterials.6.044403. S2CID 248011242.
22. ^ Wilkinson R (2022-04-06). "Elusive Polar Magnetic Metal Found". Physics. 15. Bibcode:2022PhyOJ..15..s44W. doi:10.1103/Physics.15.s44. S2CID 249250986.
23. ^ Zhang H, Raftrey D, Chan YT, Shao YT, Chen R, Chen X, et al. (March 2022). "Room-temperature skyrmion lattice in a layered magnet (Fe0.5Co0.5)5GeTe2". Science Advances. 8 (12): eabm7103. Bibcode:2022SciA....8M7103Z. doi:10.1126/sciadv.abm7103. PMC 8942374. PMID 35319983.
24. ^ Anderson PW, Blount EI (15 February 1965). "Symmetry Considerations on Martensitic Transformations: "Ferroelectric" Metals?". Physical Review Letters. 14 (7): 217–219\. Bibcode:1965PhRvL..14..217A. doi:10.1103/PhysRevLett.14.217.