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

1. ↑ "Researchers open path to finding rare, polarized metals". 2 April 2014. http://phys.org/news/2014-04-path-rare-polarized-metals.html.
2. ↑ "'Ferroelectric' metals reexamined: fundamental mechanisms and design considerations for new materials". Journal of Materials Chemistry C 4 (18): 4000–4015. 2016. doi:10.1039/C5TC03856A.
3. ↑ "Review on ferroelectric/polar metals". Japanese Journal of Applied Physics 59 (SI): SI0802. 2020-06-01. doi:10.35848/1347-4065/ab8bbf. ISSN 0021-4922.
4. ↑ "Pyroelectric and piezoelectric effects in single crystals of YBa2Cu3O7−d". Solid State Communications 75: 319. 1990. doi:10.1016/0038-1098(90)90904-P.
5. ↑ "Pyroelectric effect measurements in YBa2Cu3O6+y and La2CuO4 materials". Ferroelectrics 128 (1): 197. 1992. doi:10.1080/00150199208015091. Bibcode: 1992Fer...128..197P.
6. ↑ "Ferroelectricity in YBa2Cu3O7−δ and La2CuO4+δ single crystals". Physica C: Superconductivity 185-189: 781. 1991. doi:10.1016/0921-4534(91)91614-A.
7. ↑ "Characterization of the pyroelectric effect in YBa2Cu3O7- delta". Physical Review B 48 (22): 16634–16640. December 1993. doi:10.1103/PhysRevB.48.16634. PMID 10008248. Bibcode: 1993PhRvB..4816634M.
8. ↑ "Pyroelectric and piezoelectric effects in single crystals of YBa2Cu3O7−d". Solid State Communications 75: 319. 1990. doi:10.1016/0038-1098(90)90904-P.
9. ↑ "Ferroelectricity in underdoped La-based cuprates". Scientific Reports 5. October 2015. doi:10.1038/srep15268. PMID 26486276.
10. ↑ "Micromachined YBaCuO capacitor structures as uncooled pyroelectric infrared detectors.". J. Appl. Phys. 84 (3): 1680. 1998. doi:10.1063/1.368257. Bibcode: 1998JAP....84.1680B.
11. ↑ 11.0 11.1 "Polar metals by geometric design". Nature 533 (7601): 68–72. May 2016. doi:10.1038/nature17628. PMID 27096369. Bibcode: 2016Natur.533...68K.
12. ↑ 12.0 12.1 "Effect of Surface Termination on the Electronic Properties of LaNiO3 Films". Physical Review Applied 2 (5). 6 November 2015. doi:10.1103/PhysRevApplied.2.054004. Bibcode: 2014PhRvP...2e4004K.
13. ↑ "When is a ferroelectric not a ferroelectric?". 2013. http://www.isis.stfc.ac.uk/science/physics/magnetism/when-is-a-ferroelectric-not-a-ferroelectric14488.html.
14. ↑ 14.0 14.1 "A ferroelectric-like structural transition in a metal". Nature Materials 12 (11): 1024–1027. November 2013. doi:10.1038/nmat3754. PMID 24056805. Bibcode: 2013NatMa..12.1024S.
15. ↑ "Pressure-induced enhancement of non-polar to polar transition temperature in metallic LiOsO3" (in en). Applied Physics Letters 113 (1): 012902. 2018-07-02. doi:10.1063/1.5035133. ISSN 0003-6951. Bibcode: 2018ApPhL.113a2902P.
16. ↑ "A ferroelectric-like structural transition in a metal". Nature Materials 12 (11): 1024–1027. November 2013. doi:10.1038/nmat3754. PMID 24056805. Bibcode: 2013NatMa..12.1024S.
17. ↑ "Artificial two-dimensional polar metal by charge transfer to a ferroelectric insulator" (in en). Communications Physics 2 (1): 125. December 2019. doi:10.1038/s42005-019-0227-4. ISSN 2399-3650. Bibcode: 2019CmPhy...2..125Z.
18. ↑ "A room-temperature ferroelectric semimetal". Science Advances 5 (7). July 2019. doi:10.1126/sciadv.aax5080. PMID 31281902. Bibcode: 2019SciA....5.5080S.
19. ↑ "Ferroelectric switching of a two-dimensional metal". Nature 560 (7718): 336–339. August 2018. doi:10.1038/s41586-018-0336-3. PMID 30038286. Bibcode: 2018Natur.560..336F.
20. ↑ "Origin of Two-Dimensional Vertical Ferroelectricity in WTe2 Bilayer and Multilayer". The Journal of Physical Chemistry Letters 9 (24): 7160–7164. December 2018. doi:10.1021/acs.jpclett.8b03654. PMID 30540485.
21. ↑ "A room temperature polar magnetic metal". Physical Review Materials 6 (4). 2022-04-06. doi:10.1103/PhysRevMaterials.6.044403. Bibcode: 2022PhRvM...6d4403Z.
22. ↑ "Elusive Polar Magnetic Metal Found" (in en). Physics 15. 2022-04-06. doi:10.1103/Physics.15.s44. Bibcode: 2022PhyOJ..15..s44W. https://physics.aps.org/articles/v15/s44.
23. ↑ "Room-temperature skyrmion lattice in a layered magnet (Fe0.5Co0.5)5GeTe2". Science Advances 8 (12). March 2022. doi:10.1126/sciadv.abm7103. PMID 35319983. Bibcode: 2022SciA....8M7103Z.
24. ↑ "Symmetry Considerations on Martensitic Transformations: "Ferroelectric" Metals?". Physical Review Letters 14 (7): 217–219. 15 February 1965. doi:10.1103/PhysRevLett.14.217. Bibcode: 1965PhRvL..14..217A.

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