View source for Physics:Polar metal

A '''polar metal''', '''metallic ferroelectric''',<ref>{{cite web|author1=Drexel University|title=Researchers open path to finding rare, polarized metals|url=http://phys.org/news/2014-04-path-rare-polarized-metals.html|website=phys.org|access-date=23 April 2016|date=2 April 2014}}</ref> or '''ferroelectric metal'''<ref>{{cite journal| vauthors = Benedek NA, Birol T |title= 'Ferroelectric' metals reexamined: fundamental mechanisms and design considerations for new materials|journal=Journal of Materials Chemistry C|date=2016|doi=10.1039/C5TC03856A|volume=4|issue=18|pages=4000–4015|arxiv=1511.06187|s2cid= 59457320}}</ref> is a [[Chemistry:Metal|metal]] that contains an [[Physics:Electric dipole moment|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.<ref>{{Cite journal| vauthors = Zhou WX, Ariando A |date=2020-06-01|title=Review on ferroelectric/polar metals |journal=Japanese Journal of Applied Physics|volume=59|issue=SI|pages=SI0802|doi=10.35848/1347-4065/ab8bbf|issn=0021-4922|arxiv=2007.11200|s2cid=219092553}}</ref> Probably the first report of a polar metal was in single crystals of the cuprate superconductors YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−δ</sub>,.<ref>{{cite journal | vauthors = Mihailović D, Heeger AJ |title=Pyroelectric and piezoelectric effects in single crystals of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−d</sub> |journal=Solid State Communications |date=1990 |volume=75 |page=319|doi=10.1016/0038-1098(90)90904-P }}</ref><ref name = "Poberaj_1992">{{cite journal | vauthors = Poberaj I, Mihailovic D |title=Pyroelectric effect measurements in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>6+y</sub> and La<sub>2</sub>CuO<sub>4</sub> materials |journal=Ferroelectrics |date=1992 |volume=128 |page=197|doi=10.1080/00150199208015091 }}</ref> 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.<ref>{{cite journal | vauthors = Mihailovic D, Poberaj I |title=Ferroelectricity in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−δ</sub> and La<sub>2</sub>CuO<sub>4+δ</sub> single crystals |journal=Physica C: Superconductivity |date=1991 |volume=185-189 |page=781 |doi=10.1016/0921-4534(91)91614-A}}</ref> The polarization was found to disappear in the superconducting state.<ref>{{cite journal | vauthors = Mihailovic D, Poberaj I, Mertelj A | title = Characterization of the pyroelectric effect in YBa2Cu3O7- delta | journal = Physical Review B | volume = 48 | issue = 22 | pages = 16634–16640 | date = December 1993 | pmid = 10008248 | doi = 10.1103/PhysRevB.48.16634 | bibcode = 1993PhRvB..4816634M }}</ref>  The lattice distortions responsible were considered to be a result of oxygen ion displacements induced by doped charges that break inversion symmetry.<ref>{{cite journal | vauthors = Mihailovic D, Heeger AJ |title=Pyroelectric and piezoelectric effects in single crystals of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−d</sub> |journal=Solid State Communications |date=1990 |volume=75 |page=319|doi=10.1016/0038-1098(90)90904-P }}</ref><ref>{{cite journal | vauthors = Viskadourakis Z, Sunku SS, Mukherjee S, Andersen BM, Ito T, Sasagawa T, Panagopoulos C | title = Ferroelectricity in underdoped La-based cuprates | journal = Scientific Reports | volume = 5 | pages = 15268 | date = October 2015 | pmid = 26486276 | pmc = 4614081 | doi = 10.1038/srep15268 }}</ref> The effect was utilized for fabrication of pyroelectric detectors for space applications, having the advantage of large pyroelectric coefficient and low intrinsic resistance.<ref>{{cite journal | vauthors = Butler DP, Çelik-Butler Z, Jahanzeb A, Gray JE, Travers CM |title=Micromachined YBaCuO capacitor structures as uncooled pyroelectric infrared detectors. |journal=J. Appl. Phys. |date=1998 |volume=84 |issue=3 |page=1680|doi=10.1063/1.368257 |bibcode=1998JAP....84.1680B }}</ref>
Another substance family that can produce a polar metal is the nickelate perovskites. One example interpreted to show polar metallic behavior is lanthanum nickelate, LaNiO<sub>3</sub>.<ref name=Kim>{{cite journal | vauthors = Kim TH, Puggioni D, Yuan Y, Xie L, Zhou H, Campbell N, Ryan PJ, Choi Y, Kim JW, Patzner JR, Ryu S, Podkaminer JP, Irwin J, Ma Y, Fennie CJ, Rzchowski MS, Pan XQ, Gopalan V, Rondinelli JM, Eom CB | display-authors = 6 | title = Polar metals by geometric design | journal = Nature | volume = 533 | issue = 7601 | pages = 68–72 | date = May 2016 | pmid = 27096369 | doi = 10.1038/nature17628 | bibcode = 2016Natur.533...68K | s2cid = 518197 }}</ref><ref name="Kumah">{{cite journal| vauthors = Kumah DP, Malashevich A, Disa AS, Arena DA, Walker FJ, Ismail-Beigi S, Ahn CH |title=Effect of Surface Termination on the Electronic Properties of LaNiO<sub>3</sub> Films|journal=Physical Review Applied|date=6 November 2015|volume=2|issue=5|page=054004|doi=10.1103/PhysRevApplied.2.054004|bibcode = 2014PhRvP...2e4004K |doi-access=free}}</ref> A thin film of LaNiO<sub>3</sub> grown on the (111) crystal face of [[Chemistry:Lanthanum aluminate|lanthanum aluminate]], (LaAlO<sub>3</sub>) was interpreted to be both conductor and a polar material at room temperature.<ref name=Kim/> 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&nbsp;nm) on the (100) crystal face of LaAlO<sub>3</sub>, the LaNiO<sub>3</sub> can be a polar insulator or polar metal depending on the atomic termination of the surface.<ref name=Kumah/> Lithium osmate,<ref>{{cite web|title=When is a ferroelectric not a ferroelectric?|url=http://www.isis.stfc.ac.uk/science/physics/magnetism/when-is-a-ferroelectric-not-a-ferroelectric14488.html|website=www.isis.stfc.ac.uk|access-date=21 April 2016|date=2013}}</ref> LiOsO<sub>3</sub> also undergoes a ferrorelectric transition when it is cooled below 140K. The [[Point group|point group]] changes from ''R{{overline|3}}c'' to ''R3c'' losing its centrosymmetry.<ref name=Shi/><ref>{{Cite journal| vauthors = Paredes Aulestia EI, Cheung YW, Fang YW, He J, Yamaura K, Lai KT, Goh SK, Chen H |date=2018-07-02|title=Pressure-induced enhancement of non-polar to polar transition temperature in metallic LiOsO3|journal=Applied Physics Letters|language=en|volume=113|issue=1|pages=012902|doi=10.1063/1.5035133|issn=0003-6951|arxiv=1806.00639|bibcode=2018ApPhL.113a2902P|s2cid=119058864}}</ref> 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.<ref name=Shisup>{{cite journal | vauthors = Shi Y, Guo Y, Wang X, Princep AJ, Khalyavin D, Manuel P, Michiue Y, Sato A, Tsuda K, Yu S, Arai M, Shirako Y, Akaogi M, Wang N, Yamaura K, Boothroyd AT | display-authors = 6 | title = A ferroelectric-like structural transition in a metal | journal = Nature Materials | volume = 12 | issue = 11 | pages = 1024–1027 | date = November 2013 | pmid = 24056805 | doi = 10.1038/nmat3754 | arxiv = 1509.01849 | bibcode = 2013NatMa..12.1024S | s2cid = 27226642 }}</ref> Artificial two-dimensional polar metal by charge transfer to a ferroelectric insulator has been realized in LaAlO<sub>3</sub>/Ba<sub>0.8</sub>Sr<sub>0.2</sub>TiO<sub>3</sub>/SrTiO<sub>3</sub> complex oxide heterostructures.<ref>{{Cite journal| vauthors = Zhou WX, Wu HJ, Zhou J, Zeng SW, Li CJ, Li MS, Guo R, Xiao JX, Huang Z, Lv WM, Han K | display-authors = 6 |date=December 2019|title=Artificial two-dimensional polar metal by charge transfer to a ferroelectric insulator|journal=Communications Physics|language=en|volume=2|issue=1|pages=125|doi=10.1038/s42005-019-0227-4| arxiv=2007.05903| bibcode = 2019CmPhy...2..125Z |issn=2399-3650|doi-access=free}}</ref>

Native metallicity and ferroelectricity has been observed at room temperature in bulk single-crystalline [[Chemistry:Tungsten ditelluride|tungsten ditelluride]] (WTe<sub>2</sub>); a transition metal dichalcogenide (TMDC). It has bistable and electrically switchable spontaneous polarization states indicating ferroelectricity.<ref>{{cite journal | vauthors = Sharma P, Xiang FX, Shao DF, Zhang D, Tsymbal EY, Hamilton AR, Seidel J | title = A room-temperature ferroelectric semimetal | journal = Science Advances | volume = 5 | issue = 7 | pages = eaax5080 | date = July 2019 | pmid = 31281902 | pmc = 6611688 | doi = 10.1126/sciadv.aax5080 | bibcode = 2019SciA....5.5080S }}</ref> Coexistence of metallic behavior and switchable electric polarization in WTe<sub>2</sub>, which is a [[Physics:Two-dimensional materials|layered material]], has been observed in the low-thickness limit of two- and three-layers.<ref>{{cite journal | vauthors = Fei Z, Zhao W, Palomaki TA, Sun B, Miller MK, Zhao Z, Yan J, Xu X, Cobden DH | display-authors = 6 | title = Ferroelectric switching of a two-dimensional metal | journal = Nature | volume = 560 | issue = 7718 | pages = 336–339 | date = August 2018 | pmid = 30038286 | doi = 10.1038/s41586-018-0336-3 | arxiv = 1809.04575 | bibcode = 2018Natur.560..336F | s2cid = 49907122 }}</ref> Calculations suggest this originates from vertical charge transfer between the layers, which is switched by interlayer sliding.<ref>{{cite journal | vauthors = Yang Q, Wu M, Li J | title = Origin of Two-Dimensional Vertical Ferroelectricity in WTe<sub>2</sub> Bilayer and Multilayer | journal = The Journal of Physical Chemistry Letters | volume = 9 | issue = 24 | pages = 7160–7164 | date = December 2018 | pmid = 30540485 | doi = 10.1021/acs.jpclett.8b03654 | s2cid = 56147713 }}</ref> In April 2022 another polar metal at room temperature was reported which was also magnetic, [[Physics:Skyrmion|skyrmions]] and the [[Physics:Rashba–Edelstein effect|Rashba–Edelstein effect]] were observed.<ref>{{Cite journal | vauthors = Zhang H, Shao YT, Chen R, Chen X, Susarla S, Raftrey D, Reichanadter JT, Caretta L, Huang X, Settineri NS, Chen Z  |date=2022-04-06 |title=A room temperature polar magnetic metal |journal=Physical Review Materials |volume=6 |issue=4 |pages=044403 |doi=10.1103/PhysRevMaterials.6.044403|arxiv=2106.00833 |bibcode=2022PhRvM...6d4403Z |s2cid=248011242 }}</ref><ref>{{Cite journal | vauthors = Wilkinson R |date=2022-04-06 |title=Elusive Polar Magnetic Metal Found |url=https://physics.aps.org/articles/v15/s44 |journal=Physics |language=en |volume=15 |bibcode=2022PhyOJ..15..s44W }}</ref><ref>{{cite journal | vauthors = Zhang H, Raftrey D, Chan YT, Shao YT, Chen R, Chen X, Huang X, Reichanadter JT, Dong K, Susarla S, Caretta L, Chen Z, Yao J, Fischer P, Neaton JB, Wu W, Muller DA, Birgeneau RJ, Ramesh R | display-authors = 6 | title = Room-temperature skyrmion lattice in a layered magnet (Fe<sub>0.5</sub>Co<sub>0.5</sub>)<sub>5</sub>GeTe<sub>2</sub> | journal = Science Advances | volume = 8 | issue = 12 | pages = eabm7103 | date = March 2022 | pmid = 35319983 | pmc = 8942374 | doi = 10.1126/sciadv.abm7103 | bibcode = 2022SciA....8M7103Z }}</ref>

[[Biography:Philip Warren Anderson|P. W. Anderson]] and E. I. Blount predicted that a ferroelectric metal could exist in 1965.<ref name="Shi">{{cite journal | vauthors = Shi Y, Guo Y, Wang X, Princep AJ, Khalyavin D, Manuel P, Michiue Y, Sato A, Tsuda K, Yu S, Arai M, Shirako Y, Akaogi M, Wang N, Yamaura K, Boothroyd AT | display-authors = 6 | title = A ferroelectric-like structural transition in a metal | journal = Nature Materials | volume = 12 | issue = 11 | pages = 1024–1027 | date = November 2013 | pmid = 24056805 | doi = 10.1038/nmat3754 | bibcode = 2013NatMa..12.1024S | arxiv = 1509.01849 | s2cid = 27226642 }}</ref> They were inspired to make this prediction based on superconducting transitions, and the ferroelectric transition in [[Chemistry:Barium titanate|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 [[Chemistry:Sodium tungsten bronze|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 [[Physics:Ferroelectricity|ferroelectricity]].<ref>{{cite journal| vauthors = Anderson PW, Blount EI |title=Symmetry Considerations on Martensitic Transformations: "Ferroelectric" Metals?|journal=Physical Review Letters|date=15 February 1965|volume=14|issue=7|pages=217–219|doi=10.1103/PhysRevLett.14.217|bibcode = 1965PhRvL..14..217A }}</ref>

== References ==
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[[Category:Metals]]
[[Category:Ferroelectric materials]]

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