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"Materialuniverse"yieldssurprisingnewparticle Aninternationalteamofresearchershaspredictedtheexistenceofanewtypeofparticle calledthetype-IIWeylfermioninmetallicmaterials.Whensubjectedtoamagneticfield, thematerialscontainingtheparticlebehaveasinsulatorsforcurrentappliedinsome directionsandasconductorsforcurrentappliedinotherdirections.Thisbehaviorsuggestsa rangeofpotentialapplications,fromlow-energydevicestoefficienttransistors. Theresearcherstheorizethattheparticleexistsinamaterialknownastungstenditelluride (WTe2),whichtheresearcherslikentoa"materialuniverse"becauseitcontainsseveral particles,someofwhichexistundernormalconditionsinouruniverseandothersthatmay existonlyinthesespecializedtypesofcrystals.TheresearchappearedinthejournalNature thisweek. ThenewparticleisacousinoftheWeylfermion,oneoftheparticlesinstandardquantum fieldtheory.However,thetype-IIparticleexhibitsverydifferentresponsesto electromagneticfields,beinganearperfectconductorinsomedirectionsofthefieldandan insulatorinothers. TheresearchwasledbyB.AndreiBernevigofPrincetonUniversity,MatthiasTroyerand AlexeySoluyanovofETHZurich,andXiDaioftheChineseAcademyofSciencesInstituteof Physics.TheteamincludedPostdoctoralResearchAssociatesZhijunWangatPrincetonand QuanShengWuatETHZurich,andgraduatestudentDominikGreschatETHZurich. Theparticle'sexistencewasmissedbyphysicistHermannWeylduringtheinitial developmentofquantumtheory85yearsago,saytheresearchers,becauseitviolateda fundamentalrule,calledLorentzsymmetry,thatdoesnotapplyinthematerialswherethe newtypeoffermionarises. Particlesinouruniversearedescribedbyrelativisticquantumfieldtheory,whichcombines quantummechanicswithEinstein'stheoryofrelativity.Underthistheory,solidsareformed ofatomsthatconsistofanucleisurroundedbyelectrons.Becauseofthesheernumberof electronsinteractingwitheachother,itisnotpossibletosolveexactlytheproblemof many-electronmotioninsolidsusingquantummechanicaltheory. Instead,ourcurrentknowledgeofmaterialsisderivedfromasimplifiedperspectivewhere electronsinsolidsaredescribedintermsofspecialnon-interactingparticles,called quasiparticles,thatmoveintheeffectivefieldcreatedbychargedentitiescalledionsand electrons.Thesequasiparticles,dubbedBlochelectrons,arealsofermions. Justaselectronsareelementaryparticlesinouruniverse,Blochelectronscanbeconsidered theelementaryparticlesofasolid.Inotherwords,thecrystalitselfbecomesa"universe," withitsownelementaryparticles. Inrecentyears,researchershavediscoveredthatsucha"materialuniverse"canhostall otherparticlesofrelativisticquantumfieldtheory.Threeofthesequasiparticles,theDirac, Majorana,andWeylfermions,werediscoveredinsuchmaterials,despitethefactthatthe lattertwohadlongbeenelusiveinexperiments,openingthepathtosimulatecertain predictionsofquantumfieldtheoryinrelativelyinexpensiveandsmall-scaleexperiments carriedoutinthese"condensedmatter"crystals. Thesecrystalscanbegrowninthelaboratory,soexperimentscanbedonetolookforthe newlypredictedfermioninWTe2andanothercandidatematerial,molybdenumditelluride (MoTe2). "One'simaginationcangofurtherandwonderwhetherparticlesthatareunknownto relativisticquantumfieldtheorycanariseincondensedmatter,"saidBernevig.Thereis reasontobelievetheycan,accordingtotheresearchers. Theuniversedescribedbyquantumfieldtheoryissubjecttothestringentconstraintofa certainrule-set,orsymmetry,knownasLorentzsymmetry,whichischaracteristicofhighenergyparticles.However,Lorentzsymmetrydoesnotapplyincondensedmatterbecause typicalelectronvelocitiesinsolidsareverysmallcomparedtothespeedoflight,making condensedmatterphysicsaninherentlylow-energytheory. "Onemaywonder,"Soluyanovsaid,"ifitispossiblethatsomematerialuniverseshostnonrelativistic'elementary'particlesthatarenotLorentz-symmetric?" Thisquestionwasansweredpositivelybytheworkoftheinternationalcollaboration.The workstartedwhenSoluyanovandDaiwerevisitingBerneviginPrincetoninNovember2014 andthediscussionturnedtostrangeunexpectedbehaviorofcertainmetalsinmagnetic fields(Nature514,205–208,2014,doi:10.1038/nature13763).Thisbehaviorhadalready beenobservedbyexperimentalistsinsomematerials,butmoreworkisneededtoconfirmit islinkedtothenewparticle. TheresearchersfoundthatwhilerelativistictheoryonlyallowsasinglespeciesofWeyl fermionstoexist,incondensedmattersolidstwophysicallydistinctWeylfermionsare possible.Thestandardtype-IWeylfermionhasonlytwopossiblestatesinwhichitcan resideatzeroenergy,similartothestatesofanelectronwhichcanbeeitherspin-upor spin-down.Assuch,thedensityofstatesatzeroenergyiszero,andthefermionisimmune tomanyinterestingthermodynamiceffects.ThisWeylfermionexistsinrelativisticfield theory,andistheonlyoneallowedifLorentzinvarianceispreserved. Thenewlypredictedtype-2Weylfermionhasathermodynamicnumberofstatesinwhichit canresideatzeroenergy–ithaswhatiscalledaFermisurface.ItsFermisurfaceisexotic,in thatitappearsalongwithtouchingpointsbetweenelectronandholepockets.Thisendows thenewfermionwithascale,afinitedensityofstates,whichbreaksLorentzsymmetry. Thediscoveryopensmanynewdirections.Mostnormalmetalsexhibitanincreasein resistivitywhensubjecttomagneticfields,aknowneffectusedinmanycurrent technologies.Therecentpredictionandexperimentalrealizationofstandardtype-IWeyl fermionsinsemimetalsbytwogroupsinPrincetonandonegroupinIOPBeijingshowed thattheresistivitycanactuallydecreaseiftheelectricfieldisappliedinthesamedirection asthemagneticfield,aneffectcallednegativelongitudinalmagnetoresistance.Thenew workshowsthatmaterialshostingatype-IIWeylfermionhavemixedbehavior:Whilefor somedirectionsofmagneticfieldstheresistivityincreasesjustlikeinnormalmetals,for otherdirectionsofthefields,theresistivitycandecreaselikeintheWeylsemimetals, offeringpossibletechnologicalapplications. "Evenmoreintriguingistheperspectiveoffindingmore'elementary'particlesinother condensedmattersystems,"theresearcherssay."Whatkindofotherparticlescanbe hiddenintheinfinitevarietyofmaterialuniverses?Thelargevarietyofemergentfermions inthesematerialshasonlybeguntobeunraveled." ResearchersatPrincetonUniversityweresupportedbytheU.S.DepartmentofDefense,the U.S.OfficeofNavalResearch,theU.S.NationalScienceFoundation,theDavidandLucile PackardFoundationandtheW.M.KeckFoundation.ResearchersatETHZurichwere supportedbyMicrosoftResearch,theSwissNationalScienceFoundationandtheEuropean ResearchCouncil.XiDaiwassupportedbytheNationalNaturalScienceFoundationof China,the973programofChinaandtheChineseAcademyofSciences. Thearticle,"TypeIIWeylSemimetals,"byAlexeyA.Soluyanov,DominikGresch,Zhijun Wang,QuanShengWu,MatthiasTroyer,XiDai,andB.AndreiBernevigwaspublishedinthe journalNatureonNovember26,2015. Image: E ky E ky kx kx Left:Allowedstatesforthestandardtype-IWeylfermion.Whenenergyistunedfrom below,atzeroenergy,apinchinthenumberofallowedstatesguaranteestheabsenceof many-bodyphenomenasuchassuperconductivityorordering. Right:Thenewlydiscoveredtype-IIWeylfermion.Atzeroenergy,alargenumberofallowed statesarestillavailable.Thisallowsforthepresenceofsuperconductivity,magnetism,and pair-densitywavephenomena.