Download "Material universe" yields surprising new particle An international

"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.
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Left:Allowedstatesforthestandardtype-IWeylfermion.Whenenergyistunedfrom
below,atzeroenergy,apinchinthenumberofallowedstatesguaranteestheabsenceof
many-bodyphenomenasuchassuperconductivityorordering.
Right:Thenewlydiscoveredtype-IIWeylfermion.Atzeroenergy,alargenumberofallowed
statesarestillavailable.Thisallowsforthepresenceofsuperconductivity,magnetism,and
pair-densitywavephenomena.