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1、Low-energyacousticplasmonsatmetalsurfacesBogdanDiaconescu1,KarstenPohl1,LucaVattuone2,LetiziaSavio2,PhilipHofmann3,Vyach-eslavM.Silkin4,JoseM.Pitarke5,EugeneV.Chulkov4,PedroM.Echenique4,DanielFar´ıas6,MarioRocca71DepartmentofPhysicsandMaterialScienceProgram,Univ
2、ersityofNewHampshire,DurhamNH03824,USA2CNISMandDipartimentodiFisica,UniversitadiGenova16146Genova,Italy`3InstituteforStorageRingFacilitiesandInterdisciplinaryNanoscienceCenter(iNANO),Univer-sityofAarhus,8000AarhusC,Denmark4DonostiaInternationalPhysicsCenter(DIPC
3、),Depto.deFisicadeMaterialesandCentroMixtoCSIC-UPV/EHU,FacultaddeCienciasQuimicas,UPV/EHU,20018SanSebastian,Spain5CICnanoGUNEConsoliderandMateriaKondentsatuarenFisikaSaila,UPV/EHU,MikeletegiPasealekua56,E-2009Donostia,BasqueCountry,Spain6DepartamentodeF´ısicadel
4、aMateriaCondensada,UniversidadAutonomadeMadrid,28049´Madrid,Spain7IMEM-CNRandDipartimentodiFisica,UniversitadiGenova16146Genova,Italy`Nearlytwo-dimensionalmetallicsystemsformedinchargeinversionlayers1andartificiallayeredmaterials2,3permittheexistenceoflow-energyc
5、ollectiveexcitations4,5,so-called2Dplasmons,whicharenotfoundinathree-dimensionalmetal.Theseexcitationshavecausedconsiderableinterestbecausetheirlowenergyallowsthemtoparticipateinmanydynamical1processesinvolvingelectronsandphonons3andaspossiblecandidatestomediate
6、theattrac-tiveinteractionresponsiblefortheformationofCooperpairsinhighTsuperconductors6.cMetalsoftensupportelectronicstatesthatareconfinedtothesurfaceforminganearly2Delectrondensitylayer.However,itwasarguedthatthesesystemscouldnotsupportlow-energycollectiveexcita
7、tionsbecausethesewouldbescreenedoutbytheunderlyingbulkelectrons7.Infact,metallicsurfacesshouldonlysupporttheconventionalsurfaceplasmons8,modeswithenergiesofafeweV,dependingonlyontheelectrondensity,withimportantapplicationsinsurface-plasmonresonancemicroscopy9,10
8、,photonicsandsub-wavelengthoptics11–13,butnorelevancetothelow-energydynamics.Hereweshowthat,incontrasttothiswell-establishedbelief,alow-energycollectiveexcitationmode
