Electronic structure of the quasi-one-dimensional

arXiv:cond-mat/0304283v1[cond-mat.str-el]11Apr2003Electronicstructureofthequasi-one-dimensionalorganicconductorTTF-TCNQM.Sing,U.Schwingenschl¨ogl,andR.ClaessenExperimentalphysikII,Universit¨atAugsburg,D-86135Augsburg,GermanyP.BlahaInstitutf¨urPhysikalischeundTheoretischeChemie,TechnischeUniversit¨atWien,A-1060Wien,AustriaJ.M.P.CarmeloandL.M.Martelo*GCEP-CenterofPhysics,UniversityofMinho,CampusGualtar,P-4710-057Braga,PortugalP.D.SacramentoDepartamentodeF´ısicaandCFIF,InstitutoSuperiorT´ecnico,P-1049-001Lisboa,PortugalM.Dressel1.PhysikalischesInstitut,Universit¨atStuttgart,D-70550Stuttgart,GermanyC.S.JacobsenDepartmentofPhysics,TechnicalUniversityofDenmark,DK-2800Lyngby,Denmark(Dated:February2,2008)Westudytheelectronicstructureofthequasi-one-dimensionalorganicconductorTTF-TCNQbymeansofdensity-functionalbandtheory,Hubbardmodelcalculations,andangle-resolvedphotoelectronspectroscopy(ARPES).Theexperimentalspectrarevealsignificantquantitativeandqualitativediscrepanciestobandtheory.Wedemonstratethatthedispersivebehavioraswellasthetemperature-dependenceofthespectracanbeconsistentlyexplainedbythefinite-energyphysicsoftheone-dimensionalHubbardmodelatmetallicdoping.Themodeldescriptioncanevenbemadequantitative,ifoneaccountsforanenhancedhoppingintegralatthesurface,mostlikelycausedbyarelaxationofthetopmostmolecularlayer.WithinthisinterpretationtheARPESdataprovidespectroscopicevidencefortheexistenceofspin-chargeseparationonanenergyscaleoftheconduc-tionbandwidth.Thefailureoftheone-dimensionalHubbardmodelforthelow-energyspectralbehaviorisattributedtointerchaincouplingandtheadditionaleffectofelectron-phononinteraction.PACSnumbers:71.20.Rv,79.60.Fr,71.10.PmI.INTRODUCTIONTheelectronicstructureofone-dimensional(1D)con-ductorsprovidesavaluabletestinggroundforthestudyofthequantum-mechanicalmany-bodyproblem.Onthetheoreticalsidethereexistvariousmodelsfor1Dinteractingelectronsystems,whichpredicthighlyun-usuallow-energyexcitationsduetodynamicaldecou-plingofchargeandspindegreesoffreedom.Asacon-sequence,thelow-energyparadigmaticFermiliquidpic-turefailsfor1Dmetalsandanewgenericmany-bodyquantumstateemergeswhichiscommonlyreferredtoasTomonaga-Luttingerliquid(TLL).1Experimentally,quasi-1Dmetalsareindeedfoundtodisplaymarkedde-viationsfromconventionalmetallicbehavior,suchastheabsenceofFermi-Diracedgesinthesingle-particleexcita-tionspectraprobedbyangle-resolvedphotoelectronspec-troscopy(ARPES).2,3However,anunambiguousspec-troscopicidentificationofspin-chargeseparationandtheexistenceoflow-energyTLLbehaviorin1Dmetalsisstilllackingsofar.Additionalinterestin1Delectronsystemsarisesfromthesuggestionthattheirphysicsmayalsoberelevanttotheelectronicstructureofthecuprate-basedhigh-temperaturesuperconductors,4relatedtotherecentdiscoveryofchargeorderinginthesematerialsintonarrowmetallic1Dstripesseparatedbyinsulatingregions.5,6Againstthisbackgroundthesearchforpositivespectroscopicsignaturesofunusualelectroniccorrelationeffectsin1Dmetalsremainstobeoftopicalimportance.Inthesearchforpromisingrealizationsofaproto-typical(quasi-)1Dconductortheorganicchargetransfersaltsappearasinterestingcandidates.Duetothefor-mationoflinearmoleculestacksinthecrystalstructureandanelectronicchargetransferfromcationictoan-ioniccomplexestheydisplaystronglyanisotropicmetal-licconductivities.7,8PhotoemissionexperimentsonsuchmaterialsoftenfindunusualspectralbehaviorliketheabsenceofametallicFermiedge.2However,thelackofinformationonsurfacequality,especiallywithre-gardtotherapidphoton-induceddecompositionofor-ganiccompoundsinthevacuumultraviolet,9castsse-riousdoubtstowhatextenttheseobservationsreflectintrinsicelectronicpropertiesorratherastronglydis-turbedsurface.Thisisfurthercorroboratedbythefail-2ureofARPEStodetectspectralenergy-vs.-momentumdispersionsinmostchargetransfersalts.10,11,12AnotableexceptionisTTF-TCNQ(tetrathiafulvalene-tetracyanoquinodimethane),beingthefirst(andsofaronly)organicconductorforwhichdispersing1DbandshavebeenobservedbyARPES.13,14Thisindicatesawell-orderedperiodicsurfacestructureandthuslendsmuchenhancedsignificancetotheobservationofadeeppseu-dogaparoundtheFermienergy,whichevenincreasesuptoroomtemperature.Thisspectralbehaviorhasrecentlybeeninterpretedaspossibleindicationofahighlyun-usualnormalstateinthis1Dconductor.13,15InthispaperwepresentacomprehensiveexperimentalandtheoreticalstudyoftheelectronicstructureofTTF-TCNQ,elaboratingonourearlierARPESresultspub-lishedinRef.15.Thecomparisonbetweenexperimentandbandtheoryrevealssignificantdiscrepancies,con-cerningboththewidthoftheconductionbandsaswellastheirqualitativedispersion.Whilethebandwidthrenormalizationcanbeattributedtoamolecularsurfacerelaxation,theremainingdiscrepanciesindicateafail-ureofthebarebandpicture.Rather,weareabletodemonstratethattheTCNQ-derivedpartoftheARPESfinite-energydispersionscanbeconsistentlymappedontotheelectronremovalspectrumofthe1DHubbardmodelatfinitedoping.16,17Theimportanceofelectroniccor-relationsisfurthercorroboratedbyapeculiartemper-aturedependenceofthephotoemissionspectra.Basedonthesefindingsthespectra