Temperature dependent non-monotonic bands shift in ZrTe₅

Temperature dependent non-monotonic bands shift in ZrTe ₅

G. Manzoni

, A. Crepaldi

, G. Autès

, A. Sterzi

, F. Cilento

, A. Akrap

, I. Vobornik

, L. Gragnaniello

, Ph. Bugnon

, M. Fonin

, H. Berger

, M. Zacchigna

, O.V. Yazyev

, F. Parmigiani

aUniversitádegliStudidiTrieste,ViaA.Valerio2,Trieste34127,Italy

bElettra–SincrotroneTriesteS.C.p.A.,StradaStatale14,km163.5,Trieste,Italy

cInstitueofPhysics,EcolePolytechniqueFédéraledeLausanne(EPFL),CH-1015Lausanne,Switzerland

dNationalCentreforComputationalDesignandDiscoveryofNovelMaterialsMARVEL,EcolePolytechniqueFédéraledeLausanne(EPFL), CH-1015Lausanne,Switzerland

eDQMP,UniversityofGeneva,CH-1211Geneva4,Switzerland

fCNR-IOM,StradaStatale14,km163.5,34149Trieste,Italy

gUniversityofKonstanz,78457Konstanz,Germany

hInternationalFaculty,UniversityofKöln,50937Köln,Germany

a b s t r a c t

TheelectronicstructureofZrTe5hasbeenmatterofrenewedinterestaimedatclarifying,alongwithits topologicalcharacter,thetemperaturedependenceoftheunusualtransportpropertiesofthismaterial.

Here,wereportanextensivehighresolutionAngleResolvedPhotoelectronSpectroscopy(ARPES)study unveilinganon-monotonicshiftofthebands,whenthesampletemperatureisvariedbetween16Kand 300K.Moreover,thepresentconventionalARPESandcircularlydichroicARPESmeasurementsreveal thepresenceoftwostatesatthetopofthevalenceband.ThestrongARPESdichroicsignaldetectedin proximityoftheFermienergyhasbeeninterpretedastheindicationofthepresenceofspinpolarized states,inagreementwiththepredictedstrongtopologicalcharacterofthismaterial.

1. Introduction

TherecentdebateaboutthetopologicalcharacterofZrTe₅[1–6]

hastriggerednovelstudiesaimedatunderstandingtheoriginsof itsexotictransportproperties[7–9].

Theunusualtemperatureevolutionoftheelectronictransport propertiesofZrTe5consistsinaresisitivitypeakatT*accompanied byasignreversaloftheSeebeckcoefficient.However,duetothe presenceofimpuritiesanddefectsderivingfromthesamplegrowth conditions[10,11],T*isstronglysampledependentanditcanvary from∼60Kto∼170K[1,7,12,13].

Theoriginoftheseanomalouspropertieshasbeensubjectof debateanddifferentmechanismsincludingastructuralphasetran- sition[11,14],formationofchargedensitywaves(CDW)[7,15]and thepresenceofpolaronicchargecarriers[10]havebeenconsidered.

However,directexperimentalevidencessupportingthesetheories

∗Correspondingauthorat:UniversitádegliStudidiTrieste,ViaA.Valerio2,Trieste 34127,Italy.

arestilllacking[11,15],andanunanimousconsensusaboutthe bandstructurebehaviourversustemperaturehasnotbeenreached.

Ourpreviousbandstructureinvestigation[16]hasrevealeda bandshifttowardslowerE−EFenergies,whichwasmonotonicin theinvestigatedtemperaturerangeof300–125K.Inparticular,the bindingenergyofthecharacteristicDiracconeatthepointofthe Brillouinzonereachesitsmaximumat∼T*[16].

RecentARPESdata[12],performedbetween∼300Kand∼2K, haveshownthatthebandshiftismonotonictowardhigherbinding energies,whenthesampleiscooled.Asaconsequence,theDirac pointand thebottomoftheconduction band(CB) aredetected belowtheFermienergy(E_F)at∼2K[12].Conversely,otherARPES experiments,doneat24K[5]andat20K[1],haveshowntheBulk ValenceBand(BVB)ofZrTe5crossingEF.

In this work,wehave performedAngleResolved Photoelec- tronSpectroscopy(ARPES)measurementsinatemperaturerange between16Kand 300K,revealing a non-monotonic bandshift acrosstheFermilevelatk_a=k_c=0 ˚A,withtheinversionpointin proximityof T*.The shiftof theDiracpoint acrossE_F seemsto beconsistentwiththeresistivitypeakdetectedatT*,aswehave reportedinourpreviousstudy[16].However,thediscoveryofa

Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-0-415008

Erschienen in: Journal of Electron Spectroscopy and Related Phenomena ; 219 (2017). - S. 9-15 https://dx.doi.org/10.1016/j.elspec.2016.09.006

Fig.1.(a)ElectronicbandstructureofZrTe5alongthekadirectionmeasuredat∼16Kwithphotonenergyof22eV.Theblacklines,h1andh2,indicatetwobandsdispersing withsimilargroupvelocity:V_h1∼8±1×10⁵m/sandV_h2∼7±1×10⁵m/s.(b)Constantenergymapsobtainedintegrating∼45meVaround(i)EF,(ii)−0.24eV,(iii)−0.46eV, (iv)−0.72eV,(v)−0.88eV,(vi)1.15eVand(vii)−1.30eV.

non-monotonicbehaviouroftheenergybandshiftcallsforadif- ferentexplanationthanwhatwehavereportedbefore[16]about thecarriersignchange,aswewillfurtherdiscussinthefollowing.

Dependingonthecrystallatticeparameters,theoreticalcalcu- lationshavepredictedZrTe₅ tobeatthevergeofatopological phase transition between strong and weak topological insula- tor (STI-WTI) [3]. Scanning tunneling microscopy/spectroscopy (STM/STS)experimentshaveproposedtheexistenceofunidimen- sionaltopologicallyprotectedstatesatstepedges[4,5],suggesting thepossibilitythatthematerialisintheWTIphase.Thepredic- tionoftheSTIphaseofZrTe₅hasbeenalsorecentlysupportedby acombinedARPESandSTM/STSstudy[6].

InordertobetterclarifythetopologicalphaseofZrTe₅,theSTI characterof thismaterial is discussedherein thelight ofhigh resolution(HR)ARPESdataandcirculardichroic(CD)ARPESmea- surements.Inparticular,thestrongCDsignalobservedinproximity oftheFermienergyisproposedtobeafingerprintofthepresence ofspinpolarizedstates.ThissupportstheSTInatureofZrTe₅.

2. Methods

HighqualityZrTe5 singlecrystalshavebeengrownbyvapor transporttechniquewithiodinemethods[17].ZrTe₅presentsan orthorombicstructureandbelongstotheCmcm(D¹⁷_2h)pointgroup symmetry.PrismaticZrTe3chainsareconnectedbyTeatomsalong thecaxis,witha=3.99 ˚Aandc=13.73 ˚AasdeterminedbyX-ray powderdiffraction[18].EachcrystalcellcontainstwoZrTe₅planes piledalong the baxis,withan interlayer distance,at 300K, of 7.23 ˚AasdeterminedbypreviousXRDstudy.Theplanesareweakly boundedbyvanderWaalsforces.Thecleansurfaceexposedafter acleaveisthea−csurface.

ARPESmeasurementshavebeencarriedoutattheAPEbeam- line, Elettra, with linear horizontal(LH) polarization at a fixed photonenergy of22eV.Valence bandsand Fermisurfacemea- surementswereperformedusingahighresolution VG-SCIENTA

DA30electronanalyzer,withenergyandangularresolutionbetter than20meVand0.2^◦,respectively.TheVG-SCIENTADA30elec- tronanalyzerhasthecapabilitytomapthemomentumspacein twodimensions,k_aandk_c,withoutmovingthesample.Inourcase, kawasalongtheslitdirection,kcwasscannedviathenewlydevel- oped deflectorsystem.Sampleshave been cleavedin ultrahigh vacuum(UHV,basepressurep∼1×10⁻¹⁰mbar)atroomtemper- atureandmountedonavariabletemperaturecryostat;datahave beencollectedfrom16Kto300K.Thecrystalshavebeenpreviously orientedbylow-energyelectrondiffraction(LEED).

Theresistivitywasdeterminedusingfour-pointmethod,with thecontactsmadeusingsilverpasteandgoldwires.Thecurrent wasinjectedalongtheaaxis.Themeasuredsampleswereseveral mmlong.

Densityfunctionaltheory(DFT)calculationsofbulkZrTe₅elec- tronicstructurewereperformedwithinthegeneralizedgradient approximationasimplementedintheQuantum-Espressopackage [19].Spin–orbitcoupling(SOC)istakenintoaccountwiththehelp offullyrelativisticnorm-conservingpseudopotentials.Thecalcu- lationswerecarriedoutusingan24×24×12k-pointmeshand aplanewavekineticenergycutoffof80Ryforthewavefunctions.

WeusedtheexperimentallydeterminedcrystalstructurefromRef.

[18].

3. Results

3.1. ZrTe₅bandstructureat16K

Fig.1showstheresultsofhighresolutionARPESmeasurement along theka direction(panel(a)) anddifferentconstant energy mapsintheka−kc plane(panel(b)),asobtainedbyintegrating over∼45meVatselectedbindingenergies.Thesampletempera- turewas∼16K.TheZrTe₅Fermisurfaceisshowninpanel(b–i).The hole-likestateh₁formsacircularpocketatEF,anditsevolutionis trackedinFig.1(b-i)–(b-iv).Thepocketchangesfromacircletoa

-36 Ted

Tez Tet

Ted Tez

Tet a)

b)

~0.72 eV

~0.72 eV 10

5

0

-42 -40 -38

Intensity (arb. u.)

E - EF (eV)

-40 -38

E - EF (eV)

).u .bra(SOD detaluclaC

4d3/2

4d5/2

Fig.2. (a)MeasuredTecorelevels:4d3/2and4d5/2.(b)CalculatedTe4dcorelevels.

warpedrectangle(b-i)–(b-ii),then,itevolvesinamorecomplex shape(Fig.1(b-iii)–(b-iv)).

Fig.1(a)revealsanotherband,h₂,dispersingwithsimilargroup velocityofh1andappearingathigherbindingenergy.Thisband (seeFig.1(a))hasthemaximumlocatedatk_a=k_c0 ˚A⁻¹andbind- ingenergy∼−0.72eV.ThebandsvelocitiesVh1 andVh2 havebeen estimated∼8±1×10⁵m/sand∼7±1×10⁵m/s,respectively.

Thedifferentevolutionsofthetwobandsintheconstantenergy maps(CEMs)ofFig.1(b)havegiventhepossibilitytodiscardthe idea,suggestedbythecloseV_h1 andV_h2 values,thath₂mightbe ahigherbindingenergyreplicaofh₁,asreportedforothercom- pounds havingmultiplesurfaceterminations[20].Inparticular, Fig.1(b-v)showsthath₂doesnotevolveinawarpedrectangle, ash₁inpanel(b-ii),butinanalmond-likeshape,suggestingadif- ferentoriginfortheh₁andh₂bands.Moreover,in(b-vi)and(b-vii), thequasi1Dcharacterofh₂isrevealedbythelineardispersionof thebandthatcrossesthesurfaceBrillouinzone(BZ)withnegligi- bledispersionalongthekcdirection.Thisobservationprovidesnew informationaboutthismaterial.Indeed,eventhoughthecrystalis formedbychainswithreduceddimensionality,thequasi1Dband h₂doesnotreachE_F.Thiscanjustifywhytheelectronictransport propertiesreflectthetwo-dimensionalcharacterofh1.

AnotherinterestingfeatureoftheZrTe₅ electronicproperties is thesplittingofthe 4d_3/2 and 4d_5/2 Te corelevels.TheTe 4d emissionpeaks,collectedatatemperatureof∼77Kandatpho- tonenergyh=75eV,areshowninFig.2(a).Thesespectraclearly showa replicashiftedby∼0.72eVforboth the4d_3/2 and4d_5/2 spin–orbitsplittedpeaks.Weascribethisreplicatothepresence oftwodifferentlycoordinatedTeionsatthesurfacetermination.

Tobetterclarifytheoriginofthesespectralfeaturesinthe4d corelevels,weperformeddensityfunctionaltheory(DFT)calcula- tions.ThecalculatedTecorelevelsareshowninFig.2(b).

Relativistic norm-conserving pseudopotentials were used, includingthe4dcorestatesforTe.Thedensityofstatesprojected onthe4dorbitals ofthethreetypes ofTe atomsareshown in Fig.2(b),whereTetandTe_dcorrespondtothetopandthetwobot- tomatomsoftheZrTe₃prism,whileTezcorrespondstotheatoms connectingtheZrTe₃chainsalongthecaxis.Asitcanbeseen,the

3 2 1 Resistivity Ratio 0

300 200

100 0

Temperature (K)

T

T

T

Fig.3.(a)NormalizedresitivityofZrTe5asafunctionoftemperature.

DFTresultsreproduceaccuratelythespin–orbitsplittingofthe4d Testates.Thesplittingofthecorelevelsofdifferentsites,seenin experiment,isalsoqualitativelydescribed.Inparticular,theTet

corelevelareshiftedupby∼0.6eVwithrespecttotheTezsites corelevel.Theprojecteddensityofstates,whichreliesontheposi- tionofKohn–Shameigenvaluesandconsidersonlytheinitialstate, canjustreproducequalitativelythephotoemissionspectra.Asa consequence,smalldiscrepanciesareobservedintheenergyshift estimation(∼0.72eVintheexperiment,∼0.6eVintheDFTmodel) andmoreimportantlyintheobtainedbindingenergyvalues.How- ever,thecalculationsprovideuswithvaluableinformationabout themechanismattheoriginofthechemicalshift.

Thecalculatedcorelevelsplittinggivesanenergeticpositionof theTezpeakswhichhavenotbeenobservedexperimentally.Within DFT,amoreaccurateestimationoftheXPSspectracanbeobtained byfollowingthemethodproposedinRef.[21].Withinthisscheme, thecorelevelshiftsarecalculatedasthedifferenceoftotalenergy betweentheunexcitedsystemandasystemwithacoreholeonan excitedTeatom.TosimulateexcitedTeatoms,weproduceapseu- dopotentialforTewithaholeinthe4dshelltreatedasacorelevel.

Oneexcitedatomisthenintroducedasanimpurityina2×2×2 supercellforeachofthethreeinequivalentTepositions.Thesecal- culationsshow,inagreementwiththeexperiments,thattheTe_d andTe_z corelevelsareseparatedbyonly∼8meVwhiletheTe_t corelevelisshiftedupwardby∼515meVwithrespecttotheTe_d level.ThestrongchemicalshiftoftheTetcorelevelwithrespect totheotherTesitescanbeascribedtothedifferentcoordination oftheTeions.Inparticular,theTetatomatthetopoftheZrTe₃ prismhaslongerboundswiththeneighboringZrand Teatoms whencomparedtothetwoothersites.

3.2. Temperaturedependentnon-monotonicbandsshift

Havingdetailedthedispersionofsomesignificativefeaturesof theZrTe5bandstructureat∼16K,wecannowlookattheevolu- tionoftheelectronicpropertieswiththetemperature.Thisstudyis motivatedbytheanomalousresistivitypeak(Fig.3)reportedvary- ingthesampletemperature.Forthesampleusedinthepresent experiment,theresistivitypeakislocatedatT*150K,butmea- surementscarriedoutondifferentsamplesfromthesamebatch, i.e.grownunderthesameconditions,revealthatT*canvaryfrom 140Kto160K.

Wehaveinvestigatedindetailsthetemperatureevolutionof theZrTe5electronicbandstructurealongthekadirection,collecting ARPESdatabetween16Kand300K.Threerepresentativetempera- tureshavebeenconsideredhere,T₁=16K,T₂=150KandT₃=300K, showninFig.4(a)–(c),respectively.Fig.4(d)–(f)showsthediffer- entialbanddispersionsresultingfromthesubtractionoftheARPES dataasfollow:(d)T₃−T₂,(e)T₂−T₁and(f)T₃−T₁.

Aremarkabletransferofspectralweightisobservedinallthe three differential images. By cooling thesample from 300K to

∼150K(d),thebandstructuremovestowardslowerE−E_Fvalues, consistentlywithourpreviousresults[16].However,bylower- ingthesampletemperature,downto∼16K,thebandshiftinverts

Fig.4.(a–c)ARPESdata,takenath=22eV,at(a)T1=16K,(b)T2=150Kand(c) T3=300K.(d–f)DifferentialARPESimagesobtainedbysubtractingthelowtemper- aturedispersionfromthehightemperatureone.Blue(<0)andred(>0)features inthedifferentialimagesmarkthetemperaturedrivenspectralweighttransfer.

(g)EDCcurvesextractedatka0 ˚A⁻¹integratingover0.025 ˚A⁻¹fromARPESdata takenatdifferenttemperatures.

directionbetween∼150Kand∼120Kunveilinganonmonotonic temperaturedependence.

ThespectralweighttransferobservedinFig.4(d)–(f)isinter- pretedasabandshiftacrossE_F.Thisbandshiftcanbebetterseenin Fig.4(g)thankstotheenergydistributioncurves(EDCs)extracted fromARPESdatatakenatdifferenttemperaturesbetween16Kand 300K.TheEDCshavebeentakenatk_a0 ˚A⁻¹andintegratedover 0.025 ˚A⁻¹,theEDCsextractedfromdataatthethreecharacteris- tictemperaturesareshownrespectivelyingold(T1),black(T2)and violet(T₃).

AquantitativeestimationoftheenergyshiftisshowninFig.5(a) andithasbeenobtainedusingthefollowingprocedure.Amomen- tumdistributioncurve(MDC),integratedover20meV,hasbeen extractedat−0.3eVfromtheARPESmeasurementtakenat300K, and thewavevectorska ofthetwo peakshave beenevaluated.

Successively,MDCshavebeenextractedforallthemeasuredtem- peratures.Inthesecases,thebindingenergieshavebeenchosen inordertopreservethepeakpositionsatthesamewavevectors, k_a.TheenergypositionoftheMDCextractedatroomtemperature,

E−E_F−0.3eV,hasbeentakenasreference.Thereportedener- giesshiftisthedifferencebetweentheMDCenergyatdifferent temperaturesandthereferenceone.

Theminimumoftheshiftisfoundat∼150K,henceatapprox- imatelythesametemperatureoftheresistivitymaximum,T*.The wholetrendoftheenergyshift,asobtainedinthepresentstudy, isshowninFig.5(bluesquares)anditiscomparedtothedata reportedinourpreviouslaser-ARPESwork(yellowsquares)[16].

The Fermi surface evolution with temperature brings com- plementaryinformationaboutthenon-monotonicbandshift. In panels(b–j)ofFig.5,wereporttheFermisurfacesanddifferent CEMstakenatT116K(b–d),T2150K(e–g)andT3300K(h–j).

Panels(b),(e)and(h)displaytheFermisurfaces(FSs)ofthematerial atdifferenttemperatures.

Thebandstructureshiftstowardhigherbindingenergies,by loweringthetemperaturefromT₃ toT*.AtT*,theDiracpointis locatedatapproximatelyE_F(Fig.5,panel(e)).Thisdeterminesa verylowdensityofchargecarriersavailableandtheconsequent riseoftheresisitivity.Coolingthesamplefrom∼150Kto∼16K, thebandstructureshiftinvertsitstrend,i.e.thebandsmovetoward lowerbindingenergies.AtT₁thebandstructurereachesthelargest shiftvalue,withrespecttoroomtemperature.Thisobservationis supportedbyconsideringthattheareaoftheholepocketattem- peratureT₁ (Fig.5panel(b))islargerthantheoneatT₃ (Fig.5 panel(h)).AccordingtotherawdatashowninFig.4(a)–(c),the bandstructureshiftconsistsinarigidchangewithoutvariationof theFermivelocity.

Thesefindingsextendourpreviousinvestigation[16],where weobservedamonotonicshiftofthebandstructureinthetem- peraturerangebetween300Kand∼125K.Byloweringfurtherthe sampletemperaturedownto∼16K,weobservethatthebinding energyshiftisclearlynon-monotonic.Thisfindingisincontrast withthedatareportedinRef.[11],howeveritisconsistentwith otherworks[1,5]showingthatatlowtemperature(20K[1]and 24K[5])thebulkvalenceband(BVB)iscrossingE_FandtheDirac pointisunoccupied.

AlthoughtheseARPESstudycouldjustifytheresisitvitypeakat T*,asimplesignreversaloftheSeebeckcoefficientisstillunex- plainedbyarigidandnon-monotonicbandshift,asobservedin thepresentexperiments.Hence,ournewdata,takenonawider temperaturerangethenbefore,callforanovelanddifferentexpla- nationforthechargecarriersign,withrespecttowhatwehave previouslyproposed[16].

3.3. Topologicalcharacter

Inordertocontributetothedebateaboutthetopologicalchar- acterofZrTe₅[1–3,5,6],wepresentinFig.6highresolutionARPES data,asmeasuredat16Kandataphotonenergyof∼22eV.

Fig. 6(a) shows the Fermi surface of ZrTe5, where the blue andgreenmarkershighlighttheconcentricholepocketscontours.

Thesetwoquasi-degeneratebandscrossingtheFermiLevelcan beeasilyobservedinFig.6(b), whereanARPES imagetakenat k_c=0 ˚A⁻¹ is shown. The presence of thetwo bands have been alreadyreportedintheliterature[6,22]andtheyhavebeeninter- preted in terms of bands witha bulk and a surface dominant character,respectively[6].

Inordertoprovideclearevidenceaboutthepresenceoftwo distinctstatesinproximityofEF,wealsoanalyzeamomentum distributioncurve (MDC), integrated over 15meV, as shown in Fig. 6(b). The resulting MDC intensity is reported in Fig. 6(c).

Eachbranchatbothpositiveandnegativewavevectorsisclearly doubled,thusconfirming thepresenceof two distinctstates in proximityofE_Fat∼16K.

Wehaveshownthattheh₁ bandiscomposed bythesuper- positionoftwodifferentstateswhich,inanotherwork[6],have

Fig.5.(a)Evolutionoftheextractedenergyshiftasafunctionofthetemperature(blue)comparedtotheoneofRef.[16](yellow).(b–j)ConstantenergymapstakenatEF, at−60meVandat−120meV,atthreedifferenttemperatures(T116K,T2150KandT3300K).

Fig.6.(a)FermiSurfacecollectedattemperature16Kandphotonenergyof22eV.Theblueandgreenmarkersindicatethetwoconcentricholepocketsposition.(b)Electronic bandstructureatkc=0 ˚A⁻¹.(c)MDCextractedinproximityofEF.Thebandappearstobedoublepeakedindicatingthepresenceoftwoquasi-degeneratestates.

beeninterpretedasasurfacestate(SS)andabulkvalenceband (BVB).TheproposedexistanceofaSSandaBVBsuggeststhatZrTe₅ belongstotheSTIphase.IntheSTIphasetheSSisexpectedtobe spinpolarized.

The need to study the spin texture of strongly spin–orbit- coupledmaterialshasrecentlypromotedcirculardichroicangular resolvedphotoelectronspectroscopy(CD-ARPES)asanindirectbut

powerfultooltodetectthespinpolarization.Although,aquantita- tiveanalysisrequirestoaccountforfinalstateandphotoelectron interferenceeffects[23].

Fig.7(a)reportstheARPESdatameasuredat22eV,withlinear horizontalpolarizationat∼210K.Thesedataareusedforcompar- isonwiththeCD-ARPESdatatakenat∼210KshowninFig.7(b).

ThisfigurehasbeenobtainedfromthedifferenceI^L−I^R between

Fig.7. (a)ARPESimagetakenath=22eVandT=210K.Thephotonenergywaslinearhorizontal.(b)DichroicdifferentialimageobtainedfromthedifferenceI^L−I^Rwhere I^LandI^Rhavebeenobtainedwithcircularpolarizedlight(leftandright,respectively).TheimagehasbeendividedinfourenergyregionsofinterestA,B,CandD.

thetwoARPESdataobtainedwithleft(I^L)andright(I^R)circularly polarizedlight.

Fig.7(b)canbedividedinfourdifferentenergeticregions.In regionA,BandCtheSSandtheBVBhavebeentakeninexam.In particular,AandCpresentthesamebehaviour,i.e.positivesignal fornegativekaandviceversa.Importantistonotethatintheregion AtheBVBandtheSSstatesarewellseparatedinenergy,whereas theyarealmostdegenerateintheregionC.

RegionBpresentsanoppositebehaviour.Thesignalisnegative fornegativevaluesofk_aandviceversa.InthisregiontheCDsignal isclearlydistributedalongtheM-likeshapeofBVB,thusresulting mainlyfromthebulkstate.

InaSTI,theexistenceofatopologicallyprotectedsurfacestate isaconsequenceofaninversionintheenergyorderingofthestates formingthebandgap.Thesearealsocharacterizedbydifferentpar- itiesattimereversalinvarianthighsymmetrypoints(inparticular, inthepresent caseat)[24].In thecaseofZrTe₅,theopening ofaninvertedbandgaphasbeenalsointerpretedastheoriginof theM-likeshapeattheBVBtop.WeproposethattheCDsignal signreversalatthetopofBVBmightbethesignatureofthisband inversionbetweenthebottomoftheCBandthetopoftheBVB.

Finally, in theregionD,the h₂ bandpresentsa verystrong dichroicsignal.Thisbandexhibitsanoppositecharacter,compared tothebehavioroftheBVBandtheSSinregionC.Thisconfirmsthat h₂ isnotareplicaoftheBVB.Hence,theCDARPESsignalindi- catesadifferentorbitalcharacterofh₂comparedtotheBVBand, eventually,aboutthedifferentspintexture,comparedtotheSS.

4. Conclusions

Inthepresentwork,wehaveperformedhighresolutionARPES measurementsandcorelevelanalysisonZrTe₅.

Wehaveobservedadoublingofthe4dTecorelevelswhich,on thebasisofDFTcalculations,hasbeenattributedtothechemical shiftduetothepresenceofnon-equivalentTeatomsinthecrystal surface.

OurARPESresultsshowatemperatureevolutionoftheZrTe5

bandstructure,revealinganon-monotonicenergyshiftofthestates acrosstheFermiLevel.Thisenergybandshiftreachesitsminimum at∼T*.ThesefindingscouldexplaintheresistivityanomalyofZrTe5. Unfortunately,thenon-monotoniccharacterofthebindingenergy

shiftoftheDiracconereopensthequestionaboutthesignreversal oftheSeebeckcoefficient.

Highresolution ARPESmeasurementsreveal thepresence of two quasi-degeneratebands,forming theh1 state, crossingthe Fermi level at very low temperature, 16K. The band structure presentalsoaband,h₂,withFermivelocitycomparabletoh₁.The twobandsh1andh2revealastrongandoppositedichroicsignal andtheassociatedCEMsshowthattheyhaveverydifferentcon- stantenergycontours.Inparticular,h₂displaysaone-dimensional character,withlittledispersionalongkc.

CD ARPES measurements bringnovel information aboutthe bandinversionatthetopofthevalenceband.Thestrongdichroic signaldetectedinproximityofEFhasbeeninterpretedastheindi- cation ofthepresence ofspin polarizedstates, asalsorecently observedinspinresolvedARPESmeasurements[25],suggesting thatZrTe5isastrongtopologicalinsulator.

Acknowledgments

We acknowledge G.Miceli forhelpfuldiscussions aboutthe computationofcore-levelshifts withintheDFTframework. We alsoacknowledgeF.Giustiforthegraphicalsupport.Thisworkwas supportedinpartbytheItalianMinistryofUniversityandResearch underGrantNos.FIRBRBAP045JF2andFIRB-RBAP06AWK3andby theEuropean CommunityResearch Infrastructure Action under theFP6StructuringtheEuropeanResearchAreaProgramthrough the Integrated Infrastructure Initiative Integrating Activity on SynchrotronandFreeElectronLaserScience,ContractNo.RII3-CT- 2004-506008.G.A.andO.V.Y.acknowledgesupportbytheNCCR MarvelandtheERCStartinggrantTopoMat(GrantNo.306504).

A.A.acknowledgesfundingfromtheAmbizionefellowshipofthe SwissSNF.Firstprinciplescalculationshavebeenperformedatthe SwissNationalSupercomputingCentre(CSCS)underprojects675.

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