Dynamical properties of the single--hole $t$--$J$ model on a 32--site square lattice

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arXiv:cond-mat/9509087v1 15 Sep 1995Dynamicalpropertiesofthesingle–holet–Jmodelona32–sitesquarelattice

P.W.Leung∗PhysicsDepartment,HongKongUniversityofScienceandTechnology,ClearWaterBay,HongKong

R.J.Gooding†DepartmentofPhysicsandtheCentreforMaterialsScienceandEngineering,MassachusettsInstituteofTechnology,Cambridge,MA02139(February6,2008)

AbstractWepresentresultsofanexactdiagonalizationcalculationofthespectralfunctionA(k,ω)forasingleholedescribedbythet–Jmodelpropagatingona32–sitesquarecluster.Theminimumenergystateisfoundatacrystalmomentumk=(π2),consistentwiththeory,andourmeasureddisper-sionrelationagreeswellwiththatdeterminedusingtheself–consistentBornapproximation.Incontrasttosmallerclusterstudies,ourspectrashownoevidenceofstringresonances.WealsomakeaqualitativecomparisonofthevariationofthespectralweightinvariousregionsofthefirstBrillouinzonewithrecentARPESdata.

PACS:71.27.+a,74.25.Jb,75.10.Jm

TypesetusingREVTEX1Thet–Jmodelhasreceivedalotofattentioninrecentyears.Itisbelievedtobethesimpleststrong–couplingmodelofthelowenergyphysicsoftheanomalousmetallicstateofhigh–temperaturesuperconductors[1,2].TheHamiltonianofthemodelis

H=−t󰀁󰀅ij󰀆σ(˜c†iσ˜cjσ+H.c.)+J󰀁󰀅ij󰀆(Si·Sj−1

2,πThus,thispaperrepresentsamajoradvanceintheexact,unbiased,numericaltreatmentofanimportantstrong–couplingHamiltonian.Inorderforustocompletetheexactdiagonalizationonsuchalargelattice,weusetranslationandonereflectionsymmetrytoreducethetotalnumberofbasisstatestoabout150million.Atk=(3π4),noreflectionsymmetrycanbeusedandthetotalnumberofbasisstatesisabout300million.Tostudytheeffectoffinitesystemsizes,wewillsupplementourresultswithdataobtainedfromsmallersystems:theN=16(4×4)cluster,aswellasa24–site(√32)clusterthatincludesmanyoftheimportantwavevectors[7].Theelectronspectralfunctionisdefinedby

A(k,ω)=󰀁n|󰀊ψN−1n|˜ck,σ|ψN0󰀋|2δ(ω−EN0+EN−1n),(2)whereEN0andψN0arethegroundstateenergyandwavefunctionofthemodelathalffilling,respectively,andEN−1nandψN−1naretheenergyandwavefunctionofthentheigenstateofthesingle–holestate,respectively.A(k,ω)iscalculatedusingacontinuedfractionexpansion[8]with300iterationsandanartificialbroadeningfactorǫ=0.05.WeobtainA(k,ω)thatarewellconvergedusingthesequantities.Figure2(a)showsA(k,ω)atJ=0.3from(0,0)to(π,π).At(0,0),thespectrumhasaquasiparticlepeakatω∼1.34andabroadfeatureatlowerenergies.Askmovesawayfrom(0,0)alongthe(1,1)directiontowards(π,π),spectralweightshiftsfromthebroadfeaturetobothahigherenergyquasiparticlepeakandthelowenergytailofthespectrum.Thequasiparticlepeakincreasesinintensityandshiftstohigherenergies,reachingthevalencebandmaximumat(π2).Whenkgoesfurthertowards(π,π),thequasiparticlepeakmoves

tolowerenergiesanditsintensitydropssignificantly.Spectralweightmovetowardsthecentralpartofthespectrumagain,eventuallyleavingonlyaverysmallquasiparticlepeakatω=1.2313,andabroadlowenergystructure.Figure2(b)showsA(k,ω)atotherdistinctk.FromFig.2oneseesthatalongtheABZedgeA(k,ω)arequalitativelysimilar[9].Theyhavestrongquasiparticlepeakswhichdonotdispersemuch.Theintensityofthe

3quasiparticlepeakisthelargestat(π,0).Askmovesfrom(π,0)to(π2),theintensityof

thequasiparticlepeakdecreasesandisthesmallestat(π2)alongtheABZedge.Thiscanbemadequantitativebycalculatingthequasiparticleweight,whichisdefinedby

Zk=|󰀊ψN−1m|˜ckσ|ψN0󰀋|2

2,π

2,π2,πthe32–siteresults,exceptfordetailedvaluesoftheintensity.Weconcludethatthe24–sitelatticeislargeenoughtocapturetheessentialshapeofthespectralfunctionalongthe(1,1)direction,whilethe16–sitelatticeistoosmall,especiallywhenthelowerenergyfeaturesareconcerned.Inparticular,thewell–definedsecondarypeaksfoundat(π2)onthe16–sitelattice,whichwereinterpretedtoberelatedtothe“stringpicture”[12],arenotfoundinour32–sitesystem.Hencewefindnoevidencesupportingthestringpicture(atleastforthisvalueofJ).Further,thesingle–holeenergy,definedasEh=EN−10−EN0=−E(π2),

iscalculatedat0.1≤J≤0.8forthe32–sitesystem.FittingtotheformEh−J=a+bJνgivesa=−3.24,b=2.65,andν=0.72.Thisisconsistentwiththe16–siteresults[12],a=−3.17,b=2.83,andν=0.73,andalsowiththelargeclusterestimateoftheSCBAcalculations[11].However,νisnot2

2,π

2,π

2,π2,π

2,π