Effects of high magnetic field and substitutional doping on optical properties of cuprate superconductors

280 Pages·1997·9.7 MB·English

by  Liu, Hsiang-Lin

EFFECTSOFHIGHMAGNETICFIELDAND
SUBSTITUTIONALDOPINGONOPTICAL
PROPERTIESOFCUPRATESUPERCONDUCTORS
By
HSIANG-LINLIU
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
^.
UNIVERSITYOFFLORIDA
1997
ACKNOWLEDGMENTS
Itismygreatplecisuretothankmysupervisor,ProfessorDavidB.Tanner,for
givingmetheopportunitytobepartofhisreseaxchgroupandstudyalotofvery
excitingprojectsdetailedinthisdissertation. Hisconfidenceandcontinuedsupport
areaverymuchvaluedcontribution,asarethemanysuggestionsandhelpfuldiscus-
sions. Heisalsoagoodrolemodelasascientistandaneducator. Iwillkeephis
attitudeinmind: positivethinking,self-motivated,andenjoysworking. Ihavebeen
trulyprivilegedtoworkwithhim.
IwouldliketothankProfessorsP.J.Hirshfeld,J.M.Graybeal,D.H.Reitze,and
J.R. Reynoldsforreadingthisdissertationandfortheirinterestinservingonmy
supervisorycommittee.
ThanksalsogotoallmypastandpresentcolleaguesinTanner'sgroupfortheir
friendship,usefulconversationandcooperationthroughoutmygraduatework,M.A.
Quijada,Y.-D.Yoon,A.Zibold,J.L.Musfeldt,K.Kamaras, U.Akito,A.Memon,
CD.Porter,L.Tache,D.Stark,D.John,J.Laveigne,V.Boychev,andS.-K.Hong,
Iwouldliketoacknowledgethosewhohavegenerouslysuppliedhigh-qualitysam-
pleswhichwereessentialtothecompletionofthisdissertation. TheyareDrs. M.Y.
LiandM.K.Wu(TsiangHuaUniversity,Taiwan),M.J.BurnsandK.A.Delin(Cali-
forniaInstituteofTechnology)forstudiesofYBa2Cu307_5films,Beom-Hoan0and
J.T.Markert(UniversityofTexas),G.CaoandJ.E.Crow(NationalHighMagnetic
FieldLaboratory)forstudiesYBa2Cu307_«-familysinglecrystals,R.J.KellyandM.
Onellion(UniversityofWisconsin),H.Berger,G.Margaritondo,andL.Forro(Ecole
ii
Polytechnique Federale de Lausanne, Switzeland) for studies ofBi2Sr2CaCu208-
familysinglecrystals,A.E.PuUenandJ.R.Reynolds,L.-K.ChouandD.R.Talham
(DepartmentofChemistry,UniversityofFlorida)forstudiesoforganicconductors.
Inaddition, Ihavegreatlyappreciated assistancefromDr. Y.J.Wang atthe
NationalHighMagneticFieldLaboratory. IamalsointhedebtofProfessorM.W.
Meisel,Dr. J.S.KimandT.Stevefortheacsusceptibilitymeasurementsoforganic
conductorsandhigh-Tcmaterials. IwouldliketothankDr. K.A.AbboudforX-ray
experimentsoforganicconductors. Itheinkthetechnicalstaffmembersinthephysics
departmentmachineshop,electronicshop,andcryogenicfortheirefforts.
Iwouldalsoliketotakethisopportunitytothankmyparentsfortheirlovecind
support. Finally,butmostimportantly,veryspecialandsincerethanksgotomywife
Yu-HueiChu,whoseconstantlove,dedication,encouragementandguidanceledme
tothepathandhelpedmetostaythere.
FinancialsupportfromtheNSF(GrantNo. DMR-9403894)isgratefullyacknowl-
edged.
iii
TABLEOFCONTENTS
Page
ACKNOWLEDGMENTS
ii
ABSTRACT
vii
CHAPTERS
L INTRODUCTION 1
II. FAR-INFRAREDPROPERTIESOFSUPERCONDUCTING
YBa2Cu307-*FILMSINZEROANDHIGHMAGNETICFIELDS 3
Introduction 3
Experimental 6
Results 9
Zero-fieldspectra 9
Magneticfieldstudies 10
Discussion 12
Dielectricfunctionmodels 12
Thevortexdynamics 15
Summary 21
HI. DOPING-INDUCEDCHANGEOFOPTICALPROPERTIESIN
UNDERDOPEDCUPRATESUPERCONDUCTORS
37
Introduction 37
Experiment 39
Samplecharacteristics 39
Opticalmecisurements 40
Results 4j
Room-temperaturespectra 41
iv
Temperaturedependenceintheinfraxed 43
Discussion 44
Low-frequencyspectralfunctions 44
Dielectricfunctionmodels 46
Quasiparticlescatteringrate 49
Superconductingresponse 54
Summary 61
IV. OPTICALSTUDYOFUNTWINNED(Bii.57Pbo.43)Sr2CaCu208+i
SINGLECRYSTAL:AB-PLANEANISOTROPY 84
Introduction 84
Experimental 86
ResultsandDiscussion 89
Polarizedreflectance 89
Opticalconductivity 90
Oscillatorstrengthsumrule 92
Dielectricfunctionmodels 93
Superconductingstate 98
Summary 103
V. AB-PLANEOPTICALSPECTRAOFIODINE-INTERCALATED
Bii.9Pbo.iSr2CaCu208+«: NORMALANDSUPERCONDUCTING
PROPERTIES 113
Introduction 113
Experimental 115
ResultsandDiscussion 115
Reflectancespectrum 116
Opticalconductivity 117
Oscillatorstrengthsumrule 120
Quasiparticlescatteringrate 122
Spectralweightinthecondensate 126
Summary 127
VL AB-PLANEOPTICALPROPERTIESOFNI-DOPEDBi2Sr2CaCu208+«137
Introduction 137
ExperimentalTechniquesandDataAnalysis 138
ComparisonofNi-DopedtoPureBi2Sr2CaCu208+5 140
TemperatureDependenceofNi-Doped 141
Reflectancespectrum 141
Conductivityanddielectricfunction 142
Two-componentmodel 143
One-componentanalysis 147
Spectralweight 149
Superconducting-stateproperties 150
Simunaxy 152
VIII. CONCLUSIONS 165
APPENDICES
A STRUCTUREANDPHYSICALPROPERTIESOF
ANEW1:1CATION-RADICALSALT,C-(BEDT-TTF)PF6 169
B OPTICALANDTRANSPORTSTUDIEDOFNi(dmit)2
BASEDORGANICCONDUCTORS 213
REFERENCES
249
BIOGRAPHICALSKETCH
268
vi
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
EFFECTSOFHIGHMAGNETICFIELDAND
SUBSTITUTIONALDOPINGONOPTICAL
PROPERTIESOFCUPRATESSUPERCONDUCTORS
By
Hsiang-LinLiu
August1997
Chairman: DavidB.Tanner
MajorDepartment:Physics
Infraredandopticalspectroscopieshavebeenappliedtostudyboththenormal
state and superconducting state electronicpropertiesofcupratesuperconductors.
Twoimportantparametersusedinourexperimentsaretheappliedmagneticfield
andsubstitutionaldoping.
Varyingthemagneticfieldatconstanttemperatureallowsustostudythevortex
d3mainicsinthehigh-temperaturesuperconductors. Incontrasttoseveralprevious
reports,wedonotseeanyfield-sensitivefeaturesineitherfar-infraredreflectanceor
transmittancespectraofYBa2Cu307_5thinfihnsatlowtemperatures. Onlyatfields
andtemperatureswherethedcresistanceisnotzero-onaccoimtofdissipativeflux
motion-thereisafield-inducedeffectinfar-infraredtransmittance.
Westudiedthea6-planeopticalresponseofY-dopedBi2Sr2CaCu208 andPr-
dopedYBa2Cu307_5singlecrystalsfromunderdopedtooptimallydopedregimes.It
vii
isdifficulttorelateourresultsoflow-frequencyspectralfunctionstothenormal-state
pseudogap. Wehavealsofoundthatthereisa"universalcorrelation"betweenthe
numbersofcarriersandthetransitiontemperature. Thiscorrelationholdswhether
oneconsidersthemmiberofcarriersinthesuperfluidorthetotalnumberofcarriers.
ThepolarizedspectraofPb-dopedBi2Sr2CaCu208+iwerealsoinvestigated.We
findthat although Pb dopingremoves the 6-axissuperlatticestructure, thereisa
definitea6-planeanisotropyofopticalpropertiesaboveandbelowTc. Ourresults
provideevidencethatwithintheCu02planetheelectronicstructuresoftheseBi-
basedcupratesuperconductorsareanisotropic,irrespectiveofthesuperstructurein
theBi-0layer.
Intheiodine-intercalatedBi2Sr2CaCu208+« sample, wefind that theinfrared
phononspectrumandthevisible-ultravioletinterbandtransitionsaxemodifiedafter
intercalation. Theaft-planeopticalconductivityresultssupporttheideathatinter-
CcilatediodineincreasestheholeconcentrationintheCu02 planes. Theiodineis
presentasions,mostlikelyasIj.
Finally, thea&-planeopticalspectraofNi-doped Bi2Sr2CaCu208+5 weremea-
sured. Wesuggestthatthelow-frequencyfeatureinthenormal-stateai{(jj)isassoci-
atedwiththesignificantdisorderinducedbyNiimpurities. Inthesuperconducting
state,asmallersuperfluiddensitythanimpliedbyTcfromtheUemuralinecouldbe
connectedwiththeeffectofimpurity-induceddisorder.
viii
CHAPTER
I
INTRODUCTION
ElevenyearshavepassedsincethefirstcupratesuperconductorWeisdiscovered.
Althoughwehavenotyetreachedafinalgoaloftheelucidationofthehigh-Tcmech-
anism,atremendousnumberofexperimentaltechniquesandtheoreticalworkhave
revealeditselectronicproperties.Infraredandopticalspectroscopieshaveplayedan
importantroleinclarifyingtheelectronicstateofcupratesuperconductors.^"* In-
fraredlightprobesthedynamicsofthefreecarriers,thenatureofcharge-transferand
otherlow-energyexcitations,thesuperconductinggap (inprinciple),phonons,and
otheraspectsoftheelectronicstructure. Becauseinfraredpenetratesasubstcintial
distance(~0.2-1/xm)intothematerial,itislesssensitivethanotherprobes(tun-
nelingandphotoemission,forexample)tosurfacedamage,especiallyifthesurface
layersarenonconducting.
Thisdissertationisorganizedinsuchawaythateachchapterisessentiallyself-
completeandcanbereadwithoutassumingspecialknowledgefromanotherchapter.
Wepresentanexperimentalstudyoftheinfraredpropertiesofhigh-Tcsuperconduc-
tors. Thereflectanceand/ortreinsmittanceofthesematerialshas beenmeeisured
attemperatures between4.2 K and300 Koverwavelengthsfromthefar-infrared
(35cm~^ or4.5meV)tothenearultraviolet (40000cm~^ or5eV).Furthermore,
twoimportantparametersusedinourmeasurementsaretheappliedmagneticfield
andsubstitutionaldoping. Promtheseexperimentstheopticalconductivity,(Ti(u;),
dielectricfunction,ei(u;),andotheropticalpropertiescanbeestimated.
Whenamagneticfieldisappliedtothehigh-Tcsuperconductors, theirmixed
statepropertiesarecharacterizedbytheexistenceofvortices. Therehasbeencon-
1
2
siderableinterest in vortex dynamicsin thehigh-Tc materials, including theirac
properties.Inourwork,wereportthefar-infraredreflectanceandtransmittancemea-
surementsofYBa2Cu307_ifilms. Weusereflectance<indtransmittancetoextract
frequency-dependentopticalconductivityasafunctionoftemperatureandapplied
magneticfield. Ourdiscussionwillconcentrateontherelationshipbetweenthespec-
trumchangesinamagneticfieldandthetwodistinctparts(vortexsolidandvortex
liquidstates)ofthemagneticphasediagram.
Changeofpropertieswithvaryingcarrierdensityisalsoofcentralconcernfor
theexperiments and theories aiming to understand the high-Tc mechanism. We
have examined the optical properties ofY-doped Bi2Sr2CaCu208 and Pr-doped
YBa2Cu307_5singlecrystalsfromunder-tooptimally-dopedregimes. Wewillad-
dressseveralinterestingissues. Amongthemarethefollowing: Isthereapseudogap
inthenormalstate? Whatarethenatureofthelow-lyingelectronicstates? What
governsthelifetimeofthechargecarriers? Doesthelow-frequencyspectralweight
evolvewithdoping?
Inordertoprovidefurtherinformationonthenatureofthea&-planeanisotropy,
theroleofinterlayercoupling, andlocalizationeffectsinthechargetransport,we
haveinvestigatedtheopticalpropertiesofseveralBi2Sr2CaCu208-fainilymaterials.
WechosePb-dopedBi2Sr2CaCu208+isamplesbecausePbdopingchangestheperi-
odicityofthesuperstructurealongthe6axis. Byperformingthepolarizedreflectance
measurements,wewillshowthatthereisdefiniteaft-planeanisotropyaboveandbe-
lowTc. Theeffectofiodineintercalationistoincreaseinterlayercoupling. Wewill
discussthechangeofa6-planeopticalpropertiesafterintercalationandthevalence
stateofintercalatediodineatoms. Niisadopant,whichsubstitutesfortheCuatoms
intheCu02planes. Wewillstudytheinfluenceofimpurity-induceddisorderonthe
a6-planeopticalpropertiesintheNi-dopedBi2Sr2CaCu208+«.