Search for an infrared electro-optic effect in thin high temperature superconducting films PDF
Preview Search for an infrared electro-optic effect in thin high temperature superconducting films
SEARCHFORANINFRAREDELECTRO-OPTIC EFFECTINTHINHIGHTEMPERATURE SUPERCONDUCTINGFILMS By ANDREWC.WINT ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 2004 ACKNOWLEDGMENTS ThroughoutmytimeasagraduatestudenthereattheUniversityofFlorida, manyspecialpeoplehavehadaprofoundimpactonmydevelopment—bothasan experimentahstandasaperson. Itgivesmeenormouspleasuretopayhomageto thosewhohavechallenged,inspiredandencouragedmeduringthisexcitingperiod ofmyacademiclife. Firstly,Iwouldliketoexpressmysinceregratitudetomyadvisor.Professor DavidB.Tanner,forallowingmetobeapartofhisresearchteam,andforhis beliefandconfidenceinme. Ihavebenefitedgreatlyfromhistimelyadviceandhis willingnesstohelpwheneverhishelpwasneeded. IamalsogreatlyindebtedtoProfessorsA.F.Hebard,K.Ingersent,C.J. StantonandP.HoUowayforservingonmysupervisorycommittee. IwouldalsoliketothankmycolleaguesL.Gasprov,N.Tache,J.Hwang,V. Boychev,A.Ugawa,M.Chen,K.Kamaras,CD.Porter,K.Woo,M.Nikolou,H. Tashiro,N.Margankunte,H.Zhang,V.Long,L.BooshehriandN.Nemesfortheir insightfuldiscussionsandinspirationduringmygraduatework. ItismygreatpleasuretothankConductusforsupplyingthethinYBa2Cu306+x filmsonsapphiresubstrates,andZ.ZhangandR.Hadfieldforsupplyingsimilar filmsonmagnesiumoxidesubstrateswhichwereusedinthiswork. Iwouldalsoliketoacknowledgethetechnicalstaffatthemachineshop, electronicsshopandcryogenicslabwhowerealwaysveryhelpful. Itwouldberemissofmenottomentionthetwomostimportantpeople,my parents,whoseselflesssacrifice,loveandencouragementhavemadeitpossiblefor metobeheredoingthiswork. ii Finally,IwouldliketothanktheNSF(GrantNo. CTS-0082969)forits financialsupportofthisproject. iii TABLEOFCONTENTS page ACKNOWLEDGMENTS ii ABSTRACT vi CHAPTER 1 INTRODUCTION 1 2 OPTICALTHEORY 4 2.1 Introduction 4 2.2 LinearIsotropicHomogeneousConductingMedia 4 2.2.1 WavePropagation 4 2.2.2 ExtractionofOpticalConstants 7 2.3 StratifiedMedia 11 2.4 AnisotropicMedia 14 2.5 Drude-LorentzModel 20 2.6 SumRules 21 3 INTERFEROMETRICSPECTROSCOPY 23 3.1 Introduction 23 3.2 BasicTheory 23 3.2.1 InterferogramandSpectrum 24 3.2.2 TruncationandApodization 26 3.2.3 Sampling 27 3.2.4 PhaseErrors 28 3.2.5 Noise 29 3.3 Bruker113vSpectrometer 32 4 INFRAREDPROPERTIESOFHIGH-T,SUPERCONDUCTORS ... 37 4.1 Introduction 37 4.2 GenericTrends 38 4.2.1 NormalStatePhaseDiagram 38 4.2.2 CrystalStructure 38 4.2.3 DopingEffects 39 4.2.4 OpticalAnisotropy 40 4.3 ModelsofOpticalConductivity 41 4.3.1 Two-ComponentModel 42 iv 4.3.2 One-ComponentModels 43 4.4 SuperconductingStateElectrodynamics 45 4.5 SurveyofExperimentalResults 47 4.5.1 DopingDependenceofOpticalConstants 47 4.5.2 MidinfraredBand 49 4.5.3 RelaxationRate 50 4.5.4 One-ComponentAnalysis 51 4.5.5 c-AxisConductivity 52 4.6 Conclusion 54 5 SAMPLEPREPARATIONANDEXPERIMENTALTECHNIQUES . . 56 5.1 Introduction 56 5.2 YBasCugOe+xFilmonMgO 56 5.3 YBa2Cu306+xFilmsonSapphire 57 5.4 Experiment 58 5.4.1 CryogenicApparatus 58 5.4.2 ReflectivityMeasurements 59 5.4.3 TransmittanceMeasurements 60 6 FAR-INFRAREDSTUDIESOFYBazCuaOe+xFILMS 66 6.1 Introduction 66 6.2 ResultsandDiscussion 67 6.2.1 MgOSubstrate 67 6.2.2 SapphireSubstrate 68 6.2.3 YBazCuaOg+xonMgO 71 6.2.4 YBa2Cu306+xonSapphire 72 6.2.5 Two-FluidModel 73 6.2.6 FreeCarrierRelaxationandCondensate 75 6.3 Summary 77 7 FAR-INFRAREDELECTRO-OPTICSTUDIESOFYBaaCugOe+xFILMS 92 7.1 Introduction 92 7.2 ResultsandDiscussion 93 7.2.1 Electro-opticTransmittanceofFilmonMgO 93 7.2.2 Electro-opticTransmittanceofFilmonSapphire 95 7.2.3 Two-FluidSimulations 100 7.2.4 CurrentDensityDistributionsinSuperconductingThin Films 105 7.3 Summary Ill 8 CONCLUSION 113 REFERENCES 116 BIOGRAPHICALSKETCH 122 V AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy SEARCHFORANINFRAREDELECTRO-OPTIC EFFECTINTHINHIGHTEMPERATURE SUPERCONDUCTINGFILMS By AndrewC.Wint December2004 Chair: DavidB.Tanner MajorDepartment: Physics Infraredspectroscopyisusedtoinvestigatetheeffectoftransportcurrentson theab-planesuperconductingstatepropertiesofthinc-axisorientedYBa2Cu306+x filmsonsapphireandmagnesiumoxidesubstrates. Thefilmswerefirstcharacterizedinboththenormalandsuperconducting states,throughopticalreflectivityandtransmittancemeasurementsoverawidefire- quencyrange(40-350cm~^)inthefar-infrared. Weobservedfairlyhighreflectivity andlowtransmittanceinthesuperconductingstateatfirequenciesbelow150cm~^ Thisisincontrasttothelowerreflectivitiesandhighertransmittanceswhichare observedinthenormalstateoverthesamefrequencyrange. ItisweUknownfromnonlineartransportstudiesonthinsuperconductingfilms belowtheirtransitiontemperature,Tc,thatlargecurrentsexceedingthecritical currentIcdestroysuperconductivity,triggeringatransitiontotheohmiclossy regimetypicalofthenormalstate. Westudiedtheeffectoftransportcurrentson thesuperconductingstatetransmittanceinthefarinfraredregionbelow350cm~^ vi Thefirstsetofspectrawastakenat75K—about10Kbelow forbothfilms— andasetofspectrawasalsotakenatlowtemperature(40K)forthefilmon sapphiresubstrate. Thefilmsonmagnesiumoxideshowednoelectro-opticresponseuptovery highcurrents,whileathighcurrents,thefilmonsapphireshowedanincreasein transmittanceatlowfrequencyaccompaniedbyasuppressionofthetransmittance athigherfrequencies. Atlowertemperatureasimilareffectisobservedinthefilm onsapphire,evenatcurrentsmuchlowerthanthecriticalvalue. Webelievethe changesobservedarespurious,andmaybeattributedtoohmicheating. This hasbeenfurtherverifiedbysimulationsofthedielectricresponseusingasimple two-fluidmodelofthesuperconductingstatewithacurrentdependentsuperfluid oscillatorstrength. vii CHAPTER 1 INTRODUCTION Hightemperaturesuperconductivity(HTSC)inthelayeredcuprateshas beenacentralissueofcontemporarycondensedmatterphysicsformorethan17 years[1]. Thesematerialsareinterestingnotonlybecauseoftheirnovelelectronic properties,butalsobecauseoftheenormouspotentialwhichexistsforfabricating manypracticaldevicesincorporatingthem. Infrared,microwave,terahertzand opticalspectroscopieshavebeenusedtostudythenatureofchargetransfer,the dynamicsoffreecarriers,phonons,theJosephsonplasmaresonanceandmany othercollectivephenomenawhichareobservedinthisclassofcompounds[2,3]. Despiteconsiderableeffortbytheoristsandexperimentalistsalike,thepairing mechanismresponsibleforHTSCinthesematerialsstillremainsamystery. Hightemperaturesuperconductorsappeartobeattractivecandidatesforthe fabricationofopticalmodulatorswhichareusedinfiberopticcommunications. Atpresent,fiberoptictransmissionratesarelimitedbytherateatwhichoptical datacanbemodulated. Thecurrentmodulationtechniqueswhichincludedirect modulation,electro-absorptionmodulation,andMachZendermodulation,hmit thetransmissionratestoabout40Obitpersecond. Directandelectro-absorption modulationarehmitedbythecarriermobilityofgalliumarsenide(GaAs). The modulationrateofMach-Zendermodulatorsislimitedbythedifficultiesin producingrapidhighvoltagepulsesrequiredtoswitchtheelectro-opticlithium niobiate(LiNbOa)materialusedinthesemodulators. HTSCcaneasilybeswitched fromasuperconductingstatetoaresistivestatebypassingacriticalamountof current. Theresistivestateischaracterizedbyhightransmissionatfarinfrared frequencies,ascomparedtotheopaquesuperconductingstate. Itisbelievedthat 1 2 therateofswitchingmaybeashighasaterabitpersecond. Thishighrateof switchingwouldvastlyimprovethetransmissionrateofcurrentmodulatorsinfiber opticnetworks. Anotherfigureofmeritofanyopticalmodulatorisitsextinctionratio. The extinctionratioforasuperconductingmodulatoristheratiooftransmittedlightin theresistivestatetothetransmittedlightinthesuperconductingstate. Itwillbe shownthatasuperconductingmodulatorbasedonHTSCfilmsoffersnoadvantage overthecurrentmodulatorswhichareavailablecommercially. Thisdissertationdescribesasemi-systematicstudyoftheeffectsoftrans- portcurrentonthefarinfraredtransmittancespectraofthinsuperconducting YBa2Cu306+xfilms. Ihaveconfinedtheinvestigationstotransmittancemeasure- mentsbelow350cm~^ Therearetwomainreasonsforthis. Thefirstisbecause theeffectofthedephasingandsubsequentweakeningofsuperconductingorderis expectedtobeseeninthefarinfraredregion. Secondly,thechangesareexpected tobequitesmallandopticaltransmittanceisfarmoresensitivethanreflectivityto suchsmallchanges. Thenextsectiongivesabriefdescriptionoftheorganizationoftheremaining chaptersofthistreatise. Thisdissertationconsistsofeightchaptersincludingtheintroduction. Ihave triedtoorganizethechaptersinalogicalsequence,sothatalltheconceptsneeded areeasilyaccessible. Chapter2describesthebasicopticaltheoryofthepropagationoflight throughopticalmedia. Ishalldiscussthevarioustechniqueswhichareused toextracttheopticalconstantsofamaterialfrommeasurementsofoptical reflectivityandortransmittance. Chapter3givesadiscussionofthebasictheory ofinterferometricspectroscopyandadescriptionoftheinterferometerusedin thiswork. Areviewofourcurrentunderstandingoftheelectrodynamicsofhigh 3 Tcsuperconductors,bothinthenormalandsuperconductingstates,isgivenin chapter4,wherevariousmodelsusedtoanalyzeopticaldataareintroduced. Abriefdescriptionofthefilmpreparationtechniquesandtheircharacteri- zationbyastandard4-probemethodispresentedinchapter5. Chapter6deals withtheresultsoftheopticalreflectivityandtransmittancemeasurementsandthe extractedopticalconstantsforthethinYBagCuaOe+xfilmsusedinthiswork. In chapter7Ipresenttheresultsofelectroopticmeasurementsinthesuperconducting stateofthesamefilmsusedinchapter6. Thedifficultiesencounteredininterpret- ingthedataarediscussed. Finally,chapter8concludesthisdissertationwitha reviewoftheresults.
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