Search for an infrared electro-optic effect in thin high temperature superconducting films

2004·4.9 MB·English

by  Wint, Andrew C

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.