High Tc superconductors and related transition metal oxides: special contributions in honor of K. Alex Muller on the occasion of his 80th birthday PDF
Preview High Tc superconductors and related transition metal oxides: special contributions in honor of K. Alex Muller on the occasion of his 80th birthday
HighT Superconductorsand c RelatedTransitionMetalOxides Annette Bussmann-Holder Hugo Keller (Eds.) High T Superconductors c and Related Transition Metal Oxides Special Contributions in Honor of K. Alex Müller on the Occasion of his 80th Birthday 123 Dr.AnnetteBussmann-Holder Max-Planck-InstitutfürFestkörperforschung Heisenbergstraße1 70569Stuttgart,Germany [email protected] Dr.HugoKeller UniversitätZürich Physik-Institut WinterthurerStraße190 8057Zürich,Switzerland [email protected] LibraryofCongressControlNumber:2007922939 ISBN978-3-540-71022-6SpringerBerlinHeidelbergNewYork DOI10.1007/978-3-540-71023-3 Thisworkissubjecttocopyright.Allrightsarereserved,whetherthewholeorpartofthematerial isconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation,broad- casting,reproductiononmicrofilmorinanyotherway,andstorageindatabanks.Duplicationof thispublicationorpartsthereofispermittedonlyundertheprovisionsoftheGermanCopyrightLaw ofSeptember9,1965,initscurrentversion,andpermissionforusemustalwaysbeobtainedfrom Springer.ViolationsareliableforprosecutionundertheGermanCopyrightLaw. SpringerisapartofSpringerScience+BusinessMedia springer.com (cid:1)c Springer-VerlagBerlinHeidelberg2007 Theuseofregisterednames,trademarks,etc.inthispublicationdoesnotimply,evenintheabsence ofaspecificstatement,thatsuchnamesareexemptfromtherelevantprotectivelawsandregulations andthereforefreeforgeneraluse. CoverDesign:WMXDesignGmbH,Heidelberg,Germany TypesettingandProduction:LE-TEXJelonek,Schmidt&VöcklerGbR,Leipzig Printedonacid-freepaper 2/3100YL–543210 UmeintadellosesMitgliedeinerSchafherdeseinzukönnen, mussmanvorallemeinSchafsein. Inordertobeanirreproachablememberofaflockofsheep, mostnotablyonehastobeasheep. AlbertEinstein K.AlexMüller Preface WheninitiatingthisbookandinvitingfriendsofK.AlexMüllertosendcon- tributions to it in order to honor his 80’th birthday, we frequently were told that it would be difficult to write anything related to high temperature su- perconductivity. It seemed that most of his friends and colleagues associate almost exclusively high temperature superconductivity with his name and have forgotten that K. Alex Müller’s scientific career is more tightly bound to phase transitions, critical phenomena, electron paramagnetic resonance (EPR),andferroelectricitythantosuperconductivity,eventhoughhecaused –togetherwithGeorg Bednorz–arevolutioninthisfield. Morethanhalfof his research life concentrates on the above mentioned phenomena and ma- jorbreakthroughshavebeenachievedbyhiminthisarea.Startingpointwas the investigation of the electron paramagnetic resonance of MnIV in SrTiO 3 whereheobservedthatdistortedregimesappearinthenominallycubicphase. A homogeneous state is only realized when the crystal is in the tetragonal phase. Even though SrTiO remained in the center of his research interests 3 earlyon,theJahn–Tellereffectwasfascinatinghim.Experimentallyandtheo- reticallyhestartedvariousprojectsinthisfieldwhichhastheuniqueproperty thatavibronicstatedominatesthephysics,whereelectronicandionicdegrees offreedomareundistinguishable.Thisstateisaconsequenceofacompetition between enhancing the entropy and lowering the energy of the system and intimatelyrelatedtoferrodistortivetransitions.Thesehavebeeninvestigated indetailbyhimconcentrating ontherotationalinstability oftheoxygenoc- tahedralinperovskiteswhereheshowedthattherotationangleplaystherole ofan order parameter. Together withHarryThomas he established atheory ofphasetransitions, wheresoftmodedynamicsarepredicted incoexistence withelasticinstabilitiesstemmingfromthecouplingtostrainfields.However, inferrodistortivetransitionstheJahn–Tellereffectistheimportantone,since here electron-lattice interactions play a crucial role. Since phase transitions are intimately related to critical exponents, EPR techniques have been used by him in order to determine these for the rotational instabilities in various perovskites.Theessenceofthisworkwasthatinthecriticalregionphysically measurablevariablesbecomeindependentofinteratomicdistances,sincedue to cooperative phenomena the correlation lengths are much larger than the range of forces between the individual particles. The phenomenon of phase VIII Preface transitions hasbeen hisfocusformany years, where especially thoserelated toferroelectricityandzoneboundaryinstabilitieshaveattractedhisinterest. Besides of EPR also electron spin resonance (ESR) methods have been used byhim.ItshouldbeemphasizedherethatK.AlexMüller’sworkalwaysrepre- sentedamixtureoftheoreticalconsiderationsandexperimentalresults.Inthe followingyearsheconcentrated ondetailsrelated tostructuralphasetransi- tions,wherehewasabletoshowthatrotationalfluctuationsarepresentalready above the instability, and a crossover from a fast to slow fluctuation regime sets in upon approaching the critical temperature . This notion is nowadays reinvented, since high resolution local probes enable to detect the onset of fluctuationsfaraboveT .Besidesofperovskites,healsoworkedonhydrogen- c bonded ferroelectrics, where for the first time evidence for the existence of locallypolarizedclusterswasobtainedwhichformfarabovetheactualphase transition temperature and exhibit distinct life times. Importantly, these ex- tradynamicsarepresentindependentofthetunnelingmotionoftheprotons and are related to the heavy ion sublattice. We believe this work is the first toprovideinsight into thephase transition mechanism ofhydrogen-bonded ferroelectrics which is beyond the concept of purely hydrogen ordering in- duced.Intimatelyrelatedtothisistheclassificationofferroelectricsintoeither order/disorder or displacive ones. From the very beginning of the research in ferroelectricity this scheme has been used to classify this material class. Itisbasedontheassumptionthatinanorder/disorderdominatedtransition theinvolvedionstunnelbetweenequivalentlatticesitesinarandommanner aboveT andordercooperativelyintooneoftheminimaatT .Inadisplacive c c transition, ontheotherhand, themotionoftwosublattice ionsagainst each other slows down with decreasing temperature to freeze out at T , and thus c determinesthelowtemperaturepolarstructure.FromhisEPRmeasurements K.AlexMüllercametotheconclusionthatsuchastrictdistinctionofthetwo picturesisnotpossible,butthatalwaysacoexistenceregimemustbepresent. Interestingly,heresolvedwiththisproposalalongstandingcontroversy,since longwavelengthexperimentsshowed–incontrasttolocalprobes–displacive characteristics,whereasthelocalprobesprovidedevidencefororder/disorder dynamics.Thisexperimentaldiscrepancyhasbeenshowntobeaconsequence ofdifferenttimeandlengthscalesinvolvedinthedifferentexperiments.Theis- sueoftimeandlengthscaleshasbeenfollowedbyhimuntilnow.Animportant point was addressed by K. Alex Müller withinthe context of ferroelectricity. While he concentrated for a while on the structural instability observed in SrTiO at104K,whichisdrivenbythecondensationofazoneboundarymode, 3 hegotmoreinterestedinthedielectricbehavioratlowertemperatureswhere thedielectricconstantcontinuouslygrowswithdecreasingtemperature.Such a feature is reminiscent of a polar instability. However, and opposite to e.g. BaTiO and KNbO no instability sets in, since quantum fluctuations sup- 3 3 pressthephasetransition.Thisledhimtointroducetheconceptofquantum paraelectricity which is a rare phenomenon and only observed in one other Preface IX perovskite, namely KTaO . Ferroelectricity can be induced in these systems 3 byproperdoping. Especially, Cadoping ofSrTiO showsinteresting aspects, 3 sinceadisplacivetypetransitioncanbeinducedbyaaddingasmallamount ofCa.Thenatureofthetransitionchangessubstantiallywithincreasingdop- ingfromaXYquantumferroelectrictoarandom-fieldinduceddomainstate. Usingphotoelectronandinversephotoemissionspectroscopythebandstruc- tureofSrTiO hasbeeninvestigatedbyhimwhichshowsthepeculiarfeature 3 that the valence band states are not of purely oxygen 2p character, but have smalladmixturesofTid-states. Thisp-dhybridizationhasbeen shownthen tobeabasicingredientforphasetransitionmechanisminperovksiteoxides. Sincelatticedynamicsisacentralissuetoferroelectricsasitcandiscriminate between order/disorder and displacive transitions, he used EPR in order to answerthisquestion.EspeciallyinthecaseofBaTiO itremainedunclearhow 3 toclassifythiscompound,sinceagaindifferentexperimentsreporteddifferent dynamics. K. Alex Müller was able to measure the degree of anharmonic- ity which is a direct probe of the transition dynamics. He observed that the anharmonicity of BaTiO is 34% larger than the one of SrTiO and KTaO . 3 3 3 Correspondingly, thedouble-wellpotential hasalargepotential barrier asis typicalfororder/disordertransitions.Thesameconclusionshavebeenreached onlyrecentlybyNMRtechniqueswhichprobelocalproperties. WhileK.AlexMüllerisverywellknownforhisextendedresearchinphase transitions, critical phenomena, and ferroelectricity, it has frequently been overlookedthatheworkedalsoinsuperconductivityconcentratingonnowvery popularaspects.Heobservedthatthesuperconductingpropertiesofgranular aluminum are highly inhomogeneous and characterized by local transition temperatures. Using microwavemeasurements of granular aluminum sheets in a cavity he obtained direct information on the number and temperature dependenceofsuperconductingCooperpairs.Specifically.inhomogeneityhas importantinfluencesontheresistivetransitionswhichtakeplacebytheforma- tionofsuperconducting regionsbelowacertaincriticaltemperature. Butthe lockingofphasesbetweenthegrainsproducesacontinuoussuperconducting pathonlywhenalowertemperatureisreached.Thenotionofinhomogeneity isnowadaysanimportantissueinhightemperaturesuperconductivitywhere substantialevidenceformulti-componentsexists. The discovery of high temperature superconductivity in cuprates did not end his interests in phase transitions, but was actually a result of hisformer research in transition metal oxides where electron lattice interaction effects havebeen showntobecrucial.Hisdetailed knowledgeoftheJahn–Telleref- fect played here an important role, since from his collaboration with Harry ThomasinBaselandHeinzBilzinStuttgartthepossibilityseemedtobegiven that polaronformationcouldbe a new glue in Cooper pair formation. Since thisprocessisnottiedtotheenergyscaleofatypicallatticevibration,there wasachanceofraisingthesuperconductingtransitiontemperatureinanun- expectedway.Hissuccessjustifiedthisassumption.However,theresearchin X Preface superconductivitytookanunexpectedway,sincetheantiferromagneticprop- erties of the parent compounds have frequently been taken as evidence that purelyelectronicmechanismsareatwork.TheseproposalsintensifiedK.Alex Müller’s work in the field of high temperature superconductivity, since they were against his own conviction. His scientific focus after the discovery was consequently onexploring the microscopicoriginofthepairing mechanism incuprates.SinceinconventionalsuperconductorstheisotopeeffectonT has c proventhatelectron-latticeinteractionsarethesourceofCooperpairforma- tion,heinitiatedavastandextremelysuccessfulresearchattheUniversityof Zürichonisotopeeffectsincuprates.Inordertoidentifythelocusofpairing, carefully prepared site-selective oxygen isotope experiments have been per- formedshowing thatthelargesteffectonT stems fromtheplanes, whereas c only a negligible isotope effect can be attributed to the out of plane oxygen ions.Besidesofthisisotopeeffect,variousothershavebeendiscoveredwhich aremostlyhighlyunconventional,unexpectedandbeyondaBCSframework. Ithastobementionedhere,thatasubstantialandsignreversedisotopeeffect has been observed by different methods and in various cuprate families on ∗ theso-calledpseudogaptemperatureT .Furthermorethesuperfluiddensity has been shown to be isotope dependent, thereby showing that the conven- tionalBCSpairing mechanism canbeexcluded, since withinBCStheorythe penetrationdepthshowsnoisotopeeffect.UsinghispreferredmethodofEPR hecouldshowthattheEPRlinewidthinMn2+ dopedLa Sr CuO islarger 2–x x 4 for 18O than for 16O, being pronounced at low temperatures and decreasing withincreasing Srcontent.Thiseffecthasbeenexplained intermsCuS=1/2 relaxationcoupledlinearlytolocalQ /Q tiltingmodesoftheCuO octahedra. 4 5 6 SincethesemodesarecoupledstericallytotheQ typeJahn–Tellermode,the 2 resultssupporttheideaofpolaronformationasbeingarelevantingredientto superconductivity.AnimportantissuehasbeenraisedbyK.AlexMüllerearly on,namely,thatthepairingstateisnotofd-wavesymmetryalone,butcarries asubstantials-wavecomponent.Thisimpliesthatcupratesaremulticompo- nentsystemswherethedynamicalinterplaybetweenthecomponentsgivesrise tomulti-bandsuperconductivity.Theseideashaverecentlybeenconfirmedex- perimentallybymuonspinrotation,whichunambiguouslydemonstratesthat besidesofaleadingd-wavecomponent,alsoapronounceds-wavecomponent ispresentincuprates.Thisobservationprovidesfurtherevidencethatcuprates areinhomogeneous anda composite ofat least twocomponents, aholerich polaronicdistortedoneandaholepoorundistortedmatrix,asconfirmedearly onbyhisEPRexperiments. Eventhoughtheoriginaboutthepairinggluein cupratesisnotgenerallyagreedon,manyfactssupporthisoriginalconceptof Jahn–Tellerpolaronformation. In addition to cuprates also manganites have been of extreme interest to himinthemorerecent past. Their structuralsimilarity tocuprates andper- ovskiteferroelectricssuggeststhatelectron-latticeinteractionsareimportant inunderstanding theirrichphenomenology.ThathereagaintheJahn–Teller
The list of books you might like
