High Tc superconductors and related transition metal oxides: special contributions in honor of K. Alex Muller on the occasion of his 80th birthday

327 Pages·2007·7.7 MB·English

by  A. S. Alexandrov (auth.)

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
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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