Unconventional superconductors : experimental investigation of the order-parameter symmetry PDF
Preview Unconventional superconductors : experimental investigation of the order-parameter symmetry
Gernot Goll Unconventional Superconductors Experimental Investigation of the Order-Parameter Symmetry With67Figures ABC GernotGoll UniversitätKarlsruhe PhysikalischesInstitut Wolfgang-Gaede-Str.1 76128Karlsruhe Germany E-mail:[email protected] LibraryofCongressControlNumber:2005933767 PhysicsandAstronomyClassificationScheme(PACS): 74.70.Tx,74.70.Pq,74.20.Rp,74.50.+r,74.25.Bt,74.25.Nf ISSNprintedition:0081-3869 ISSNelectronicedition:1615-0430 ISBN-10 3-540-28985-2SpringerBerlinHeidelbergNewYork ISBN-13 978-3-540-28985-2SpringerBerlinHeidelbergNewYork Thisworkissubjecttocopyright.Allrightsarereserved,whetherthewholeorpartofthematerialis concerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation,broadcasting, reproductiononmicrofilmorinanyotherway,andstorageindatabanks.Duplicationofthispublication orpartsthereofispermittedonlyundertheprovisionsoftheGermanCopyrightLawofSeptember9, 1965,initscurrentversion,andpermissionforusemustalwaysbeobtainedfromSpringer.Violationsare liableforprosecutionundertheGermanCopyrightLaw. SpringerisapartofSpringerScience+BusinessMedia springeronline.com (cid:1)c Springer-VerlagBerlinHeidelberg2006 PrintedinTheNetherlands Theuseofgeneraldescriptivenames,registerednames,trademarks,etc.inthispublicationdoesnotimply, evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfromtherelevantprotectivelaws andregulationsandthereforefreeforgeneraluse. Typesetting:bytheauthorusingaSpringerLATEXmacropackage Coverconcept:eStudioCalamarSteinen Coverproduction:design&productionGmbH,Heidelberg Printedonacid-freepaper SPIN:11010715 56/TechBooks 543210 OLord,howmanifoldareyourworks! Inwisdomhaveyoumadethemall; theearthisfullofyourcreatures. (Psalm104,24) ToClaudia,AnneandJulia S -asinglewordthatstillcausesexcitement,morethan90 yearsafteritsdiscovery S -anamazingphenomenonoflossfreetransport andlevitationinamagneticfield S -a challenging field of scientific activities between fundamental condensed-matter research and industrialapplications S -fascinating both experimentalists andtheorists S -some- times uncon- ven- tional S S Preface In the past decades a growing number of metals exhibited evidence for exotic types of superconductivity. The superconductivity is often called unconventional inordertoclassifysuperconductorswithrespecttotheorder-parametersymmetry. Manyintriguingexperimentshavebeencarriedouttoelucidatetheorder-parameter symmetry. Thecharacterizationoftheorderparameterofsuperconductingmaterialsneeds informationon(i)thepairingmechanism,(ii)theparityandspinstate,(iii)thesize and the nodal structure of the energy gap, and (iv) the macroscopic superconduc- tive phase. Each property can be accessed by different experiments. For example, the nodal structure can be investigated by measurements of the temperature de- pendenceofthermodynamicandtransportproperties,byangular-resolvedthermal- conductivity measurements, and by the directional dependence of current–voltage characteristics in point-contact and tunnelling measurements, though the latter ex- perimentshavebeenmainlyappliedforthedeterminationofgapsizeandthedensity ofstatesaround E .Ontheotherhand,theobservationofapowerlawinthelow- F temperaturebehaviourofoneproperty,forexampleofthepenetrationdepth,isnot sufficientforthecharacterizationofanunconventionalsuperconductorandmustbe treatedwithsomecautionsincetheremightbeother,quitedifferentandrathercon- ventionalexplanationsofthedata.Therefore,itisimportanttohavecorroborating evidencefromanumberofdifferentexperimentsbeforeaconclusivepictureofthe order-parametersymmetryisobtained.Asnoneoftheexperimentalmethodsalone can definitely reveal the order-parameter symmetry, manifold experiments will be discussed,eachofwhichaccessesacertainaspect.Inparticular,someemphasiswill beputontheinvestigationoftheenergygapofunconventionalsuperconductorsby point-contactspectroscopy.Thepoint-contactandtunnelspectrayieldinformation on the size and the anisotropy of the superconductive energy gap, its temperature, anditsfielddependence. This book is subdivided into three parts. In part I, a brief overview on possi- bleunconventionalsuperconductorsisfollowedbyashorttheoreticalintroduction ofthemainconceptsofunconventionalsuperconductivitycombinedwithageneral definition of often-used terms and notations. Part II presents several experimental methods that allow a characterization of the order-parameter symmetry. This in- clude measurements of specific heat, thermal conductivity, penetration depth, and ultrasound attenuation for information on the nodal structure (Chap. 3), nuclear X Preface magnetic resonance and muon spin rotation experiments for information on the parity and spin state (Chap. 4), point-contact and tunnelling spectroscopy for in- formation about the energy gap (Chap. 5), and phase-sensitive probes, namely the Josephsoneffect(Chap.6).Thispartisconcludedwithanoverviewofexperimental methodsthatprobetheeffectsofanunconventionalorderparameteronsinglevor- ticesandthesymmetryoftheflux-linelattice(Chap.7).PartIIIreviewsthreeclasses ofunconventionalsuperconductors,namelytheCe-basedheavy-fermionsupercon- ductors (Chap. 8), the U-based heavy-fermion superconductors (Chap. 9), and the metal-oxide superconductors (Chap. 10), especially Sr RuO . This book is not in- 2 4 tendedtoreviewthehigh-temperaturesuperconductors.Thereexistavastnumber ofexcellentreviewarticlesandmonographsthatgiveaquitecompleteoverviewof thestate-of-the-artresearchonthesematerials.Nevertheless,thehigh-temperature superconductorswillbementioned,asthehugeamountofresearchonthesecom- poundshaspushedthewholefield,boththetheoreticalandexperimentalactivities, andhasshedlightontheunderstandingofmanyothertopicsrelatedtounconven- tionalsuperconductivity. Finally,Ithankallpeoplewhocontributedtothisworkinmanyrespectsduring the past years. First of all, I am grateful to Prof. H. v. Lo¨hneysen for continuous support,encouragement,andmanystimulatingdiscussions.Itisapleasuretothank Prof.I.K.Yansonforinitiatingtheinterestintheexperimentaltechniqueofpoint- contact spectroscopy, and Dr. Y. Naidyuk for helpful discussions and correspon- dence.IthankProf.L.Taillefer,Prof.Y.Maeno,Dr.Z.Q.Mao,Dr.F.Lichtenberg, Dr.V.Zapf,andDr.E.Bauerforprovidingthesamplesnecessaryforthesestudies andforusefuldiscussions.IwouldalsoliketoacknowledgeProf.M.B.Maple,Prof. E.Dormann,Prof.C.Bruder,Prof.P.Wo¨lfle,Prof.J.Wosnitza,Dr.M.Eschrig,Dr. M.Fogelstro¨m,Dr.R.Werner,andDr.O.Stockertformanyfruitfulandilluminat- ingdiscussions.IthankDr.C.Obermair,Dr.F.Laube,T.Brugger,S.Kontermann, andM.Marzfortheircontributionstothepoint-contactdatareviewedhereandall my colleagues for good collaboration. I thank L. Behrens for drawing, scanning, andeditingmanyofthefigures.Last,butnotleast,IthankmywifeClaudiaandmy daughtersAnneandJuliafortheirpatienceandsympathyduringmyweekendwrit- ingofthismanuscript.ThisworkwaspartlysupportedbytheDeutscheForschungs- gemeinschaftthroughSonderforschungsbereich195“LokalisierungvonElektronen in makroskopischen und mikroskopischen Systemen” and Graduiertenkolleg “An- wendungenderSupraleitung.” Karlsruhe,October2005 GernotGoll Contents PartI IntroductiontoUnconventionalSuperconductivity 1 ABriefOverview .............................................. 3 References..................................................... 7 2 BasicTheoreticalConcepts...................................... 11 References..................................................... 18 PartII ExperimentalMethods 3 ProbingtheNodalStructure..................................... 21 3.1 SpecificHeatandThermalConductivity ....................... 21 3.2 MagneticPenetrationDepth.................................. 23 3.3 UltrasoundAttenuation...................................... 24 References..................................................... 26 4 ProbingtheParityandSpinState................................ 27 4.1 H Measurements.......................................... 27 c2 4.2 NuclearMagneticResonanceandKnightShift .................. 28 4.3 MuonSpinRotation ........................................ 30 References..................................................... 32 5 ProbingtheEnergyGap ........................................ 35 5.1 PlanarTunnellingSpectroscopy .............................. 35 5.2 ScanningTunnellingSpectroscopy(STS) ...................... 37 5.3 Point-ContactSpectroscopy.................................. 39 References..................................................... 53 6 ProbingthePhase.............................................. 55 6.1 JosephsonEffect ........................................... 55 6.2 Zero-BiasAnomalies ....................................... 59 References..................................................... 59 XII Contents 7 ProbingtheVortices:LatticeSymmetry andInternalStructure.......................................... 61 7.1 ScanningTunnellingSpectroscopy ............................ 62 7.2 Small-AngleNeutronScattering(SANS)....................... 62 7.3 MuonSpinRotation ........................................ 63 References..................................................... 63 PartIII PossibleUnconventionalSuperconductors 8 Ce-BasedHeavy-FermionSuperconductors ....................... 67 8.1 CeCu Si ................................................. 67 2 2 8.2 CeCoIn andCeIrIn ........................................ 72 5 5 8.3 CePt Si................................................... 81 3 References..................................................... 84 9 U-BasedHeavy-FermionSuperconductors ........................ 89 9.1 UPt ..................................................... 89 3 9.2 UBe ....................................................103 13 9.3 URu Si ..................................................107 2 2 9.4 UNi Al andUPd Al ......................................111 2 3 2 3 References.....................................................115 10 Metal-OxideSuperconductors ...................................121 10.1 Sr RuO ..................................................122 2 4 10.2 High-TemperatureSuperconductors ...........................140 References.....................................................147 A Appendix .....................................................153 References.....................................................161 Index .............................................................169
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