Muon spin rotation measurements of the magnetic field distribution in the vortex state of the oxygen deficient high-Tc superconductor YBa{2}Cu{3}O{6.60} reveal a vortex-lattice melting transition at much lower temperature than that in the fully oxygenated

MeltingandDimensionalityoftheVortexLatticeinUnderdopedYBa2Cu3O6.60J.E.Sonier1,J.H.Brewer2,R.F.Kie 2,D.A.Bonn2,J.Chakhalian2,S.R.Dunsiger2,W.N.Hardy2,R.Liang2,

W.A.MacFarlane3,R.I.Miller2,D.R.Noakes4,T.M.Riseman5andC.E.Stronach4

2

arXiv:cond-mat/9906449v1 [cond-mat.supr-con] 30 Jun 1999LosAlamosNationalLaboratory,LosAlamos,NewMexico,USA87545TRIUMF,CanadianInstituteforAdvancedResearchandDepartmentofPhysicsandAstronomy,UniversityofBritishColumbia,Vancouver,BritishColumbia,CanadaV6T1Z13LaboratoiredePhysiquedesSolides,UMR8502,Universit´edeParis-Sud,91405OrsayCedex,France4DepartmentofPhysics,VirginiaStateUniversity,Petersburg,Virginia238065SuperconductivityResearchGroup,UniversityofBirmingham,Birmingham,B152TT,UnitedKingdom()1Muonspinrotation(µSR)measurementsofthemagnetic elddistributioninthevortexstateoftheoxygende cienthigh-TcsuperconductorYBa2Cu3O6.60revealavortex-latticemeltingtransitionatmuchlowertemperaturethanthatinthefullyoxygenatedmaterial.Thetransitionisbestdescribedbyamodelinwhichadjacentlayersof“pancake”vorticesdecoupleintheliquidphase.Evidenceisalsofoundforapinning-inducedcrossoverfromasolid3Dtoquasi-2Dvor-texlattice,similartothatobservedinthehighlyanisotropicsuperconductorBi2+xSr2 xCaCu2O8+y.74.60.Ge,74.25.Dw,76.75.+i,74.72.BkInthehigh-temperaturesuperconductorsthereexistseveralexoticvortexlattice(VL)phasesowingtotheweakcouplingbetweenthesuperconductingCuO2layers(whichgivesrisetohighly exiblevortices),arelativelyshortcoherencelength(whichenhancestheirsusceptibil-itytopinning)andhighvaluesofTc(whichpermitlargethermalenergiestobereachedinthevortexstate).TodatemuchattentionhasbeenpaidtoVLmeltingandpinninge ectsinthehighlyanisotropicsuperconductorBi2+xSr2 xCaCu2O8+y(BSCCO)andinYBa2Cu3O7 δ(YBCO)fornearoptimaloxygenconcentrations(i.e.δ≈0.05,givingthehighestvalueofTc).ThevorticesinBSCCOareoftendescribedasstacksoftwo-dimensional(2D)“pancakes”,whichcanbecomemisalignedatele-vatedtemperaturesand/orinthepresenceofstrongran-dompinningsites.Ontheotherhand,in“optimally”dopedYBCOthevorticesbehaveinathree-dimensional(3D)manner.Thisdi erenceisattributedtothemassanisotropyγ=(mc/mab)1/2=λc/λab≈5-7inYBCOandγ≈50-250inBSCCO[1],whereλabandλcarethemag-neticpenetrationdepthsdescribingthescreeningof uxbysupercurrents owinginandoutoftheCuO2layers,

respectively.Anaddedcomplicationinmanystudiesof

YBCOisthepresenceoftwin-planeboundarieswhich

mayactasextendedpinningsitesforvortices.

Themuonspinrotation(µSR)techniquemeasuresthe

muonprecessionfrequencydistributionandthusthein-

ternalmagnetic elddistributionn(B),alsoknownasthe

µSRlineshape,whichhasproventobeapowerfultool

forinvestigatingVLphasesInoptimallydoped

YBCO,stronglocalpinningbroadenstheµSRlineshape

1aspredictedforsmallrandomdisplacementsof3DvortexlinesBycontrast,randompinninginBSCCOleadstoa eld-inducedreductionoftheµSRlinewidthassociatedwithadimensionalcrossoverfroma3D-VLtoaquasi-2DsystemconsistingofindependentweaklyinteractingVLsindi erentCuO2layers.Inthequasi-2Dregion,therepulsiveinteractionbetweenpan-cakevorticesinthelayersexceedsthestrengthoftheirinterlayercoupling,sothatrandompinningresultsinamisalignmentofthepancakevorticesinthe elddirec-tion.Thee ectontheµSRlineshapeofthermal uctua-tionsofthevortexpositionshasbeenthefocusofnu-merousstudiesinBSCCOAsexplainedinRef.[10],thetypicaltimescaleforvortex uctuationsisshortenoughthatthemuondetectsa uctuation-averaged eld.ThisresultsinareductionofboththewidthandtheskewnessoftheµSRlineshape.IncleansamplestheVLmeltsina rst-orderphasetransition(see,forexample,Ref.[12]).RecentµSRmeasurementsinBSCCOhavebeeninterpretedasevidenceforatwo-stageVLtransition: rsttheintralayercouplingofthepancakevorticesisovercomebythermal uctuations,thentheirinterlayercouplingislostInthelessanisotropiccompoundYBCO,ameltingtransitionhasbeenobservedbysmall-angleneutronscat-tering,magnetization,transport,speci cheatandHallprobeacsusceptibilitymeasurements(see,forexample,RefHowever,unlikeinBSCCO,thevortex-liquidphaseisfoundonlyinaverynarrowregionbelowthesecond-orderphasetransitionatBc2(T).Ameltingtran-sitionnearBc2(T)isdi culttostudywiththeµSRtech-nique,becauseevenfora3DVL,theµSRlineshapenar-rowsandbecomesmoresymmetricduetotheoverlapofthevortices.ItiswellestablishedthatremovalofoxygenfromtheCuOchainlayersinYBCOweakensthecouplingbetweenthesuperconductingCuO2layers.HerewereportµSRmeasurementsoftheVLintheoxygen-de cientcompoundYBa2Cu3O6.60,whichhasanin-creasedanisotropyratioγ≈22-36(see,Ref.[15]).Weobserveanexpandedvortex-liquidregionandapinning-inducedcrossoverfroma3Dtoaquasi-2Dsystem,quali-tativelyresemblingthevortexphasediagramofBSCCO.TheµSRexperimentswereperformedontheM15and