Non-Fermi liquid behavior in a fluctuating valence

arXiv:0708.0412v1[cond-mat.str-el]2Aug2007Non-Fermiliquidbehaviorinafluctuatingvalencesystem,thefilledskutteruditecompoundCeRu4As12R.E.Baumbach,P.C.Ho,T.A.Sayles,andM.B.MapleDepartmentofPhysicsandInstituteforPureandAppliedPhysicalSciences,UniversityofCaliforniaatSanDiego,LaJolla,CA92093,USAR.Wawryk,T.Cichorek,A.Pietraszko,andZ.HenkieInstituteofLowTemperatureandStructureResearch,PolishAcademyofSciences,50-950Wroclaw,Poland(Dated:February1,2008)AbstractElectricalresistivityρ,specificheatC,andmagneticsusceptibilityχmeasurementsmadeonthefilledskutteruditeCeRu4As12revealnon-Fermiliquid(NFL)T-dependencesatlowT,i.e.,ρ(T)∼T1.4andweakpowerlaworlogarithmicdivergencesinC(T)/Tandχ(T).MeasurementsalsoshowthattheT-dependenceofthethermoelectricpowerS(T)deviatesfromthatseeninotherCesystems.TheNFLbehaviorappearstobeassociatedwithfluctuationsoftheCevalencebetween3+and4+ratherthanatypicalKondolatticescenariothatwouldbeappropriateforanintegralCevalenceof3+.PACSnumbers:71.10.Hf,71.28.+d1I.INTRODUCTIONThefilledskutteruditecompoundsoftheformMT4X12whereM=alkalimetal,alka-lineearth,lanthanide,actinide,T=Fe,Ru,OsandX=P,As,Sbhavebeenshowntoexhibitawealthofstronglycorrelatedelectronphenomenaincluding;spinfluctuations,itin-erantferromagnetism,localmomentferromagnetismandantiferromagnetism,conventionalBCSsuperconductivity,unconventionalsuperconductivity,heavyfermionbehavior,andnon-Fermiliquidbehavior.1,2Manyofthesephenomenadependonhybridizationbetweentherareearthoractinidef-electronstatesandtheconductionelectronstates.ThistrendisevidentintheCeT4X12systems,forwhichtheexpectedlatticeconstantvalueduetotheLn3+latticecontractionatroomtemperatureisstronglydepressedforthephosphides,isnotdepressedatallfortheantimonides,andisintermediateforthearsenides.3,4TheaffectsofhybridizationareparticularlydramaticforCeRu4Sb12which,untilnow,wastheonlyfilledskutteruditeknowntoshownon-Fermiliquid(NFL)behavior.5,6,7Inthiscompound,theNFLbehaviorischaracterizedatlowTasfollows:resistivity-ρ(T)∼T1.4,specificheatdividedbytemperature-C(T)/T∼-lnTorT−0.58,andmagneticsusceptibility-χ(T)∼T−0.35,allofwhichsuggestthatCeRu4Sb12islocatednearaquantumcriticalpoint(QCP).ThevalueofthelatticeconstantforCeRu4Sb12atroomtemperatureindicatesthattheCeionsarenearlytrivalent,suggestingaKondolatticeistheappropriatedescriptionofthephysics.However,itshouldbenotedthattheunusualevolutionofχ(T)withTsignalsamorecomplicatedscenariowheretheCeionsmayundergoacontinuousvalencetransitionfrom3+towards4+withdecreasingT.Inthispaper,wereportNFLbehaviorinthefilledskutteruditeCeRu4As12,indicatingthatthiscompoundisalsonearaQCP.However,inthiscase,thelatticeconstantatroomtemperatureandtheTdependenceofχ(T)revealthattheCeionshaveanintermediatevalencebetween3+and4+.Therefore,CeRu4As12isoneofonlyafewcompoundswheretheappropriatedescriptionoftheNFLbehaviormaybeavalencefluctuationratherthanaKondolatticepicture.Wepresentmeasurementsofρ(T),C(T),χ(T),andthermoelec-tricpowerS(T)forthecompoundCeRu4As12,characterizetheNFLbehavior,discusstheevidenceforintermediatevalenceontheCeions,andcomparetootherCebasedsystemswheretheNFLbehaviormayberelatedtointermediateCevalence.2II.EXPERIMENTSinglecrystalsofCeRu4As12weregrownfromelementswithpurities≥99.9%byamoltenmetalfluxmethodathightemperaturesandpressures,thedetailsofwhichwillbereportedelsewhere.8Afterremovingthemajorityofthefluxbydistillation,CeRu4As12singlecrystalsofanisometricformwithdimensionsupto∼0.7mmwerecollectedandcleanedinacidinanefforttoremoveanyimpurityphasesfromthesurfacesofthecrystals.ThecrystalstructureofCeRu4As12wasdeterminedbyX-raydiffractiononacrystalwithdimensionsof0.17×0.18×0.23mm.Atotalof5714reflections(447unique,Rint=0.0772)wererecordedandthestructurewasresolvedbythefullmatrixleastsquaresmethodusingtheSHELX-97programwithafinaldiscrepancyfactorR1=0.0273[forI2σ(I),wR2=0.0619].9,10Electricalresistivityρ(T)measurementsfor50mKT290Kwereperformedinafour-wireconfigurationinzeromagneticfieldusingaconventional4Hecryostatanda3He-4Hedilutionrefrigerator.Magneticsusceptibilityχ(T)measurementsfor1.9KT300KwereconductedusingaQuantumDesignMagneticPropertiesMeasurementSystem(MPMS)onamosaicofcrystals(m=49mg)whichweremountedonasmallDelrindiscusingDucocement.SpecificheatC(T)measurementsfor650mKT10Kweremadeusingastandardheatpulsetechniqueonacollectionof11singlecrystals(m=45mg)attachedtoasapphireplatformwithasmallamountofApiezonNgreaseina3Hesemiadiabaticcalorimeter.ThethermoelectricpowerS(T)for0.5KT350KofCeRu4As12singlecrystalswithlengthlessthan1mmwasdeterminedbythemethoddescribedinref.11III.RESULTSSinglecrystalstructuralrefinementshowsthattheunitcellofCeRu4As12hastheLaFe4P12-typestructure(IM3)spacegroupwithtwoformulaunitsperunitcell,andalatticeconstanta=8.5004(4)˚A,inreasonableagreementwithearliermeasurementsofa=8.4908˚Aanda=8.4963˚A.3,4,12OthercrystalstructureparametersaresummarizedinTableI.ThedisplacementparameterUeqrepresentstheaveragedisplacementofanatomvibratingarounditslatticepositionandisequaltoitsmean-squaredisplacementalongtheCartesianaxes.ThedisplacementparametersdeterminedforCeRu4As