NaxCoO2: Enhanced low-energy excitations of electrons on a 2D triangular lattice

0.2 MB·English

by  M. Brühwiler

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n
Na CoO :Enhancedlow-energyexcitationsofelectronsona2D
a x 2
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triangularlattice
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M.Bru¨hwilera, B.Batlogga S.M.Kazakova Ch.Niedermayerb J.Karpinskia

l] aLaboratoryforSolidStatePhysics,ETHZu¨rich,CH-8093Zu¨rich,Switzerland
e bLaboratoryforNeutronScattering,PaulScherrerInstitute,CH-5232VilligenPSI,Switzerland
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r
t
s
.
at Abstract
m
Toelucidatethelow-energyexcitationspectrumofcorrelatedelectronsona2Dtriangularlattice,wehavestudied
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d theelectricalresistanceandspecificheatdownto0.5Kandinmagneticfieldsupto14T,inNaxCoO2sampleswith
n aNacontentrangingfromx≈0.5to0.82.Twodistinctregimesareobserved:forxfromabout0.6tox≈0.75the
o specificheatisstronglyenhanced,withapronouncedupturnofCp/T belowabout10K,reaching47mJ/(molK2).
c This enhancement is suppressed in a magnetic field indicative of strong low-energy spin fluctuations. At higher
[ Na content the fluctuations are reduced and µSR data confirm the SDW ground state below 22K and the much

reducedheatcapacityisfieldindependent.
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8 Keywords: NaxCoO2,spinfluctuations,thermodynamicproperties,triangularlattice,SDW,muonspinrotation
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In light of the complexity of the ground state due namicsoftheitinerantelectrons.Thisadditionalcom-
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togeometricfrustrationamonglocalizedmagneticmo- plexitybecomesevenmorerelevantbecauseofthepar-
0
mentsresidingonatriangularlattice,[1]itisofpartic- tialmobilityoftheNaionsevenatambientandmod-
/
t ularinterest tostudythelow-energy physicsof corre- estlyelevatedtemperature,andtheresultingpossibil-
a
m lated electrons as they acquire some itineracy on the ityfor theNa ions toorder. The CDW-likeelectronic
triangularlattice.NaxCoO2 ismetallic, crystallizesin stateisaclearexampleasithasbeenassociatedwitha
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d alayeredstructurewithedge-sharingCoO2octahedra, stripe-likeorderingoftheNaions,[7]andseveralindi-
n andCooccupiesatriangularlattice.Originallyknown cationsforNa-orderingatothervaluesofxhavebeen
o foritsunusuallylargethermoelectriceffect[2]thiscom- observed(e.g.Ref.[8]).Thusitwillbedesirabletoiden-
c pound exhibits a rich phase diagram as a function of tifytheinfluenceoftheFermilevelshiftandthatofthe
: temperatureandNacontent.Particularly notableare interactions with the Na ions, even as there is reason
v
i the superconductivity in water-intercalated material to expect some interplay between them.[9,10] In this
X
(xnear0.3to0.4),[3]aCDW-likestatenearx=0.5,[4] reportwefocusonthecompositionrangewhereNaor-
r andaSDW-likestatenearx=0.8.[5,6] dering appears not to dominate the electronic states,
a
WhilechangingtheelectroncountintheCoO2layer atleastnotonthelevelasitdoesatx=0.5.
byvariationoftheNacontentappearsbothsimpleand Thespecificheatandelectricalresistivityweremea-
effective,itisalsotobeexpectedthattheCoulombin- sured in aQuantumDesign PPMS setup with a He-3
teractionwiththeNaionswillinfluencethechargedy- insertanda14Tmagnet.Polycrystallinesampleswere
synthesized in a traditional way and the Na content
wasconfirmedbytheunitcellparameters,inparticu-
∗
Corresponding author. Tel: [phone] fax: +41 44 lar by thec/a ratio. µ+SR studies were performed at
6331072 thePaulScherrerInstituteandwereparticularlyhelp-
Emailaddress: [e-mail]
fultocheckthemagnetichomogeneityofthesamples.
(M.Bru¨hwiler).
PreprintsubmittedtoElsevierScience 6February2008
the measured electronic Cp/T for the superconduct-
ing compound at lower x,[13] or the x = 0.82 sam-
90
70 plealso shown inthefigure.Forcomparison, anearly
60 LDAcalculationforNa0.5CoO2gaveaCp/T ofroughly
80 50 11mJ/(molK2).[14]ThemeasuredCp/T thussuggests
2 ) 40 two contributions to the low-energy excitation spec-
/K 70 30 trum.Onewithatypicalenergyscaleofabout1meV,
mol 20 which is suppressed in a magnetic field, and an other
J/ 60 10 thatextendstoasignificantlyhigherenergy.
m The situation is different at higher Na composition
0
( 50 0 T 0 5 10 15 20 25 whereaSDWdevelops,markedbyadistinctanomaly
T
/ at 22K.[6,12] In the present study we have focused
)uc 40 x = 0.64 on the low-T region and the dependence on a mag-
n
C netic field. The µ+SR results of such a sample reveal

-p 30 theexcellent qualityin terms of themagnetic volume
C 14 T fraction. Afitoftheµ+SRdatausingthreeexponen-
(
20 tial relaxation functions and a KuboToyabe function
x = 0.82 indicates that over 80% of the muons experience a
10 staticmagneticfield.Belowabout2KCp/T saturates
andat0.5Kamountsto11mJ/(molK2).Interestingly,
0 within theexperimentaluncertaintyof a few percent,
1 10 this value remains constant in fields up to 14T. The
T (K)
same also holds truefor theentiretemperaturerange
covered:It isworth notingthat neitherthetransition
Fig.1. Low-temperaturespecificheatCp/T oftwoNaxCoO2 temperature at 22K nor the broad hump near 5 to
samplesrepresentingtheregionsofcontrastingelectroniclow-T
10K,whichhasbeenassociatedwithanothermagnetic
states.Thenuclearcontributionhasbeensubtractedforclar-
transition,[12]aremodifiedinafieldupto14T.
ity.Thex=0.64sampleisfromthebroadcompositionregion
where Cp/T increasesbelowabout 10K, indicativeof strong The low-temperature electrical resistivity ρ in the
lowenergyspinfluctuations,whicharesuppressedbyamag- spinfluctuationregion(x=0.7)significantlydeviates
neticfield.The x=0.82 sample undergoesaSDW-like tran- from a T2 behavior. At low T, it follows a tempera-
isnititohneaspti2n2-flKu.ctTuhaetiolonwr-eTgiovna,luaendofisCnpo/tTiniflsumenucchedlobwyearmthaagn- ture dependence ρ = ρ0+A˜Ta, with a smaller than
neticfield.TheinsetshowsthesamedataonalinearT scale. 2.Theexponentastartsatabout1.3inzerofieldand
increases rapidly with increasing field reaching about
Theessential features of thelow-T specificheat for 1.8at14T.
two representative compositions are shown in the fig- This study was partly supported by the Swiss Na-
ure.Awelldefinednuclearcontributionstemmingpre- tionalScienceFoundation.
dominantly from Co has been subtracted for clarity.
Thesamplewithx=0.64isrepresentativeofsamples
with a sodium content in the range from x ≈ 0.6 to
x ≈ 0.75. Zero field µ+SR spectra down to 1.7K in a
x=0.7sampleindicatetheabsenceofmagneticorder. References
Forthiscomposition range,thespecificheatrevealsa
[1] A.P.Ramirez,Annu.Rev.Mater.Sci.24(1994)453–480.
highdensityoflowenergyexcitations,givingrisetoan
increase of Cp/T below about 10K to reach values as [2] I.Terasakietal.,Phys.Rev.B56(1997)R12685.
large as 47mJ/(molK2), which has been observed in [3] K.Takadaetal.,Nature422(2003)53–55.
variousstudies(e.g.[11,12]).Additionalinsightscome [4] M.L.Fooetal.,Phys.Rev.Lett.92(2004)247001.
frommeasurementsinanexternalmagneticfield.Ina
[5] J.Sugiyamaetal.,Phys.Rev.B67(2003)214420.
magnetic field,these excitations aresignificantly sup-
pressed, shown in thefigure as the set of data for 0T [6] T.Motohashietal.,Phys.Rev.B67(2003)064406.
to 14T with 2T intervals. The suppression tends to [7] H.W.Zandbergenetal.,Phys.Rev.B70(2004)024101.
saturateinhighfieldsandfromanextrapolationato- [8] J.L.Gavilanoetal.,Phys.Rev.B69(2004)100404.
tal reduction by about 20mJ/(molK2), correspond-
[9] M.Indergandetal.,cond-mat/0502116.
ingtoasmuchas43%,canbeestimated.Apparently,
[10]P.Zhangetal.,cond-mat/0502072.
thelow-energyexcitationsareofmagneticorigin.The
residualvalueofCp/T is26to28mJ/(molK2)andit [11]M.Bru¨hwileretal.,cond-mat/0309311.
isstillenhancedbyaboutafactoroftwocomparedto [12]H.Sakuraietal.,J.Phys.Soc.Japan73(2004)2393.
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[13] R.Jinetal.,Phys.Rev.Lett.91(2003)217001.
[14] D.J.Singh,Phys.Rev.B61(2000)13397.
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