59Co Nuclear Quadrupole Resonance Studies of Superconducting and Non-superconducting Bilayer Water Intercalated Sodium Cobalt Oxides NaxCoO2.yH2O

0.17 MB·English

by  C. Michioka

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59Co Nuclear Quadrupole Resonance Studies of Superconducting and
Non-superconducting Bilayer Water Intercalated Sodium Cobalt Oxides
Na CoO ·yH O
x 2 2
6
C. Michioka, H. Ohta, Y. Itoh, and K. Yoshimura
0
0
Department of Chemistry, Graduate School of Science,
2
KyotoUniversity,Kyoto 606-8502, Japan

n
(Received February6,2008)
a
J We report 59Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated
0 sodium cobalt oxides NaxCoO2·yH2O (BLH) with the superconducting transition tempera-
3 tures, 2 K < Tc ≤ 4.6 K, as well as a magnetic BLH sample without superconductivity. We
obtained a magnetic phase diagram of Tc and the magnetic ordering temperature TM against
] thepeakfrequencyν3 ofthe59CoNQRtransitionIz =±5/2↔±7/2andfoundadomeshape
n superconducting phase. The 59Co NQR spectrum of the non-superconducting BLH shows a
o broadening below TM without the critical divergence of 1/T1 and 1/T2, suggesting an uncon-
c ventional magnetic ordering. The degree of the enhancement of 1/T1T at low temperatures
- increases with the increase of ν3 though the optimal ν3 ∼ 12.30 MHz. In the NaxCoO2·yH2O
r
p system,theoptimal-Tc superconductivityemergesclosetothemagneticinstability. Tc issup-
u pressed near the phase boundary at ν3 ∼ 12.50 MHz, which is not a conventional magnetic
s quantumcritical point.
.
t
a
KEYWORDS: superconductivity,sodium cobaltoxide,nuclearquadrupole resonance
m
-
d
n Thediscoveryofsuperconductivityofbilayerwaterin- The59Co(nuclearspinI=7/2)NQRexperimentshave
o tercalated sodium cobalt oxides Na CoO ·yH O (BLH) beendoneforthepowdersamplesbyutilizingacoherent-
x 2 2
c has renewed our interests in itinerant triangular lattice type pulsed NMR spectrometer. The 59Co nuclear spin-
[
systems.1) After much experimental efforts have been latticerelaxationtime59T wasmeasuredbyaninversion
1
1 devotedtofindthechemicalparametersonthesupercon- recoverytechnique. The spin-echosignalM(t)wasmea-
v
ducting transiton temperature T , the complicated and suredasafunctionoflongdelaytimetafteraninversion
9 c
delicate structural parameters on the Na vacancies, the pulse, and M(∞)[≡M(t>10T )] was recorded. All the
5 1
6 intercalated water molecules and oxonium ions, which recovery corves were measured at the peak freqency of
1 partially occupy the sodium site,2) have been revealed. ν ,whichcorrespondstothethetransitionI =±5/2↔
3 z
0 However, the microscopic roles of these parameters in ±7/2. In order to estimate 59T , the recovery curves
1
6
characterizing the superconductors are still poorly un- p(t)≡1−M(t)/M(∞)werefittedbyatheoreticalp(t)=
0
derstood. p(0)[(3/7)e−3t/T1+(100/77)e−10t/T1+(3/11)e−21t/T1]for
/
at Recently we successfully synthesized powder samples the transition (Iz = ±5/2 ↔ ±7/2), where p(0) and T1
m of Na CoO ·yH O with different superconducting tran- are fit parameters.5) Agreement of the theoretical and
x 2 2
sitiontemperature T by a softchemicalmethod by uti- experimentalrecoverycurveswastotallysatisfactoryex-
- c
d lizing so-called duration effect.3) The T of a series of cept for magnetically ordered states and also except for
c
n
samples shows a systematic change with the increase of the samples near the boundary between the supercon-
o
time duration after synthesis of bilayer Na CoO ·yH O ducting and magnetic phases in Fig. 2. The tempera-
c x 2 2
: being kept in a humidified chamber (75 % relative hu- ture dependence of the transverse relaxation curves was
v
midity atmosphere). The another conditions of humid- also measured with using a spin echo technique for non-
i
X ity and temperature were independently found to lead superconductingBLH.AproductionofLorentzian(T )
2L
r to different duration effects, where Tc also shows a sys- and Gaussian (T2G) terms with double time constants,
a
tematic change.4) In contrast to the X-ray and neutron m(2τ)=m(0)exp[−(2τ/T )−1/2(2τ/T )2],wasfitted
2L 2G
diffractiontechniques,the59Conuclearquadrupolereso- to the data.
nance(NQR)isapowerfultechniquetoinvestigatelocal Figure1(a)showstheNQRspectraat10Konsuper-
electronicstate aroundcobaltatoms. Thuswehaveper- conducting (T = 2, 3.1, 3.7, 4.4, and 4.6 K) and non-
c
formed59Co nuclearquadrupole resonance(NQR) stud- superconducting BLH as well as MLH Na CoO ·yH O.
x 2 2
ies onbilayerNa CoO ·yH O preparedby using the du- These resonance lines arise from the transition, I =
x 2 2 z
rationeffecttocharacterizetheelectronicstatesofCoO ±5/2 ↔ ±7/2. In the superconductors BLH, the peak
2
planes and the electron spin dynamics microscopically. freqency of ν decreases as temperature increases as
3
The powder samples of bilayer water intercalated shown in Fig. 1 (b).
sodium cobalt oxides were prepared by the same way TheMLHcompoundhasanalternatingstackingstruc-
as mentioned in ref. 3. A monolayer water intercalated ture of Na/H O and CoO layers. Four peaks in the
2 2
sodium cobalt oxide (MLH) was synthesized by a dein- NQRspectrumcorrespondingtoν suggestthatatleast
3
tercalation method of water molecules from BLH in a four cobaltsitesexistinthe MLH.Butwecannotassign
dry N gas flowing atmosphere. thesesitesbecauseoflackofinformationontheNa/H O
2 2
2 Online-JournalSubcommittee
s) ν3 (a) 12.5 non-superconducting 7 T : duration experiment 1
t c
uni BTLcH T =10 K 12.4 6 TTc :: dreuproarttieodn experiment 2
arb. 44..64 KK Hz) 12.3 K ) 5 Nor mTcMa l(cid:13)
nsity ( 33..271 K KK ν (M312.2 T T ( , cM 43 state M
e 12.1 Tc = 4.6K
nt nonSC (b) 2 SC
I MLH 12.0
1
12.0 13.0 0 50 100150200
ν (MHz) T (K) 0
12.00 12.25 12.50
ν ( MHz)
Fig. 1. (a) 59Co NQR spectra of ν3 in superconducting BLH 3
NaxCoO2·yH2O with Tc = 2, 3.1, 3.7, 4.4, and 4.6 K, non-
superconducting BLH and MLH measured at 10 K. These Fig. 2. MagneticphasediagramofBLHNaxCoO2·yH2Odecided
resonance lines correspond to the transition (Iz = ±5/2 ↔ byNQRmeasurements. TcandTMareplottedagainstν3at10
±7/2). (b) Temperature dependence of ν3 in BLH with non- K.Filledandopencirclesrepresentthedataforthesamplessyn-
superconducting (rightsideinthedomeshapesuperconducting thesizedbydurationeffectfromtwodifferentinitialconditionsof
phase in Fig. 2) and Tc = 4.6 K (left side in the dome shape theparentcompound. Crossmarksarethedatareportedinref.
superconducting phaseinFig. 2)samples. 7. TrianglesarethemagneticorderingtemperatureTM decided
fromthetemperaturedependence ofthenuclearspin-latticere-
laxation rate (1/T1). SC and M denote superconducting and
magneticstates,respectively.
localstructure. Thesodiumionsandwatermoleculesoc-
cupy just upper sites of oxygen sites along the c-axis in
the MLH.2) The more than four cobalt sites with differ-
cause we study the samples with systematically chang-
ent NQR frequency ν may be related with Na vacancy
Q ing T in this work, the consistency with the previous
c
superlattice and oxonium ordering.
work suggests that the samples in ref. 7 are involved
In the case of BLH, only a broad single peak of ν
3 in the series of our samples. We found that T is sup-
c
appears in the NQR spectrum. The intercalated wa-
pressedneartheboundarybetweenthe superconducting
ter molecules smear out the local electric field gradient
and the magnetic phases. The optimal T and the high-
c
aroundseveralcobaltsites. TheBLHcompounds,whose
est suprconducting volume fraction are realized in the
NQR spectra are shown in Fig. 1, were synthesized by sample with ν ∼12.30 at 10 K.
3
asingle setoftime durationtreatmentmentionedinref.
The domed superconducting phase is similar to those
3. As the time duration in the humidified chamber and
of the high T cuprates. The similar dome shape phase
c
T increase,thepeakfrequencyν oftheNQRspectrum
c 3 diagram of T as a function of the sodium content in
c
decreaseswithoutthechangeoflineshape. Iftheoxygen Na CoO ·yH O system is reported in ref. 8. However,
x 2 2
content changes with the time duration, the NQR spec-
the formal cobalt valence does not change with the dif-
trum would be expected to show asymmetrically broad-
ferent (ν , T ) samples of the same sodium content. If
3 c
ening as in the case of high-T cupper oxides. Thus the
c the sodium content can be changed from a sample to
oxygen content in BLH is thought to be stoichiometric.
another,theadditionaloxoniumionsshouldcompensate
FromtheanalysisofX-raypowderdiffractions,3) thelat-
for the loss of charge to make cobalt valence almost in-
tice constantswere nearly the same for the presentsam- variant.6) Therefore, we believe that the coordinate of
ples. The cobalt valence in the BLH system is reported
(ν , T ) specifies the degree of the distortion of CoO
3 c 6
tobe+3.4anddoesnotchangesomuch.6) Thus,notthe
octahedron, which may change the band structure.
dopedcarriersbutthelocalcrystalshrinkofCoO octa-
6 We obtained several non-superconducting BLH sam-
hedronschangeswiththe timedurationaccompaniedby
plesbyusingthedifferentinitialandhumidityconditions
the decrease of ν3. The distortion of CoO6 octahedron in the synthesis process. Figure 3 (a) shows the 59Co
may cause some change in the electronic state of CoO
2 NQR spectra in one of the non-superconducting BLH
planes and then increase T . Therefore, the value of ν
c 3 samples at 2 and 10 K. The separable peaks at about
can be a measure of the change in the electronic state.
8.15 and 12.50 MHz correspond to the transition lines
Figure 2 is a magnetic phase diagram of Tc plotted ν (I = ±3/2 ↔ ±5/2) and ν (I = ±5/2 ↔ ±7/2),
2 z 3 z
againstν at10K,whichwasdrawnfromthe results by
3 respectively. At 10 K, the line shape and width are sim-
the magnetic susceptibility and ν from the 59Co NQR
3 ilar to those of the superconducting BLH samples ex-
spectrameasurements. T wasdetermined by measure-
M cept for a slight difference in the peak position of the
mentsofthenuclearspinrelaxationratesasshownlater.
resonance line. The line shapes of the superconducting
TheclosedcirclesinFig. 2wereobtainedforthepresent
BLH samples do not change so much in the supercon-
BLH samples in Fig. 1 (a). The open circles were ob-
ducting state at 2 K. However, the line shape of the
tained for the other BLH samples with the different du-
non-superconducting sample drastically changes at 2 K;
ration effect using different initial mother compounds.
the ν line broadens both toward the lower and upper
2
The crosses in Fig. 2 denote the previous data on the
frequency sides while the ν line shows broadening only
3
accidentally obtained samples with various T ’s.7) Be-
c tothelowerfrequencyside. IfthesechangesintheNQR
Instructions forthePreparationofManuscript 3
)
s
b.unit (a) 10K ν 3 100046 nTTcco n==-s 44up..76e rKKco ( nrdeuf.c t1in0g)
nsity (ar ν 22K -1T1/ (s)1 100462 (cid:129)‘T1.5 (cid:129)‘T3 (cid:129)‘T0.7
e
nt 2 (cid:129)‘T1
I 8 ν1 0(MHz)12 14 10
5x103 2.5x104 1 2 3 4 5 67810 2 3 4 5678100
T (K)
T
4 M 2.0
-1T1/(s)2G 321 110...5052L1/ (s)T-1 -1T (s)111862000000000 T T ) ( K c , M543212.00ν3 1 (2 SM.C2H5z1) 2.M50
(b) 1/ 400
0 0.0 non-superconducting
2 4 6 8 2 4 6 8 2 200 Tc = 4.6K (b)
10 100
T (K) 0
0 20 40 60 80 100
T (K)
Fig. 3. (a) NQR spectra of non-superconducting BLH measured
at 2 and 10 K. (b) Temperature dependence of 1/T2G (open
Fig. 4. (a) Logarithmic and (b) linear scales of the temperature
circles)and1/T2L (closedcircles)innon-superconducting BLH
measuredatthepeakfrequencyν3 (12.50MHzat10K).TM is dependence ofthe nuclear spin-latticerelaxation rate(1/T1)in
amagneticorderingtemperature. non-superconducting and superconducting BLH with Tc = 4.6
K(ν3 =12.20MHzat10K,rightsideinthedomedsupercon-
ductingphaseinFig. 2). InsetinFig. (b)isasketchofFig. 2.
Notations arethesameinthemainandinsetfigures.
spectraarisefromchangesinelectricfieldgradientswith
cooling, the change in the ν line should be scaled by
3
that in the ν line. But the actual change was lager in superconductingandsuperconducting(T =4.6K)BLH
2 c
ν than that in ν . Therefore, the asymmetric broad- samples. In both cases, 1/T shows a rapid decrease be-
2 3 1
enings in the ν and ν lines indicate the occurrence of low T = 4.6 K and T ∼ 4.2 K, indicating that the
2 3 c M
an internal magnetic field. In general, the NQR spec- elementary excitation spectrum changes at T and T .
c M
trum of I = 7/2 would show broadening at the upper At lowtemperaturesin the paramagneticstate,both re-
frequency side of ν under the small internal magnetic laxation rates 1/T show non-Korringa power law be-
3 1
field parallel to the principal axis of the electric field havior (∼ Tn, n < 1) and the index n is near or less
gradient tensor (c-axis),9) suggesting that the small in- than 0.7. In the superconducting sample below T , the
c
ternal magnetic field should point in the CoO plane in power law index n is about 3 without coherence peak,
2
thepresentcase. Thebroadbutnotsplitν linesuggests indicatingthepresenceofthelinenodesinthesupercon-
2
that the internal magnetic field distributes in the plane. ducting gap parameter. The T-linear behavior of 1/T
1
The presence of such a small internal magnetic field in in the wide rangeofthe low temperatures indicates that
non-superconducting BLH is similar to that previously the residual density of states is lager than that of the
reported.7) optimal superconducting BLH samples previously inves-
Figure 3 (b) shows the temperature dependence of tigated.10,11) The appreciable residual density of states
the relaxation rates 1/T and 1/T in the non- is a characteristic of the line-node superconductivity.
2L 2G
superconductingBLHsample. Theprimarycontribution In the non-superconducting BLH,1/T does not show
1
to 1/T is a T process. 1/T is a nuclear spin-spin the critical divergence at T . The 1/T behavior near
2L 1 2G M 1
relaxation rate. 1/T increases with cooling below 50 T resembles the case of the charge-density-wave tran-
2G M
K,indicating that anindirectnuclear spin-spincoupling sition. But at least the static internal magnetic field
enhances and the coupling constant with the static spin exists below T . Below T , 1/T shows the power raw
M M 1
susceptibilityincreasesbelow50K.Athightemperatures behavior with n ∼ 1.5. The T-linear behavior is ex-
above100K,thenarrowingeffectsuchassodiummotion pected in both the itinerant ferromagnetic and itinerant
may increase 1/T and decrease 1/T . At 4.2 K, both N´eel states in T ≪ T according to the self-consistent
2L 2G M
1/T and 1/T show abrupt changes. Taking into ac- renormalizationtheory.12) Thus,thelowlyingexcitation
2L 2G
count the existence of the static internal magnetic field, spectrum of the magnetically ordered state in the non-
the abrupt change in 1/T and an anomaly in 1/T superconducting BLH is thought to be unconventional.
2L,G 1
(shown later), a kind of static magnetic ordering occurs
at T ∼ 4.2 K. The distributed internal magnetic field Figures 5 (a) and (b) show the temperature depen-
M
andtheabsenceofconventionalcriticalslowingdownef- dence of 1/T T in logarithmic and linear scales, respec-
1
fect on T and T indicate an unconventional magnetic tively. In addition to the non-superconducting and su-
2 1
ordering such as a quasi-long range ordering. perconducting (T = 4.6 K) BLH samples in, 1/T T in
c 1
Figure 4 shows the temperature dependence of the thesuperconductingBLHwithT =3.1Kisshown. The
c
nuclear spin-lattice relaxation rate 1/T in the non- samplewithT =3.1Kislocatedneartheboundarybe-
1 c
4 Online-JournalSubcommittee
35
and magnetic phase transitions in several samples near
30 non-superconducting
-1K) 25 TTcc == 34..16KK tnheeticbpouhnasdeasr.yBbeectawueseenthtehevascuapnecriecsonodfutchteinsgodainudmmioangs-,
-1s 20 thecontentsoftheoxoniumionsandthewatermolecules
(
TT 115 fluctuate structurally in the BLH NaxCoO2·yH2O sys-
1/ 10 tem, we could not confirm the coexistence of supercon-
5 (a) ductivity and magnetic ordering in a sample.
0 2 3 4 5 678 2 3 4 5 678 In summary, we have done 59Co NQR studies of BLH
1 10 100
T (K) sodium cobalt oxides NaxCoO2·yH2O with the super-
35 conducting transition temperatures 2 K < Tc ≤ 4.6 K
non-superconducting
30 Tc = 3.1K )K 5 as well as the non-superconducting but magnetic BLH.
-1-1sK) 2250 Tc = 4.6K T T ( c , M432 SC M aWgeaidnestveνl3opwehdicahmmaagynsepteiccipfyhathseeddieaggrreaemofotfhTecdiastnodrtTioMn
TT1/ (11150 12.00ν3 1 (2 M.2H5z1) 2.50 pofertchoenCduocOt6inogctpahhaesderwonit.hWtheeobopsetrimveadlaν3do∼m1e2s.3h0apMeHsuz-.
5 In the non-superconducting BLH, the 59Co NQR spec-
(b)
0 trum shows a broadening below TM without the critical
0 20 40 60 80 100 slowing down effect of 1/T and 1/T , indicating an un-
1 2
T (K) conventional magnetic ordering. From the analysis of
the temperature dependence of the relaxation rates, we
Fig. 5. (a) Logarithmic and (b) linear scales of the temperature
concluded that the magnetic fluctuations are enhanced
dependenceof1/T1T innon-superconductingandsuperconduct-
above T and T with the increase of ν . We observed
ing BLH with Tc = 4.6 K (ν3 = 12.20 MHz at 10 K) and 3.1 c M 3
K(ν3 =12.45MHzat10K,rightsideinthe domedsupercon- that Tc is suppressed near the phase boundary ν3 ∼
ductingphaseinFig. 2). InsetinFig. (b)isasketchofFig. 2. 12.50 MHz, which may arise from the additional effects
Notations arethesameinthemainandinsetfigures.
to cause an unconventional magnetic ordering.
We thank Dr. T. Waki and Dr. M. Kato for their
experimental supports, and Dr. K. Ishida, Mr. Y. Ihara
tweensuperconductingandmagneticphasesinthephase and Dr. Sakurai for fruitful discussion. This work is
diagram in Fig. 2. At low temperatures above Tc and supported by Grants-in Aid for Scientific Research on
TM, 1/T1T increases with decreasing temperature due Priority Area ”Invention of anomalous quantum mate-
to the enhancement of the magnetic fluctuations, and rials”, from the Ministry of Education, Culture, Sports,
decreases at the superconducting and magnetic phase Science and Technology of Japan (16076210).
transitions. The enhancement of 1/T T increases as ν
1 3
increases, suggesting the magnetic correlations develop
with ν3. Although Tc increases with the enhancement 1) K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi,
of 1/T T and the magnetic correlations in the left side R.A.DilanianandT.Sasaki,Nature422,(2003) 53.
1
2) K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi,
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previous report,13) the enhancement of 1/T1T becomes 372.
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side of the superconducting phase. We also observed a Jpn.74,(2005)3147.
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3
Theholepocket-likeFermisurfaceneartheK pointis gradienttensordidnotaffectsomuchtheestimationof T1.
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Q J.Phys.Soc.Jpn.74,(2005)867.
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