NMR study of the semimetallic compound UCoGa5

S. Kambe,1 H. Sakai,1 H. Kato,1,* Y. Tokunaga,1 T. Fujimoto,1 R. E. Walstedt,1,† S. Ikeda,1 T. Maehira,2

Y. Haga, and Y. Ōnuki31Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan

2Faculty of Science, University of The Ryukus, Nishihara, Okinawa 903-0213, Japan3Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

�Received 7 February 2007; revised manuscript received 6 April 2007; published 9 July 2007�

We report NMR studies on a single crystal of UCoGa5, which has the same tetragonal HoCoGa5 �115�structure as the d-wave high-Tc superconductor PuCoGa5. The spin-lattice relaxation rate �1/T1� in UCoGa5 issmall compared with those of other actinide 115 compounds. This fact is consistent with a semimetallic natureof UCoGa5 suggested by de Haas–van Alphen effect measurements and band calculations. From comparison of1/T1 at crystallografically different Ga and Co sites and the static susceptibility, an existence of two bands withrather different characters is revealed.

DOI: 10.1103/PhysRevB.76.024411 PACS number�s�: 76.60.�k, 75.30.Mb

I. INTRODUCTION

Recently, new superconductors have been discoveredamong Pu-based, tetragonal HoCoGa5�115� structure com-pounds: PuCoGa5 �Tc=18.5 K� �Ref. 1� and PuRhGa5 �Tc

=9 K�.2 These discoveries have stimulated a lot of interest inactinide compounds, since Tc for these compounds is quitehigh compared with the Ce-based 115 superconductingcompounds.3 This indicates the superconductivity of thesecompounds to be rather unconventional. In fact, spin-latticerelaxation time �T1� measurements in PuCoGa5 �Ref. 4� andPuRhGa5 �Ref. 5� suggest d-wave superconductivity forthese systems. Since unconventional superconductivity in Pu115 compounds may be mediated by magnetic interactions, itis important to clarify the nature of the magnetic fluctuations.In order to investigate the magnetic fluctuations in actinide�U, Np, and Pu� 115 compounds, systematic NMR measure-ments have been performed recently.6 The actinide 115 com-pounds studied can be categorized as itinerant antiferromag-nets �UPtGa5, NpFeGa5, and NpCoGa5�, a Pauli paramagnet�UFeGa5�, or a superconductor �PuRhGa5�, except forUCoGa5, which has somewhat peculiar properties as wedemonstrate in this report. Comparing the series of AnCoGa5compounds with An=U, Np, and Pu, we find that UCoGa5 isa semimetal, NpCoGa5 is a metallic antiferromagnet �TN

=47 K�, and PuCoGa5 is a superconductor. The differencesbetween the respective ground states can clearly be seen inthe NMR results on NpCoGa5,7 on PuCoGa5,4 and this reporton UCoGa5. These differences will be analyzed and dis-cussed.

UCoGa5 has a rather small electronic specific heat coeffi-cient �=3.3 mJ/mol K2.8 Furthermore, no magnetic orderinghas been observed down to 15 mK.8 Moreno et al. have re-ported the thermal and transport properties of UCoGa5.9

They have found somewhat different behaviors for the spe-cific heat and magnetic susceptibility from our previousresults,8 which may be due to a difference between samples.Present measurements are performed on the same singlecrystal samples reported in Ref. 8. The compensated, semi-metallic nature of this compound has been revealed by deHaas–van Alphen �dHvA� effect measurements results,8

which are consistent with band-structure calculations.10,11

In this paper we report 69,71Ga and 59Co NMR Knightshift and spin-lattice relaxation measurements which havebeen carried out on high-quality, single-crystal samples ofUCoGa5. Our present results support the existence of twobands. The character of these two bands is revealed to berather different.

II. EXPERIMENTAL

High-quality single-crystal samples have been preparedby the Ga-flux method.8 The NMR measurements were car-ried out using a 12 T superconducting magnet and a phasecoherent pulsed spectrometer. The NMR spectra which weanalyze are field-sweep spectra taken at constant frequency,using periodic digital averaging of the nuclear spin-echo sig-nals. Spin-lattice relaxation time �T1� data were also obtainedusing digital averaging of the spin-echo signals. These relax-ation times have been determined using both the central �m= 1

2 ↔− 12

� and the satellite �m=− 12 ↔− 3

2� transitions, yielding

results which agree within expected errors.

III. EXPERIMENTAL RESULTS

A. Static susceptibility and Knight shift

Figure 1 shows the T dependence of the static susceptibil-ity for an applied magnetic field H parallel to the a��100��axis and the c��001�� axis �� ��=a ,c�. The NMR measure-ments were performed in the same sample. �� decreases withdecreasing T, becoming constant at low temperatures. Intypical localized and itinerant systems, Curie-Weiss �CW�and T-independent susceptibility behaviors are expected forthe static susceptibility, respectively. Thus, it is difficult toexplain the observed T dependence of �� in UCoGa5 basedon such simple models. Based on a density of states derivedfrom the dHvA measurements,8 the Pauli-type static suscep-tibility is estimated to be quite small ��5�10−5 emu/mol�compared with the observed one, indicating that some othertype of susceptibility, possibly the Van Vleck term, is makinga large contribution here.

PHYSICAL REVIEW B 76, 024411 �2007�

1098-0121/2007/76�2�/024411�6� ©2007 The American Physical Society024411-1

NMR measurements were performed on two crystallo-graphically inequivalent Ga sites �Ga�1�: the 1c site andGa�2�: the 4i site� and on the Co site.12 The Ga�1� and Cosites have a tetragonal local symmetry, while the Ga�2� site isorthorhombic. Since Ga and Co nuclei have a nuclear spin Ilarger than 1/2 �I=3/2 for 69Ga and 69Ga, I=7/2 for 59Co�,the nuclear quadrupolar interaction should be considered atthese sites, which have noncubic local symmetry. Althoughresults for both 69Ga and 71Ga nuclei have been obtained,only results for 69Ga NMR are presented in this report.

Figure 2 shows a field-sweep NMR spectrum for H �c at10 K in UCoGa5. This figure shows only the central transi-tion region in order to present the seven transitions of 59CoNMR clearly. The linewidth of each peak is quite small, i.e.,�10 Oe, which confirms the high quality of the single crys-tal sample.

From the NMR spectrum, the nuclear quadrupolar fre-quency �Q at the Ga�1�, Ga�2�, and Co sites, and the asym-metry factor � at the Ga�2� site ��=0 at the Ga�1� and Cosites� have been determined �Table I�.6 Since the principalaxis n�ZZ of the electric field gradient tensor �EFG� is perpen-dicular to the �100� plane at the Ga�2� site in the �100� plane,

two different Ga�2� sites are observed when H �a��100��:12

the Ga�2a� site �n�ZZ �H� and Ga�2b� site �n�ZZ�H�. For theGa�2� site, the subscripts �=a and �=b in this report indi-cate measurements at the Ga�2a� and Ga�2b� sites, respec-tively. For the tetragonal Ga�1� and Co sites, Knight shiftsand spin-lattice relaxation time has only one component forH �a��100��.

Figure 3 shows the T dependence of the Knight shifts K���=a ,c� at the Ga�1�, Ga�2�, and Co sites for H �a �Ka� andH �c �Kc�. Results only for the Ga�2a� site are presented,since results for the Ga�2b� site �Kb� are similar to those forKa. The Knight shifts decrease slightly with decreasing Talong with ��. In order to compare the Knight shifts with thestatic susceptibility, plots of K� versus �� �K-�� are pre-sented in Figs. 4 and 5. For all cases, good linearity has beenobtained, although some anomaly has been found for thesmall �� regime at low T, which is due perhaps to magneticimpurity contributions on ��. The good linearity observed upto 250 K indicates that the excited crystal field energies arelarge compared with 250 K in UCoGa5.12

K� and �� ��=a ,c for the Ga�1� and Ga�2� sites, �=a ,b ,c for the Ga�2� site, �a=�b� are found to obey therelation

�� = ���T� + ��const,

K� = A����T� + K�0, �1�

where ���T� is the T-dependent �� term, whereas ��const is a

constant �� term, A� is the hyperfine coupling constant for

1.0

0.5

0.0

χ(1

0-3em

u/m

ol)

3002001000T(K)

UCoGa5

H // a

H // c

FIG. 1. �Color online� T dependence of the static susceptibilityfor H �a axis ��a� and H �c axis ��c�.

Spin

Ech

oIn

ten

sity

(arb

.un

its)

10098969492H (kOe)

FIG. 2. Field-sweep spectrum of 69Ga and 59Co NMR forH �c��100�� axis at 10 K in UCoGa5. The nine peaks shown corre-spond to the central �m= 1

2 ↔− 12

� transitions for the Ga�1� and Ga�2�sites, and all seven transitions for the Co site �I=7/2 for 59Co�.Since the EFG principal axis for the Co site is along the �001�direction, the seven peaks are uniformly spaced, i.e., by the quad-rupole splitting �Q=0.93 MHz �see Table I�. Quadrupolar satellitepeaks for the Ga lines are not presented, although they are as clearlyobserved as the others.

TABLE I. Nuclear quadrupolar frequency �Q in MHz at 100 K.Hyperfine coupling constant A in kOe/�B for the 69Ga and 59Cosites in UCoGa5 are obtained from K-� plots. From their tetragonallocal symmetry Aa=Ab at the Ga�1� and Co sites

Ga�1� Ga�2� Co

�Q 10.2 32.5 0.93

Aa 15.6 12.4 4

Ab 12

Ac 33 20 17.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

K(%

)

3002001000T(K)

Ga2Ka

Ga2Kc

Ga1Ka

Ga1Kc

CoKa

CoKc

FIG. 3. �Color online� T dependence of the Knight shift at theGa�1� �triangle� and Ga�2� �square� and Co �circle� sites for H �aaxis �open symbols� and H �c axis �closed symbols�. ForH �Ga�2�-site a axis, the Knight shift for the Ga�2a� is presented.

KAMBE et al. PHYSICAL REVIEW B 76, 024411 �2007�

024411-2

���T�, K�0 is a T-independent term which is usually due toorbital and Van Vleck contributions. From the slope of theK-� plots in Figs. 4 and 5, A� has been estimated �Table I�.Since the magnitude of A��10 kOe/�B, which cannot beexplained by the usual dipolar field from 5f moment at the Usite, the hyperfine fields at the Ga and Co sites are thought tobe transferred hyperfine fields due to hybridization betweenU-5f and Ga-4s ,4p, Co-3d ,4s orbitals. At the Ga�2� site, forH � �100�, Aa for the Ga�2a� site is found to be almost thesame as Ab for the Ga�2b� site.

B. Spin-lattice relaxation time T1

Because of the tetragonal local symmetry of the U site,which makes the principal contributions to the dynamicalsusceptibility, Im ��q ,n�, Im ���q ,n� ��: a= �100�, b= �010�, and c= �001� axes� has only two components:Im �a�q ,n�=Im �b�q ,n� and Im �c�q ,n�. Taking into ac-count this fact and the observed relation Aa�Ab at the Ga�2�site, 1 /T1T for H �c and H �a at the all sites are expressed,

�1/T1T�H�c =�n

2kB

2 �q

Aa�q�2 Im �a�q,n�n

, �2�

�1/T1T�H�a =�n

2kB

4 �qAa�q�2 Im �a�q,n�

n

+ Ac�q�2 Im �c�q,n�n

, �3�

where Aa,c�q� is a q-dependent hyperfine coupling constant�Aa�0�=Aa, Ac�0�=Ac�. For the orthorhombic Ga�2� site, theT1 value obtained for H � �100� at the Ga�2a� site is almostthe same as that at the Ga�2b� site, in agreement with Aa

�Ab.Therefore, two relaxation rates parallel to the a and c axes

Ra,c can be defined at the all sites

Ra � �1/T1T�H�c ��n

2kB

2 �q

A�q�a2 Im �a�q,n�

n�4�

and

Rc � 2�1/T1T�H�a − �1/T1T�H�c ��n

2kB

2 �q

Ac�q�2 Im �c�q,n�n

.

�5�

Figure 6 shows the T dependence of Ra,c at the 69Ga�1�,69Ga�2�, and 59Co sites. The magnitude of Ra,c is �102 timessmaller than magnetic actinide 115 compound, e.g., UPtGa5�TN=26 K�.13 1 /T1T decreases with decreasing T. It shouldbe noted that the T1 process is magnetic up to 300 K, accord-ing to the ratio of T1 values for the 69Ga and 71Ga isotopes.Usually 1/T1T1/T behavior is expected for localized sys-tems, whereas one expects 1 /T1T�constant behavior for or-dinary itinerant system. Along with the T dependence of ��

and K�, the T dependence of 1 /T1T in UCoGa5 cannot beexplained with either the simple localized or itinerant pic-ture.

Figure 7 shows Ra at the Ga�2� site as a function of Ra atthe Ga�1� site in UCoGa5 and UPtGa5. Since the q depen-dences of A��q� for the Ga�1� and Ga�2� sites are different,6

i.e., these two sites are sensitive to fluctuations in differentregions of q space, this plot would not be linear ifIm �a�q ,n� had a strong q dependence, as observed near TN

in UPtGa5.13 Thus, the observed linear relation suggests that

0.80

0.75

0.70

0.65

0.60

0.55

0.50

K(%

)

1.00x10-3

0.900.800.70

χ a, c(emu/mol)

Ga1Kc Ga2

Kc

Ga1KaGa2

Ka

FIG. 4. Knight shift versus static susceptibility �K�-��� is plot-ted for the Ga�1� and Ga�2� sites. The same symbols are used as inFig. 3. The solid lines shown are least-squares fits.

K(%

)

χ a, c(emu/mol)

CoKc

CoKa

FIG. 5. Knight shift versus static susceptibility �K�-��� plot forthe Co sites. The same symbols are used as in Fig. 3. Solid linesshown are least-squares fits. Deviations from the linear plots areattributed to impurity contributions to ��.

1.0

0.8

0.6

0.4

0.2

0

Ra,

c(s

-1K

-1)

3002001000T(K)

Ga(1) Ra

Ga(2) Ra

Co Ra

Ga(1) Rc

Ga(2) Rc

Co Rc

FIG. 6. T dependence of spin-lattice relaxation rate R� inUCoGa5.

NMR STUDY OF THE SEMIMETALLIC COMPOUND UCoGa5 PHYSICAL REVIEW B 76, 024411 �2007�

024411-3

Im �a�q ,n� has no strong q dependence, which is alsoclearly shown in the discussion of Fig. 8. It should be notedthat a linear relation has been also observed between the Rafor the Ga�1,2� and Co sites.

It is remarkable that the extrapolation of the plot does notintersect the origin in UCoGa5 shown in the Fig. 7. The samebehavior is observed in a plot of Rc at the Ga�1� versus Rc atthe Ga�2� �not shown�. From Eq. �4�, the plot should arrive atthe origin when Im �a�q ,n� is unique. The observed devia-tion from the origin indicates that Im ���q ,n� is composedof at least two contributions which are connected with 1/T1Tthrough a different A��q�. Since the plot is a straight line,only one contribution from Im ���q ,n� is considered togive the T dependence of R�. Then Im ���q ,n� is consid-ered to be composed of a T-dependent term A and aT-independent term B,

Im ���q,n� = Im ��A�q,n,T� + Im ��

B�q,n� . �6�

In this case, R� is expressed as

R� ��n

2kB

2 �qA�

A�q�2 Im ��A�q,n,T�n

+ A�B�q�2 Im ��

B�q,n�n

, �7�

where A�A�q� and A�

B�q� correspond to the hyperfine couplingconstant for Im ��

A�q ,n ,T� and Im ��B�q ,n ,T�, respec-

tively. Actually the observed A� corresponds to A�A�0�. Be-

cause of the term B in Eq. �7�, which is different for theGa�1� and Ga�2� sites, the plot does not intersect the origin ingeneral. We cannot exclude the existence of a third term, butit is not necessary to assume one to be present in order toexplain the present results. The term A generates the unusualT dependence of 1 /T1T. In contrast, the T-independent termB seems to be due to the usual metallic band, which gives aKorringa relation 1/T1T�const.

In contrast, for a large, one band case, e.g., UPtGa5 �seeFig. 10�,14 the same type of plot intersects the origin as ex-pected �Fig. 7�. From Eq. �4�, the slope of the plot in Fig. 7corresponds to the square of the A��q� ratio of Ga�1� andGa�2�: �Ga2A�

A�q� / Ga1A�A�q��2. In fact, the estimated slope is

�0.5, which is roughly consistent with �Ga2Ac / Ga1Ac�2

= �20/33�2�0.4.In Fig. 8, a plot of Ra as a function of �a is presented. As

shown in the figure, a good linear relation is obtained for allsites. A similar linearity has been obtained in the Rc versus �cplot �not shown�. Since the q dependence of A��q� is differ-ent for each site,6 R� for each site and �� would show dif-ferent T dependence if Im ���q ,n� is q dependent. In par-ticular, the good linearity observed indicates that theT-dependent part of Im ���q ,n� i.e., Im ��

A�q ,n ,T� has nostrong q dependence. The observed q-independentIm ���q ,n� indicates that magnetic correlations are notstrong in UCoGa5.

IV. DISCUSSION

In order to explain our results consistently, we should as-sume a band A which causes T dependence for �� and of1/T1T �contribution from the term A� and a typical itinerantband B which gives constant terms to �� and to 1/T1T �con-tribution from the term B�. In addition, band A has localizedcharacter since Im �A�q ,n� is q independent, although bandA does not yield CW behavior. If we estimate tentatively thevalue of the constant �a term �a

const using the extrapolation ofRa to zero in Fig. 8, we find �a

const�0.75�10−3 emu/mol.�a

const is considered to be composed of the Pauli-paramagnetic term from band B: �a

B and probably a VanVleck term �a

VV. As mentioned in Sec. III A, �aB as estimated

from the dHvA measurements is much smaller than the val-ues obtained for �a

const, indicating that the �aVV contribution

may be large. On the other hand, the T-dependent term fromband A, �a

A=�a−�aconst, may be small ��0.1

�10−3 emu/mol� at T�0 K, and then increase rapidly as Tincreases.

From the dHvA measurements8 and band calculations,10,11

this compound has a compensated semimetallic nature with

0.20

0.15

0.10

0.05

0.00

Ra

atG

a(2)

(s-1

K-1

)

0.50.40.30.20.10.0

Ra at Ga(1) (s-1

K-1

)

25

20

15

10

5

0

3020100

UCoGa5

UPtGa5

FIG. 7. �Color online� The spin-lattice relaxation rate Ra at theGa�2� site as a function of Ra at the Ga�1� site in UCoGa5 andUPtGa5. Dashed and solid lines shown are a least-squares fit. Atlow temperatures in UPtGa5, Ra at the Ga�1� site increases morerapidly than that at the Ga�2� site as T approaches TN, because ofenhancement of in-plane ferromagnetic fluctuations around TN, towhich the A��q� for the Ga�1� is most sensitive �Ref. 13�.

0.8

0.6

0.4

0.2

0

Ra

(s-1

K-1

)

1.2x10-3

1.00.8χ a(emu/mol)

Co

Ga(1)

Ga(2)

FIG. 8. The spin-lattice relaxation rate Ra is plotted as a func-tion of �a. Solid lines shown are least-squares fits. Extrapolation ofthe lines to 0 intersect near �a�0.75�10−3 emu/mol.

KAMBE et al. PHYSICAL REVIEW B 76, 024411 �2007�

024411-4

small electron and hole Fermi surfaces. The existence of twodifferent bands is consistent with our current results. Thevalue of 1 /T1T is quite small compared with those frommetallic actinide 115 compounds,6 which is consistent withthe small Fermi surfaces, i.e., low carrier density, in thiscompound.8,10,11 The present results indicate that the densityof states increases strongly with increasing energy around theFermi level of band A, thus the effective number of carriersincreases with increasing T, which gives the observed in-crease of � and 1/T1T as T increases. In contrast, the struc-ture of the density of states for band B around the Fermilevel is suggested to be rather flat.

From the dHvA measurements and the band calculations,the two bands are composed of heavy electrons and lightholes, respectively. U-5f electrons are the main occupants ofthe heavy band, whereas Co-3d and Ga-4s, p are the consid-erable contributors to the light band.15 The heavy band andthe light band may correspond to bands A and B, respec-tively. In addition, the band calculations suggest, in fact, thatthe heavy band has a sharp increase in the density of states atthe Fermi level, whereas the light band has a flat structure asshown in the Fig. 9.15

The present NMR study reveals the existence of a pecu-liar band A, which has a localized character, but does notyield a CW-type magnetic susceptibility. This peculiaritycomes from the small Fermi surface character of the band A.In typical localized systems, constant localized magnetic mo-ments can give rise to CW behavior. However, in the presentcase, the effective localized moment decreases rapidly withdecreasing T, thus, the C.W. behavior cannot be observed. Inother actinide compounds, the two-bands effect has appar-ently not been observed,6 although multibands should con-tribute to the Fermi surface in these compounds. In these

compounds, perhaps the largest �heaviest� band dominatesthe physical properties. As a result the multibands effect isdifficult to observe. In the case of UCoGa5, the two-bandseffect can be observed, since small, but similar-size Fermisurfaces are involved. In this sense, the detection reportedhere of a two-bands effect by means of NMR should beconsidered as a somewhat particular case.

Based on dHvA measurements16 and bandcalculations,11,17 schematic band structures for AnCoGa5�An: U, Np, and Pu� and UPtGa5 are presented in Fig. 10.Naively, the number of 5f electrons increases from U to Pu.The difference between bands in these compounds is under-stood qualitatively by considering the changing numbers of5f electrons. The existence of a large density of states at EFin NpCoGa5 is consistent with a short T1 in this compound.7

For the superconductor PuCoGa5, a two-bands structure isexpected, although no experimental evidence has been ob-tained for this so far. As the density of states in PuCoGa5 islarge ���100 mJ/mol K2�, it may be difficult to detect atwo-bands effect if one heavy band dominates the physicalproperties as discussed above.

In summary, the T dependence and anisotropy of Knightshifts and spin-lattice relaxation times T1 have been deter-mined for all of the Ga and Co sites in UCoGa5. The relationbetween T1 behaviors at different sites can be explained interms of two bands with markedly different character. Thepicture obtained for the two bands is consistent with dHvAmeasurements and with band calculations. One band is ausual metallic band which gives a constant static susceptibil-ity and a Korringa relation �1/T1T�const�, while the otherband has a localized character, thus giving a T-dependentcharacter to the static susceptibility and to 1/T1T.

ACKNOWLEDGMENTS

We thank D. Aoki and T. Hotta for stimulating discus-sions.

*Present address: Faculty of Science, Kochi University, Kochi 780-8520, Japan.

†Present address: Physics Department, The University of Michigan,Ann Arbor, MI 48109, USA.

1 J. L. Sarrao, L. A. Morales, J. D. Thompson, B. L. Scott, R.Stewart, F. Wastin, J. Rebizant, P. Boulet, E. Colineau, and G. H.Lander, Nature �London� 420, 297 �2002�.

2 F. Wastin, P. Boulet, J. Rebizant, E. Colineau, and G. H. Lander,

Band A

Band B

Energy

DOS

EF

FIG. 9. Schematic picture of the two bands of UCoGa5. Band A�heavy� has strong U-5f character, whereas band B �light� has moreCo-3d and Ga-4s, p character �Ref. 15�.

EF

UCoGa5

EF

UCoGa5

PuRhGa5

EF

PuCoGa5

EF

E

k

EF

NpCoGa5 UPtGa5

EF

NpCoGa5 UPtGa5

FIG. 10. �Color online� Schematic band structures for UCoGa5,NpCoGa5 �UPtGa5�, and PuCoGa5. The position of the Fermi levelEF increases with an increasing number of 5f electrons. ForNpCoGa5 and UPtGa5, EF is located at the same level since thetotal number of 5f and 3�5�d electrons are the same.

NMR STUDY OF THE SEMIMETALLIC COMPOUND UCoGa5 PHYSICAL REVIEW B 76, 024411 �2007�

024411-5

J. Phys.: Condens. Matter 15, S2279 �2003�.3 H. Hegger, C. Petrovic, E. G. Moshopoulou, M. F. Hundley, J. L.

Sarrao, Z. Fisk, and J. D. Thompson, Phys. Rev. Lett. 84, 4986�2000�.

4 N. J. Curro, T. Caldwell, E. D. Bauer, L. A. Morales, M. J. Graf,Y. Bang, A. V. Balatsky, J. D. Thompson, and J. L. Sarrao,Nature �London� 434, 622 �2005�.

5 H. Sakai, Y. Tokunaga, T. Fujimoto, S. Kambe, R. E. Walstedt, H.Yasuoka, D. Aoki, Y. Homma, E. Yamamoto, A. Nakamura, Y.Shiokawa, K. Nakajima, Y. Arai, T. D. Matsuda, Y. Haga, and Y.Ōnuki, J. Phys. Soc. Jpn. 74, L1710 �2005�.

6 S. Kambe, H. Sakai, Y. Tokunaga, H. Kato, T. Fujimoto, R. E.Walstedt, S. Ikeda, T. D. Matsuda, Y. Haga, D. Aoki, Y. Homma,Y. Shiokawa, and Y. Ōnuki, J. Magn. Magn. Mater. 310, 176�2007�.

7 H. Sakai, S. Kambe, Y. Tokunaga, T. Fujimoto, R. E. Walstedt, H.Yasuoka1, D. Aoki, Y. Homma, E. Yamamoto, A. Nakamura, Y.Shiokawa, and Y. Ōnuki, Phys. Rev. B �to be published�.

8 S. Ikeda, Y. Tokiwa, T. Okubo, Y. Haga, E. Yamanoto, Y. Inada,R. Settai, and Ōnuki, J. Nucl. Sci. Technol. 3, 206 �2002�.

9 N. O. Moreno, E. D. Bauer, J. L. Sarrao, M. F. Hundley, J. D.Thompson, and Z. Fisk, Phys. Rev. B 72, 035119 �2005�.

10 T. Maehira, M. Higuchi, and A. Hasegawa, Physica B 329-333,574 �2003�.

11 I. Opahle, S. Elgazzar, K. Koepernik, and P. M. Oppeneer, Phys.Rev. B 70, 104504 �2004�.

12 H. Kato, H. Sakai, Y. Tokunaga, Y. Tokiwa, S. Ikeda, Y. Ōnuki, S.Kambe, and R. E. Walstedt, J. Phys. Soc. Jpn. 72, 2357 �2003�.

13 S. Kambe, H. Sakai, Y. Tokunaga, T. Fujimoto, H. Kato, R. E.Walstedt, S. Ikeda, T. D. Matsuda, Y. Haga, and Ōnuki, J. Phys.Soc. Jpn. 75, 127 �2005�.

14 S. Ikeda, Y. Tokiwa, Y. Haga, E. Yamanoto, T. Okubo, M. Ya-mada, N. Nakamura, K. Sugiyama, K. Kindo, Y. Inada, H.Yamagami, and Y. Ōnuki, J. Phys. Soc. Jpn. 72, 576 �2003�.

15 T. Maehira �unpublished�.16 D. Aoki, E. Yamamoto, Y. Homma, Y. Shiokawa, A. Nakamura,

Y. Haga, R. Settai, and Y. Ōnuki, J. Phys. Soc. Jpn. 73, 2608�2004�.

17 T. Maehira, T. Hotta, K. Ueda, and A. Hasegawa, Phys. Rev. Lett.90, 207007 �2003�.

KAMBE et al. PHYSICAL REVIEW B 76, 024411 �2007�

024411-6