Localized Character of 4f Electrons in CeRh$_x$(x=2,3) and CeNi$_x$(x=2,5)

4 Pages·0.25 MB·English

by  Ran-Ju Jung

Localized Character of 4f Electrons in CeRhx(x=2,3) and CeNix(x=2,5)
Ran-Ju Jung,1 Byung-Hee Choi,1 Hyeong-Do Kim,1 S.-J. Oh,1,∗ En-Jin
Cho,2 T. Iwasaki,3 A. Sekiyama,3 S. Imada,3 S. Suga,3 and J.-G. Park4
1School of Physics & Center for Strongly Correlated Material Research,
Seoul National University, Seoul 151-742, Korea
2Department of Physics, Chonnam National University, Kwangju 500-757, Korea
3 3Department of Material Physics, Osaka University, Osaka 560-8531, Japan
0 4Department of Physics, Sungkyunkwan University, Suwon 402-751, Korea
0
2 We have measured Ce 4f spectral weights of extremely α-like Ce-transition metal intermetallic
compounds CeRhx (x=2,3) and CeNix (x=2,5) by using the bulk-sensitive resonant photoemission
an technique at the Ce M5(3d5/2 →4f)-edge. Unprecedentedly high energy resolution and longer es-
capedepthofphotoemittedelectronatthisphotonenergyenabledustodistinguishthesharpKondo
J
resonancetailsattheFermilevel,whichcanbewelldescribedbytheGunnarsson-Sch¨onhammer(GS)
4
calculationbasedontheAndersonImpurityHamiltonian. Ontheotherhand,theitinerant4f band
1
description shows big discrepancies, which implies that Ce 4f electrons retain localized characters

even in extremely α-likecompounds.
]
l
e PACSnumbers: 79.60.-i,71.20.Eh,71.28.+d
-
r
t
s
. Cerium is the first element of the 4f rare-earth series states[5]. Hence quite distinct pictures on the electronic
t
a in the periodic table, and occupies a special place in the structure of α-Ce have emerged from these two models.
m condensed matter physics in that its 4f electron is be-
These contrasting views on the nature of Ce 4f elec-
- lieved to lie on the borderline between localization and tron,i.e. “localized”vs“itinerant”,extendtotheunder-
d
itinerancy. Itsoccupied4f orbitalismoreextendedthan
n standing of electronic structures of Ce compounds. For
those of heavier rare earths, and it is generally believed
o example, when cerium alloys with transition metal(TM)
c that an appropriate description of the interaction be- element to formintermetallic compounds,the hybridiza-
[ tween the 4f state and the conduction bands is essential
tion between the 4f state and the d state of TM can be
to understand the physics of cerium metal and cerium-
1 much larger than the corresponding hybridization in Ce
v basedcompounds. Thefamous‘γ−α’phasetransitionin metal[9, 10]. In these so-called extremely α-like Ce-TM
9 cerium metal is a case in point. This isostructural tran-
compounds, it was suggested that the itinerant 4f pic-
0 sition is associated with a large volume change (≈ 15%)
ture forming a narrow band is rather proper, and that
2
and loss of magnetism, and although it has been investi-
1 this itinerant description makes correct predictions on
gated for more than 50 years the nature of phase transi-
0 the equilibrium lattice constant and magnetic moment
3 tion still remains controversial. The early “promotional in agreement with experiments for such compounds as
0 model”[1] where one Ce 4f electron is presumed to go
CeFe , CeRh (x=2,3), and CeNi (x=2,5)[11, 12]. On
/ into the 5d−6s conduction bandin the α phase wasnot 2 x x
t theotherhand,thesecompoundshavealsobeenanalyzed
a supported by many experiments, and two other models
m within the Anderson impurity model[13].
based on quite different starting points have emerged.
- Photoelectron spectroscopy directly probes the elec-
d One is the Mott transition model advocated by B. tronic structure (single-particle excitation spectrum in
n
Johansson[2], which supposes that Ce 4f electron is lo- the many-body description) of solids, and can in princi-
o
calized and non-bonding in the γ phase, but becomes ple distinguish between these two contrasting pictures.
c
v: itinerant and forms a 4f band in the small volume α Indeed for CeRh3, it was claimed that photoemission
phase. The recent calculation on the ground state prop- and inverse photoemission spectra are consistent with
i
X erties of Ce metalbased onthe self-interactioncorrected the 4f band picture[10, 14]. But this interpretation
r local (spin) density approximation (SIC-L(S)DA) seems was challenged later[15], while many other Ce-TM com-
a
to support this idea[3], and even the phase diagram has pound photoemission data had been successfully inter-
been calculated [4]. The other model is the Kondo vol- pretedwithintheAndersonimpuritymodel[13]. Oneim-
umecollapsemodel[5,6],whichproposesthat4f electron portantfactorcontributingtothiscontroversyisthefact
is localized in both γ and α phases and the phase tran- that most high-resolutionphotoemission experiments on
sitionis causedby the changein the conductionelectron Ce compounds so far have been performed with low en-
screening of the 4f electron. In this model the Ander- ergy photons (hν ≤ 150 eV), which makes the spectra
son impurity hamiltonian is used to describe both spec- quite surface-sensitive[16]. Since the coordination num-
troscopic and thermodynamic properties[7, 8], and the ber becomes smaller at the surface, the hybridization of
‘γ−α’phasetransitionwasexplainedasduetothevaria- Ce 4f states with TM d levels is reduced, favoring lo-
tionofhybridizationstrengthbetween4f andconduction calized4f electronpicture[17]. Therefore,to understand
2
the bulk electronic structures it is necessary to separate
outthe surfacecontributionfromthe bulk, butthereare
manyuncertaintiesandambiguitiesinthisprocedure[18].
Hence it is desirable to obtain bulk-sensitive 4f spectra
from direct experiments to resolve this controversy.
In this Letter, we report such study of the bulk-
sensitive 4f spectral weights in extremely α-like Ce-
TM intermetallic compounds CeRh (x=2,3) and CeNi
x x
(x=2,5), which are believed to be most likely to form
itinerant 4f band among Ce compounds. Such bulk-
sensitivehigh-resolutionphotoemissionexperimentswere
made possible in a recently developed synchrotron radi-
ation beamline[19], where high energy photons are in-
cident to utilize the longer escape depth of the emitted
photoelectrons. Sincethephotoionizationcrosssectionof
Ce 4f electron is usually much less than those of TM d
FIG.1: TheCeM5 XASspectraofCeNix (x=2,5). Thein-
electrons, we used the resonance photoemission (RPES)
setshowsoff-andon-resonancephotoemissiondataonCeNi5.
techniqueattheCe3d-edgetoobtainthepartial4f spec-
tral weights, where the 4f emission is enhanced relative
to other conduction electron emissions by the process structure near hν = 882.3 eV is primarily determined
by the multiplet structures of the 3df2 electronic config-
3d104f1+h¯ω →3d94f2 →3d104f0+photoelectron uration, where the underline represents a hole[22]. The
slight change of this lineshape and the satellite struc-
Similar resonance photoemission technique has been ture near ∼885eVcan be understood as the effect of hy-
extensively used at the Ce 4d-edge to obtain 4f spectral bridizationbetweenCe4f electronandthevalenceband
weights of many Ce compounds[13]. The difference be- withinAndersonimpuritymodel[23,24]. Intheinset,we
tweenRPESatthesetwoedgesisthatthekineticenergy show the photoemission spectra of CeNi taken at pho-
5
of the valence photoelectron is ∼ 880 eV at the Ce 3d- ton energy below this edge (hν = 870 eV; off-resonance)
edge, whereas it is ∼ 120 eV at the 4d-edge. Hence the and very close to the maximum (hν = 881.9 eV; on-
escape depth of photoemitted electron at the M5-edge resonance). We can see the drastic change of spectral
is much longer than 4d RPES, and the spectra becomes shapes due to the much enhanced Ce 4f emissions on
bulk sensitive with less than ∼ 15 % of total weights resonance. Wethenextractthebulk-sensitive4f spectral
contributing from the surface region[16]. This technique weights of each compound by subtracting off-resonance
has proven to be effective in elucidating bulk electronic data fromthe on-resonancedata. In this process,we use
structures of several Ce compounds[20, 21]. theon-resonancespectraatslightlylower(∼0.4eV)inci-
All samples of CeRh , CeRh , CeNi and CeNi were dentphotonenergythantheM maximumpeakofXAS,
2 3 2 5 5
polycrystalline made by argon arc-melting followed by since the spectra taken at the M maximum peak posi-
5
annealing, and their crystal structures were checked by tion are often found to be contaminated by incoherent
x-ray diffraction. The Ce 3d → 4f RPES and x-ray ab- Auger emissions[25].
sorption spectroscopy (XAS) measurements at the Ce The experimental bulk-sensitive 4f spectral weights
M4,5 edge were performed in the beamline BL25SU of of CeNi2 and CeNi5 thus obtained are shown as black
SPring-8 in Japan. The energy resolution of the pho- dots in Figure 2. Previously reported high resolution
tonsourcearoundCeM4,5 edgewasbetter than80meV (∆E ∼ 50 meV) 4d → 4f resonant photoemission[28]
(Full width at half maximum : FWHM) and the overall observed the 4f-driven spectral feature less clear due to
experimentalresolution∼100meVFWHMwasobtained the dominant TM 3d emission. However, present work
by SCIENTA SES200electronanalyzer. The pressurein evidently shows the 4f-driven peak at the Fermi level
thevacuumchamberwasbetterthan4×10−10Torrdur- in both Ce-Ni compounds. To determine whether “lo-
ing the measurements. The dataweretakenat20K and calized” or “itinerant” picture is more appropriate for
temperature was controlled by closed-cycle He cryostat. these4f spectralweights,weplotandcomparewithboth
Sample surfaces were cleaned by filing with a diamond the Ce 4f density-of-states (DOS) from linear muffin-
file in situ and we checked the cleanliness of the surface tin orbital (LMTO) calculations with local density ap-
by monitoring O 1s level. The EF of the sample was proximation (thin solid line)[10], and the Gunnarsson
referred to that of surface-cleaned Pd metal. Scho¨nhammer(GS)calculationfit(thicksolidline)based
Figure 1 shows the x-ray absorption spectra of CeNi on the Anderson single impurity model[7]. The one-
x
(x= 2, 5) at the Ce M edge corresponding to the tran- electron 4f DOS were taken from the published band
5
sition 3d → 4f. The lineshape of the main peak structure calculations[10], which were convoluted with
5/2
3
FIG.2: ComparisonbetweentheexperimentalCe4f spectral
weights (dots) with band structure calculations (thin solid
line) and with GS fits(thick solid line) for CeNi (x = 2, 5).
x
The dashed line in the top panel is another band calculation
by Harima [29].
FIG. 3: The comparisons of experimentally extracted bulk-
sensitive 4f weights of CeRh2 and CeRh3 with the one-
TABLE I: The parameter values used for GS fitting and the electron band calculations and theGS fit.
resulting4f-level occupancynumbern andtheKondotem-
f
peratureTK. CoulombenergyUff issetto6.0eV.χm(0)’sare
magneticsusceptibilityatT=0predictedfromGSfittingand
χ∗ (0)’s are experimental values in units of 10−3emu/mol. spectroscopicinvestigations[23],andcanbeattributedto
m the shift of the Ni 3d valence band toward to the Fermi
ε (eV) ∆(meV) T (K) n χ (0) χ∗ (0) level at high Ni concentration[28]. We also find that the
f K f m m
CeRh2 1.30 95 1335 0.76 0.92 0.6 χm(0) deduced from GS calculation gives very close val-
ues to measured ones for both Ce-Ni compounds.
CeRh3 1.20 110 1350 0.70 0.54 0.4
CeNi2 1.13 89 570 0.78 1.13 0.9 On the other hand, the band structure calculation
CeNi5 1.00 90 3300 0.69 0.62 0.7 gives rather poor agreement with the experimental data
for both compounds, especially in CeNi . The calcula-
5
tion does not reproduce the peak near the Fermi level
properly, and the predicted strongest peak around 1 eV
Lorentzian broadening in the form of α|ω−ω | and the from the Fermi level is absent in the experimental data.
0
experimental resolutionof 100 meV. For the GS calcula- This discrepancy is not due to the particular calculation
tions the lowest order f0, f1, f2, and the second-order methodormisplacementoftheFermilevel,sincemorere-
f0 statesareemployedasbasisstates,andthespin-orbit cent band calculation from other group (dot-dashed line
splittingofthe4f levelisincluded. WhenGScalculation in the figure)alsoshows similar discrepancy[29]. For the
isexecuted,itisknowntobequiteessentialtoemployre- casesofCeNi2,thebandcalculationsreproducethepeak
alisticV2(ǫ)inordertofullyinterpretexperimentalspec- at the Fermi levelproperly,but the features between1-3
tra and fit the thermodynamic quantities [26]. Hence we eV from the Fermi level shows appreciable discrepancy.
used the 3d→4f off-resonancespectrum for the valence Thesedisagreementsofbanddescriptionsimply thatthe
band shape to be hybridized with Ce 4f state. correlation between the 4f electrons should not be ne-
Fromthesecomparisons,wecanseethattheGScalcu- glected in these CeNi compounds, even in the extremely
lationsprovidequitegoodfitsfortheexperimentalCe4f α-like compound CeNi5.
spectralweightsforbothCeNi andCeNi . Theirresult- The same phenomena happen in CeRh (x=2,3) com-
2 5 x
ing parametervalues are presentedin Table I along with pounds,ascanbeseeninFigure3. Inthisfigure,theex-
the Kondo temperature T , the 4f electron occupation perimental bulk-sensitive 4f spectral weights [30] (dots)
K
numbern ,themagneticsusceptibilitiesatT =0,χ (0) are compared with the one-electron band structure cal-
f m
deducedfromtheseparametervaluesandtheexperimen- culation(thinsolidline)andthefitoftheGScalculation
tally measured χ∗ (0) [27]. We find that T increases (thick solid line). In both CeRh and CeRh the experi-
m K 2 3
and n becomes smaller as the Ni content is increased, mentaldatashowalargepeakneartheFermilevelremi-
f
which is consistent with the findings of XAS and other niscentoftheKondotail,whichcanbewelldescribedby
4
theGScalculation. However,thebandstructurecalcula- [1] W.H.Zachariasen (unpublished)quoted in A.W. Lawson
tionsdonebytwoindependent groups[10,29]completely and T.Y. Tang, Phys. Rev. 76, 301 (1949); L. Pauling,
miss this feature in CeRh , and predict instead a strong J. Am.Chem. Soc. 69, 542 (1947).
3 [2] B. Johansson, Philos. Mag. 30, 469 (1974).
peak near 2 eV from E , which is absent in the experi-
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2 Svane, Phys.Rev.Lett. 72, 1248 (1994).
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below2eVfromEF showssubstantialdisagreement. On Jarlborg et al.,Phys. Rev.B 55, 1288 (1997).
the other hand, GS calculationsgivegoodgeneralagree- [5] J.W.AllenandR.M.Martin,Phys.Rev.Lett49,1106
mentsinbothCe-Rhcompounds,althoughsomediscrep- (1982); L.Z.Liuet al.,Phys.Rev.B45,8934(1992); J.
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210 (1982); J. Phys.F 13, 1007 (1983).
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(1994).
It may not be surprising that the band calculation
[11] O. Eriksson et al., Phys. Rev. Lett. 60, 2523 (1988); J.
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Trygg et al.,Phys.Rev. B 50, 4200, (1994).
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[15] D. Malterre et al., Phys. Rev. Lett. 73, 2005 (1994); D.
picturefailstodescribethesingle-particleexcitationsuch
Malterre et al.,Adv.Phys.45, 299 (1996).
as photoemission implies that it will not be able to un-
[16] StefanHu¨fner,“PhotoelectronSpectroscopy”,(Springer-
derstandmanyphysicalpropertiesofthesystem. Onthe
Verlag, Berlin, 1995) p.8.
other hand, Anderson impurity model gives consistent [17] L. Du`o, Surf. Sci. Rep.32, 233 (1998).
description of both 4f spectral weights and thermody- [18] Hyeong-DoKim et al.,Phys. Rev.B 59, 12294 (1999).
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Soc. Jpn. 69, 2771 (2000); Solid State Commun. 121,
electroncorrelationeffectis importantevenfor these ex-
561 (2002).
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In summary, the bulk-sensitive Ce 4f spectral weights [24] Ran-JuJung,Ph.Dthesis,SeoulNationalUniversity,Ko-
obtained by 3d → 4f RPES of CeNi (x=2, 5) and rea (2001).
x
CeRh (x=2, 3) show remarkably enhanced the f1 fi- [25] E.-J. Cho et al., submitted to Phys.Rev.B.
x [26] See-Hun Yanget al.,Phys. Rev.B 61, R13329 (2000).
nal state peak near the Fermi level unlike those of 4d→
4f RPES. The Gunnarsson Scho¨nhammer calculations [27] For CeRh2 M. Ozawa et al., Physica B 206 & 207, 267
(1995); for CeRh3 T. Gambke et al., “Valence Fluctua-
basedontheAndersonsingleimpuritymodelfitverywell
tionsinSolid”,(North-Holland,Amsterdam,1981)p.447;
theexperimentalCe4f spectralweights,anditsresulting for CeNi2 J. Sakurai et al., J. Magn. Magn. Mater. 52,
parametervalues areconsistentwith other spectroscopic 205 (1985); for CeNi5 D.Gignoux et al.,J. de Phys.43,
and thermodynamic data. However, one-electron band 173 (1982). Whenever possible, the susceptibility of the
calculations fail to give plausible descriptions. This im- corresponding Y or La compound was subtracted to ob-
tain the contribution from Ce ions only.
plies that the 4f states even in extremely α-like CeRh ,
2 [28] D. Purdie et al.,Solid State Commun. 106, 799 (1998).
CeRh , andCeNi stillretain‘localized’4f character,at
3 5 [29] H. Harima, privatecommunications.
least for the excitation spectra.
[30] Our3dRPESdataonCeRh3 issomewhatdifferentfrom
This work is supported by the Korean Science and the old data[14] in that the peak at the Fermi level is
EngineeringFoundationthroughthe CenterforStrongly much stronger and quite narrow. This is probably due
CorrelatedMaterialsResearchatSeoulNationalUniver- to the difference in energy resolution and the incident
sity. Theexperimentswereperformedundertheapproval photon energy.
of the Japan Synchrotron Radiation Research Institute.
We thank Prof. Harima for sending band calculation re-
sults prior to publication.
∗ Author to whom all correspondences should be ad-
dressed. e-mail: [email protected]