Table Of ContentAPS
X X
Coexistence of localized and itinerant electrons in BaFe ( = S and Se) revealed
2 3
by photoemission spectroscopy
D. Ootsuki,1 N. L. Saini,2 F. Du,3,4 Y. Hirata,3 K. Ohgushi,5,3 Y. Ueda,6,3 and T. Mizokawa7,1,2
1Department of Physics, University of Tokyo, Kashiwa, Chiba 277-8561, Japan
2Department of Physics, Universita´ di Roma ”La Sapienza”, Piazzale Aldo Moro 2, 00185 Roma, Italy
3Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
4Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education),
College of Physics, Jilin University, Changchun, 130012, People’s Republic of China
5Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan
6Toyota Physical and Chemical Institute, Nagakute, Aichi 480-1192, Japan
5
1 7Department of Complexity Science and Engineering, University of Tokyo,
0 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
2 (Dated: January 12, 2015)
n WereportaphotoemissionstudyatroomtemperatureonBaFe2X3 (X =SandSe)andCsFe2Se3
a in which two-leg ladders are formed by the Fe sites. The Fe 2p core-level peaks of BaFe2X3 are
J broadandexhibittwocomponents,indicatingthatitinerantandlocalizedFe3dsitescoexistsimilar
9 to KxFe2−ySe2. The Fe 2p core-level peak of CsFe2Se3 is rather sharp and is accompanied by a
charge-transfer satellite. The insulating ground state of CsFe2Se3 can be viewed as a Fe2+ Mott
] insulator in spite of the formal valence of +2.5. The itinerant versus localized behaviors can be
l
e associated with thestability of chalcogen p holes in thetwo-leg ladder structure.
-
r
PACSnumbers: 74.25.Jb,74.70.Xa,79.60.-i,74.81.-g
t
s
.
t
a I. INTRODUCTION been attracting much attention due to the specific quasi
m
one-dimensionalcrystalstructurewithPnmaspacegroup
- as well as the block-type antiferromagnetic state similar
d The coexistence of itinerant superconducting phase
toK Fe Se . [7–13]Inaddition,BaFe Se ispredictedto
n andlocalizedantiferromagneticphaseinKxFe2−ySe2 [1– 2 4 5 2 3
o 4] sheds light on the intervening coupling between the be a new type of multiferroic system with magnetic and
c ferrielectricordersinarecenttheoreticalwork,[14]which
electron correlation effect and the lattice effect. When
[ certainly enhances the interest in the title system. In
the Fe 3d electrons are localized and form the antiferro-
1 magnetic insulating state with high-spin Fe2+, the insu- BaFe2Se3,FeSe4 tetrahedrasharetheiredgesandforma
v lating phase tends to expand due to Fe-Fe bond length two-legladderofFesitesasshowninFig. 1(a). Themag-
5 increase and applies a kind of pressure to the remain- neticstructureofBaFe2Se3 issimilartoK2Fe4Se5 inthat
7 four Fe spins in the two-leg ladder form a ferromagnetic
ing metallic region. [5] The antiferromagnetic insulating
0 block and the neighboring blocks are antiferromagneti-
2 phase in KxFe2−ySe2 is identified as K2Fe4Se5 with Fe
cally coupledasillustratedin Fig. 1. The two-legladder
0 vacancy order. [2, 3] On the other hand, the supercon-
isdistortedwithshorterFe-Fe bonds (ferromagneticand
. ductingphaseislikelytobeFeSewhichisunderthepres-
1 antiferromagnetic) and longer Fe-Fe bonds (ferromag-
sure from the expandedand insulating K Fe Se . In the
0 2 4 5 netic and antiferromagnetic) along the ladder direction.
5 insulating K2Fe4Se5 phase, the four Fe sites with square
Themagneticstructureisconsistentwiththetheoretical
1 geometryformaferromagneticblockwhichcouplesanti-
prediction. [15] N´eel temperature reported in the litera-
: ferromagneticallywithneighboringblocks. [3]Thetrans-
v tures varies from 240 K [9] to 255 K. [7, 8, 11] Also the
fer integrals between the neighboring blocks are reduced
i
X due to the Fe vacancy, and the Mott insulating state is magnetic moment of BaFe2Se3 ranges from 2.1 µB [9] to
2.8 µ [7, 11] depending on the growthcondition, andis
ar realized. In the XPS study on KxFe2−ySe2, two compo- smallBerthanthevalueexpectedforhigh-spinFe2+. Inter-
nents of the Fe 2p peak are assignedto the coexisting
3/2 estingly,resistivityalsodependsonthegrowthcondition.
superconducting and insulating phases in the supercon-
Lei et al. reported that resistivity exhibits activation-
ductingKxFe2−ySe2while,ininsulatingKxFe2−ySe2,the
type temperature dependence with band gap of 0.18 eV.
Fe 2p peak consists of a single component. [6] The
3/2 [8] On the other hand, one-dimensional variable range
charge-transfer energy from Se 4p to Fe 3d is estimated
hopping was reported by Nambu et al. indicating that
tobe2.3eVwhichissmallerthantherepulsiveCoulomb
somecarriersarelocalizeddue tostrongscatteringeffect
interaction between the Fe 3d electrons of 3.5 eV. [6]
in the quasi one-dimensional structure. [11] In addition,
Therefore,the insulating K Fe Se phase with high-spin
2 4 5
Fe2+canberegardedasaMottinsulatingstateofcharge- coexistence of itinerant and localized electrons was re-
cently indicated by the resonant inelastic x-ray scatter-
transfer type instead of Mott-Hubbard-type, and the Se
ing study by Monney et al. [12] This observation sug-
4p orbitals should be taken into account to explain the
gests that the itinerant electrons introduced by small Fe
magnetic interactions between the Fe spins.
vacancy or some other effects would be responsible for
Recently, another insulating Fe selenide BaFe Se has
2 3
2
FIG.1: (Color online) (a) CrystalstructureofBaFe2Se3 vi-
sualized using the software package Vesta[18]. (b) Schematic
drawing for the magnetic structure and the lattice distortion
for BaFe2Se3.
the reduction of the magnetic moment and the variable
range hopping behavior of the resistivity. [11] In con-
trast to BaFe Se , CsFe Se with formal Fe valence of
2 3 2 3
+2.5 is much more insulating. [16] Interestingly, Fei et
al. have revealed that all the Fe sites have the same
local environment in CsFe Se by means of Mo¨ssbauer
2 3
spectroscopy. [16] Below 177 K, Fe spins in the two- FIG. 2: (Color online) Fe 2p XPS of BaFe2X3 (X = S and
leg ladder of CsFe2Se3 order antiferromagnetically along Se) compared with CsFe2Se3 and KxFe2−ySe2 (metallic and
the rung and ferromagnetically along the leg with mag- insulating). [6] The solid curves indicate the results of curve
netic moment of ∼ 1.8 µB. Usually, Mott insulators fittingfor BaFe2S3 and BaFe2Se3.
with integer number of valence are expected to be more
insulating than the mixed valence systems. The situa-
tionof the two-legladder Fe chalcogenidesis opposite to UPS measurements. The XPS measurement was carried
this expectation, and CsFe Se with formal Fe valence outat300KusingJPS9200analyzer. MgKα(1253.6eV)
2 3
of+2.5ismoreinsulating thanintegervalenceBaFe Se wasused as an x-raysource. The total energyresolution
2 3
and BaFe S . [16] Such puzzling mismatch between the was set to ∼ 1.0 eV. The binding energy was calibrated
2 3
formal valence and the transport behavior indicates un- using the Au 4f core level of the gold reference sample.
usual electronic states of the title system. The UPSmeasurementwasperformedusing SES100an-
In the present work, we study fundamental electronic alyzer and a He I (21.2 eV) source. The total energy
structures of BaFe Se , BaFe S , and CsFe Se above resolution was set to ∼ 30 meV and the Fermi level was
2 3 2 3 2 3
their N´eel temperatures by means of x-ray photoemis- determined using the Fermi egde of the gold reference
sion spectroscopy (XPS) and ultra-violet photoemission sample.
spectroscopy(UPS) at room temperature. The broad Fe
2p XPS peaks of BaFe Se and BaFe S indicate coexis-
2 3 2 3
tence of localized and itinerant electrons. On the other III. RESULTS AND DISCUSSION
hand,theFe2pXPSpeakofCsFe Se isrelativelysharp
2 3
suggestingthatFevalenceishomogeneousinspiteofthe Figure 2 shows the Fe 2p XPS spectra of
expectation of a mixed valence state. The apparentcon- BaFe Se , BaFe S , and CsFe Se taken at 300 K
2 3 2 3 2 3
tradiction between the valence state and the Fe 2p peak which are compared with those of superconducting
width can be reconciled by taking account of the Se 4p and non-superconducting KxFe2−ySe2. [6] In non-
or S 3p holes. superconducting KxFe2−ySe2, the chemical composition
is close to K Fe Se with Fe2+ and its ground state is
2 4 5
a charge-transfer-type Mott insulator. The sharp Fe
II. EXPERIMENTS 2p peak of CsFe Se is very similar to that of non-
2 3
superconductingKxFe2−ySe2ortheFe2+ Mottinsulator.
The single crystals of BaFe Se , BaFe S , and ThisXPSresultindicatesthatCsFe Se wouldbeaMott
2 3 2 3 2 3
CsFe Se were grown as reported in the literatures. insulatorwithFe2+ whichis actuallyconsistentwith the
2 3
[11, 16] We cleaved the single crystals at room tempera- Mo¨ssbauer study. [16] If all the Fe sites in CsFe Se
2 3
ture(300K)undertheultrahighvacuumfortheXPSand take the high-spin Fe2+ configuration,the extra positive
3
FIG. 3: (Color online) Fe 2p XPS of CsFe2Se3 (dots) com-
paredwiththeresultofclustermodelcalculation Thedotted
curve indicates the background due to the secondary elec-
trons.
charge (+0.5 per Fe) should be located at the Se sites. FIG. 4: (Color online) (a) Valence-band XPS of BaFe2X3
Ontheotherhand,insuperconductingKxFe2−ySe2,itin- (X = S and Se) compared with CsFe2Se3 and KxFe2−ySe2
erant Fe 3d electrons coexist with the localized Fe 3d (metallic and insulating). [6] (b) UPS of BaFe2X3 (X = S
electrons in the Mott insulating phase. The Fe 2p line- and Se) compared with CsFe2Se3.
shape of BaFe Se and BaFe S is very similar to that
2 3 2 3
of superconducting KxFe2−ySe2, indicating coexistence
of itinerant and localized electrons. In case of supercon- with the result of the configuration interaction calcu-
ductingKxFe2−ySe2,thecoexistenceofthelocalizedand lation on an FeSe4 cluster model. The Fe 2p3/2 peak
itinerant electronic states is governed by the spatial dis- of CsFe Se (located around 708 eV) is accompanied
2 3
tributionoftheFevacancy. Ontheotherhand,BaFe2Se3 by a broad satellite structure (located around 714 eV)
andBaFe2S3 havenoFevacancywhichcanreducetrans- which can be assigned to the charge-transfer satellite.
fer integrals between neighboring Fe sites and cause the The energy position and the intensity of the charge-
Mott localization. Instead, in the two-leg ladder struc- transfersatellitecanbeanalyzedusingtheconfiguration-
tureofBaFe2Se3 andBaFe2S3,the electronicinteraction interaction calculation on the tetrahedral FeSe4 cluster
betweenneighboringFe sites canbe controlledby the Se model. [6] The ground state is assumed to take the
4p or S 3p holes which are indicated by the mismatch high-spin d6 state mixed with the d7L, d8L2, d9L3, and
between the formal valence and the transport behavior. d10L4 states where L represents a hole in the Se 4p lig-
Here, one can speculate that the Fe 3d electrons with and orbitals. The excitation energy from d6 to d7L cor-
the Fe2+ high-spin configurationand the Se 4p (or S 3p) responds to the charge-transfer energy ∆. The excita-
holesarelocalizedinCsFe2Se3 whereastheyarepartially tion energy from d7L to d8L2 is given by ∆+U where
delocalized in BaFe2Se3 and BaFe2S3. U represents the Coulomb interaction between the Fe
The Fe 2p and Fe 2p peaks can be decomposed 3d electrons. The transfer integrals between the dnLm
3/2 1/2
intothetwocomponentswhicharederivedfromtheitin- and dn+1Lm+1 are described by (pdσ) and (pdπ), where
erant and localized parts. In Fig. 2, the results of Ma- the ratio (pdσ)/(pdπ) is fixed at -2.16. The final states
han’s line shape fitting are indicated by the solid curves are spanned by the cd6, cd7L, cd8L2, cd9L3, and cd10L4
forBaFe2Se3 andBaFe2S3. Theitinerantcomponenthas stateswherecdenotesaholeoftheFe2pcorelevel. The
lower binding energy due to the stronger screening ef- Coulomb interaction Q between the Fe 2p hole and the
fect. The intensity ratio between the itinerant and lo- Fe 3d electron is expressed as Q which is fixed at U/0.8.
calized components is 3.0 : 5.0 for BaFe2Se3 and 2.3 : With ∆ = 2.0 eV, U = 3.5 eV, and (pdσ) = -1.2eV, the
5.0 for BaFe2S3, respectively. The relative intensity of calculatedspectrum canreproduce the Fe 2p XPS result
the “itinerant” component is much larger in BaFe2Se3 as indicated by the solid curve in Fig. 3. ∆ is smaller
and BaFe2S3 than that in KxFe2−ySe2. The energy than U, indicating that CsFe2Se3 is a Mott insulator of
splitting between the two components is ∼ 0.8 eV for charge-transfertyp instead of Mott-Hubbard type.
BaFe2Se3 andBaFe2S3 whichis comparableto the value The valence-band XPS spectra of BaFe2Se3, BaFe2S3,
for KxFe2−ySe2. and CsFe2Se3 taken at 300 K are displayed in Fig. 4(a)
Figure 3 shows the Fe 2p XPS of CsFe Se compared and are compared with those of superconducting and
2 3
4
non-superconducting KxFe2−ySe2. [6] Besides the shal- mission spectroscopy. The Fe 2p core-levelpeaks consist
low core levels such as Ba 5p, Cs 5p, and K 3p, the of the two components in BaFe X , indicating that the
2 3
valence-bandstructuresofthefive systemsaresimilarto itinerantandlocalizedFe3delectronscoexist. TheFe2p
oneanother. ThespectralweightneartheFermilevelin- andvalence-bandspectrasuggestthattheitinerantFe3d
creasesin goingfrom CsFe Se to BaFe Se to BaFe S , electronsaremorestronglyconfinedinBaFe Se thanin
2 3 2 3 2 3 2 3
consistent with their transportproperties. [8, 10, 11, 16] BaFe S . On the other hand, the Fe 2p core-level peak
2 3
Figure 4(b) shows the UPS spectra of BaFe Se , of CsFe Se exhibit the single component accompanied
2 3 2 3
BaFe S ,andCsFe Se takenat300K.BaFe S withthe with the charge-transfersatellite. The insulating ground
2 3 2 3 2 3
highestconductivityshowsthetailofthespectralweight stateofCsFe Se canbeviewedasacharge-transfer-type
2 3
up to the Fermi levelwhile BaFe Se has the finite band Mott insulator with localized Se 4p holes. In BaFe X
2 3 2 3
gapof∼0.2eV.Themagnitudeofthebandgapobserved (X = S andSe), the Se 4p orS 3p holesare partiallyde-
inBaFe Se ismoreorlessconsistentwiththeactivation localized and may cause the coexistence of the itinerant
2 3
energies obtained from temperature dependence of resis-
tivitybyLeiet al. [8]CsFe Se exhibitsthelargestband
2 3
gap of ∼ 0.8 eV consistent with the charge-transfer-type
Mott insulator deduced from the Fe 2p XPS result.
The apparently homogeneous Fe valence and the rela-
tivelylargebandgapinCsFe Se canbeexplainedbased
2 3
on the idea of ligand hole. The smallness of the charge-
transfer energy for the Fe2+ state indicates that, if Fe3+
exists in CsFe Se , it should take the d6L configuration
2 3
instead of d5. In this ligand hole picture, the two-leg
ladder in CsFe Se accommodates the d6-like and d6L-
2 3
like sites. Assuming that the d6-like and d6L-like sites
are aligned along the rung, the Se 4p hole should be lo-
cated at the Se sites sandwiched by the two legs. This
situation is schematically shown in Fig. 5 where all the
Fe sites take the high-spin Fe2+ configuration and the S FIG.5: CrystalstructureandpossibleSe4pholedistribution
4p holes are localized at the Se sites on the rungs. In forCsFe2Se3 visualized usingthesoftware packageVesta[18].
this scenario,the Se 4p holes andthe Fe 3d electronsare
partially delocalized in BaFe Se and BaFe S and may
2 3 2 3
cause the lattice (and magnetic) instability. In the low- and localized Fe 3d electrons. In future, the relationship
temperature phase with the lattice distortion and the betweenthechalcogenpholes,thelatticedistortions,and
block-type magnetic order, the Se 4p holes and the Fe themagneticinteractionsshouldbefurtherstudiedusing
3d electrons would be localized in the antiferromagnetic various experimental and theoretical approaches.
dimer with the Fe2+ high-spin configuration, and they
are rather “itinerant” in the ferromagnetic dimer. Here,
the Se 4p holes and the Fe 3d electrons in the ferromag-
netic dimer are“itinerant”ina sense that they occupy a Acknowledgements
kind of molecular orbitals.
The authors would like to thank Profs. H. Takahashi,
H. Okamura, and Y. Uwatoko for valuable discussions.
IV. CONCLUSION ThisworkwaspartiallysupportedbyGrants-in-Aidfrom
the Japan Society of the Promotion of Science (JSPS)
Inconclusion,wehavestudiedtheelectronicstructures (GrantNo: 25400356). D.O.acknowledgessupportsfrom
ofBaFe X (X =SandSe)andCsFe Se usingphotoe- the JSPS Research Fellowship for Young Scientists.
2 3 2 3
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