Mott-Kondo Insulator Behavior in the Iron Oxychalcogenides

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by  B. Freelon

Mott-Kondo Insulator Behavior in the Iron Oxychalcogenides
B. Freelon,1,2∗ Yu Hao Liu,1 Jeng-Lung Chen,1,3 L. Craco,4∗ M. S. Laad,5 S. Leoni,6 Jiaqi Chen,7 Li Tao,7
Hangdong Wang,7 R. Flauca,8 Z. Yamani,8 Minghu Fang,7 Chinglin Chang,3 J.-H. Guo1 and Z. Hussain1∗
1 Advanced Light Source Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720
2Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439
3Department of Physics, Tamkang University, Tamsui, Taiwan 250
4Instituto de F´ısica, Universidade Federal de Mato Grosso, 78060-900, Cuiaba´, MT, Brazil
5The Institute of Mathematical Sciences, C.I.T. Campus, Chennai 600 113, India
6School of Chemistry, Cardiff University, Cardiff, CF10 3AT, UK
7Department of Physics, Zhejiang University, Hangzhou 310027, P. R. China
8Canadian Neutron Beam Centre, National Research Council,
Chalk River Laboratories, Chalk River, Ontario K0J 1J0, Canada
5 (Dated: January 5, 2015)
1
We perform a combined experimental-theoretical study of the Fe-oxychalcogenides (FeOCh) se-
0
2 riesLa2O2Fe2OM2 (M=S,Se),whichisthelatestamongtheFe-basedmaterialswiththepotential
to show unconventional high-Tc superconductivity (HTSC). A combination of incoherent Hubbard
n
featuresinX-rayabsorption(XAS)andresonantinelasticX-ray scattering (RIXS)spectra, aswell
a
asresitivitydata,revealthattheparentFeOCharecorrelation-driveninsulators. Touncovermicro-
J
scopicsunderlyingthesefindings,weperformlocaldensityapproximation-plus-dynamicalmeanfield
2
theory (LDA+DMFT) calculations that unravel a Mott-Kondo insulating state. Based upon good

agreementbetweentheoryandarangeofdata,weproposethatFeOChmayconstituteanew,ideal
]
l testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott
e quantum criticality.
-
r
t PACSnumbers: 74.70.Xa,74.70.-b,74.25.F-,74.25.Jb
s
.
t
a
m Since the discovery [1] of high-temperature supercon- Na,Ba),(M=(S,Se)] [10]becauseof(i)theirsimilarities
ductivity (HTSC) in iron pnictides (FePn), a fundamen- toLaFeOAs(1111)materialsand(ii)theopportunityto
-
d tal question has been whether the iron pnictides are study materials with slightly larger electron correlation
n weaklycorrelatedmetalsorbadmetalsincloseproximity thanthepnicitides[7]. Moreover,La O Fe OM (M=S,
2 2 2 2
o
to Mott localization [2, 3]. These competing ideas must Se) offer the attractive option to tune the strength of
c
implyverydifferentmechanismsofFe-basedHTSC.The electronic correlations in one system through disorder-
[
Mottness view proposes that strong, orbitally-selective free chalcogen replacement of S by Se. The FeOCh con-

1
electron correlation is relevant to HTSC, while the sist of stacked double-layered units La O and Fe O(S,
v 2 2 2
competing itinerant view holds that antiferromagnetic Se) . Fe O(S, Se) contains an Fe layer that is similar
2 2 2 2
3 (AFM) order and superconductivity (SC) originate from to the FeAs layer in the pnictides; in the latter, Fe2+ (d6
3 aKohn-Luttinger-likeFermisurface(FS)nestingmecha- configuration) is co-ordinated by O atoms but there is
0 nismwithweakelectroniccorrelations [4]. Earlyinsights an extra Oxygen atom O(2). The rare-earth layer also
0
suggested the importance of varying the strength [2, 5] contains an additional O(1) atom. Chalcogen substitu-
.
1 of electron correlations using chemical substitutions to tion results in a slightly expanded Fe-square-lattice unit
0 tunesystemsacrossband-widthdrivenMottlocalization. cellofLa O Fe OM comparedtothatofLaOFeAs[10].
2 2 2 2
5
Sincethen,moreevidencehasaccumulatedinsupportof The consequent decrease in the electronic bandwidth W
1
this proposal; FeSe is a bad-metal without AF order [6], mustincreasetheHubbardU,offeringadisorder-freepa-
:
v whileparentFe-oxychalcogenidesareelectricalinsulators rameter to subtly tune U/W across a critical value for a
i
X [7, 8]. Mott insulator phase.
r
a Most FePn superconductors exhibit bad-metal normal In this Letter, we substantiate this reasoning by pre-
states with hints of novel quantum criticality. The dis- senting first measurements of the unoccupied and occu-
coveryofHTSCinalkalineironselenidesA Fe Se , pied Fe and Oxygen density-of-states (DOS) obtained
1−x 2−y 2
whereA=K,Tl,CsandRb(referredtoas”245s”)inten- by soft X-ray absorption (XAS) and resonant inelastic
sifiedthisbasicdebate,sinceSCwithT =30Kemerges X-ray scattering (RIXS). Strong Fe moment localiza-
c
fromdopinganAFMinsulatingstateoccuringduetoFe- tion that can be tuned with chalcogen replacement, and,
vacancyinducedbandnarrowing[9]. Studyingmoresuch more crucially, the observation of incoherent Fe spec-
materialstofurtherdevelopacompletedescriptionofthe tral weight due to Hubbard band formation marks the
genericelectronicphasediagramofironsuperconductors FeOCh as correlation-driven insulators. The incoherent
is clearly warranted. With this in mind, we investigated spectralweightundergoesaresonantenhancementinthe
iron oxychalcogenides such as X OFe O M [X=(La, RIXS data and LHB spectral weight analysis, indicat-
2 2 2 2
2
Intensity [arb. units]y [arb. units] (a553)340 . L0Laea22VOOP22FFh5eeLo242OHOt0oSSBen22 En5e5rg0y [eLLVaa522]6OO022FFee22OOSS2e5270 Resistivity [Ohm.m]654320000000000 (b) LL LaaD22OOA22+FFDee22MOOFSSTe22 (U=5eV) Intesity [arb. units] (a) 692Ph6o9to4 LLn aaE622n9OOe6r22gFFy6ee 9[22e8OOVSS]72e020 (7b0)8.8eV LFLeaa 22-OO m22FFeetea22lOOSSe22
Intensit LHB 100 707.4eV
532.0eV 690 700 710 720 730 740 685 690 695 700 705
0 Temperature [K] Photon Energy [eV]
520 524 528 150 200 250 300
Photon Energy [eV] Photon Energy [eV] FIG. 2: (Color online) The Fe L2,3-edge XAS profiles of
La O Fe OM , (M = S, Se) is shown in panel (a). The
2 2 2 2
FIG. 1: (Color online) (a) O 1s XAS and (b) O Kα RIXS arrows indicate the X-ray absorption features at the photon
data for La O Fe OM , (M = S, Se). RIXS intensity data energies,707.4and708.8eV,usedtocollecttheFeRIXSdata
2 2 2 2
wascollectedusingphotonswithincidentenergiesof532and in (b). The data are plotted with a vertical offset for clarity.
534 eV. In (c) the electrical resistivity versus temperature T
data and the simulated resistivity calculated using LDA +
DMFT is shown.
atom coordination geometry and 3d occupation (oxida-
tion state). Fig. 1, top panel (a), shows the O K XAS
profiles of La O Fe OM (M = S, Se). Incident X-ray
2 2 2 2
ing the Mott insulating character of both La O Fe OS
2 2 2 2 energiesof532and534eVwereusedtocollecttheOKα
andLa O Fe OSe . Wediscusstheimplicationsofthese
2 2 2 2 RIXSdatapresentedinFig.1(a). Thepresenceoflower
findings for possible HTSC arising from an incoherent
Hubbard band (LHB) structure in La O Fe OS RIXS
2 2 2 2
metal proximate to novel selective-Mott quantum criti-
spectra is consistent with resistivity data (Fig. 1 (b))
cality.
which shows M = S to be a better insulator than M =
We investigated well characterized La2O2Fe2OM2 Se. We note the absence of O charge transfer band, i.e.,
M=(S, Se) polycrystalline materials, with nominal com- a high energy shoulder structure for the O 2p mainband
positions,thatwerefabricatedusingsolidstatereactions peak, in the Oxygen RIXS data of La O Fe OSe mate-
2 2 2 2
methodsdescribedearlier[7]. HighpurityLa2O3,Feand rial: this suggests that the extent of Fe-O hybridization
(S, Se) powders were used as starting materials and lab- DOS is different for M = S and M=Se compounds.
oratory X-ray powder diffraction data showed that these In Fig. 2 (a), we show the Fe L -edge XAS spectra
2,3
materials had minimal impurity phases; neutron powder forbothLa O Fe OS andLa O Fe OSe . Amagnifica-
2 2 2 2 2 2 2 2
diffraction data confirmed this finding. tion (inset) of the region near the Fermi level, presented
XAS and RIXS spectra for the FeOCh were collected in Fig. 2 (a), reveal a clear difference in the iron conduc-
in a polarization averaged condition. In order to re- tion band weight of the two materials. La O Fe OSe
2 2 2 2
duce the possibility of oxidation, iron oxychalcogenide has enhanced spectral weight at pre-edge just above E ,
F
samples were sheared under a pressure of 10−6 Torr in and this must ultimately be related to details of the cor-
a pre-chamber immediately before being placed in the related electronic structure changes upon replacement of
ultra-high vacuum experimental chamber. All presen- S by Se. Fe XAS data is consistent with both RIXS
tend measurements were performed in the paramagnetic and bulk measurements that show M = Se to be less
state above 150 K. The Advanced Light Source beam- stronglycorrelatedthanM =S.ResonantinelasticX-ray
lines 7.0.1 and 8.0.1 delivered X-ray beams of 100 mi- scattering(RIXS)intensitydata,seeFig.2(b),werecol-
cron spot-sizes and energy resolutions of 0.2 eV (0.2 eV) lected using incident X-ray energies tuned to near- and
and 0.5 eV (0.6 eV) for oxygen(iron) X-ray absorption off-resonance Fe X-ray absorption features at 707.4 eV
and emission, respectively. The oxygen K-edge XAS and 708.8 eV, respectively. RIXS [12] intensity results
intensity is proportional to the unoccupied p-weighted from a second-order (i.e., two-step) photon-in, photon-
DOS resulting from the ∆l = ±1 dipole selection rule outprocessthatcanbecanbedescribedbytheKramers-
for X-ray photon absorption. If the Fe-O related bond is Heisenberg (KH) relationship [12]. The KH expression
partially covalent, O 2p states hybridize with Fe 3d or- requirestheenhancementofthescatteringintensitywhen
bitalsandhencethepre-thresholdforabsorption,i.e.,the incident photon energy matches X-ray absorption edge
lowenergyfinestructure,revealsitselfinligand-to-metal energies. The RIXS data for M = S shows correlation
chargetransferexcitations[11]. XASissensitivetometal induced broad features in the high energy spectral re-
3
0.6 0.6 0.6 0 0 0
r(w)223z-r00..42 r(w)xz00..42 00..24r(w)yz S(w)Im223z-r-1-05 S(w)Imxz-1-05 --150S(w)Imyz
0.0-4 -2 0 2 4 0.0-4 -2 0 2 4 -4 -2 0 2 40.0 -15-4 -2 0 2 4 -15-4 -2 0 2 4 -4 -2 0 2 4-15
w (eV) w (eV) w (eV) w (eV) w (eV) w (eV)
0.6 0.6 0 0
LDA - La2O2Fe2OSe2 La2O2Fe2OSe2
U=3.0 eV U=3.0 eV
r(w)22x-y00..42 r(w)xy00..42 UU==45..00 eeVV (w)SIm22x-y-1-05 S(w)Imxy-1-05 UU==45..00 eeVV
0.0-4 -2 0 2 4 0.0-2 0 2 4 6 -15-4 -2 0 2 4 -15-4 -2 0 2 4
w (eV) w (eV) w (eV) w (eV)
FIG.3: (Coloronline)Orbital-resolvedLDAdensity-of-states FIG.4: (Coloronline)Orbital-resolvedimaginarypartsofthe
(DOS) for the Fe d orbitals of La O Fe OSe as well as DMFTself-energiesfortheFedorbitalsofLa O Fe OSe for
2 2 2 2 2 2 2 2
LDA+DMFT results for different values of U (with U(cid:48) = different values of U and J =0.7 eV.
H
U−2J )andfixedJ =0.7eV.Noticethenarrowbandsin
H H
the LDA DOS. Compared to the LDA results, large spectral
weight transfer along with electronic localization is visible in
ity is not relevant. Finally, clear lower-dimensional band
the LDA+DMFT spectral functions for U ≥4 eV.
structuralfeaturesarevisibleinρxz (ω)(quasi-1D)and
LDA
ρxy (ω) (2D van-Hove singular) near the Fermi energy,
LDA
gion that form the LHB. In the RIXS data, for both EF(= 0). Enhancement of the effective U/W ratio in
M = S, Se, there is a high intensity Fe 3d metal va- such an intrinsically anisotropic setting should now nat-
lence band peak. Near 692 eV, a broad shoulder in the urally favor the Mott insulator state in FeOCh. To sub-
high energy spectral region is resonantly enhanced. The stantiate this, DMFT calculations were performed us-
spectra clearly reveal incoherent electronic excitations ing the multi-orbital iterated perturbation theory (MO-
involving 3d Fe electrons. These are correlation satel- IPT) as an impurity solver. Though not exact, MO-IPT
lites formed by multi-particle excitations that involve lo- has a proven record of providing reliable results for a
calized states, and clearly reveal the stronger electronic number of correlated systems [16], and is a very effi-
correlations that generate a correlated insulator in the cient and fast solver for arbitrary T and band-filling.
FeOCh. The full Hamiltonian is H = H0 + H1 with H1 =
Takentogether,theseresultsmarktheFeOChascorre- U(cid:80)i,ania↓nia↑ + (cid:80)i,a(cid:54)=b[U(cid:48)nianib − JHSia · Sib], with
lated,bandwidth-controldriveninsulators[13]. Inmulti- U(cid:48) =(U−2JH)beingtheinter-orbitalCoulombtermand
orbital FeOCh with strong inter-orbital charge transfer, JH the local Hund’s coupling. Though U (cid:39)4.0−5.0 eV
correlationsandcrystalfieldeffects,propercharacteriza- and JH =0.7 eV are expected, we vary 3.0≤U ≤5.0 to
tionoftheMottinsulatingstate(s)isanimportant,non- get more insight.
trivialissue. Toshedlightonmicroscopicsunderlyingthe In Fig. 3, we show the total many-body local spectral
above findings, we performed LDA+DMFT calculations function (DOS) for the M = Se case. A clear transition
fortheFeOChcloselyfollowingearlierwork[8,14]. LDA from a bad-metal state at U = 3.0 eV to a correlated
calculations were performed within the linear muffin- insulator for U =4.0, 5.0 eV occurs. Looking closely at
tin orbitals (LMTO) scheme. The one-electron Hamil- the self-energies in Fig. 4, however, a remarkable aspect
tonian reads H0 = (cid:80)k,a,σ(cid:15)a(k)c†k,a,σck,a,σ with a = stands out: ImΣa(ω = EF) vanishes in the region of
xy,xz,yz,x2−y2,3z2−r2 label (diagonalized in orbital theMottgap,insteadofhavingapole,aswouldoccurin
basis) five Fe 3d bands. We retain only the d states, atrueMottinsulator. Thisaspectismorereminiscentof
since the non-d orbital DOS have negligible or no weight acorrelatedKondoinsulator,wherethegaparisesdueto
at E . An important aspect of our study is readily vis- combinedeffectsofelectroniccorrelationsandsizablein-
F
ible in Fig. 3 which shows a sizable reduction of the terband hybridization. While this novel finding is some-
LDA bandwidth relative to that of FeSe (which is it- what unexpected, proximity to a true Mott state is seen
self an incoherent bad-metal [15]), induced by hybridiza- by observing that the pole structures in ImΣ (ω) with
a
tionwithO statesanddistortingeffectsoftheLa-layers. a = xz,yz,3z2 −r2 lie only slightly above E . Thus,
F
Another interesting aspect, in contrast to Fe-arsenides, because the system is correlated, sizable spectral redis-
is that the xz − yz orbital degeneracy in the tetrago- tribution in response to additional small perturbations
nal structure is explicitly removed at the outset. Thus, (e.g, longer-range Coulomb interactions in an insulator)
in FeOCh, an electronic nematic (EN) instability linked can drive it into a true Mott insulating phase. More
to ferro-orbital order and its potentially related critical- importantly, electron doping will also generically cause
4
their implications for possible HTSC in suitably doped
3
LaOFeOS U=5.0 eV 3 or pressurized systems. Both are expected to tune the
2 2 2 2
LaOFeOSe
2 2 2 2 Fe-O hybridization: since we find a correlated Mott-
Kondo insulator, this must non-trivially reconstruct the
s) 2
arb. unit2 r(w)total1 FHmeue-tbdablsalitrcaditt-eybsa.envdeInnspaLescatt2hrOael2lFwoece2aiOglihzStaetp2i,roenssuaopgfeptsrheteshseFioeonn3sdoeftstooaxftybegsaedins-
sity ( preserved. This may provide clues to metallize FeOCh
n
Inte1 0-6 -4 -2 0 w ( 2eV) 4 6 bstyataepsprwohpircihathelyybmridainziepuwlaitthinFget-hdesOtaxtyegse.nTanhdiscihsaalcnogeexn-
citing perspective because, if HTSC results, (as in 245s),
it would reinforce the fundamental challenge to the itin-
0
0 2 4 6 8 10 erancy perspective in pnictide HTSC research. Specifi-
w ( eV)
cally, since strong correlations have now obliterated the
LDAFSasabove,large-scaleorbital-selectiveredistribu-
tion of spectral weight upon carrier doping is generically
FIG. 5: (Color online) Total LDA+DMFT XAS spectrum of expected to lead to OSMT and FS involving only a sub-
La2OFe2O2M2 (M =S, Se), showing the changes in the up- set of original d-orbitals. The LDA FS and its nesting
per Hubbard band upon chalcogen substitution. Inset shows
features can now no longer serve as a guide for rational-
the corresponding total LDA+DMFT spectral function for
izing instabilities to novel order(s). Instead, strong elec-
U =5.0 eV and J =0.7 eV.
H
tronic scattering between the partially (Mott) localized
and itinerant subsets of the many-body spectral func-
tions generically extinguishes the Landau quasiparticle
large-scale orbital-selective (OS) spectral weight trans-
(QP) pole at E . In absence of a QP description for
F
fer, leading to orbital-selective Mott transition (OSMT),
the normal state, the very tenability of the FS nesting
wherein a subset of orbital states remain (Mott) gapped
scenario is called into doubt.
in an incoherent metallic state [8]. Such states can ex-
Thus, our work provides an impetus to consider closer
hibit novel quantum criticality associated with an end-
similarities between Fe-based systems and the cuprates.
pointoftheOSMT.WhetherHTSCnearsuchnovelcrit-
In similarity with 245 materials, we suggest that
icalityoccursinsuitablydopedFeOChisanenticingand
FeOCh are ideal candidates for testing this proposal.
open issue.
The development of incoherent scattering observed in
Remarkably, a comparison to Fe-edge XAS and dc
our XAS and RIXS spectra arises from Mott localized
resistivity data reveals good agreement, providing sup-
states. If these can be appropriately doped, it is likely
port for this novel finding. In Fig. 5, we show our
that strong orbitally-selective scattering will nearly ex-
LDA+DMFT XAS spectrum. We have incorporated
tinguish the Landau-Fermi liquid quasiparticles, leading
the relevant effects of core-hole scattering for XAS line-
to the emergence of an incoherent pseudogapped metal
shapesbyconsistentlyadaptingtheprocedureofPardini
reminiscent of underdoped cuprates [8]. Whether novel
et al. [17] within our DMFT scheme. Several important
quantumcriticalityassociatedwithaselective-Motttran-
aspects stand out: (i) there is good semi-quantitative
sition in conjunction with the development of HTSC in
accord with with the Fe-edge XAS data up to 3.0 eV
FeOChwillbeseeninthefutureisofgreatinterest. Our
above E . More importantly, details of the differences
F findings provide a compelling motivation to produce and
in spectral weight between M =S and M =Se cases is
study doped or pressurized iron oxychalcogenides.
also in very good accord with data, (ii) enhanced spec-
After the recent journal submission of this paper, we
tralweightatpre-edgejustaboveE ,justasseen,while
F learned of work by G. Giovannetti et. al. on the iron
smearing of lower energy structures (0.0 < ω < 2.0 eV)
oxychalcogenides which is complementary to ours and
andhigherenergyofthe3.0eVpeakforM =Srelativeto
gives comparable results.
M =Se is in good accord, both with XAS data, as well
The Advanced Light Source is supported by the Di-
as with resistivity data which show M =S to be more
rector, Office of Science, Office of Basic Energy Sci-
correlated than M =Se. Indeed, LDA+DMFT resistiv-
ences, of the U.S. Department of Energy (DOE), un-
ities (using the Kubo formalism and neglecting vertex
der Contract No. DE-AC02-05CH11231. ZJU work is
correctionstotheconductivity,whicharenegligible[18])
supported by the National Basic Research Program of
for M =S, Se show very good accord with experiment
China (973 Program) under Grants No. 2011CBA00103
(where ρ(dMc )(T) was measured above 150 K. This is a and 2012CB821404, the National Science Foundation
stronginternalself-consistencycheck,andputsournovel of China (No. 11374261, 11204059) and Zhejiang
proposal on stronger ground. Provincial Natural Science Foundation of China (No.
Armed with these positive features, we now discuss LQ12A04007). M.S.LthanksMPIPKS,Dresdenforhos-
5
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Feng, J. Chen, and H. Q. Yuan, Europhys. Lett. 94,
Physical Chemistry department at Technical University
27009 (2011).
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