Driving mechanisms behind the various phases in manganese perovskites revealed by Mn 2p resonance photoemission

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by  A. Sekiyama

Driving mechanisms behind the various phases in manganese perovskites revealed by
Mn 2p resonance photoemission
A. Sekiyama,1,∗ H. Fujiwara,1 A. Higashiya,1 S. Imada,1 H. Kuwahara,2 Y. Tokura,3 and S. Suga1
1Department of Material Physics, Graduate School of Engineering Science,
Osaka University, Toyonaka, Osaka 560-8531, Japan
2Department of Physics, Sophia University, Tokyo 102-0094, Japan
4 3Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
0 (Dated: February 2, 2008)
0
2 Unusual temperatureand doping dependenceof the Mn 3d spectral functions of manganese per-
n ovskitesNd1−xSrxMnO3 hasbeenrevealedbythehigh-resolutionMn2p−3dresonancephotoemis-
sion. The temperature-dependent spectra cannot be explained by any theoretical model currently
a
underdebate,whileshowingevidenceforamicroscopicanddynamicphasesegregation. Theexper-
J
imental results strongly suggest that thecompetition of both the dynamical and static Jahn-Teller
9
effectswithferromagneticorderingathighandlowtemperatures,respectively,isresponsibleforthe
2
actual electronic states.

]
PACSnumbers: 75.30.-m,79.60.-i,71.30.+h,75.50.Cc
l
e
-
r
t Manganese perovskites have extensively been studied sensitivityismuchlessatthe Mn3p−3dresonanceexci-
s
. regardingcolossalmagnetoresistance(CMR)[1,2,3]and tation, hν ∼50 eV. [12] However,it has been difficult for
t
a charge-ordering [4] from both scientific and engineering a long time to obtain the temperature-dependent Mn 3d
m viewpoints. Their electronic states have fundamentally spectraatthe2p−3dresonancewithhighresolution,be-
- been understood by the so-called double-exchange (DE) causeboth photonflux ofconventionalsynchrotronlight
d
mechanism [5, 6] for a long time. In the last decade, it sourcesandenergyresolutionofsoftx-raymonochroma-
n
has been recognized that a dynamical Jahn-Teller (JT) tors have been insufficient. It is inevitable to use a high-
o
c effect induced by a local lattice distortion also plays a brilliance and high photon flux synchrotron light source
[ substantialrole.[7,8]Nowadaystherearemanytheoret- in order to obtain the Mn 2p−3d resonance photoemis-

ical models and calculations for the manganites, which sionspectrawithsuchahighenergyresolutionandgood
1
v well reproduce the temperature dependence of the resis- statistics as presented here. This has allowed us for the
1 tivity[2,3]andtheopticalconductivity.[9,10,11]Mean- firsttimetoobservethedetailedtemperaturedependence
0 whiletheMn3dspectralfunctionsandtheirtemperature of the electronic structures of manganites. In this Let-
6
dependence obtained from angle-integrated photoemis- ter, we show a unified picture of the driving mechanisms
1
sion are thought to be crucial for the test of these mod- leading to the various aspects of the 3d electronic states
0
4 els, namely, to see whether these models really explain in the manganese perovskites.
0 the Mn 3d spectral functions.
/ HerewereportonthesystemNd1−xSrxMnO3withx=
t
a The photoemission data so far accumulated on man- 0.40,0.47, 0.50. The compounds with x = 0.40 and 0.47
m ganites [12, 13, 14, 15, 16, 17, 18, 19] could not satis- undergo a paramagnetic insulator (PI)-to-ferromagnetic
- factorily check the applicability of the theoretical mod- metal (FM) transition at Tc ∼290 and 275 K, respec-
d els since most of the valence-band photoemission spec- tively, where the FM state is stable down to the lowest
n
tra have been obtained with low-energy light sources. temperaturesbelowT .[22]ThePI-FMtransitionisalso
o c
c The low-energy photoemission (hν <∼ 120 eV) spectra of seen for x = 0.50 (Tc ∼255 K), while this compound
v: three-dimensional transition-metal (TM) oxides mainly is a charge-ordered insulator (COI) below TCOI ∼160
reflect the O 2p electronic states and moreover surface K. [4] The photoemission measurements were performed
i
X electronic structures, which might deviate strongly from atBL25SUinSPring-8.[23]Theoverallenergyresolution
r thebulk TM3dstates,becauseoftherelativelylargera- was set to ∼100 meV. The base pressure was about 4 x
a tiooftheO2p/TM3dphotoionizationcrosssections[20] 10−8 Pa. Forthe temperature-dependentmeasurements,
and a short photoelectron mean free path [21] (λ< 5 ˚A, cleansurfaceswereobtainedbyfracturingthesinglecrys-
bulkcontributionof<30%). TheMn2p−3dresonance tallinesamplesinsituat280-300KinthePIphase. Then
photoemission (hν ∼640 eV with λ ∼13 ˚A, leading to a the spectra were measured at the resonance-maximum
predominantbulkcontributionof∼60%)ispromisingfor (hν = 643 eV) on cooling. A possible Auger contribu-
a more adequate discussion of the Mn 3d bulk electronic tionisclarifiedtobenegligibleathν =643eVwithinthe
states and their temperature dependence since the Mn energyregionfromE to∼3eV.Thesurfacecleanliness
F
3dspectralweightisselectivelyanddrasticallyenhanced was confirmed before and after the measurements. We
atthe Mn2pabsorptionthresholdasshowninFig.1(a). alsocheckedthattheMn3dspectraatlowtemperatures
The Mn 3d enhancement is much weaker and the bulk with fracture at low temperatures were very similar to
2
sults [13, 17, 19] Surprisingly, however, the Mn 3d spec-
(a) (c)
x = 0.40 tral line shapes change gradually with temperature not
hν = 643 eV 20 K x = 0.40 only in the eg region but also in the t2g region. Such
hν = 636 eV spectral changes over a range of several eV within the
x = 0.47
same phase are very unusual. The t peak shifts gradu-
4 3 2 1 0 2g
ally towards E for x = 0.40 and 0.47 by about 0.15 eV
F
(b) PI
FM (170 K) between 300 and 20 K. On the contrary, the t2g as well
FM (20 K) as the e components shift abruptly to higher binding
COI (120 K) x = 0.50 g
t2g AFM (20 K) PI energies concomitant with the opening of a clear energy
FM (170 K) gapofthe orderof100meV (Fig.1(d)) forx =0.50as a
FM (20 K)
COI (120 K) result of the FM-COI transition. As shown in Fig. 1(b),
x = 0.40
units) eg 1.5 1.0 0.5 0.0 othbesegrvraeddufaolrtN2gd0p.4e5aSkr0s.h55ifMtnfoOr3x(x==00.4.505a),nwdh0i.c4h7uinsdneort-
b. (d) PI goes a transitionfrom the PI to antiferromagneticmetal
ar FM (270 K)
sity ( FFFMMM (((211272000 KKK))) (gAraFdMua)latt 2p2e0akKs.h[i2ft4]toTwhaersdesrEesulwtsithinddeiccarteeastinhgattetmhe-
n 2g F
e FM (20 K)
nt x = 0.47 COI (120 K) perature for x = 0.40 and 0.47 is characteristic of the
I
PI-FM transition.
Abroadpeakat0.8-0.9eVisobservedinthePIphase
x = 0.50 as indicated by dashed bars in Fig. 1(c). We emphasize
x = 0.40
thatthisstructuresurvivesevenintheFMphaseathigh
temperatures while it is remarkably suppressed at 20 K
for x = 0.40 and 0.47 accompanied by a change of the
x = 0.47
spectral shape. Namely, another broad peak is seen at
x = 0.55 0.5-0.6eVasindicatedbysolidbarsinFig.1(c). Accord-
hν = 643 eV x = 0.50 ingtodetailedmeasurements[seeFig.1(d)],theintensity
fromE to ∼0.3eV is graduallyenhancedwith decreas-
F
3.0 2.0 1.0 0.0 0.3 0.2 0.1 0.0 -0.1
ing temperature in the FM phase. A clear Fermi cut-off
Binding Energy (eV)
is seen at low temperatures for x = 0.40 and 0.47.
To date, there has been a controversy whether
FIG. 1: Mn 2p − 3d resonance photoemission spectra of
Nd1−xSrxMnO3 (x = 0.40, 0.47, 0.50, 0.55). (a) Mn 2p−3d the spectral function changes with temperature within
resonance-maximum (hν = 643 eV) and off-resonance (hν = the FM phase based on the low-energy photoemission
636 eV) photoemission spectra of Nd0.6Sr0.4MnO3 measured data. [13, 14, 15] Our results clearly show that the Mn
at20KintheFMphase. Thespectralintensityhasbeennor- 3d spectral functions in both the t and the e regions
2g g
malized to the photon flux. (b) High-resolution Mn 2p−3d do change within the FM phase. Furthermore, there are
resonance-maximum spectra representing the Mn 3d contri-
two additional significantfeatures in our spectra. One is
butions in the PI (300 K for x = 0.40 and 0.47, 280 K for
that the spectra from 0.3 eV to E remains essentially
x = 0.50 and 260 K for x = 0.55), FM (20 and 170 K for x F
= 0.40 and 0.47, 170 K for x = 0.50) and COI (120 K for x unchanged across the PI-FM transition [see Fig. 1(d)],
= 0.50) phases. The spectrum for x = 0.55 measured in the in clearcontrastto the remarkablechange ofthe spectra
antiferromagneticmetal(AFM,20K)phaseisalsoaddedfor acrossthe FM-COI transition for x = 0.50. The other is
comparison. (c) Same as (b), but focused on the eg spectral thatthephotoemissionintensityatEF isdistinctlyfinite
weight. (d) Detailed temperature dependence of the Mn 3d even in the PI phase, which has never been seen before
spectral weights near EF for x = 0.40, 0.47 and 0.50 in the inthelow-energyphotoemissiondata.[13,14,15,16,18]
PI, FM and COI (120 K for x = 0.50) phases. The spec-
We emphasize that the observed finite intensity at E
tral weights at different temperatures in (b)-(d) have been F
normalized to the integrated intensity from EF to ∼4 eV for in the PI phase is intrinsic and cannot be reproduced
each compound. by an instrumental resolution broadening of any plau-
sible spectral line shape assumed to become zero in-
tensity at E . [25] The finite intensity at E indicates
F F
these results. that the compounds in the PI phase are neither simple
The temperature dependence of the Mn 2p − 3d bandinsulatorsnorMottinsulatorsasinducedbyanon-
on-resonance spectra (Mn 3d spectra hereafter) of site Coulomb interaction between the Mn 3d electrons.
Nd1−xSrxMnO3 is summarized in Figs. 1(b)-(d). The It is thus found that the 3d electron correlation effects
t -derivedelectronicstatesappearasastrongpeaknear alone,whichthemselves arestrongas previouslypointed
2g
2.0 eV. The spectral weight from E to ∼1.5 eV orig- out,[12,14,17]cannotaccountfor the PI-FMtransition
F
inates dominantly from the e states. These qualita- in the manganites.
g
tive spectral features are in agreement with previous re- We now turn to a discussion of the existing theoret-
3
ical models against this anomalous temperature depen-
dence of the Mn 3d spectral functions. The key issues
of our data are summarized as follows: (a) finite inten-
sity at E even in the PI phase, (b) t peak shift with
F 2g
temperature in the FM phase, (c) presence of a peak
at 0.8-0.9 eV in the PI phase, and (d) gradual intensity
reduction of this peak and the appearance of another s) 300 K
peak at 0.5-0.6 eV with decreasing temperature in the nit
u
FM phase. Although the calculations based on the DE b.
r
mechanism [26, 27] are not contradictory to (a), they (a 270 K
cannot predict the e -derived broad peak away from E y
g F sit
at all, namely, fail to explain (c) and (d). The dynam- n 220 K
e
ical JT model [7, 8] and a model including the orbital nt
I
degreesoffreedom[28,29]seemto fairlyexplain(a)and
170 K
(c) since they show the broad-peak structure away from
E . However,suchapeakishardlyshiftedwithtemper-
F 120 K
ature according to these models, which is contradictory hν = 643 eV
to (d). In addition, these models are not likely to ex- Nd0.6Sr0.4MnO3 20 K
plain (b) because the center of gravity in the calculated
e spectral weights is hardly shifted with temperature. 3.0 2.0 1.0 0.0
g
Binding Energy (eV)
To conclude, all theoreticalapproachesmentioned above
are still insufficient even for a qualitative explanation of
thetemperaturedependenceoftheMn3dspectra. Afur- FIG. 2: Detailed temperature dependenceof theMn 2p−3d
ther elaborated theoretical model is required to explain resonancephotoemissionspectraofNd0.6Sr0.4MnO3. Thered
solid lines show the spectra in the FM phase while the black
our experimental findings.
dashed line is in the PI phase at 300 K. Proper linear com-
In order to understand the temperature-dependent
binationsofthetwoexperimentalspectraat20 Kand300K
spectralfunctionsasmuchaspossiblefromexperimental are shown by the black thin solid lines at intermediate tem-
viewpoint, we have tried to reproduce the spectra for x peratures,whichcanwellreproducetheexperimentalresults.
= 0.40 and 0.47 measured at 120, 170, 220, and 270 K In the linear combinations, the relative contributions of the
in the FM phase by a linear combination of the spectra 20 K spectrum are 68, 54, 43 and 7 % at 120, 170, 220 and
270 K, respectively, with an accuracy of ±7 %. We have
at 20 K in the FM phase and those at 300 K in the PI
also successfully reproduced the temperature-dependent Mn
phase. AsshowninFig.2forx=0.40,thelinearcombi-
3dspectraofNd0.53Sr0.47MnO3 intheFMphase(notshown)
nation of the spectra well reproduces the Mn 3d spectra
by the linear combination of the spectra at 20 K and 300 K,
in both the eg and the t2g regions at intermediate tem- where the contributions of the spectrum at 20 K are 50, 29
peratures. Thisanalysisindicatesthatthespectrainthe and 4 % at 170, 220 and 270 K,respectively.
FM phase still include the spectral component of the PI
phase, whose weight is gradually reduced with decreas-
ing temperature. Such a scenario that a dynamic and x = 0.50 and 0.55 than for x = 0.40 and 0.47 at low
microscopic phase segregation grows in the FM region temperatures. [4, 22, 24] It is thought that the e elec-
g
with increasing temperature, as has been proposed from tronpartiallyoccupiesthelowerJTsplitbandandhence
inelastic neutron scattering on La0.7Ca0.3MnO3, [30] is providesmore spectralweight at higher binding energies
the most likely story to explain our data. in the e region. Accordingly the observed temperature
g
In the following we show a unified picture, which ex- dependence andthe dynamic andmicroscopicphaseseg-
plains the various electronic phases in the manganites. regation scenario are well understood as follows: The
We focus now on the e spectral weight and its temper- static JT distortion (i.e., the degree of freedom in the e
g g
ature and doping dependence. The spectral shape in the orbitals) mainly controls the 3d electronic states in the
PI phase is essentially similar for different x as shown in ground states at low temperatures. In particular, the
Fig. 1(b) and 1(c). In the ground states at the lowest completely developed FM phase without phase segrega-
temperature, on the other hand, the peak in the e re- tioncanberealizedatlowtemperaturesforthecomposi-
g
gion becomes more prominent for x = 0.50 (COI) and tions x = 0.40and 0.47in whichthe static JT distortion
0.55 (AFM) at higher binding energies compared with is weak, while the FM ordering is suppressed for x =
the PI phase, while it becomes broader for x = 0.40 0.50 and 0.55 where the JT distortion is strong. In the
(FM) and 0.47 (FM) in the sense that the e states ex- PI phasenear roomtemperature, onthe other hand, the
g
tend above E (remember that the clear Fermi cut-off dynamical JT effect governs the 3d electronic states ir-
F
is seenat lowtemperatures). This tendency corresponds respective of both x and the strength of the static JT
well with the static JT distortion which is stronger for distortionatlowtemperatures. The dynamicalJT effect
4
weakensgraduallywithdecreasingtemperaturebelowTc. [1] R. von Helmolt et al.,Phys. Rev.Lett. 71, 2331 (1993).
ThentheamountoftheFMphasegrowscorrespondingly [2] Y. Tokuraet al., J. Phys. Soc. Jpn. 63, 3931 (1994).
for0.40≤x≤0.50. Inthisway,thedynamicalandstatic [3] A. Urushibaraet al., Phys.Rev.B 51, 14103 (1995).
[4] H. Kuwahara et al.,Science 270, 961 (1995).
JT effects hinder the FM ordering in the manganites at
[5] C. Zener, Phys. Rev. 82, 403 (1951).
high and low temperatures, respectively.
[6] P. W. Anderson, H. Hasegawa, Phys. Rev. 100, 675
FinallywepointoutacloserelationbetweentheCMR
(1955).
and the dynamic phase segregation. The CMR effect [7] A. J. Millis, B. I. Shraiman, R. Mueller, Phys. Rev. Lett.
is not notable near the phase boundary between the PI 77, 175 (1996).
and FM states for Nd1−xSrxMnO3 (x = 0.40-0.50)com- [8] A.J.Millis, R.Mueller, B.I.Shraiman,Phys.Rev.B54,
pared with that between the FM and COI states for 5405 (1996).
Nd Sr MnO . From the above discussion, we con- [9] S. G. Kaplan et al.,Phys. Rev.Lett. 77, 2081 (1999).
0.5 0.5 3 [10] Y. Okimoto et al.,Phys. Rev.B 55, 4206 (1997).
clude that the dynamic phase segregation originating
[11] M. W. Kim et al., Phys.Rev. Lett.89, 016403 (2002).
from the gradually changed dynamical JT effect with
[12] T. Saitoh et al.,Phys. Rev.B 51, 13942 (1995).
temperature plays an important role in reducing the [13] J. -H.Park et al.,Phys. Rev.Lett. 76, 4215 (1996).
CMR at the PI-FM transition. Namely, in such a sit- [14] D. D. Sarma et al.,Phys. Rev.B 53, 6873 (1996).
uation as the dynamical JT effect is strong in the PI [15] A. Chainani et al.,Phys.Rev. B 56, R15513 (1997).
phase and abruptly suppressed at the PI-FM transition, [16] J. -H.Park et al.,Nature392, 794 (1998).
CMR would be remarkable. [17] A. Sekiyamaet al.,Phys.Rev. B 59, 15528 (1999).
[18] Y. -D.Chuang et al.,Science 292, 1509 (2001).
In conclusion, we have clearly observed the tempera-
[19] J. -S.Kang et al.,Phys. Rev.B 68, 012410 (2003).
ture dependence of the high-resolution Mn 2p−3d res-
[20] J. J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32, 1
onance photoemission spectra of Nd1−xSrxMnO3 in the (1985).
PI and FM phases as well as evidence for the micro- [21] S.Tanuma,C.J.Powell,D.R.Penn,Surf.Sci.192,L849
scopicanddynamicphasesegregation. Itisclarifiedthat (1987).
Nd1−xSrxMnO3 in the PI phase are found to be neither [22] R. Kajimoto et al.,Phys.Rev.B 60, 9506 (1999).
a band insulator nor a Mott insulator. Although our re- [23] Y. Saitoh et al.,Rev.Sci. Instrum.71, 3254 (2000).
[24] H. Kuwahara et al., Phys.Rev.Lett. 82, 4316 (1999).
sults cannot be well explained by any available model,
[25] As for the PI phase for x = 0.40-0.55, the spectra in a
we propose a unified picture to interpret the experimen-
binding energy region from 0.05 to ∼0.5 eV can be fairly
talresults. The3delectronicstatesaremainlycontrolled
reproduced by a power-law of the binding energy (EB),
by the static JT effect at low temperatures whereas the i.e.,|EB|α whereα∼0.7-1andthespectralweightisorig-
dynamical JT effect governs the electronic states near inally zero at EF. However, the spectral weight at and
room temperature. above EF cannot bequantitativelyexplained bybroaden-
We thank K. Noda, K. Konoike, T. Satonaka, S. Ka- ing the power-law spectral weight with the experimental
resolution (∼100meV).From thesefacts, it issurelycon-
sai, A. Yamasaki, A. Irizawa, M. Tsunekawa, and the
staff of SPring-8, especially T. Muro, Y. Saitoh, and cluded that the observed finite spectral weight at EF is
intrinsic in thePI phase.
T. Matsushita for supporting the experiments. This [26] A. Kubo,J. Phys.Soc. Jpn.33, 929 (1972).
workwassupportedbyaGrant-in-AidforCOEResearch [27] N. Furukawa,J. Phys.Soc. Jpn. 64, 3164 (1995).
fromthe MinistryofEducation,Culture,Sports,Science [28] S. Yunoki,A.Moreo, Phys.Rev.B 58, 6403 (1998).
and Technology (MEXT), Japan. AS acknowledges the [29] S. Yunoki, A. Moreo, E. Dagatto, Phys. Rev. Lett. 81,
support form the Kurata foundation. The photoemis- 5612 (1998).
[30] J. Zhanget al.,Phys. Rev.Lett.86, 3823 (2001).
sion measurements were performed under the approval
of the Japan Synchrotron Radiation Research Institute
(2001A0129-NS-np,2003A0593-NS1-np).
∗
[email protected]
∗ [email protected]