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Preprint Renner et al. January 2002, Nature 416, 518-521 (2002)
1
Atomic-scale images of charge ordering in a mixed-
valence manganite.
Ch. Renner*, G. Aeppli*, B-G. Kim†, Yeong-Ah Soh* and S.-W. Cheong†
*NEC Research Institute, 4, Independence Way, Princeton, NJ 08540, USA
†Rutgers University, Department of Physics and Astronomy, Piscataway, NJ 08854, USA
Transition-metal perovskite oxides exhibit a wide range of extraordinary but
imperfectly understood phenomena. Charge, spin, orbital, and lattice degrees of
freedom all undergo order-disorder transitions in regimes not far from where the
best-known of these phenomena, namely high-temperature superconductivity of the
copper oxides1, and the ‘colossal’ magnetoresistance of the manganese oxides2,3, occur.
Mostly diffraction techniques, sensitive either to the spin or the ionic core, have been
used to measure the order. Unfortunately, because they are only weakly sensitive to
valence electrons and yield superposition of signals from distinct mesoscopic phases,
they cannot directly image mesoscopic phase coexistence and charge ordering, two key
features of the manganites. Here we describe the first experiment to image charge
ordering and phase separation in real space with atomic-scale resolution in a
transition metal oxide. Our scanning tunneling microscopy (STM) data show that
charge order is correlated with structural order, as well as with whether the material
is locally metallic or insulating, thus giving an atomic-scale basis for descriptions4 of
the manganites as mixtures of electronically and structurally distinct phases.
The material chosen for our experiments is Bi1-XCaXMnO3 (BCMO). For trivalent
Bi and divalent Ca, the Mn ions are in a mixed valence state Mn3+x. At high temperature,
Mn3+ and Mn4+ randomly occupy the manganese sites. Upon reducing the temperature,
these cations are believed to order, yielding an increased lattice periodicity visible to X-ray
and neutron diffraction5. For our nominally x=0.76 samples, grown from a BiO flux, this
occurs at TCO=250K, as established using SQUID magnetometry. We performed the STM
experiments in ultrahigh vacuum at a base pressure of 5·10-10 Torr. Previously published
STM investigations of manganites primarily focused on spectroscopy of the density of
states averaged over many atoms6,7, and demonstrated phase separation into metallic and
insulating regions on submicron8, but not atomic length scales. In contrast, STM with
atomic resolution has already been achieved for cuprates and revealed inhomogeneities in
the superconducting order on atomic length scales9,10.
BCMO single crystals do not cleave naturally, and preparing flat and atomically
clean surfaces suitable for STM is a major challenge. It turned out that cleaning the as-
grown samples in ethanol with a cotton stick shortly before loading into the vacuum
chamber, allowed us to obtain reproducible topographic images with atomic resolution. The
STM tips were made of etched tungsten wires. Typical tunneling parameters during
Preprint Renner et al. January 2002, Nature 416, 518-521 (2002)
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imaging were 0.2nA and 0.7V for the set current and sample bias respectively. The
piezoelectric scan actuators were calibrated to an accuracy of 4% by imaging layered
graphite crystals at different temperatures.
As one might expect based on the rather crude surface preparation, STM failed to
yield atomic resolution over large portions of the surface. We mainly saw a broad range of
different nanometer scale textures. However, we repeatedly found clean terraces a few
100nm2 wide where atomic resolution was routinely achieved. The room temperature image
in Fig. 1A clearly shows the square lattice expected for the (010) face of BCMO (simple
cubic notation). The corresponding lattice constant a0=3.8±0.6Å is in good agreement with
the value of 3.77Å determined by X-ray diffraction11,12. At room temperature, we
sometimes also observe a ‘√2a0x√2a0’ lattice, with the unit cell doubled along [101],
coexisting with the ordinary square lattice (Fig. 2A). The cut, shown in Fig. 2B, along one
crystallographic axis in Fig. 2A suggests a homogeneous charge distribution among the
atomic sites in the cubic (disordered) region, whereas charges appear redistributed among
alternating atomic sites in the ordered region, generating the doubled unit cell. The most
natural interpretation is that there is local charge ordering, namely the alternation of Mn3+
and Mn4+ ions, as posited in the simplest descriptions of mixed valence manganites.
Conductance spectra acquired in all areas where the doubled unit cell is stable
systematically show more insulating characteristics than those measured in cubic regions
(Fig. 2C). This validates the association of charge ordering with a metal-insulator transition
– the carriers are localized and the material is insulating when charges are distributed in a
set spatial pattern rather than fluctuating with time at each site. The metallic state is not an
especially good metal, with a semi-metallic spectrum superposed on a small ohmic term
(inset to Fig. 2C). The energy at which the deviations from ohmic behavior become
apparent is around 0.1 eV, which is also where a strong non-Drude contribution begins to
dominate the optical conductivity for similar samples at room temperature13. The gap for
the insulating spectrum collected for the regions at room temperature where the doubled
unit cell occurs, is ~0.7 eV. This is very similar to the gap one can deduce from the optical
conductivity measured at 150K, when the bulk of the crystal is charge-ordered13. At a
qualitative level, the spectra of Fig. 2C look strikingly similar to those measured by Fäth et
al.8 on the isostructural compound (La,Ca)MnO3. While their lower hole doped samples
lack a charge-ordering phase transition, their finding of spectroscopic inhomogeneities near
the metal-insulator transition is very consistent with our results. Fig. 2 is remarkable
because it shows the first data to connect the different tunneling spectra to distinct phases at
the atomic scale.
When the sample is cooled below TCO, the √2a0x√2a0 lattice becomes the dominant
structure (Fig. 3). The current-voltage spectra of these regions always show insulating
characteristics strikingly similar to those measured in the ordered regions above TCO, as
illustrated in Fig 4. Fig. 3A displays a large region of uninterrupted √2a0x√2a0 order of
precisely the type seen at the upper right of Fig. 2A. There is a clear modulation of the
height as well as the positions of the atoms away from the vertices of a square lattice. A
more thorough analysis of Fig. 3A reveals a finer structure, characterized by alternating
Preprint Renner et al. January 2002, Nature 416, 518-521 (2002)
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long and short interatomic distances (ID) along the main crystallographic axes (Fig. 3B).
The short ID form regular zigzag chains across the entire scanned area (yellow lines in
Figs. 3A and 3D), thus breaking the crystal symmetry. The distortion amplitude, which is
the difference between long and short ID, can be inferred directly from histograms of all ID
measured along the cubic unit cell vectors (Fig. 3C). This distortion, generally in excess of
0.5Å, is too large to ascribe to the Mn atoms alone. On the other hand, the bright spots in
STM images such as Fig. 3A, match the apical oxygens sitting over the manganese sites on
the (010) surface according to X-ray experiments11 (Fig. 3D). The X-ray data11 show that
the MnO6 octahedra are tilted by an angle θ of about 10°, hence shifting the apical oxygen
by a distance d//=0.34Å away from the vertices of the square lattice, where d//=aMOsin(θ),
and aMO=1.94Å is the Mn-O bond length. The tilting is not uniform, rather the octahedra
form a zigzag pattern where neighbouring oxygens are either separated or brought closer by
roughly 2d//. For a lattice constant of 3.77Å, these shifts should yield a lattice with
alternating interatomic distances of 3.1Å and 4.4Å, in excellent agreement with our STM
images (Fig.3C). Interestingly, the room temperature images bear a similar distribution of
short and long ID. However, in contrast to the low temperature images, the short ID are
more randomly distributed (Fig. 1B). Static disorder in the oxygen tilts14,15 therefore
appears to be annealed on cooling through TCO, yielding a regular zigzag pattern. Although
the precise temperature where this happens at the surface has not been identified in the
present experiments, these results illustrate on a local scale the idea that ordered lattice
distortions, especially tilts of the MnO6 octahedra, are important for stabilizing the charge-
ordered state in manganites16.
Fig. 3A contains not only information about the geometry of the crystal face
exposed, but also about the ‘heights’ of its features. It is natural to ask whether the apparent
‘height’ modulation is derived simply from a modulation of atomic coordinates (possible
surface reconstruction), or whether it indicates a modulation of the electronic
wavefunctions centered at the atom cores. The last paragraph’s analysis of the in-plane
displacements shows that the STM contrast consistently reflects the tilting of the MnO6
octahedra. The apex oxygens illustrated as the larger blue spheres in Fig. 3D, move a
certain distance perpendicular to the (010) plane when the octahedra distort and tilt due to
the Jahn-Teller (JT) effect induced by a central Mn3+. Since all octahedra are tilted by
approximately the same angle (θ≈10°), the apex oxygens are all displaced by roughly the
same distance d⊥=aMO(1-cos(θ)). Thus, the tilting of the octahedra cannot account for the
observed corrugation. On the other hand, the JT distortion of the order 0.05-0.1Å11,17
happens exclusively for octahedra centered at Mn3+. Taking this together with the fact that
the octahedra are tilted, the JT distortion will then account for at most 0.1Å of the height
modulation at the apical oxygen sites. What we see instead – as clearly demonstrated by the
histogram in Fig. 3E – are apparent height differences between neighbouring atomic sites
which are frequently larger than 0.1 Å. Thus, the measured corrugation needs an additional
contribution, which is most likely due to different wavefunctions responsible for the
tunneling between the two neighbouring octahedra and the STM tip. The most natural
explanation is that the Mn ions at the centers of these octahedra have different charges and
Preprint Renner et al. January 2002, Nature 416, 518-521 (2002)
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therefore different orbital occupancy, which affects the tunneling through the apical
oxygens that cap the planes exposed to our STM experiments.
The charge-ordered STM images discussed so far consistently show a Mn3+ to Mn4+
ratio of 1:1, in apparent contradiction with diffraction data5,12 (which show a periodicity of
~4√2a0 rather than the √2a0 we see) for crystals prepared following the same procedures
and with the same nominal doping. There are many possible explanations for this
discrepancy. The first is to appeal to the surface sensitivity of STM, and assert that the
surfaces behave differently from the bulk. Indeed, such an argument has been made for
Sr2RuO4, the only other transition metal oxide without copper for which atomic resolution
STM data have been reported18. The agreement of our atomic coordinates with those
deduced from bulk X-ray crystallography (Fig. 3D) speaks against a surface reconstruction
on the scale of that seen for Sr2RuO4. Furthermore, the agreement of the tunneling gap
shown in Fig. 4 with the optical gap13 (another bulk probe) also speaks against a surface
effect. This leaves the possibility that even though the bulk charge ordering transition for
our crystals is as sharp as any previously reported, the effective doping may not be x=0.76
at all surfaces. Besides the surface, which may affect the actual local doping, there is the
possibility of phase separation into (commensurate) charge-ordered phases upon cooling
through TCO19. It is worth noting that what distinguishes our experiments from previous
STM work on manganites is that we have atomic resolution for a small, rather than zero
fraction of the crystal surface exposed. This leaves the possibility that the remaining
fraction would exhibit structures consistent with bulk diffraction data for the nominal
composition of the sample. The fact that the measured optical gap13 corresponds to a
smooth rise in conductivity rather than a hard onset is consistent with coexistence, even in
the bulk, of distinct ordered phases with different periodicities and different gaps.
We have presented the first atomic-scale images of any manganese oxide using
scanning tunneling microscopy. The images display many of the phenomena that have been
posited for the manganites, most notably charge ordering, of which STM is the most direct
probe because of its sensitivity to the outer valence electrons – diffraction and transmission
electron microscopy, which have been the tools for the initial exploration of the charge
ordering phenomena, are all primarily sensitive to atomic core positions rather than the
outer electrons. We have been able to associate the metallic and insulating current-voltage
characteristics with distinct atomic-scale structures.
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Acknowledgements. We are grateful to N. Wingreen and J. Chadi for comments on the manuscript, and
acknowledge helpful discussions with D. Khomskii, A . Millis, C. de Morais Smith, and A. Yazdani. BGK
and SWC are supported by the National Science Foundation.
Correspondence should be addressed to C.R. (e-mail: renner@research.nj.nec.com).
Preprint Renner et al. January 2002, Nature 416, 518-521 (2002)
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Figure 1 STM mapping of the paramagnetic phase of
Bi0.24Ca0.76MnO3 at 299 Kelvin. A 3.4x3.1 nm2 image
with well resolved square lattice (a0=3.8±0.6Å). B The
intensity profile reveals a random distribution of short
and long interatomic distances (indicated by arrows)
along the main crystallographic axes.
Figure 2 Topographic and spectroscopic atomic
scale signatures of phase separation into metallic
and insulating regions in the paramagnetic phase of
Bi0.24Ca0.76MnO3 at 299 K. A 3.5x3.5 nm2 STM image
of a grain boundary (yellow line) between an
insulating √2a0x√2a0 charge-ordered region (upper
right) and a more metallic homogeneous cubic region
(lower left). B Intensity profile extracted along the
orange line in A. Note the larger amplitude
modulation in the ordered region due to charge
ordering. C Charge-ordered regions with the
√2a0x√2a0 lattice (purple) yield insulating dI/dV(V)
characteristics, while the disordered cubic regions
(green) are characterized by more metallic dI/dV(V)
characteristics (numerical derivatives normalized to
the metallic junction resistance R=V/I at 0.7V). The
low bias part of the corresponding I(V) data are
shown in the inset. The spectra were taken at the
yellow crosses on the 3.7x2.9 nm2 STM images
(white squares = cubic unit cell).
Preprint Renner et al. January 2002, Nature 416, 518-521 (2002)
8
Figure 3 Atomic scale STM
mapping of the charge ordered
phase of Bi0.24Ca0.76MnO3 at 146
Kelvin. A 4.8x3.6 nm2 image of
the √2a0x√2a0 lattice (white
square = cubic unit cell). B
Intensity profiles extracted along
the main crystallographic
directions in A. They show the
distortion of the atomic positions
away from the cubic vertices with
alternating short and long
interatomic distances (arrows). C
Bimodal distribution into short
(3.0±0.2Å) and long (4.5±0.3Å)
interatomic distances along [100]
and [001]. D X-ray refined crystal
structure of the (010) plane of the
isostructural compound La1-
xCaxMnO3 (x=½) adapted from
Ref.11 superposed on a
magnified area of A (1.7x1.7 nm2).
For purposes of illustration, the
apical oxygens are sketched larger than the in-plane oxygen. E The amplitude
difference between neighbouring atomic sites peaks above 10pm, too large to be
solely due to structural distortions determined by X-ray diffraction.
Figure 4 Identical lattice and electronic structures of
the insulating √2a0x√2a0 regions observed in the
paramagnetic room temperature phase and the
charge-ordered low temperature phase. The gap in
the differential tunneling conductance (numerical
derivatives normalized to the 299 Kelvin junction
resistance R=V/I at 0.8V) is essentially the same
(~0.7eV) at 299 Kelvin (purple) and at 146 Kelvin
(blue) . The STM constant current images (4.5x3.5
nm2) reveal the same lattice structures in regions
where the insulating spectra were measured (yellow
crosses). The white squares correspond to the cubic
unit cell. For the 8 (out of 34) W tips for which atomic
resolution was obtained, there was 100% correspondence between the different
structural phases and the conducting or insulating nature of the spectra.
