PHYSICAL REVIEW B 91, 125110 (2015)

STM observation of charge stripe in metallic phase of α-(BEDT-TTF)2 I3
K. Katono,1 T. Taniguchi,1 K. Ichimura,1,2 Y. Kawashima,1 S. Tanda,1,2 and K. Yamamoto3
1

2

Department of Applied Physics, Hokkaido University, Sapporo 060-8628, Japan
Center of Education and Research for Topological Science and Technology, Hokkaido University, Sapporo 060-8628, Japan3
Department of Applied Physics, Okayama University of Science, Okayama 700-0005, Japan(Received 29 July 2014; revised manuscript received 24 December 2014; published 4 March 2015)
We performed scanning tunneling microscopy at room temperature on the conducting surface of α-(BETDTTF)2 I3 , which undergoes charge ordering below TCO = 135 K. An α-type BEDT-TTF molecular arrangement
was clearly observed. We observed a charge stripe structure with some inhomogeneity at room temperature.
The horizontal charge stripe developed in most of the observed area. However, the coherence was restrictedby the presence of charge uniform regions. The dimerization was found only between A and A . The charge
disproportionation state developed on the donor arrangement, which differs from that in the charge ordered state
below TCO . A certain amount of stripe structure inhomogeneity is expected to be caused by the absence of the
dimer of B and C BEDT-TTF molecules. We also found a local diagonal stripe that formed the boundary between
two types of horizontal stripes.
DOI: 10.1103/PhysRevB.91.125110

PACS number(s): 71.30.+h, 61.66.Hq, 64.70.kt, 68.37.Ef

I. INTRODUCTION

Charge-transfer salts based on organic donor molecules
such as BEDT-TTF and TMTSF have been regarded asa strongly correlated electron system. Organic conductors
exhibit various electronic phases [1]. Some of these, including spin-density wave, spin Peierls, and antiferromagnetic
phases, result from a strong on-site Coulomb interaction. In
κ-(BEDT-TTF)2 X, the superconductivity competes with theMott insulating phase. These phases have been well describedin terms of the on-site Coulomb interaction.
On the other hand, a long-range Coulomb interaction is also
important in organic conductors. In fact, the charge ordered(CO) state has been observed [2–4]. The CO state, which issimilar to a Wigner crystal, has been discussed theoreticallyin the extended Hubbard model with the off-site Coulombinteraction [5]. The charge distribution of a CO state is affectedby both the long-range Coulomb interaction and the donor
arrangement. In α- and θ -type donor arrangements, three typesof charge stripe order are plausible. Figure 1 shows schemaof the charge stripe patterns. The circles represent donor
molecules and their diameters indicate the amount of charge on
the donor site. The charge stripe structures are classified by the
direction in which the charge aligns, based on donor stacking orcolumn direction. The horizontal stripe has a charge structurein which the charge aligns in the direction perpendicular
to the donor stacking. The horizontal stripe structures havebeen observed with nuclear magnetic resonance (NMR) [6],x-ray diffraction [7], and infrared spectroscopy [8]. However,
the results of the observations were macroscopic although
they were site-selective measurements. There have been few
position sensitive measurements of the CO states in organic
conductors [9].
The intermolecular Coulomb interaction also results incharge disproportionation above TCO , and this has beenreported in θ -(BEDT-TTF)2 RbZn(SCN)4 and α-(BEDTTTF)2 I3 [6,7,10,11]. In θ -(BEDT-TTF)2 RbZn(SCN)4 , thediffuse spots of q = (1/3 k 1/4) corresponding to charge
disproportionation were observed above TCO . The charge
disproportionation is replaced by a horizontal stripe structure1098-0121/2015/91(12)/125110(5)with which structural change occurs with long coherent length
when the cooling rate is less than 0.2 K/min. When coolingrate exceeds 5 K/min, the charge disproportionation still
remains below TCO . In α-(BEDT-TTF)2 I3 , the reported charge
disproportionation among A, B, and C BEDT-TTF molecules
is far above TCO [7]. The ratio of the charge disproportionation
increases with decreasing temperature on the column with
B and C BEDT-TTF molecules.
The structural change is associated with the transition fromthe charge disproportionation state to the CO state. The resistivity [12], charge distribution [7], and permittivity [11] change
greatly at TCO , although the long-range Coulomb interaction
is independent of temperature. Carriers are localized, andthe system becomes an insulator with ferroelectricity below
TCO . Therefore, we must clarify the CO state and the charge
disproportionation to understand the role of the intermolecular
Coulomb interaction and the structural change at the COtransition. The charge disproportionation state without any
structural change is very suitable for studying the role ofthe intermolecular Coulomb interaction. However, the charge
distribution of the charge disproportionation, which reflects theeffect of the lattice and intermolecular Coulomb interaction,
has yet to be clarified in contrast to the CO state. We performed
the STM observation on the charge disproportionation in real
space on an atomic scale.
Scanning tunneling microscopy (STM) is a powerful toolwith which to study the local electronic state, since it probes the
local electronic density of states on a nanometer scale without
damaging the sample. In fact, it has been used to study the
charge modulation of charge density waves (CDWs) in lowdimensional conductors [13] and the checkerboard pattern incuprate superconductors [14]. In organic conductors, the CDWof TTF-TCNQ has been clearly observed with STM [15]. As
the tunneling current gives the local electron density, STM is
suitable for studying the CO state in organic conductors. In thisstudy, we focus on α-(BEDT-TTF)2 I3 since the height variationof BEDT-TTF molecules is negligibly small on the α type
BEDT-TTF layer in contrast to θ -(BEDT-TTF)2 RbZn(SCN)4 .
α-(BEDT-TTF)2 I3 undergoes a CO transition at TCO =135 K [16]. Figure 2(a) shows the layered crystal structure

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PHYSICAL REVIEW B 91, 125110 (2015)

FIG. 1. Schema of the three types of charge order: (a) horizontal
stripe, (b) vertical stripe, and (c) diagonal stripe. The circles represent
the BEDT-TTF molecules and their diameters express the charge
amount on the BEDT-TTF molecules.

of α-(BEDT-TTF)2 I3 . Metallic BEDT-TTF and insulating I−
3layers are stacked alternately along the c axis. Figure 2(b)shows the BEDT-TTF donor layer in the a-b plane. The
rectangle in Fig. 2(b) represents the unit cell of the α-type
BEDT-TTF molecule arrangement. There are two types ofdonor stacking columns, which are denoted by I and II,
along the a axis. The unit cell contains four BEDT-TTF
molecules: A,A ,B, and C. Column I consists of A and A ,and column II consists of B and C [12]. It should be notedthat A and A are crystallographically equivalent, and B and
C are independent. The structure of CO has been explained asa horizontal stripe with 13 C-NMR [6,17], x-ray diffraction [7],and Raman spectroscopy [8]. The first-principle calculation
was performed to study the driving force of the CO state [18].
However, the structure of the charge disproportionation has
not yet been clarified. X-ray diffraction [7] and Raman
spectroscopy [8] suggested that the charge distribution incolumn I is uniform. On the other hand, 13 C-NMR suggested
that there is also charge disproportionation on column I [17].
In this paper, we report the charge disproportionation in
α-(BEDT-TTF)2 I3 at room temperature that we observed using
STM.
II. EXPERIMENTAL

Single crystals of α-(BEDT-TTF)2 I3 were grown with anelectrochemical method. Typical dimensions of the single
crystals were 1 × 1 × 0.5 mm3 . The a-b surface was studied

FIG. 2. (Color online) (a) Crystal structure of α-(BEDT-TTF)2 I3viewed from the b axis. (b) BEDT-TTF arrangement of this salt.
Column I consists of BEDT-TTF labeled A and A . These are
crystallographically equivalent. Column II consists of BEDT-TTF
labeled B and C.

FIG. 3. (Color online) Current image of an a-b plane at bias voltage V = 50 mV and tunneling current I = 50 pA. A white rectangle
indicates the unit cell of the BEDT-TTF molecule arrangement in theimage.

using STM at room temperature. Tunneling current images
were mainly obtained at shiny surfaces. The STM tips weremade by cutting Pt-Ir wire, and the resolution was confirmed
by observing atomic images of graphite.
III. RESULTS AND DISCUSSION

Figure 3 shows an STM current image of an a-b plane withbias voltage V = 50 mV and tunneling current I = 50 pA. The
scanning area is 9.4 × 6.2 nm2 . The face-centered rectangularpattern was clearly observed. The pattern corresponds toan α-type donor arrangement. The periodicity of the spots
is consistent with the lattice parameters, a = 9.187 Å, b =10.793 Å, of α-(BEDT-TTF)2 I3 . The a and b directions are
assigned as indicated in Fig. 3. From the shape of the spots, theslant alternation was along the b axis. Then we can distinguishcolumns I and II.
We observed that the intensity varied depending on thedonor site. The tunneling current depends on both the electronic density of states and the distance from the sample tothe tip. In α-(BEDT-TTF)2 I3 , BEDT-TTF donors are locatedat the same height on the a-b surface unlike the case ofθ -(BEDT-TTF)2 RbZn(SCN)4 [9]. Therefore, the difference inthe tunneling current originates from that in the electronic
density of states. X-ray-diffraction [7] and 13 C-NMR [6,17]
measurements showed that the charge on the B molecule isricher than that on the C molecule above TCO . Taking theseresults into account, we identified brighter (darker) spots as B
(C) along the columns indicated by arrows, and the column
indicated by arrows is column II and the other is column I. The
spots on the diagonally higher (lower) side of the B molecule
are temporarily assigned as A (A) for the discussion.
To discuss the charge disproportionation in detail, we tookline profiles along columns I and II. The profiles were averaged
over the width of a column (4.6 Å) to evaluate the charge
distribution in each column. Figure 4 shows line profiles.
Red and blue lines represent columns I and II, respectively.
The twofold periodicity was clearly observed in the line

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PHYSICAL REVIEW B 91, 125110 (2015)

FIG. 4. (Color online) (a) STM image with the same bias voltage and tunneling current. (b) Line profiles along the columns. The arrow on
the left in (a) represents the direction of the horizontal axis of (b). Arrows in (b) indicate charge rich molecules. Gray rectangles in (a) and (b)indicate the charge uniform part.

profiles of column II: a, c, and e. Twofold periodicity withcharge rich A molecules was also observed in the profilesof column I. The charge disproportionation along columns
I and II is consistent with previous work [19]. The twofold
periodicities in both columns I and II form a horizontal typestripe structure, which is similar to the structure in the CO state,at room temperature. Moreover, in this study, we found thatthe stripe structure includes some inhomogeneity by observing
the charge distribution on the BEDT-TTF layer in detail. The
amplitude of the modulation in column I is weaker than that incolumn II. The charge amount ratio of the charge rich and poor
molecules depends on the position in contrast to the twofold
periodicity of column II. A uniform charge distribution domain
was observed in the region indicated by the rectangle.
It is possible that the observed stripe structure originatesfrom surface reconstruction. Although there might be thesurface effect such as antisymmetry of the anion potential
on BEDT-TTF, the surface reconstruction can be excluded.
In organic conductors, both intermolecular and intramolecularsurface reconstruction are expected to occur, since the donor
and anion molecules also consist of many atoms. Ethylenegroups of a BEDT-TTF molecule in bulk are distorted due tothe anion layer. On the other hand, the ethylene groups closest
to the surface are not distorted, since they are not affectedby anion molecules. Such surface reconstructions have beenreported by an STM study of an organic conductor β-(BEDTTTF)2 PF6 [20]. The intermolecular case was observed as anarrangement of alternating bright and less bright columns.
The surface reconstruction can be explained as resulting froma mechanism similar to that of Si(100) and GaAs(100). For thehigher columns, a large spot was observed that corresponds toa BEDT-TTF molecule with a relaxed ethylene group closest
to the surface. For the lower columns two spots with different
sizes and brightness levels were observed as a nonrelaxed
BEDT-TTF molecule. One is a large bright spot, and the otheris a small and less bright spot. In our STM results, only relaxed

BEDT-TTF molecules were observed. If the horizontal stripe
is the pattern of the height variation due to intermolecularsurface reconstruction, the spots of the nonrelaxed BEDT-TTF
molecules must be also observed at the less bright peakof twofold modulation. Therefore, the horizontal stripe was
expected to originate from charge.
Next, we note the dips along column I. The shallow dips
along column I are indicated by green circles in Fig. 4(b). The
shallow and deep dips are aligned alternately along column I.
This indicates the dimerization between A and A . On theother hand, such an alternation of dips was not observed alongcolumn II. We emphasize that the dimerization observed with
STM reflects the electronic state. The electron density between
BEDT-TTF molecules corresponds to the transfer integral. In
an α-type donor arrangement above TCO , while molecules A
and A are weakly dimerized, molecules B and C are not. At
TCO , x-ray-diffraction measurements suggested a rearrangement of donor molecules that results in dimerization evenbetween B and C, whose transfer integral is almost equal tothat of the A-A dimer [3,7]. Our STM observation of the donor
arrangement is consistent with crystallographical results [12]above TCO and indicates that there is no donor rearrangement
in contrast to that below TCO . We conclude that the charge
disproportionation state develops on the donor arrangement,
which is unlike that at the CO state below TCO . We notice thatthe horizontal stripe observed at room temperature exhibitedsome inhomogeneity. From the permittivity measurementin θ -(BEDT-TTF)2 RbZn(SCN)4 [11], TCO decreases as thecooling rate increases, which suppresses the coherence of thestructural change such as the dimerization between B and
C. Moreover, x-ray-diffraction measurements suggested thatthe diffuse spots of q = (1/3 k 1/4) corresponding to charge
disproportionation still remain below TCO for a fast cooling rate
(9–10 K/min) in contrast to the case with a slow cooling rate
(0.1–0.2 K/min). These results suggested that the dimerization
plays an important role in stabilizing the charge stripe with the125110-3

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PHYSICAL REVIEW B 91, 125110 (2015)

FIG. 5. (Color online) (a) STM image around column I developing twofold periodicity with charge rich A molecules. (b) Line profiles in(a). The arrow with the broken line represents the direction of the horizontal axis of (b). The arrows in (b) indicate charge rich molecules.

long-range order below TCO and therefore a certain amountof the charge stripe inhomogeneity expected to originate fromthe absence of the dimerization between B and C.
X-ray-diffraction measurements suggest that the charge
distribution on column I is uniform at room temperature [7].
NMR measurements [17] revealed only a slight difference
between the charge amounts of the A and A sites. Our
STM method could probe a locally ordered charge stripe
with inhomogeneity. The stripe structure in the figure, inwhich molecules A are charge rich, was also observed ina different area. Figure 5 shows the line profiles in the area
in which we observed the stripe structure with charge rich A
molecules. The twofold periodicity develops over the entire
area in Fig. 5(a).
We noticed that the phase of the charge modulation in
profile “j” is an inversion of that in profile “h.” The charge
modulation in profile “h” is twofold periodicity with chargerich A molecules, which is the same as that in Fig. 4, while
the A site is rich in profile “j.” The twofold periodicity withthe charge rich A molecules forms the stripe structure. A
horizontal stripe structure with charge rich A molecules wasformed in columns i, j, and k. In the area surrounded by thegreen line in Fig. 5(a), which corresponds to the boundary
between A rich and A rich horizontal stripes, A (column h), B(column i), and A (column j) molecules are charge rich sites.
They form a diagonal stripe [Fig. 1(c)].
We mapped the charge distribution to evaluate the stripe
pattern. We reexamined the current image (Fig. 3) in Figs. 6(a)and 6(b). Figure 6(a) shows an STM current image. We
mapped the donor sites in Fig. 6(b) and classified them into
three types, namely, charge rich, poor, and unclassified sites.
Red, blue, and white ellipses represent charge rich, poor, andunclassified sites, respectively. In some parts of column I, thecharge is regarded as being uniform as indicated by the white
rectangle with broken lines in Fig. 6(a) and the white ellipses inFig. 6(b). This corresponds to an unclassified site. The twofold
periodicity along column II with charge rich B molecules
develops over the entire observed area. We note again that thealternation in column II is strict, i.e., site B is charge rich andC is poor. The horizontal stripe structure with charge rich A
molecules develops in most of the observed area. In column I,
the modulation of the twofold periodicity is weaker than that incolumn II as described above. The ratio of the electron density
on the charge rich and poor molecules varies depending on

FIG. 6. (Color online) (a) Schema of domain structure. The domain boundaries between stripe structures are projected onto the STM current
image of Fig. 3. (b) Schema of donor sites. Red, blue, and white ellipses represent the charge rich, poor, and unclassified sites, respectively.
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PHYSICAL REVIEW B 91, 125110 (2015)

position in contrast to that in column II as shown in Figs. 4
and 5. In the parts of column I denoted by white ellipses inFig. 6(b), the alternation between charge rich and poor is notclear. Then it appears that although an A site rich horizontal
stripe order develops over almost the whole observed area thecoherence is divided by nonclassified sites, in which no charge
disproportionation occurs. Additionally, in the green rectangle
in Figs. 6(a) and 6(b), diagonal stripes are formed locally.
Both A rich and A rich horizontal stripes are degenerated
since A and A molecules are crystallographically equivalent.
It is possible that an A rich horizontal stripe develops inother areas. Diagonal stripes are formed on the boundary
between A rich and A rich horizontal stripes. The domains
with different charge rich molecules have been observed below
TCO as the domains with different polarization with opticalsecond-harmonic generation measurement [21]. Even in the
CO state, it is expected that a diagonal stripe exists as a domain
wall between the domains of the horizontal stripe structureswith charge rich A and A .
We note that the donor arrangement is different from that
below TCO . As we mentioned above, B and C moleculesalong column II form dimers in a CO state. In θ -(BEDTTTF)2 RbZn(SCN)4 , which is a typical CO compound, charge
disproportionation was observed by x-ray diffraction [10]. It
is also reported that the TCO of θ -(BEDT-TTF)2 RbZn(SCN)4
is closely related to the coherence length of the superstructure
along the c axis [11]. From these results, we can expect that
there is charge disproportionation with some inhomogeneity
above TCO , and the charge pattern becomes stable as aresult of donor rearrangement. Therefore, our data suggestthat the stripe structure observed with STM is caused byan electron-electron Coulomb interaction between nearestneighbor molecules, and some inhomogeneity of the stripe
structure is caused by the absence of the dimers of B and C
molecules along column II.
To investigate this stripe structure and evaluate the charge
disproportionation in detail, we must determine the local density of states by performing scanning tunneling spectroscopyon each BEDT-TTF molecule.

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IV. CONCLUSION

STM measurement was performed on the a-b plane of
α-(BEDT-TTF)2 I3 at room temperature. The α-type BEDTTTF donor arrangement was clearly observed. We found a
stripe structure with some inhomogeneity in the charge disproportionation phase. The horizontal charge stripe develops
in most of the observed area. Dimerization was found between
the A and A molecules. The charge disproportionation statedevelops on the donor arrangement, which differs from thatat the CO state below TCO . The absence of the dimerization
between B and C causes some inhomogeneity of the stripe
structure. We also found a local diagonal stripe that formed
the boundary between two types of horizontal stripe.
ACKNOWLEDGMENTS

This work was partially supported by Grants-in-Aid for
Challenging Exploratory Research (Grant No. 24654095) fromthe Japan Society for the Promotion of Science.

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