VOLUME 83, NUMBER 3

PHYSICAL REVIEW LETTERS

19 JULY 1999

Surface Charge-Density Wave on the One-Dimensional Organic Conductorb-共共BEDT-TTF兲兲2PF6
Masahiko Ishida,1 Takehiko Mori,2 and Hidemi Shigekawa1,3, *1

Institute of Applied Physics, CREST, Japan Science and Technology Corporation (JST), University of Tsukuba,
Tsukuba 305-8573, Japan2
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152-8552, Japan3
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo,Hongo, Tokyo 113-8656, Japan(Received 2 April 1999)
We performed STM and theoretical studies of the surface of b-共BEDT-TTF兲2 PF6 which is known
to show a metal-insulator transition at 297 K with the nesting vector of Qbulk 苷 aⴱ 1 0.5cⴱ asa bulk phase. Two typical charge-density wave (CDW) phases with QS1 苷 0.13aⴱ 1 0.26cⴱ andQS2 苷 0.19aⴱ 1 0.20cⴱ were observed at 285 K on the surface. The observed structures werecorrelated well with the analysis of the susceptibility x共Q兲, given the incomplete surface-charge transfer
in the polar surface of this material. This is the first observation of surface CDW phases that reflect thecharacteristics of the electronic structure. These results suggest the possible discovery of other surface
properties, such as surface superconductivity, in organic materials with a polar surface.
PACS numbers: 71.45.Lr, 61.16.Ch, 71.15.Fv, 73.20.Dx

face molecule ionicity. These materials consist of alternately stacked BEDT-TTF and anion layers (Fig. 1), and0.5 electrons transfer from each BEDT-TTF molecule tothe anion layers, which makes the BEDT-TTF and anion
layers conductive and insulating, respectively. Therefore,
a surface terminated with either a BEDT-TTF or cation
layer begins to exhibit characteristics of a polar surface.
On the polar surface, the net surface charge is adjusted tocancel the potential gradient throughout the crystal. Such achange is expected to have considerable influence on surface geometric and electronic structures. In fact, recent
studies revealed that surface reconstruction was induced on
the molecular crystal surface of BEDT-TTF compounds[7]. These results again raise interest in the study of thephase transitions induced in the surface layers. Since thecharge transfer is incomplete in the BEDT-TTF molecularlayer terminating the surface, a change in the band filling isexpected to significantly influence the characteristic properties such as the CDW, on the surface.

Charge-density waves (CDWs), formed in lowdimensional conductors, have attracted considerable
attention recently due to their relation to the metalinsulator (MI) phase transitions in strongly correlated
electronic systems. In order to understand the characteristic properties of CDWs, information about theirlocal structures is extremely important. As a result,numerous studies have been performed using scanningtunneling microscopy (STM) and related techniques[1–6]. For example, from a local spectroscopic studyof the CDW phases of the TaSx Se22x , MI transition due
to the Mott-Hubbard localization was directly shownon an atomic scale, and analyzed with respect to theband structure [3]. Recently, a STM study was performed on the surface of a quasi-one-dimensional organic
conductor, tetrathiafulvalene-tetracyanoquinodimethane(TTF-TCNQ), and the incommensurate 2kF CDW,driven by Peierls instability, was directly observed inreal space with molecular resolution [4]. Thus, interesting characteristics have been successfully observed, asdescribed. However, despite the early expectations, theCDW phases observed on surfaces have been understood
to be identical to those in the bulk states. For example,
the modulated CDW phase observed on the 1T -TaS2
surface was explained by the structure of bulk phases[5]. Furthermore, a specific structure observed on theTTF-TCNQ surface, i.e., a CDW chain aligned along thea axis with an anomalous rise in the tunneling current,
was attributed to be the result of local perturbation due toimpurities or surface defects [4]. Basically, no surface
CDW phases which have characteristics which differ
from the bulk phase have previously been observed.
On the other hand, bis(ethylenedithio)tetrathiafulvalene(BEDT-TTF)-based organic compounds are distinct fromother low-dimensional materials with respect to their sur-FIG. 1. Crystal structure of b-共BEDT-TTF兲2 PF6 determined by x-ray diffraction (a 苷 1.4960, b 苷 3.2643, c 苷0.6664 nm).

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© 1999 The American Physical Society

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VOLUME 83, NUMBER 3

PHYSICAL REVIEW LETTERS

From the standpoint described, we performed STM andtheoretical studies on b-共BEDT-TTF兲2 PF6 [8]. This material is the first organic conductor recognized as one dimensional along the side-by-side (transverse) array. The
largest conductivity is exhibited parallel to the molecular plane (the c axis in Fig. 1), and this salt undergoes
a MI transition at 297 K. Below this temperature theemergence of a twofold modulation along the conductive c axis was confirmed by x-ray diffraction [9]. On
the basis of ESR measurements [8], spin susceptibility
measurements [9], and tight-binding energy band calculations [10], the twofold modification is understood tobe a MI transition associated with 2kF CDW. Single
crystals of b-共BEDT-TTF兲2 PF were grown by the standard electrochemical oxidation method and then fixed tometallic plates with conductive silver epoxy. The STM
measurements were performed on the crystal a-c planeterminated with the BEDT-TTF molecular layer (Fig. 1)
in a constant-height mode under ultrahigh-vacuum conditions using electrochemically etched tungsten tips. Sample temperatures ranged from ⬃80 K to room temperature, and were measured using a calibrated thermocouple
placed near the sample holder.
At temperatures above 290 K, BEDT-TTF molecularrows were observed without any additional structures.
This indicates that the metallic phase remains even below
the transition temperature, ⬃297 K. Since the CDW
phase was observed above the transition temperature in
the case of TTF-TCNQ [4], the transition to the CDW
phase may have been slightly disturbed on the surface of
this material, suggesting a difference from the bulk phase.
At temperatures below 280 K, it was difficult to observea detailed surface structure because of the considerable
reduction in conductivity [8].
In the temperature range from 280 to 290 K, however,new structural modulations along the molecular rows
emerged on the surface. Figures 2(a) and 2(b) show thetwo typical superstructures stably observed on the surface
at 285 K. In these images, the molecular rows along thec axis are arrayed with a separation of ⬃1.5 nm in thea-axis direction. This value of separation corresponds
well with the lattice constant of 1.49 nm in the a axis.
Moreover, noteworthy features exist in the structure ofmolecular rows in these images. Figures 2(c)–2(e) show
the schematic structures of the bulk CDW phase and thetwo observed superstructures in Figs. 2(a) and 2(b).
First, the observed structures have considerably larger
modulations compared to the structure of the bulk CDW
phase that has a twofold modulation along the c axis.
The new lattice constants observed along the c axis are
2.6 6 0.1 nm [Fig. 2(a)] and 3.3 6 0.05 nm [Fig. 2(b)],and are close to fourfold and fivefold longer, respectively,
than the original lattice constant of 0.67 nm. A second
interesting feature of the modulated structures is the phaseshift. As the schematic in Fig. 2(c) shows, according toboth experimental and theoretical studies, the bulk CDW

19 JULY 1999

FIG. 2 (color). (a),(b): STM images obtained on the a-cplane of b-共 BEDT -TTF 兲2 PF 6 at 285 K (Vt 苷 1.0 V , It 苷
0.3 nA ). (c) – (e) are schematic models of the bulk CDW
and the observed superstructures of (a) and (b), respectively.
Original and the observed unit cells are represented by solidand dotted lines in (c) – (e), respectively.

phase does not exhibit periodic modulations along thea axis. On the other hand, the observed structures exhibit periodic modulations along the a axis, as shownin Figs. 2(d) and 2(e). Their periodicities along the aaxis are 11 6 1 nm and 8.0 6 0.7 nm, and are close tosevenfold–eightfold and fivefold longer than the a-axis
lattice constant, respectively. Therefore, they are completely different from the structure of the bulk CDW phasedetermined by x-ray diffraction. However, these structures emerged only below the critical temperature of MI
transition. As is known, the formation of CDW depends
strongly on the geometry of the Fermi surface (FS), i.e.,the degree of FS nesting. Therefore, in order to understand the observed modulations, we calculated the energy
band structure using the tight-binding approximation. Theparameters for the tight-binding calculation of the twodimensional cation (BEDT-TTF) sheet were estimatedfrom the overlaps of the highest-occupied molecular orbitals (HOMOs) of the neighboring cations [11]. According to STM and atomic-force microscopy (AFM) studies,
the displacement of the surface molecules from the crystal structure was estimated to be 0.02 nm at most, which
is not very large with respect to the molecular size of597

VOLUME 83, NUMBER 3PHYSICAL REVIEW LETTERS⬃1.4 nm. Therefore, we adopted the overlap parametersof the bulk crystal for the calculation [10].
The primary factor that governs the FS geometry isthe change in the band filling [6]. Band filling of thecharge-transfer molecular crystal depends on the amountof molecular charge. In the case of b-共BEDT-TTF兲2 PF6 ,if the charge transfer is completed, the BEDT-TTF
molecules which organize the conductive band retain auniform positive charge of d 苷 0.5. For the surface layer,
however, the molecular charge is reduced to d0 苷 0.25
due to the missing anion layer on one side. Therefore, theHOMO band filling in the surface layer changes from 6兾8(half filled as represented by a hole in the upper band) to7兾8 (quarter filled).
Figure 3(a) shows the calculated HOMO band structure,
where the two marked Fermi levels have been determinedby taking into account the change in the molecular charge
between the bulk and the surface layer. The structureof the FSs corresponding to the bulk and the surface
Fermi levels is shown in Fig. 3(b) by the solid anddotted lines, respectively. The FSs are depicted using
the extended zone scheme with regard to the degeneracy
on X-U. As shown in Fig. 3(b), the FSs in the bulk
layer have a sinusoidal shape and can be nested by
the transfer vector depicted by Qbulk . The change inthe Fermi level results in two major changes in thegeometry of the FS as shown in Fig. 3(b): (1) closer
distance between the neighboring FS sheets, and (2) anonsinusoidal shape of the FS. While the former change
clearly indicates a longer periodic modulation in real
space, the latter complicates the estimation of the desired
nesting vector. In order to clarify this point, we calculated
the susceptibility x共Q兲 for the bulk and the surface layer,
which represents the degree of FS nesting resulting in theformation of CDW phase [12,13].
Figure 4 shows the contour plot of x共Q兲 with bulkphase band filling, the structure of which is characterized by the steep increase of x共Q兲 around Qbulk 苷 aⴱ 1

FIG. 3. (a) Calculated HOMO band structure and (b) thestructure of the FSs corresponding to the bulk (solid line) andthe surface (dotted line) Fermi levels shown in (a).

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19 JULY 19990.5cⴱ . The inset in Fig. 4 shows the structure of FS nesting. The solid line in Fig. 4 represents the FS translatedby the vector Qbulk from the lower side, which is perfectly nested with the original FS in the upper side, represented by the dashed line. This result agrees well withthe twofold CDW structure shown along the c axis, which
was determined by x-ray diffraction for the bulk phase.
On the other hand, Fig. 5(a) shows the contour plot ofx共Q兲 with surface-phase band filling. The structure ofx共Q兲 differs markedly from that in Fig. 4: the peak became broader and its position shifted toward the smaller
Qc direction. The nesting vector, which corresponds tothe peak position, is Qsurf 苷 aⴱ 1 0.27cⴱ , which is expected to produce a 3.7-fold periodic modulation along
the c axis. However, as shown in the inset of Fig. 5(a),the nesting vector Qsurf does not perfectly nest the FS
sheets, suggesting, i.e., the possibility of the appearanceof other CDW phases, as observed experimentally. The
incompleteness of FS nesting is the result of the nonsinusoidal shape of the FS, which makes the x共Q兲 peakbroader because partial nesting of FS sheets is expectedaround the Qsurf . Therefore, as shown in Fig. 5(a), x共Q兲does not decrease steeply and has a relatively high susceptibility even around the peak position, and particularly
along the two ridge lines denoted by the dotted lines.
In Fig. 5(a), the wave numbers corresponding to the experimentally obtained periodic modulations in Figs. 2(a)and 2(b) are marked by QS1 苷 0.13aⴱ 1 0.26cⴱ andQS2 苷 0.19aⴱ 1 0.20cⴱ , respectively. They are locatedaround the ridge lines described above, and have relatively
high values of x共Q兲. In addition, their periodicities along
the c axis are 2.6 6 0.1 nm and 3.3 6 0.05 nm, and areclose to being commensurate with the underlying lattice;fourfold and fivefold, respectively. Furthermore, the STM
observations were performed near the critical temperature.

FIG. 4. Contour plot and side view of x共Q兲 with bulk-phaseband filling (see text).

VOLUME 83, NUMBER 3PHYSICAL REVIEW LETTERS

19 JULY 1999

to show a MI transition at 297 K with the nesting vector of Qbulk 苷 aⴱ 1 0.5cⴱ as a bulk phase. Two typical
CDW phases with QS1 苷 0.13aⴱ 1 0.26cⴱ and QS2 苷0.19aⴱ 1 0.20cⴱ were observed at 285 K on the surface. The observed structures were well correlated withthe analysis of the susceptibility x共Q兲 in consideration of
the incomplete surface-charge transfer in the polar surfaceof this material. This is the first observation of surface
CDW phases which reflects the characteristics of the surface electronic structure. Furthermore, slightly loweredtransition temperature and the nonsinusoidal FS sheet in
the surface layer suggested interference of the FS nesting. These results suggest the possible discovery of othersurface properties, such as surface superconductivity, inorganic materials with a polar surface.
This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science,Sports and Culture of Japan. One of the authors (M. I.)
was financially supported by the Japan Society for thePromotion of Science (JSPS).

FIG. 5. (a) Contour plot and side view of x共Q兲 with surfacephase band filling. QS1 and QS2 are the nesting vectors
corresponding to the observed CDW phases in Figs. 2(a) and2(b), respectively. (b) Cross sections of x共Q兲 in (a) along
Qc 苷 0.27cⴱ , 0.25cⴱ , and 0.20cⴱ .

Therefore, the incommensurate CDW expected from theQsurf is not stable enough to be observed due to the sliding motion over the underlying lattice, and the commensurate CDW phases, such as the observed QS1 and QS2 ,might have been realized.
Next, we consider the modulation along the a axis. Figure 5(b) shows the cross sections of x共Q兲 along Qc 苷0.27cⴱ , 0.25cⴱ , and 0.20cⴱ . As shown in Fig. 5(b), vectors
QS1 苷 0.13aⴱ 1 0.26cⴱ and QS2 苷 0.19aⴱ 1 0.20cⴱ arelocated near the peaks and have high x共Q兲 values. Therefore, the observed CDW phase shift is also consistent withthe calculation. Instability induced in the surface layer
might have caused the phase shift between two CDW
chains, as it did in the CDW structure of the TTF-TCNQ
crystal. Further study is necessary to clarify the interaction between CDW chains.
In summary, we performed STM and theoretical studies of the surface of b-共BEDT-TTF兲2 PF6 which is known*Email address: hidemi@ims.tsukuba.ac.jp(http://dora.ims.tsukubas.ac.jp)[1] R. V. Coleman, B. Drake, P. K. Hansma, and G. Slough,Phys. Rev. Lett. 55, 394 (1985).
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