Solid State Communications 167 (2013) 10–13

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Solid State Communications
journal homepage: www.elsevier.com/locate/ssc

Charge stripe structure in Fe1+xTe by STM
Yuki Kawashima a,n, Koichi Ichimura a,b, Junya Ishioka a, Tohru Kurosawa c, Migaku Oda c,b,
Kazuhiko Yamaya a, Satoshi Tanda a,ba

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, Japanc
Department of Physics, Hokkaido University, Sapporo 060-0810, Japanb

art ic l e i nf o

a b s t r a c t

Article history:
Received 30 November 2012
Received in revised form
12 April 2013
Accepted 25 April 2013
by Xianhui Chen
Available online 23 May 2013

The electronic state of Fe1.17Te was investigated with atomic resolution by using STM. We discovered acharge stripe structure at 7.8 K for the ﬁrst time. The wave vector of the charge stripe structure is along
the a-axis or b-axis. Furthermore we found that the charge stripe occurred at the Fe layer. We also founda 9 meV gap structure on the Fermi surface by STS. The gap size is consistent with the mean ﬁeld
approximation with SDW transition temperature of 58 K.
& 2013 Elsevier Ltd. All rights reserved.

Keywords:
A. Iron-based superconductor
D. Charge stripe
E. STM

1. Introduction
How do we form a stripe structure using an isotropic interaction? An anisotropic interaction can create a stripe pattern byordering in the same direction. There are many anisotropicinteractive elements such as electric dipoles in ferroelectricmaterials, directors in liquid crystals and magnetic dipoles. Whenthe anisotropic interaction is composed of an attractive direction
and a repulsive direction, the elements that have the anisotropicinteraction form a stripe structure. Can an isotropic interaction
form a stripe pattern? Theoretically, isotropic particles can form a
stripe pattern [1]. A coulomb interaction between charges is an
isotropic interaction. The charge stripe structure was discovered in
an organic conductor as charge ordering [2]. The electronic band
structure of organic conductors is formed by anisotropic lattice
structure. The charge order in an organic conductor is formed byelectronic structural anisotropy. A cuprate superconductor also hasa charge stripe structure [3,4]. The charge stripe structure had
been predicted theoretically [5] as the pattern formed by carriersin Mott-insulator. The stripe structure on the cuprate superconductor was found as doped carrier with insulating antiferromagnetic domain [3]. Similar to cuprate superconductors, the charge
stripe was found in other transition metal oxides such as nickelates [6], manganites [7,8] and cobaltates [9]. In these materials,
holes form a stripe structure to minimize antiferromagneticnCorresponding author. Tel.: +81 11 706 7818.
E-mail address: yuki-k@eng.hokudai.ac.jp (Y. Kawashima).

0038-1098/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ssc.2013.04.033

domain wall. It is important that the stripe structure in Mottinsulator is stabilized only at commensurate doping. To study theisotropic interactive stripe structure, we need more simple charge
stripe structure.
Recently, the iron-based superconductor was discovered[10,11]. Iron-based superconductors have different types of crystal
structures including RFePnO (R ¼rare earth metals, Pn ¼P, As)[10,11], (Ba, K)Fe2As2 [12], LiFeAs [13] and FeSe1−δ [14]. The
superconducting transition temperature of the iron-based superconductor is higher than 40 K for RFeAsO1−xFx [15–17]. Therefore,
the iron-based superconductor is called a new high T c superconductor. The iron-based superconductor has a two-dimensional
crystal structure and Fermi surfaces [18]. It is said that the holeFermi surface at the Γ point and the electron Fermi surface at the
M point induce a spin density wave (SDW) transition as a result of
Fermi surface nesting [19,20]. It is noteworthy that direct evidences of SDW such as collective excitations have not been
observed yet. The phase diagram of the iron-based superconductor
reveals the coexistence of antiferromagnetism and superconductivity [11,21–23]. It is important that superconductivity is very
close to magnetism in the iron-based superconductor [24]. The
phase diagram of the iron-based superconductor is similar to thatof an organic conductor and a cuprate superconductor. An organic
conductor and a cuprate superconductor forms a charge orderwhich usually competes with superconductivity. Then iron-based
superconductors may form a charge ordering structure. FeX (X ¼S,
Se, Te) family has the simplest composition and crystal structure
among iron-based superconductors. It is easy to control the carrier
concentration. Since FeX has no blocking layer, it is suitable toY. Kawashima et al. / Solid State Communications 167 (2013) 10–13

11

study the electronic state of the conducting layer. In this letter, we
report the charge stripe structure discovered in the iron-based
superconductor by STM measurement.

2. Experimental
A single crystalline sample of Fe1+xTe was grown by chemical
vapor transport using iodine as the transport agent. Fe and Te wereplaced in an evacuated quartz tube with small amount of iodine.
The quartz tube was heated at 700 1C for 1 week. The electronic
properties of the sample were characterized by measuring thetemperature dependence of resistivity and magnetic susceptibility.
The DC resistivity was measured with the four probe method. The
magnetic susceptibility was measured with a SQUID magnetometer while applying a magnetic ﬁeld of 0.5 T. Temperature
variable UHV-STM was used in the STM/STS study. Pr–Ir alloy wire
was used as the scanning tip. To avoid absorption of gas, thesample was cleaved at 80 K in an ultra high vacuum. The crystal
orientation was determined with an X-ray diffraction measurement, which shows that the crystal cleaves at the a–b plane.

Fig. 2. (Color online) Temperature dependence of magnetic susceptibility of Fe1+xTe (x ¼ 0.17).

3. Results and discussion
Temperature dependence of the resistivity and the magnetic
susceptibility was measured to characterize the present sample.
Fig. 1 shows temperature dependence of resistivity. The resistivity
increases slightly from room temperature to 58 K with decreasing
temperature. The resistivity exhibits a sharp peak at 58 K, and thenit decreases from 58 K to 50 K. The resistivity increases slightly
between 50 K and 0.5 K. The slope of the resistivity from room
temperature to 58 K is larger than that from 50 K to 0.5 K. Fig. 2
shows temperature dependence of the magnetic susceptibility.
The magnetic susceptibility obeys the Curie–Weiss law above 58 K
and then rapidly decreases below 58 K. The temperature dependence shows a clear antiferromagnetic transition. The antiferromagnetic transition temperature is the same as the resistive peaktemperature. By comparing temperature dependence of resistivity
and antiferromagnetic transition temperature with the previous
reports [25,26], the excess iron x is estimated as 0.17 for Fe1+xTe.
The composition was conﬁrmed approximately by energy dispersive X-ray spectroscopy.
Fig. 3 shows an STM current image of the a–b surface taken at
T ¼7.8 K with a bias voltage V ¼700 mV and a tunneling current
I ¼0.9 nA. The crystal was cleaved at the a–b plane as mentioned
above. The scanning area of the image is 10  10 nm2. Atoms are

Fig. 1. Temperature dependence of resistivity of Fe1+xTe (x¼ 0.17).

Fig. 3. (Color online) STM current image of Fe1+xTe (x ¼ 0.17) at bias voltage V ¼ 700
mV and tunneling current I¼ 1.0 nA. The image size is 10  10 nm2. The charge
stripe wave vector is along the X arrow direction. Atoms connect and separate in
the Y direction and X directions, respectively.

clearly resolved as bright spots, that form a square lattice withsides of 0.38 nm. The length of the lattice side corresponds to a Te
lattice on an a–b layer. The bright spots in Fig. 3 are assigned as Te
atoms. We found that the topmost layer was a Te layer that
resulted from the cleaving. We discovered the stripe structure in
the image. The atoms connect with each other along the Y arrow
direction in Fig. 3. In contrast, no atoms connect with each otheralong the X arrow direction in Fig. 3. We ﬁrst observed the charge
stripe structure for the iron-based superconductor. The charge
stripe wave vector direction is along the a-axis or b-axis. Thischarge stripe was observed at bias voltage in a range from 50 mV
to 1000 mV.
To analyze the stripe structure in detail, we obtained a current
image with a small scanning area. Fig. 4 shows a high-resolution
current image obtained at 7.8 K. We took line proﬁles along lines
A, B and C in Fig. 4. Fe atoms beneath a Te layer can be seen in thisimage. The image was taken at a bias voltage V¼700 mV and atunneling current I ¼1.0 nA. The image size is 3.3  3.3 nm2. In theimage, there is a stripe structure of which wave vector is parallel tothe lines A and B. The line proﬁle A, which was taken along theblue line in the image, was obtained for both the direction of the
stripe wave vector and on the Te atoms. There are other peaks
indicated by blue arrows between the Te peaks in the line proﬁle12

Y. Kawashima et al. / Solid State Communications 167 (2013) 10–13

Fig. 4. (Color online) STM current image of Fe1+xTe (x¼ 0.17) at bias voltage V ¼700 mV and tunneling current I¼ 1.0 nA. The image is 3.3 nm square. There are line proﬁles in
the STM image. A, B and C in the STM image correspond to the line proﬁles signs. Peaks indicated by green and blue arrows correspond to the tunneling current from Fe
atoms. The unlabeled peaks correspond to the tunneling current from Te atoms. The kind of atoms are decided by atomic distance.

A. It is likely that the top surface is Te layer as described above. The
strong peaks correspond to the tunneling current from the Te
atoms. The Fe layer is located 0.174 nm below the surface Te layer.
We concluded that the weak peaks between the Te peaks were thetunneling current from the Fe layer beneath the Te layer. There are
Fe atoms and Te atoms in the image. Line proﬁle B is taken along
the stripe structure between the Te atoms. There is a tunnelingcurrent only from the Fe layer in the line proﬁle B. Line proﬁle B
shows that there is a tunneling current beneath the Fe layer. Fe
atoms form a square lattice. The lattice side is 0.27 nm long. The
lattice shape is rotated 451 in plane from the Te lattice. These Fe
lattice details are consistent with the crystal structure. Line proﬁleC is taken along the side of the Fe lattice. There are two types oftunneling current from the Fe atoms. This result shows that there
are two types of Fe site. Each site has a different electronic state.
The clear stripe structure is formed of two electronic states on the
Fe layer. One is rich in charge and another is poor. We ﬁnd that the
stripe structure is caused by Fe atoms.
Fig. 5(a) shows the tunneling differential conductance on Fe1+xTe (x¼0.17) at 7.8 K and 80 K. At 80 K, the tunneling differential
conductance curve is almost ﬂat. At 7.8 K, the conductance curve
has a gap structure at 79 mV indicated by black arrows in Fig. 5(a). The tunneling spectra, where the differential conductance at
V¼ 0 is well reduced, show a gap structure at the Fermi level. Fig. 5(b) shows d2I/dV2 curve which is a numerical differential of thedifferential conductance at 7.8 K. The kink positions indicated byblack arrow were determined as ﬁrst peak (V 4 0) and dip (V o 0)
in d2I/dV2 curve [27]. We assign the structure at 7 9 mV as the
SDW gap. It is noteworthy that the shape of the differential
conductance curve on the present material is similar to that ofother iron-based superconductors such as the LaFeAsO1−xFx [28]and SmFeAsO1−xFx [29]. The existence of the pseudogap is suggested in these materials [28,29]. In Fig. 5, there are kink structurearound 730 mV indicated by green arrows was determined asﬁrst dip (V 4 0) and peak (V o0) in d2I/dV2 curve. The kinkstructure at 730 mV is similar to the pseudogap structure
observed in other iron-based superconductors [28,29].
The SDW transition temperature TSDW of 58 K is determined byresistivity and magnetic anomalies. Correspondingly, the 2ΔSDW =kT SDW
is 3.60 which is almost consistent with the mean ﬁeld value of 3.52.

Fig. 5. (Color online) (a) Tunneling differential conductance on Fe1+xTe (x¼ 0.17) at
7.8 K and 80 K indicated by red and blue line, respectively. Black and green arrows
indicate the SDW gap and the pseudogap, respectively. (b) Numerical differential ofthe differential conductance on Fe1+xTe (x ¼0.17) at 7.8 K. Black arrows and green
arrows correspond to the SDW gap and the pseudogap at the differential
conductance curve.

The gap structure is caused by the SDW transition. We conﬁrmed thatthe gap structure does not exist at 80 K. This gap structure is anindication of the SDW.
As a mechanism of the charge stripe in the present system, it
might be a possible scenario in which the symmetry breaking of

Y. Kawashima et al. / Solid State Communications 167 (2013) 10–13

the spin, charge and crystal structure occurs cooperatively [30]. In
the model, SDW causes the charge stripe.
Organic conductors and cuprate superconductors have a strong
coulomb interaction. Although they are called strong correlated
electron systems, electrons in organic conductors have different
characters from that in cuprate superconductors. Organic conductors are characterized by a small electron kinetic energy since theconduction is born by the overlap of molecular orbitals. The band
width of organic conductors is about 0.5 eV [31]. Charge localization tends to occur when the electron kinetic energy is small. It isdifﬁcult to maintain itinerancy in a small kinetic energy. On theother hand, a cuprate superconductor is Mott-insulator basedmaterial which has strong onsite coulomb interaction. Strong
onsite coulomb interaction makes charges localize with antiferromagnetic order. Carriers form stripe structure to minimize theantiferromagnetic domain wall at commensurate ratio of thedoping. The driving force of the charge stripe in cuprate superconductors is the strong onsite coulomb interaction. An iron-based
superconductor is based on metallic material which has several
conduction bands. Therefore mechanism of forming stripe structure in iron-based superconductor is different from that in Mottinsulator materials. An iron-based superconductor has a large
offsite coulomb interaction. This leads to coexistence of magnetism and superconductivity. We argue that the itinerancy and theoffsite coulomb interaction play important role in forming charge
stripe in iron-based superconductors. Moreover, we think thatcharge strip order can be formed in other iron-based superconductors as well as Fe1+xTe with other iron concentration. The
charge order structure and spin order structure in iron-based
superconductors is similar to that of cuprate conductors. Theseorder is important to study superconducting mechanism in the
cuprate superconductor. Therefore, iron-based superconductors
and cuprate superconductors have similar superconducting
mechanism. We think that iron-based superconductor is a new
type of low dimensional material, and it falls between a magnetic
material and a superconducting material.
4. Conclusions
We discover the charge stripe structure on iron-based using
STM/STS measurement. In Fe1.17Te, the stripe structure is caused by
the Fe atoms having different electronic states. With STS measurement, an SDW gap structure is observed at 7.8 K. We have aperspective that the charge stripe is occurred by the symmetry
breaking as a result of SDW, and charge and spin order coexist.
Acknowledgment
The authors are grateful to Dr. K. Inagaki for valuable discussion. The authors also thank Professor S. Takayanagi for usefuladvices on resistivity measurements. This work was partially13

supported by Grant-in-Aid for Scientiﬁc Research on Innovative
Areas (No. 21110501) from Ministry of Education, Culture, Sports,
Science and Technology of Japan.

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