Solid State Communications 204 (2015) 33–36

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

STM/STS study of the superconducting gap in SmFeAsO1  xFx
Yuki Kawashima a,n, Koichi Ichimura a,b, Kazuhiro Katono a, Tohru Kurosawa b,c,Migaku Oda b,c, Satoshi Tanda a,b, Yoichi Kamihara d, Hideo Hosono ea

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, JapandDepartment of Applied Physics and Physico-Informatics, Keio University, Yokohama 223-8522, Japane
Frontier Research Center, Tokyo Institute of Technology – Yokohama 226-8503, Japanb

art ic l e i nf oa b s t r a c t

Article history:
Received 17 October 2014
Received in revised form
26 November 2014
Accepted 4 December 2014
Communicated by H. Akai
Available online 11 December 2014

We report an electron tunneling study of SmFeAsO1  xFx in the low doping region (x ¼ 0, 0.045, 0.046,0.069) by low temperature UHV-STM/STS. Superconducting gaps are observed for each superconducting
sample x ¼ 0.045 (Tc ¼12.9 K), x ¼0.046 (Tc ¼ 32.9 K) and x ¼0.069 (Tc ¼46.9 K). We obtained corresponding superconducting gap size of ΔSC ¼ 9:5 7 0:5 meV, 9.75 7 0.25 meV and 11 7 1 meV. While Tc
increases, ΔSC is kept the same. This suggests that the effective attractive interaction is the same andthat there is some mechanism that suppresses the superconductivity in the low doping region. On theother hand, similar gap structures were found in a non-superconducting sample with x ¼0 at 7.8 K. The
obtained gap size was ΔN ¼ 8:5 7 1:5 meV, which is almost the same as the superconducting gap in thesuperconducting samples (x ¼ 0.045, 0.046, 0.069).
& 2014 Published by Elsevier Ltd.

Keywords:
A. Iron-based superconductor
D. Superconducting gap
E. STM

1. Introduction
Iron-based superconductors were discovered as a new class ofhigh critical temperature Tc superconductor with a two-dimensional
structure [1,2]. These superconductors have a high critical temperature and offer the possibility of a new superconducting mechanism
thus making them a high impact material. Iron-based superconductors have different types of crystal structures including RFePnO(R¼ rare earth metals, Pn¼P, As) [1,2], BaFe2As2 [3], LiFeAs [4] andFeSe 1  δ [5]. The RFePnO group has the highest Tc among iron-based
superconductors. The phase diagram has been studied over a wide
range of doping levels by employing hydrogen substitution on
LaFeAsO [6]. Iron-based superconductors generally have the magnetic phase next to the superconducting phase. One can imagine thatthe superconductivity has a magnetic origin in iron-based superconductors. The itinerant antiferromagnetic ordered phase is stable
for the mother compound of iron-based superconductors [7,8],
unlike high-Tc cuprates, whose magnetic phase is a Mott insulator.
It is important to study the magnetic phase to clarify the origin of thesuperconductivity in iron-based superconductors. With RFePnO, theground state is controlled by the carrier concentration. SmFeAsO1 xFx has the highest Tc of RFeAsO1 xFx system materials [9,10].
The Tc of SmFeAsO1 xFx increases to 58.1 K and is very sensitive tonCorresponding author. Tel.: þ 81 11 706 7818
E-mail address: yuki-k@eng.hokudai.ac.jp (Y. Kawashima).

http://dx.doi.org/10.1016/j.ssc.2014.12.003
0038-1098/& 2014 Published by Elsevier Ltd.

ﬂuorine substitution [11,12]. Therefore, investigating the superconductivity of SmFeAsO1 xFx is a valuable method for understandingiron-based superconductors. SmFeAsO1 xFx is useful when discussing the difference between the electronic states of the magnetic andsuperconducting phases because its higher Tc increase the superconducting order parameter. The doping dependence of the crystal
structure on SmFeAsO1 xFx was studied by using high resolution
X-ray diffraction at several ﬂuorine concentrations [13]. Spectroscopic studies have been needed to investigate the superconductivity
with the systematic substitution of ﬂuorine in SmFeAsO1 xFx.
Electron tunneling is useful for revealing the mechanism of thesuperconductivity [14]. It is a direct method for obtaining the energy
spectrum of the electronic density of states. Because scanningtunneling microscopy/spectroscopy (STM/STS) observes the electronic state in a local area within a few atoms, STM can probe a single
crystalline domain in a polycrystalline sample. Using STM, we canobtain the energy spectra of the density of states with high energy
resolution by measuring the tunneling differential conductance, andthe electron density with a nano-scale spatial resolution. Therefore,
we can obtain the spatial variation of the superconducting gap withatomic resolution. The gap anisotropy can be studied directly withthe STM/STS technique by probing the local density of states atvarious crystal surfaces [15]. Since the tip of the probe does not comein contact with the sample, STM/STS is a nondestructive method.
Various STM studies have been undertaken on the surface structure
and the gap symmetry in iron-based superconductors [16]. Recently,
an STM study revealed a charge stripe structure on FeTe [17].

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Y. Kawashima et al. / Solid State Communications 204 (2015) 33–36

STM/STS [18] and photoemission spectroscopy [19,20] have beenused to observe the superconducting gap and pseudogap ofLaFeAsO1 xFx. The superconducting gap of F-doped SmFeAsO wasobserved by point contact measurement [21] and of SmFeAsO1  δby STM [22]. Tunneling spectroscopy is needed for SmFeAsO1 xFx inthe lower ﬂuorine concentration region, where the boundary
between the superconducting and non-superconducting phases islocated. We study the electronic state of SmFeAsO1 xFx with several
ﬂuorine concentrations by using STM/STS and discuss the superconducting gap depending on Tc.

2. Experimental
The procedure for synthesizing polycrystalline SmFeAsO1 xFx(x¼0.045, 0.046, 0.069) is reported in reference [11]. The F content(x) of samples was obtained according to Vegard's volume rule [23].
The superconducting transition temperatures for x¼0.045, 0.046 and0.069 were determined as 12.9 K, 32.9 K and 46.9 K, respectively, fromresistivity measurements. An undoped SmFeAsO sample does notexhibit superconductivity in a resistivity measurement. Temperature
variable ultra high vacuum (UHV)-STM was used in the STM/STS study.
Pr–Ir alloy wire was used as the scanning tip. Block shaped samples
were cracked just before being transferred into the UHV-STM chamber
to obtain a fresh surface. The tunneling differential conductance at 7 Kand 80 K was directly measured with the standard lock-in techniquewith a 5 kHz ac modulation of 3 mV added to the bias voltage.

regardless of the distance between the tip and the sample at thesame position. As shown in Fig. 1(a), the tunneling differential
conductance is reduced around the zero bias voltage, and this isassociated with the superconducting state. The conductance peaks
at gap edges do not appear in the tunneling spectra. There is a kinkstructure around ∣V∣ ¼ 12 mV indicated by blue arrows, which
2corresponds to the peak and dip in the d I=dV 2 curve. We conﬁrmedthat the differential conductance curve at 80 K is featureless. It
means that the kink structures around ∣V∣ ¼ 12 mV do not exist at80 K. We deﬁne the kink structures indicated by the blue arrows as
the superconducting gap. We also found the pseudogap structure in
the form of other kinks around ∣V∣ ¼ 36 mV and shown by green
arrows. These structures were determined in the same manner aswith LaFeAsO1  x Fx [18]. We obtained the superconducting gap
Δsc ¼ 12 meV and the pseudogap ΔPG ¼ 36 meV for these data. For
several data, we obtained Δsc ¼ 117 1 meV for SmFeAsO1  x Fx
(x¼0.069), correspondingly 2Δsc =kT c ¼ 5:417 0:52. The pseudogap
was obtained as ΔPG ¼ 25–42:5 meV.
Fig. 2(a) and (b) shows typical normalized tunneling conductance
dI=dV curves for x¼ 0.046 ðT c ¼ 32:9 KÞ at 7.5 K and x¼0.045
ðT c ¼ 12:9 KÞ at 7.8 K, respectively. Tunneling spectra for x¼0.045
and x¼0.046 samples have similar structures to that for the x¼0.069
sample. We deﬁne the superconducting gap and pseudogap in thesame manner as described above. The kink structures corresponding
to the superconducting gap can be seen around ∣V∣ ¼ 9:5 mV in Fig. 2(a) and (b) and are indicated by blue arrows. We obtained Δsc as
9.7570.25 meV and 9.570.5 meV for x¼0.046 and x¼0.045,

3. Results and discussion
Fig. 1(a) and (b) shows a typical normalized tunneling conductance dI=dV curve for SmFeAsO1 xFx (x¼0.069, Tc ¼ 46.9 K) at 7.3 K
2and its numerical differential d I=dV 2 , respectively. The conductancecurves have a slight spatial dependence. They are almost identicalFig. 1. (Color online) (a) Tunnel differential conductance of SmFeAsO1  x Fx
(x¼ 0.069, T c ¼ 46:9 K) at 7.3 K. (b) Numerical differential of the conductance curveshown in (a).

Fig. 2. (Color online) (a) and (b) show tunnel differential conductances for x¼ 0.046
ðT c ¼ 32:9 KÞ at 7.5 K and x¼ 0.045 ðT c ¼ 12:9 KÞ at 7.8 K, respectively.

Y. Kawashima et al. / Solid State Communications 204 (2015) 33–36

35

Table 1
T c , ΔSC , 2ΔSC =kT c and ΔPG for x¼ 0, 0.045, 0.046 and 0.069.
x
T c ðKÞΔSC ðmeVÞ
2ΔSC =kT c
ΔPG (meV)0—
ðΔN ¼ 8:5 7 1:5Þ
—
16–48

0.045
12.9
9.5 7 0.5
17.08 70.9
25–700.046
32.99.75 7 0.25
6.87 7 0.18
22.5–94

0.069
46.9
117 1
5.417 0.52
25–42.5

Fig. 3. (Color online) (a) Tunnel differential conductance of undoped SmFeAsO
at 7.8 K. (b) Numerical differential of the conductance curve shown in (a).

respectively. The corresponding 2Δsc =kT c values were estimated to be
6.8770.18 and 17.0870.9 for x¼0.046 and x¼0.045, respectively.
Pseudogaps of ΔPG ¼ 22:5–94 meV and 25–70 meV were obtained forx¼ 0.046 and x¼0.045, respectively.
Fig. 3(a) and (b) shows a typical normalized tunneling conductance dI=dV curve for undoped SmFeAsO and its numerical differ2
ential d I=dV 2 at 7.8 K, respectively. The conductance at V¼0 islarger than that of superconducting samples (x¼0.045, 0.046,0.069), suggesting greater lifetime broadening [24]. It is noteworthy
that the anomalies indicated by the blue arrows are similar to thesuperconducting gap structure observed in the superconducting
samples. However, undoped SmFeAsO does not exhibit superconductivity in the bulk resistivity measurement. Then we deﬁne thegap ΔN of undoped SmFeAsO. The pseudogap indicated by the green
arrows was also found. We obtained ΔN ¼ 8:5 7 1:5 meV andpseudogap ΔPG ¼ 16–48 meV for undoped SmFeAsO.
Our obtained T c ; ΔSC ðΔN Þ, 2ΔSC =kT c and ΔPG values are listed
in Table 1. Fig. 4 shows ΔSC ; ΔN and T c as a function of x. When wecompare results for x¼0.045 and x¼ 0.046, the T c changes rapidly
from 12.9 K to 32.9 K, while the superconducting gap remains
almost the same. The T c increases linearly at x Z 0:046. ΔSC forx¼ 0.069 is larger than that for x¼0.046. However, there is only aslight change in the superconducting gap. The 2ΔSC =kT c values for
SmFeAsO1  x Fx are 17.0870.9, 6.8770.18 and 5.4170.52 forx¼ 0.045, 0.046 and 0.069, respectively. Our results show that2ΔSC =kT c decreases with increasing x and is larger than the BCS
value of 3.52 for x¼0.045, 0.046 and 0.069. On the other hand, the2ΔSC =kT c value at a higher doping level is reported to be 3.68 for Fdoped SmFeAsO [21] and 3.55–3.8 for SmFeAsO1  δ [22]. Thesevalues are fairly consistent with the BCS value. We deduce that2ΔSC =kT c merges with the BCS value as the carrier concentration
increases. Our results show that both ΔSC and T c increase withincreasing x. However, the variation in ΔSC is smaller than that of T c .
As a consequence, 2ΔSC =kT c decreases as T c increases. We emphasize that the variation in ΔSC is very small. ΔSC generally corresponds

Fig. 4. (Color online) Δsc;N and T c as a function of x. Red, green and blue dots show
Δsc , ΔN and T c , respectively.

to the strength of the attractive interaction. We suggest that theattractive interaction is slightly changed by the substitution level,although T c varies signiﬁcantly. In other words, there is somemechanism that suppresses the superconductivity while the interaction remains almost constant.
A superconducting gap like structure ΔN exists on undoped
SmFeAsO, where no superconducting transition was observed inthe resistivity measurement. There is a possibility that ΔN isassociated with the antiferromagnetic order [11,25,26]. From theobserved ΔN ¼ 8:5 meV, the antiferromagnetic transition temperature T N is expected to be 55.7 K based on the mean ﬁeld theory.
However, the observed T N ¼ 144 K [11] is far higher than the value
expected from ΔN . Then it is difﬁcult to relate ΔN to the antiferromagnetic order. The energy scale of ΔN can be reasonably associatedwith the maximum superconducting transition temperature onSmFeAsO1  x Fx ðT c ¼ 58:1 KÞ. This should be noted that the gap
ΔN ¼ 8:5 71:5 meV found for x¼0 has the same energy scale asthe superconducting gap for x¼0.045, 0.046 and 0.069. Thissuggests that the attractive potential exists even in the nonsuperconducting phase with the same strength as that in thesuperconducting phase. The substitution of ﬂuorine leads to thebulk superconductivity of SmFeAsO1  x Fx .
The pseudogap structure was observed for all SmFeAsO1  x Fx
(x¼0, 0.045, 0.046, 0.069) as listed in Table 1, which is similar to theresult for LaFeAsO1  x Fx [18]. For some reason the obtained values
are scattered. However, the magnitude of the pseudogap is tens ofmeV for all x in the present study. This is largely consistent with thevalue ΔPG ¼ 60 meV for F-doped SmFeAsO1  δ obtained with timedomain spectroscopy [27]. The pseudogap is thought to be acommon feature in iron-based superconductors. The relationship36Y. Kawashima et al. / Solid State Communications 204 (2015) 33–36

between the pseudogap and the superconductivity remains still anopen problem.

4. Summary
We undertook a systematic study of the electronic state of
SmFeAsO1  x Fx at a low doping level. The superconducting gap andthe pseudogap structure were observed for superconducting samples(x¼0.045, 0.046, 0.069). We found the size of the superconducting
gap to be almost the same irrespective of the ﬂuorine concentration,
while T c increased as the doping level increased. The result suggeststhat the attractive interaction is constant irrespective of the doping
level and a mechanism that suppresses T c . Fluorine substitution
weakens the suppression of the superconducting transition. In a nonsuperconducting sample (x¼0), we found the gap structure ΔN ,which is similar to the gap ΔSC in the superconducting samples. Theenergy scale of ΔN can be reasonably associated with the maximum
superconducting transition temperature of SmFeAsO1  x Fx ðT c ¼58:1 KÞ. The ﬁnding suggests that an attractive interaction potentiallyexists even in the non-superconducting phase.

Acknowledgments
This work was supported by the 21COE program on “TopologicalScience and Technology” of the Ministry of Education, Culture, Sport,Science and Technology of Japan. This work was partially supportedby the Asahi Glass Foundation, and the Japan Society for Promotionof Science (JSPS) KAKENHI Grant no. 26400337.
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