Photon Factory Activity Report 2017 #35 (2018)

BL-28A/2015S2-003, 2016G096

ARPES study of Te-annealed 11-type iron-based superconductor FeTe1-xSex
Keisuke KOSHIISHI1,*, Takumi. OTSUKA2, Suguru. NAKATA1, Kenta. HAGIWARA1, Chun. LIN1, Yuxuan. WAN1,Hiroshi. KUMIGASHIRA3, Kanta. ONO3, Koji. HORIBA3, Takao. WATANABE2 and Atsushi. FUJIMORI1
12

Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, JapanGraduate School of Science and Technology, Hirosaki University, 3, Bunkyo-cho, Hirosaki-shi, Aomori-ken, 036-8561,
Japan
3

Photon Factory, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan

1 Introduction
11-type iron-based superconductors have the simplest
crystal structure in which layers consisting of Fe atoms
tetrahedrally coordinated by chalcogens (e.g. Te, Se)
stack. The parent compound FeTe shows a bi-collinear
double-stripe antiferromagnetic (AFM) order with thewave vector Q=(π/2, π/2), which is distinct from the usualcollinear magnetic structure observed in other parent
compounds like BaFe2As2 [1]. As tellurium in the parent
compound is gradually replaced by selenium, the AFM
order is suppressed and superconductivity is observed. It
has been reported that the superconductivity is affected byexcess Fe located at interstitial sites and very sensitive toits stoichiometry. By annealing a specimen, the excess Fe
can be removed, and then the superconductivity can beimproved. [2,3] The superconductivity with thesuperconducting transition temperature (TC) of ~10 K is
observed in a wide low Se-concentration region from x =0.05 up to 0.5 and the maximum bulk TC reaches 14.5 K
at x ~ 0.4. According to recent transport measurement for
Te-annealed sample [4], although Hall coefficient (RH) ofannealed FeTe0.6Se0.4 is almost independent of
temperature above 50 K, it decreases with decreasing
temperature below 50 K and eventually changes their signfrom positive to negative around 30 K, indicating multicarrier features of holes and electrons in optimally doped
samples. Moreover, the strong temperature dependencewith the sign change suggests that band-specific pseudo
gaps may exist. On the other hand, for FeTe0.8Se0.2, RH isalways positive similar to the case of as-grown samples.
Naively, this behavior indicates that contribution to thetransport properties dominantly comes from hole carriersin contrast to the optimally doped sample. However, since
Fe-based superconductor has multi-orbital nature because
the 3d electrons of Fe form bands near the Fermi level, it
is not clear whether such simple speculation is valid ornot. Therefore, we have performed angle-resolved
photoemission spectroscopy (ARPES) measurements on
FeTe1-xSex in order to investigate differences in theelectronic structure between the optimally doped and low
Se-doped samples and the temperature dependence of
their electronic structure.
2 ExperimentHigh-quality single crystals of FeTe1-xSex were grownusing the Bridgman method and annealed in a tellurium
vapor (“Te-anneal”). To check the Tc and thesusceptibility, magnetization measurements were carriedout by using a Magnetic Property Measurement System5S (Quantum Design, Co., Ltd.) at The University ofTokyo
CryogenicResearchCenter.
ARPESmeasurements were performed at beamline 28A ofPhoton Factory using circularly polarized light with thephoton energy of 54 eV corresponding to kz  0. A
SIENTA SES-2002 electron analyzer was used with thetotal energy resolution of ~ 20 meV. The crystals werecleaved in situ below T = 20 K, and the measurements
were carried out in an ultrahigh vacuum of ~ 9×10-11 Torr.

Fig. 1: ARPES results for FeTe1-xSex (a), (b) Fermi
surface mapping for x = 0.4 and 0.2, respectively. (c), (d)Energy-momentum plots for x = 0.4 divided by the FermiDirac function along cut 1 and cut 2 depicted by red
arrows in panel (a), respectively. (e), (f) Second
derivative of (c) and (d) with respect to energy,respectively. (g), (h), (i), (j) Same as (c), (d), (e) and (f),respectively, except for x = 0.2.
3 Results and Discussion
ARPES results are shown in Fig. 1. Intensity map at
the Fermi level for x = 0.4 integrated from EB = -10 meV
to EB=10 meV is shown in Fig. 1(a). The electron pocketPhoton Factory Activity Report 2017 #35 (2018)

around the M point and the strong intensity around the Γpoint coming from hole bands can be observed. ARPES
data for x = 0.4 are shown in Figs. 1(c) and (d). We also
show their second derivative images with respect toenergy in Figs. 1(e) and (f), respectively, in order tohighlight the band structure. Three hole bands and one
electron band can be observed. At least one of the threehole bands seems to cross the Fermi level, forming a tinyhole pocket around the Γ point as mentioned above. For x= 0.2 (Figs. 1(g)-(j)), one can observe not only the holebands around the Γ point but also the electron bandaround the M point similar to the band structure for x =0.4, and the electron band forms a sizable electron pocket
at the corner of the Brillouin zone (Fig. 1(b)).
Interestingly, the heaviest hole band assigned to the dxyband by the previous calculation [5] is crossing the Fermi
level only in the case of x = 0.2, and then it is supposed tomake a large Fermi surface around the Γ point although it
is difficult to observe that Fermi surface in the intensity
map of Fig. 1(b). This results suggest that the large hole
pocket appears and that mainly hole carriers play a role in
conductivity only in the case of x = 0.2, which isconsistent with the tendency of RH. Moreover,considering that the superconducting transition
temperature scarcely depends on Se concentration over awide doping region although there are large differences in
the Fermi surfaces, one can suggest that the large hole
pocket consisting of the dxy orbital may possibly makeonly minor contribution to the superconductivity in FeTe1xSex.

Fig. 2: Temperature evolution of ARPES spectra ofFeTe1-xSex (a) Energy-momentum plots for x = 0.2 alongcut 2 depicted by red arrows in Fig.1 (a). They are
symmetrized with respect to the Fermi level. (b)Temperature evolution of EDC around kF of x = 0.2. They
were obtained by integrating EDCs in the shaded area inpanel (a). (c) Temperature dependence of the intensity
around the Fermi energy (blue area in panel (b)). Red andblue markers correspond to x = 0.2 and 0.3, respectively.

We have also measured the temperature dependence ofelectronic states near the Fermi level. Figure 2(a) shows
the temperature evolution of symmetrize ARPES spectra
around the M point for x = 0.2. One can see the electronband near the Fermi level and identify the position of kF
at all temperatures. To see the energy gap features, EDCs
integrated around kF are plotted in Fig. 2(b). Due to thesmall size of the superconducting gap of ~1.6 meV at 2.5
K [6] compared to the energy resolution of ~17 meV, thecoherence peak cannot be distinguished even at 10 K.
However, the intensity proportional to the density ofstates around EF seems to be enhanced with increasing
temperature, indicating an energy gap closure. For morequalitative comparison, we have plotted the intensity near
EF as a function of temperature in Fig. 2(c). Red and Bluecircles correspond to x=0.2 and 0.3, respectively. The RHof x = 0.3 shows temperature dependence similar to x =0.4 although RH does not change sign at low temperatures.
As for x = 0.2, the intensity grows monotonically withincreasing temperature, and eventually saturates around150 K. This result indicates that pseudo gap opens above
Tc for x = 0.2. On the other hand, behavior for x = 0.3 is
not monotonic. The intensity increases with temperaturein the low temperature region but drastically drops around
50 K, suggesting that somehow a pseudo gap on theelectron pocket opens only above ~50 K. Assuming apseudo gap opening on the electron Fermi surface above~50 K, the strong temperature dependence of RH can beexplained. Namely, if a pseudo gap opens only on theelectron Fermi surface above ~50 K, hole carriers would
be dominant in the high temperature region and electron
carriers may recover with the closure of the pseudo gap
below ~50 K. Thus one can understand why RH takes
positive values above ~50 K and undergoes strong
temperature dependence below ~50 K.
The same temperature dependence of RH has beenreported even for FeSe under high pressure [7]. In thatcase, the dominant carriers can be changed from electrons
to holes by applying pressure even though the applicationof pressure does not introduce extra carriers into thesystem, and the authors have concluded that thetemperature dependence of Hall resistivity is due to Fermi
surface reconstruction with antiferromagnetic ordering.
However, in the present case, we could not observe any
evidences for antiferromagnetic order such as bandfolding, and so far antiferromagnetic nature has been
observed by ARPES only in FeTe [8]. Therefore, it might
be difficult to conclude that the pseudo gap on theelectron pockets of superconducting compounds isderived from antiferromagnetic order. On the other hands,
antiferromagnetic spin correlations with stripe-type
antiferromagnetic wave vector QSAF = (π, 0) in FeTe1-xSex
has been observed by inelastic neutron scattering
measurements [9]. In order to clarify the origin of pseudo
gap and investigate effects of magnetic fluctuation onelectronic state near Fermi level in FeTe1-xSex, further
work is necessary
References[1] S. Li et al., Phys. Rev. B 79, 054503 (2009).

Photon Factory Activity Report 2017 #35 (2018)[2] Y. Sun et al., Sci. Rep. 6, 32290 (2016)[3] Y. Koshika et al., J. Phys. Soc. Jpn. 82, 023703 (2013)[4] T. Otsuka et al., 2016 JPS autumn meeting[5] L. C. C. Ambolode, II et al., Phys. Rev. B 92, 035104(2015).
[6] K. Okazaki et al., Phys. Rev. Lett. 109, 237011 (2012).
[7] J. P. Sun et al., Phys. Rev. Lett. 118, 147004 (2017).
[8] Y. Zhang et al., Phys. Rev. B 82, 165113 (2010).
[9] Z. Xu et al., Phys. Rev. B 93, 104517 (2016).
* keisuke@wyvern.phys.u-tokyo.ac.jp