Photon Factory Activity Report 2018 #36 (2019)

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

Angle-resolved photoemission spectroscopy study ofTe-annealed superconducting FeTe1-xSexKeisuke KOSHIISHI1,*, Takumi OTSUKA2, Chun LIN1, Masahiro SUZUKI1, Yuxuan WAN1,Hiroshi KUMIGASHIRA3, Koji HORIBA3, Takao WATANABE2 and Atsushi FUJIMORI1
1
2Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan

Graduate School of Science and Technology, Hirosaki University, 3, Bunkyo-cho, Hirosaki-shi, Aomori-ken, 036-8561,Japan
3Photon 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 doublestripe antiferromagnetic (AFM) order with the wave vector
Q=(π/2, π/2), which is distinct from the usual collinear
magnetic structure observed in other parent compounds
like BaFe2As2 [1]. As tellurium in the parent compound isgradually replaced by selenium, the AFM order issuppressed and superconductivity is observed. It has beenreported that the superconductivity is affected by excess Fe
located at interstitial sites and very sensitive to its
stoichiometry. By annealing a specimen, the excess Fe canbe removed, and then the superconductivity can beimproved [2,3]. The superconductivity with thesuperconducting transition temperature (Tc) of ~10 K is
observed in the 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 a recent transport measurement for
Te-annealed samples [4], although the Hall coefficient (RH)of annealed FeTe0.6Se0.4 is almost independent oftemperature 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 open. 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 states of Fe form bands near the Fermi level, it is notclear whether such simple scenario is valid or not.
Therefore,
we
have
performed
angle-resolved
photoemission spectroscopy (ARPES) measurements onFeTe1-xSex in order to investigate differences in theelectronic structure between the optimally doped and low
Se-doped samples and the temperature dependence of theirelectronic structures.
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, magnetization
measurements were carried out by using a Magnetic
Property Measurement System-5S (Quantum Design, Co.,
Ltd.) at The University of Tokyo Cryogenic ResearchCenter. ARPES measurements were performed atbeamline 28A of Photon Factory using circularly
polarized light with the photon energy of 54 eVcorresponding to kz  0. A SIENTA SES-2002 electronanalyzer was used with the total energy resolution of ~ 20
meV. The crystals were cleaved in situ below T = 20 K,
and the measurements were carried out in an ultrahighvacuum of ~ 9×10-11 Torr.

Fig. 1: Temperature evolution of the electron bands aroundthe zone corner of Te-annealed FeTe1-xSex (x=0.2 and 0.4).
(a) In-plane Fermi surface of x=0.2. (b), (c) Temperature
evolution of the energy-momentum plots around the M
point and those symmetrized with respects to the Fermi
level to highlight Fermi momentum positions, respectively.
(d), (e), (f) The same as (a), (b) and (c) for x=0.4,

Photon Factory Activity Report 2018 #36 (2019)respectively. Cuts are along red arrows in (a) and (d).
3 Results and Discussion
Figure 1 shows the temperature evolution of the electronband around the zone corner of Te-annealed FeTe1-xSex(x=0.2 and 0.4) from 10 K up to 200 K. The cuts are along
the red arrows in Figs. 1(a) and 1(d). Intensity plots areshown in Figs. 1(b) and 1(e). One can see that the electronband is broadened and becomes indistinct with increasing
temperature due to temperature broadening. To highlightthe kF positions, the ARPES spectra symmetrized along theenergy direction with respects to EF are shown in Figs. 1(c)and 1(f). Interestingly, in the case of x=0.2, distance
between the different kF’s seems to expand with increasing
temperature, suggesting the expansion of the electron
pocket. For x=0.4, as temperature increases, spectral
intensity is weakened and eventually disappears. One may
suspect the possibility of electron doping or impurity
scattering on the sample surface due to contamination of
the sample surface. However, in our measurements, wehave also measured ARPES spectra with cooling from thehighest temperature (180 K for x=0,2 and 200 K for x=0.4)
to the lowest temperature (10 K for x=0.2 and 8 K for
x=0.4) after heating, and similar spectra were obtained at
the lowest temperature. This reproducibility proves that thetemperature evolution of the observed spectra is intrinsic.

Fig. 2: Temperature evolution of the spectra of the electronband in Te-annealed FeTe0.8Se0.2. (a) ARPES spectra at 10K. (b) Temperature evolution of the momentum
distribution curve (MDC) integrated within EF ±10 meVfrom 10 K up to 180 K. (c) Temperature evolution of theenergy distribution curve (EDC) obtained by integrating
MDCs within momentum region of kF ±0.05π/a indicatedby shaded section in (a). (d) Symmetrized EDCs shown in(c) with respects to EF. (f) Temperature dependence ofspectral weight around EF. The intensity has been byintegrating the symmetrized EDCs within the energy rangeof EF±10 meV, and normalized to that at the lowest
temperature (10 K).
For more detailed analysis, we have plotted themomentum distribution curve (MDCs) and EDCs atdifferent temperatures in Figs. 2 and 3. As has been pointed
out above, for x=0.2, an MDC-peak shift with temperature
can be recognized (Fig. 2(b)). In Fig. 2(c), a small shoulder
structure around -10 meV arising from the electron bandnear EF was observed at 10 K, and it is attenuated at hightemperatures due to the temperature broadening. For x=0.4,
although MDC and EDC peaks can be recognized in thelow temperature region, both intensities were too weak to
examine peak positions above 100 K. Their trends are
similar to each other. Namely, as temperature increases,
the peak structure is obscured. EDCs that have beensymmetrized in order to eliminate the temperaturebroadening are also shown in Figs. 2(d) and 3(d).

Fig. 3: Temperature evolution of the spectra of the electronband in Te-annealed optimally-doped FeTe0.6Se0.4. (a)ARPES spectra at 8 K. (b) Temperature evolution of theMDC integrated within EF ±10 meV from 8 K up to 200 K.
(c) Temperature evolution of the EDC obtained byintegrating MDCs within momentum region of kF ±0.05π/a
indicated by shaded section in (a). (d) Symmetrized EDCs
shown in (c) with respects to EF. (f) Temperaturedependence of spectral weight around EF. The intensity
ratio has been obtained by integrating the symmetrized
EDCs within the energy range of EF±10 meV, andnormalized to that at the lowest temperature (8 K).
For quantitative comparison of the density of states(DOS) near EF, in Figs. 2(f) and 3(f), we have plotted
intensities near EF obtained by integrating symmetrized
EDC within the energy range of EF±10 meV as a functionof temperature. They have been normalized to the intensity
at the lowest temperature (10 K for x=0.2 and 8 K for
x=0.4). In the case of the Se-underdoped sample, theintensity near the Fermi level is nearly constant or grows
slightly with heating, indicating the insensitivity of theDOS near EF to temperature. On the other hand, theintensity near EF of the optimally-doped =0.4 sample
depends on temperature in a characteristic way (Fig. 3(f)).
The intensity decreases with increasing temperature in thetemperature region below 100 K although it is independentof temperature above l30 K, which suggests that the DOSof the electron band around the zone corner decreases withincreasing temperature, indicating the suppression of thecoherent peak of the electron band and the disappearanceof the electron pocket, in contrast to Se-underdopedcompound. Therefore, we consider that spectral weight istransferred from the coherent states to incoherent stateswith temperature, similar to the case of the dxy band around [5]. According to the magneto-transport measurements,
the Hall coefficient of the optimally-doped compound also
depends on temperature only blow ~150 K and increases
with heating in the temperature region below ~75 K,
similar to the temperature dependence of the coherent peakof the electron band in the optimally doped compound. For
the relationship between the Hall coefficient and theelectronic states, we have depicted schematic pictures forPhoton Factory Activity Report 2018 #36 (2019)

the Fermi surfaces predicted by present results in Fig. 4. In
the low Se-doped compound, the electron band does notexhibit drastic changes within the temperature range below
200 K except for the slight kF shift. If the hole surfaces also
do not change, the Fermi surfaces should not change. Inthat case, one cannot attribute the temperature dependenceof the Hall coefficient to the evolution of the Fermi surface.
One possibility is that antiferromagnetic fluctuation
corresponding to the antiferromagnetic order observed in
the parent compound FeTe affects charge carriers incompounds close to the parent compound. In the case ofcuprates [6], it has been theoretically suggested that
antiferromagnetic fluctuations can give rise non-trivial
temperature dependence of the Hall coefficient, which
differs from the prediction from the Fermi surface
topology. On the other hand, for the optimally-dopedcompound, we could not observed the electron band at hightemperatures, indicating the disappearance of the electron
pocket. Assuming that the hole bands in the Te-annealed
sample exhibit the temperature dependence similar to asgrown samples [5], the optimally doped compound would
have only a small hole pocket centered at . Taking into
account the charge carrier concentration estimated fromthe enclosed area of the Fermi surface at low temperature,
the charge carrier concentration is 0.03 holes/unit cell in
x=0.4 in the high temperature region, which is comparable
to that of x=0.2. The similar carrier concentrations may
cause similar magnitudes of Hall coefficient between themat high temperature. In order to completely clarify thetemperature evolution of the electronic states of Teannealed FeTe1-xSex, the hole bands at high temperatures
remain to be studied systematically.

Fig. 4: Schematic picture of the temperature evolution ofthe Fermi surface in Te-annealed FeTe1-xSex.
In conclusion, in order to clarify the temperature
evolution of the band structure and the relationshipbetween the Hall coefficient and the electronic structure, inFeTe1-xSex, in which the excess Fe atoms are completelyremoved, we have carried out ARPES measurements ofTe-annealed FeTe1-xSex (x=0.2 and 0.4). Upon increasing
temperature, the coherent peak of the electron band aroundthe zone corner weakens and disappears above 100 K in

the optimally-doped compound in contrast to its
temperature independent behavior for the Se-underdoped
compound, indicating the disappearance of the electron
pockets. Moreover, the possible changes of the Fermi
surface topology predicted from the present results explain
the temperature dependence of the Hall coefficient.
However, since we have clarified only the temperaturedependence of the electron band and the hole bands at hightemperature is still uncertain, further work is necessary inorder to fully clarify the temperature evolution of theelectronic states in Te-annealed FeTe1-xSex.
References[1] S. Li et al., Phys. Rev. B 79, 054503 (2009).
[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., Phys. Rev. B 99, 184505 (2019)[5] M. Yi et al., Nat. Commun. 6, 7777 (2015).
[6] H. Kontani, Reports Prog. Phys. 71, 026501 (2008)* keisuke@wyvern.phys.u-tokyo.ac.jp