Photon Factory Activity Report 2016 #34 (2017)

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

Angel-resolved photoemission spectroscopy of Te-annealed
11-type iron-based superconductor FeTe1-xSex
Keisuke Koshiishi1,*, Takumi Otsuka2, Masafumi Horio1, Suguru Nakata1, Kenta Hagiwara1, Chun Lin1, Yuxuan Wan1,
Horoshi Kumigashira3, Kanta Ono3, Takao Watanabe2 and Atsushi Fujimori1
1 Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
2 Graduate School of Science and Technology, Hirosaki University, 3, Bunkyo-cho, Hirosaki, Aomori, 036-8561, Japan
3Photon Factory, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan

1 Introduction
11-type iron-based superconductor has the simplest
crystal structure in which layers consisting of Fe atoms
tetrahedrally coordinated by chalcogens (e.g. Te, Se)
stack altanatively. The parent compound FeTe has a bicollinear double-stripe antiferromagnetic (AFM) orderwith the wave vector Q=(π/2, π/2) that is distinct from theusual collinear magnetic structure observed in otherparent compounds like BaFe2As2 [1]. As tellurium isgradually replaced by selenium in the parent compound,
AFM order is suppressed and superconductivity isobserved. It has been reported that the superconductivity
is disrupted by excess Fe atoms located at interstitial sites
and is very sensitive to its stoichiometry. By annealing thespecimen, the excess Fe can be removed andsuperconductivity is recovered [2,3]. Superconductivity
with the superconducting transition temperature (TC) of~10 K is observed in a wide low-concentration region andthe maximum bulk TC reaches 14.5 K at x~0.4. Accordingto recent transport measurement on Te-annealed samples[4], The Hall coefficient (RH) of annealed FeTe0.6Se0.4
changes from positive to negative with decreasing
temperature, indicating that there are both hole andelectron carriers in the optimally doped samples. On theother hand, for FeTe0.8Se0.2, RH is always positive similar
to the case of as-grown samples. Thinking naively, thisbehavior indicates that contribution to transport properties
predominantly comes from hole carriers in contrast to theoptimally doped sample. Indeed, similar behavior in RH ofFeSe under pressure has been reported [5]. However, theFe-based superconductor has multi-orbital nature because
3d electrons of Fe form energy bands near the Fermi level.
Therefore, it is not clear whether such simple picture isvalid or not. Therefore, we have performed anglephotoemission spectroscopy (ARPES) measurements on
FeTe1-xSex in order to investigate differences in theelectronic structure between the optimally doped and low
Se-doped samples.
2 ExperimentHigh-quality single crystals of FeTe1-xSex were grownusing the Bridgman method and annealed under tellurium
vapor (“Te-anneal”). To check the TC and thesusceptibility, magnetization measurements were carriedout by using a Magnetic Property Measurement System5S system (Quantum Design) at the University of Tokyo
Cryogenic Research Center. ARPES measurements wereperformed at beamline 28A of Photon Factory using
circularly polarized light with the photon energy of 54 eVcorresponding to kz = 0. A SIENTA SES-2002 electron
analyzer was used. The total energy resolution was ~ 20meV. 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: ARPES results of 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 FermiDirac function along cut 1 and cut 2, respectively,
depicted by red arrows in panel (a). (e), (f) Second
derivatives of (c) and (d), respectively, with respect toenergy. (g), (h), (i), (j) Same as (c), (d), (e) and (f),respectively, but for x=0.2. (k) Comparison of MDCpeaks for two compositions.
3 Results and Discussion
ARPES results are shown in Fig. 1. The 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). An electron pocket
around the M point and strong intensity around the Γpoint coming from hole bands can be observed. ARPES
data for x=0.4 can be seen in Figs. 1(c) and (d). We also
show the second derivatives of them with respect toenergy in Figs. 1(e) and (f), respectively, in order toPhoton Factory Activity Report 2016 #34 (2017)highlight the band dispersions. 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 atthe corner of the Brillouin zone (Fig. 1(b)). In order tocompare the band structures between the twocompositions, we plot the peak positions of themomentum distribution curves (MDC) peak in Fig. 1(k).
We find that Fermi wave vector derived from the MDC
peaks changes with varying Se contents, and that theelectron pocket of the low Se-concentration sample islarger than that of the optimally doped sample. These
results suggest that the low Se-doped compounds as wellas the optimally doped compound can be regarded as amultiband system. It has been reported that Hall
coefficient of some electron-doped cuprates undergoes asign reversal as a function of temperature in spite of thefact that the Fermi surface of the electron-doped cuprates
is composed of a large hole pocket. The phenomena is,therefore, attributed to the scattering of carriers by
antiferromagnetic fluctuations, and a theoretical study
taking into account the vertex correction due toantiferromagnetic fluctuations can reproduce thetemperature dependence of RH for the electron-doped
cuprates [6]. Actually, the parent compound has an AFM
ordering, and spin correlations with the double stripe-type
antiferromagnetic wave vector QDSAF=(π/2, π/2) and thosewith the stripe-type antiferromagnetic wave vector QSAF=(π, 0) has been observed by inelastic neutron scattering
measurements [7]. Even in a previous study on FeSe
under pressure, correlation between the tendency of RH
with a hole-dominated character and enhanced spin
fluctuations is pointed out. Thus the magnetic scattering
may play an important role in the transport properties ofFeTe1-xSex. However, it should be mentioned that the sign
reversal in the RH of FeTe0.6Se0.4 is opposite to that in thecase of the electron-doped cuprates. Furthermore,
although 11-type iron-based superconductors are thesystem close to a Mott insulator due to sizable
renormalization factor of the compounds [8], FeTe0.6Se0.4
is different from the cuprate in that both electron and hole
pockets exist. Hence, it is not clear whether magnetic
fluctuations alone can explain the temperaturedependence of the transport properties in FeTe1-xSex.
Further studies are needed to investigate the effects ofmagnetic fluctuations on the electronic states near theFermi level related to the transport properties byestimating the scattering rate of quasi-particles.
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* keisuke@wyvern.phys.s.u-tokyo.ac.jp