RAPID COMMUNICATIONSPHYSICAL REVIEW B 89, 140506(R) (2014)

Evolution from incoherent to coherent electronic states and its implications
for superconductivity in FeTe1−x Se x
E. Ieki,1 K. Nakayama,1 Y. Miyata,1 T. Sato,1 H. Miao,2 N. Xu,2 X.-P. Wang,2 P. Zhang,2 T. Qian,2 P. Richard,2,3 Z.-J. Xu,4J. S. Wen,4 G. D. Gu,4 H. Q. Luo,2 H.-H. Wen,5 H. Ding,2,3 and T. Takahashi1,612Department of Physics, Tohoku University, Sendai 980-8578, Japan
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
3
Collaborative Innovation Center of Quantum Matter, Beijing, China
4
Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
5
Center for Superconducting Physics and Materials, National Laboratory for Solid State Microstructures, Department of Physics,
Nanjing University, Nanjing 210093, China
6
WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan(Received 27 December 2013; revised manuscript received 16 April 2014; published 28 April 2014)
We have performed systematic angle-resolved photoemission spectroscopy (ARPES) of iron-chalcogenide
superconductor FeTe1−x Sex (0  x  0.45) to elucidate the electronic states relevant to the superconductivity.
While the Fermi-surface shape is nearly independent of x, we found that the ARPES spectral line shape shows
prominent x dependence. A broad ARPES spectrum characterized by a small quasiparticle weight at x =0, indicative of incoherent electronic states, becomes progressively sharper with increasing x, and a welldefined quasiparticle peak appears around x = 0.45 where bulk superconductivity is realized. The present result
suggests the evolution from incoherent to coherent electronic states and its close relationship to the emergenceof superconductivity.
DOI: 10.1103/PhysRevB.89.140506

PACS number(s): 74.25.Jb, 74.70.Xa, 79.60.−i

The discovery of superconductivity at 26 K in
LaFeAsO1−x Fx [1] and the subsequent increase of superconducting transition temperature (Tc ) up to ∼ 55 K [2–4]have evoked much attention to this new family of highTc materials called Fe superconductors. The recent reporton a possible higher-Tc superconductivity at ∼ 65 K in a
FeSe thin film [5,6] has brought further excitement andgenerated tremendous attention to this new class of Fe
superconductors, iron chalcogenide FeTe1−x Sex [7–11]. Fromthe crystallographic point of view, FeTe1−x Sex is regarded
as the simplest Fe superconductor, since it consists of only
Fe(Te,Se)-tetrahedra layers. It is also known that the end
material, FeTe (x = 0), exhibits a “bicollinear” antiferromagnetic order with the propagation vector of Q = (π/2,π/2),
which differs from that of the pnictide family exhibiting the“collinear” antiferromagnetic order with Q = (π,0) [11,12].
Another interesting aspect of FeTe1−x Sex is that the doping ofextra carriers is not necessary to induce superconductivity. The
isovalent substitution of Se for Te, instead of carrier doping,
suppresses the bicollinear antiferromagnetic order for x > 0.1,
and bulk superconductivity takes place around x = 0.3 [seeFig. 1(a)] [11]. Upon further substitution, Tc reaches the
maximum of ∼ 14 K around x = 0.5 and then decreases to
8 K at the end composition of x = 1 (FeSe).
According to band structure calculations, the Fermi surfaceof FeTe1−x Sex is basically similar to that of iron pnictides [14].
The parent antiferromagnetic and optimum superconducting
regimes have been intensively studied by angle-resolved
photoemission spectroscopy (ARPES), which has revealed
that FeTe in the normal state has holelike Fermi surfaces atthe  point and electronlike Fermi surfaces at the M point[Fig. 1(b)]. These Fermi surfaces are connected by Q = (π,0)
similarly to the case of iron pnictides [15]. Furthermore,
complex reconstruction of the electronic structure acrossthe antiferromagnetic transition temperature (TN ) has been
1098-0121/2014/89(14)/140506(5)reported [16,17]. In the superconducting samples with anearly optimum Tc value, the opening of nodeless superconducting gap has also been clarified [18–20]. The observed
Fermi-surface dependence of the superconducting gap has
been interpreted as a signature of s± -wave superconducting
pairing [18], consistent with the appearance of so-called
magnetic resonance [21,22] and the peculiar magnetic-fielddependence of Bogoliubov quasiparticle interference [23],both of which are expected for a superconducting-gap function
with a sign reversal. These ARPES studies gave important
insights into the origin of the antiferromagnetic and superconducting orders. However, due to the lack of systematic
study on the Se concentration dependence, some essential
questions remain unresolved, such as (i) how the electronic
states evolve from the parent antiferromagnet to the optimum
superconductor and (ii) which characteristic of the electronic
states triggers superconductivity. Clarifying the low-energy
electronic states as a function of Se concentration is thusof particular importance to answer these essential questions
as well as to gain further insights into the superconducting
mechanism.
In this Rapid Communication, we report high-resolution
APRES results on iron chalcogenide FeTe1−x Sex with various x
values (x = 0, 0.2, 0.3, 0.4, and 0.45) to show the evolution of
normal-state electronic states as a function of x. Our systematic
ARPES study definitely demonstrates that the shape of Fermi
surface is almost unchanged from optimum superconducting
regime (x = 0.45) to non-bulk-superconducting regime (x =0.2), in sharp contrast to the case of iron pnictides [24–31].
In addition, we found a significant spectral broadening in thevicinity of the antiferromagnetic phase, which characterizes anunusual metallic state at low Se concentrations. We compare
the present results with the reported physical properties anddiscuss the implications to the origin of the characteristic
superconducting phase diagram.

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PHYSICAL REVIEW B 89, 140506(R) (2014)(c) x = 0.45
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FIG. 1. (Color online) (a) Schematic phase diagram of FeTe1−x Sex derived from Refs. [9,11,13]. SC and AFM denote superconductivity
and antiferromagnetism, respectively. The miscible region exists at high Se concentrations due to the difficulty in growing single-phasesamples. (b) One-Fe/unit-cell Brillouin zone of FeTe1−x Sex used in this study together with the schematic hole and electron Fermi surfaces atthe  and M points, respectively. (c)–(g) Se-concentration dependence of normal-state ARPES spectra along the -M line in a wide energy
region for FeTe1−x Sex (T = 25 K for x = 0.45–0.2 and 80 K for x = 0.0) measured with the He-Iα resonance line (hν = 21.218 eV). (h)–(l)Corresponding ARPES intensity plotted as a function of binding energy and wave vector.

High-quality single crystals of FeTe1−x Sex were grownby the unidirectional solidification method. The nominal
compositions are FeTe (x = 0; TN = 67 K), FeTe0.8 Se0.2(x = 0.2), FeTe0.7 Se0.3 (x = 0.3; Tc = 13 K), FeTe0.6 Se0.4(x = 0.4; Tc = 14 K), and FeTe0.55 Se0.45 (x = 0.45; Tc =14.5 K). Energy-dispersive x-ray spectroscopy shows thatthe amount of excess iron atom residing at interstitial sites
is as low as 0.03. High-resolution ARPES measurements
were performed with a VG-Scienta SES2002 spectrometerand a He discharge lamp (hν = 21.218 eV) at Tohoku
University. ARPES measurements were also performed withsynchrotron radiation at BL-28A at Photon Factory (KEK)
using a VG-Scienta SES2002 spectrometer with circularly
polarized 44 eV photons, and at BL-7U at UVSOR using a
MBS-A1 spectrometer with linearly polarized 21 eV photons.
The energy and angular resolutions were set at 6–12 meV
and 0.2◦ , respectively. Clean sample surfaces were obtained
for the ARPES measurements by cleaving crystals in situ
in an ultrahigh vacuum of 1 × 10−10 Torr. The Fermi level(EF ) of the samples was referenced to that of a gold film
evaporated onto the sample holder. In this study, we adopt theone-Fe/unit-cell description where Q = (π,0) corresponds to
Q = (π,π ) of the two-Fe/unit-cell description adopted in ourearlier study [20].
Figures 1(c)–1(g) compare the normal-state ARPES spectra
along the -M line [Fig. 1(b)] for different Se compositions.
The corresponding ARPES-intensity plots as a function ofbinding energy and wave vector are displayed in Figs. 1(h)–1(l). As most clearly seen in the highest-Tc sample (x = 0.45),
there are holelike bands approaching EF in addition to adispersive prominent band at ∼ 300 meV binding energy
near the  point. We also find a less dispersive band at∼ 50 meV and a weak but finite intensity near EF aroundthe M point. The latter weak intensity signifies the presenceof an electron pocket as we demonstrate later. While the band

structure of the five compositions is qualitatively similar, we
definitely recognize the broadening of the peak structure andthe reduction of intensity with decreasing x. We will return tothis point later.
To investigate in more detail the low-energy electronic
states directly responsible for superconductivity, we focus ona narrower energy range in the vicinity of EF . The ARPES
intensity at EF for different compositions is displayed inFig. 2(a). We clearly identify an essentially similar intensitydistribution pattern for different x, with bright intensity aroundthe  point and relatively weak intensity around the M point,
which correspond to the hole and electron Fermi surfaces,
respectively. The near-EF band dispersions around the  point
are composed of three holelike bands labeled α, α  , and β[Figs. 2(b)–2(g)], which are assigned to the even combinationof the dxz and dyz orbitals, the odd combination of the dxz anddyz orbitals, and the dxy orbital, respectively [18]. At x = 0.45,
the α  and β bands create hole Fermi surfaces, as seen in
Fig. 2(a). While these spectral features of bands become lessclear with decreasing x, we can still trace the dispersions of
the α  and β bands until the compositions of x = 0.0 and 0.2,
respectively. The extracted data set in Fig. 2(h) reveals that theholelike bands crossing/touching EF do not exhibit significant
x dependence (note that the α band is not clearly resolved forx  0.3, probably because of the strong spectral broadening
and/or a change in the band energy).
Around the M point, a shallow electron pocket (γ ) is
observed at x = 0.45 [Fig. 2(i)]. Since the intensity ofthe γ band is already weak at x = 0.45, its dispersion is
rapidly smeared out with the reduction of Se concentration(not shown). Nevertheless, the momentum distribution curve(MDC) at EF exhibits a peak structure showing that the γband resides near EF even at x = 0.2 [see Fig. 3(b)]. The
peak position (ky ∼ 0.1 π/a) corresponding to the Fermi
wave vector (kF ) is unchanged for 0.2  x  0.45, although

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80 40 EF
40 EF
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FIG. 2. (Color online) (a) Comparison of ARPES-intensity plot at EF as a function of two-dimensional wave vector in the normal state.
The intensity is obtained by integrating the spectral intensity within ±10 meV with respect to EF . The experimental Fermi surface of x = 0.45
(black dashed curves) determined by smoothly connecting the kF points is superimposed on each figure for comparison. (b)–(d) High-resolution
ARPES spectra near EF and (e)–(g) corresponding second-derivative intensity plots around the  point at T = 25 K for x = 0.45 and 0.2, and
80 K for x = 0.0. (h) Comparison of experimental band dispersions in the vicinity of EF determined by tracing the peak position in ARPES
spectra. (i) Near-EF ARPES spectra measured around the M point at 25 K. (j) High-resolution ARPES spectra near the  point obtained in
the bicollinear antiferromagnetic phase (20 K) in FeTe (TN = 67 K). A shallow electron pocket induced by the reconstruction of the electronic
structure due to the antiferromagnetic transition is observed. Black dots in (b)–(d) and (i) are guides for eyes to trace the band dispersions.

the kF position of the γ band is no longer well defined at
x = 0, as in a previous study [16]. This result, combined withthe x-insensitive behavior of the α  and β bands near the point, suggests that the shape of the Fermi surface is approximately independent for 0.2  x  0.45, even when the Se
concentration is altered from the bulk-superconducting regime(e.g., x = 0.45) to the non-bulk-superconducting regime(x = 0.2).
The observed invariable Fermi-surface shape is a uniqueproperty of FeTe1−x Sex , in contrast to chemically doped
and isovalent-substituted iron pnictides, where a systematicand drastic change in the Fermi-surface size and topology
takes place [24–31]. In the context of the weak-coupling
approach, ill-defined nesting of Fermi surface causes thedisappearance of superconductivity. However, the present
result strongly suggests that the Tc value is not linked to thenesting condition in FeTe1−x Sex because similar Fermi-surface
nesting conditions are retained in both bulk-superconducting
and non-bulk-superconducting compositions. By taking into
account the absence of a good Fermi-surface nesting via
Q = (π,0) in Ax Fe2−y Se2 [32] and monolayer FeSe thinfilm [6] regardless of their high-Tc values, there would exist
no common relationship between Tc and the nesting conditionin iron-chalcogenide superconductors. This would suggestthat the Fermi-surface nesting is not important for superconductivity in these systems, while we cannot completely
exclude the possible role of nesting in FeTe1−x Sex becauseof the good correspondence between the development of
Q = (π,0) antiferromagnetic fluctuations and the emergenceof superconductivity [11] in the well-nested samples [20]. In
either case (the Fermi-surface nesting is important or not), thepresent results indicate that the presence of a factor which is
not directly associated with the nesting should be taken into

account to understand the suppression of superconductivity at
low Se concentrations.
To answer the above question and further evaluate theelectronic states, we turn our attention to the ARPES spectralline shape, which exhibits a striking x dependence in contrast
to the Fermi-surface shape. Figure 3(a) compares the normalstate ARPES spectrum near the  point for various x values.
While a well-defined quasiparticle peak due to the α  band
is observed for x  0.4, the peak intensity is drasticallysuppressed and simultaneously broadened with decreasing x[see inset to Fig. 3(a)], and finally almost vanishes at x = 0,
suggesting that the electronic states become incoherent at
x = 0 [33]. A similar quasiparticle-peak suppression is also
recognized for the electron band around the M point. One canclearly find in Fig. 3(b) that the MDC peak around the M point
is significantly broadened at the low Se concentrations. The
present results thus clearly show the occurrence of progressive
evolution from coherent to incoherent electronic states taking
place with decreasing x. We have confirmed the reproducibilityof such an observation by repeating the measurements at least
twice for each concentration x.
Next we discuss the relationship between the evolution ofcoherent/incoherent electronic states and the disappearanceof superconductivity. In Figs. 3(c)–3(f), we show the phase
diagram of FeTe1−x Sex compared with several characteristic
quantities extracted from the present ARPES and previous
studies [11]. We immediately notice that the increase of
the Se concentration gives rise to a marked sharpening of thequasiparticle-peak width [Fig. 3(d)] and an increase of
the quasiparticle-peak intensity [Fig. 3(e)] as explained
above. Interestingly, the x dependence of the quasiparticlepeak intensity resembles the bulk superconducting-dome
shape, suggesting that the development of well-defined

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PHYSICAL REVIEW B 89, 140506(R) (2014)

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FIG. 3. (Color online) (a) Se-concentration dependence of thenear-EF ARPES spectrum at 20 K for x = 0.2–0.45 and 80 K for
x = 0.0 measured at ky ∼ 0.2 π/a where the quasiparticle-peakintensity of the α  band is dominant (note that we did not selectthe ARPES spectrum at the kF point since it is strongly influenced by
the α band). Each spectrum is normalized to the intensity at 100 meV.
The ARPES spectra after subtracting a Shirley-type backgroundare plotted in the inset, in which each spectrum is normalized tothe intensity of the peak maximum to demonstrate the spectral
broadening. (b) MDCs at EF along the (π,0)-(π,π ) line for various
Se concentrations. Solid curves represent the numerical fittings with
two Lorentzians. MDC peak positions extracted from the fitting areshown in the inset, where blue and red circles denote the peak
position for negative and positive ky regions, respectively. (c) Phase
diagram of Fe1.02 Te1−x Sex [11]. (d) Full width at half maximum of
the EDC peak in the inset to (a) (blue filled circle) and the MDC
peak in (b) (red filled circle) plotted as a function of x. We foundthat sample-to-sample variation is much smaller than the observed
x dependence as highlighted by the comparison of open and filled
circles. (e) x dependence of the quasiparticle-peak intensity of theα  band (blue filled circle) extracted from ARPES spectra in (a).
The MDC peak intensity extracted from (b) is also plotted using
green filled triangles (ky < 0) and orange filled triangles (ky > 0).
The sample-to-sample variation is also included by open symbols. (f)The superconducting volume fraction (red circle) together with thedρ/dT value at 35 K [11].

quasiparticles in the normal state triggers bulk superconductivity in FeTe1−x Sex . The present results agree well with thetransport and superconducting properties, as explained below.
As seen in Fig. 3(f), the resistivity at 35 K (slightly aboveTc ) exhibits a metallic behavior (dρ/dT > 0) for x > 0.3 [seeblack diamond in Fig. 3(f)], whereas the negative dρ/dT value
for 0.1 < x < 0.3 signifies weak charge carrier localization.
For x < 0.1, the positive dρ/dT value is associated with theoccurrence of the antiferromagnetic transition, and the charge
carriers are still weakly localized (dρ/dT < 0) above TN , assuggested by the absence of a Drude peak in the optical conductivity spectra [34]. These characteristic physical properties are

consistent with the observed evolution of coherent/incoherent
electronic states in the normal state. We also note that a sharp
quasiparticle peak expected for the metallic transport in theantiferromagnetic phase in FeTe is clearly identified aroundthe  point below TN [Fig. 2(j)]. The good agreement of the
ARPES result with the transport properties demonstrates thatthe observed electronic states, including the incoherent featurein the normal state, certainly reflect the inherent characteristics
of FeTe1−x Sex . Most importantly, the superconducting volume
fraction, which is close to 100% (−4π χ ∼ 1) at x ∼ 0.45
[red circle in Fig. 3(f)], gradually decreases and eventuallyvanishes to ∼ 0% across the Se concentration of ∼ 0.3, at
which the dρ/dT value changes its sign. This demonstrates
the close relationship between the emergence of incoherent
normal states and the destruction of bulk superconductivity at
low Se concentrations.
The origin of the incoherent electronic states at low Se
concentrations is crucial for fully understanding the superconducting mechanism. Excess iron is known to promote
the carrier localization and suppress the bulk superconductivity, and thus it may be responsible for the incoherent
states; e.g., Fe1.03 Te0.63 Se0.37 is a bulk superconductor, while
Fe1.11 Te0.64 Se0.36 is not [35,36]. However, this would not
be the main cause of the present observation, because
the ARPES spectrum becomes drastically broadened evenwhen the amount of excess iron is as low as 0.03 [seeFigs. 3(a) and 3(b)]. We thus propose that either bicollinear
antiferromagnetic fluctuations or electronic correlations are
responsible for the incoherent states. In the former case,the bicollinear antiferromagnetic fluctuations strongly scatter
itinerant electrons to make them weakly localized. In thelatter case, the large on-site Coulomb interaction pushes
the system close to the Mott metal-insulator transition. The
electronic correlations driven by the Hund’s rule coupling
may play an additional important role like in manganites[37–39].
In conclusion, we reported high-resolution ARPES resultsof FeTe1−x Sex with various Se concentrations (0  x  0.45).
We found that the Fermi-surface shape does not show aclear difference for 0.2  x  0.45. We also observed thatthe broad normal-state ARPES spectrum characterized by asmall quasiparticle weight in FeTe progressively transforms
into a sharp well-defined quasiparticle peak upon increasing
the Se concentration. The present results suggest that the
suppression of superconductivity at low Se concentrations is
not caused by the deterioration of the nesting condition, but israther associated with the incoherent electronic states produced
by an additional factor such as bicollinear antiferromagnetic
fluctuations and electronic correlations.
We thank Y. Tanaka, H. Kumigashira, K. Ono, M.
Matsunami, and S. Kimura for their assistance in ARPES
measurements. This work was supported by grants fromthe Japan Society for the Promotion of Science (JSPS), theMinistry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, the Chinese Academy of Sciences(CAS), the National Science Foundation of China (NSFC),
the Ministry of Science and Technology (MOST) of China,
KEK-PF (Proposals No. 2009S2-005 and No. 2012S2-001),
and UVSOR (Proposal No. 22-540).

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