Photon Factory Activity Report 2017 #35 (2018)

BL-6C/2015G526

Local structures in FeSe1-xTex high-temperature superconductor
Shinya HOSOKAWA,1,* Jens Rüdiger STELLHORN,1 Tomohiro MATSUSHITA,2Naohisa HAPPO,3 Koji KIMURA,4 Kouichi HAYASHI,4 and Yoshihiko TAKANO5
1 Department of Physics, Kumamoto University, Kumamoto 860-8555, Japan
2 Japan Synchrotron Radiation Research Institute (JASRI), Sayo 679-5198, Japan
3 Graduate School of Information Sciences, Hiroshima City University, Hiroshima 731-3194, Japan
4 Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 4668555, Japan
5 MANA, National Institute for Materials Science, Tsukuba 305-0047, JapanFe Ka x-ray fluorescence holography experiments were carried out to investigate the atomic scale structure of the hightemperature superconductor FeSe0.4Te0.6. Very clear images are obtained by a sparse modeling approach to theexperimental data using a L1-regularized linear regression. The results are in good agreement with previous findings ofXRD and XAFS experiments, but furthermore indicate large fluctuations in the Fe sub-lattice.
1 Introduction
By replacing Te with Se in a FeTe crystal, FeSe1-xTex
shows a superconducting nature with a criticaltemperature of about 15 K at x = 0.4~0.5 [1], although
FeTe exhibits no superconductivity. The chalcogen atoms
are located at slightly different positions in a PbO-type
unit cell as shown in Fig. 1. The different atomicenvironments of Se and Te were intensively studied by xray diffraction (XRD) [2], neutron diffraction (ND) [3],and x-ray absorption fine structure (XAFS) [4].
Based on an ab initio structure optimization of thestructures, a direct influence of the displacements of Se
and Te on the hybridization of the Fe orbitals and thecharge distribution was found [3,5], which can lead tomagnetic instabilities in the material.

Fig. 1: Layer structure in the FeSe0.4Te0.6 tetragonal
crystal [2,3]. Large-, medium- and small-sized balls
indicate Te, Fe and Se atoms, respectively. Taken fromRef. [7].
Thus, the local structure is a key point to understandthe superconducting properties of FeSe1-xTex. A newly

developed technique for atom-resolved characterizationsof the local structure of materials is x-ray fluorescenceholography (XFH) [6], which can provide threedimensional (3D) atomic images around a specificelement emitting fluorescent x-rays. Compared with otherexperimental methods, no specific model of the atomic
structure is necessary. In this work, we will present Fe Ka
XFH measurements on the FeSe0.4Te0.6 superconductor[7].
2 Experiment
A single crystal of FeSe0.4Te0.6 was grown using aslow-cooling method from the melt. In the slow-coolingmethod, a polycrystal raw sample was placed in a BNcrucible sealed in an Ar-filled Mo capsule. It was thenheated to 1150◦C, followed by cooling to 780◦C for 80 h,
and then cooled to room temperature. The crystal wascleaved with a flat (001) surface larger of about 3 ´ 3mm2.
The XFH technique is based on the angulardependence of the x-ray fluorescence intensity from anemitter atom caused by an interference of an incident xray wave with the waves scattered from nearby atoms.
The Fe Kα (6.403 keV) XFH measurements were carriedout at BL-6C of PF-KEK and at BL39XU of SPring-8.
For this experiment, the holograms were measured in theazimuthal angular range of 0◦ ≤ f ≤ 360◦ in steps of ~0.35◦
and in the incident angular range of 0◦ ≤ θ ≤ 75◦ in stepsof 1◦. They were expanded using the fourfold rotational
symmetry along the [001] axis and the three mirror
symmetries of the (100), (010), and (001) planes. The
sample was cooled to 100 K by a liquid N2 cryo-stream.
The XFH measurements were performed at eight
different incident energies between 7.5 and 11.0 keV insteps of 0.5 keV. Further details on the XFH method andits implementation can be found elsewhere [6]. The
recorded holograms were used to reconstruct the local
atomic environment around the Fe atoms in real space bya scattering pattern matrix extraction algorithm using L1

Photon Factory Activity Report 2017 #35 (2018)

regularized linear regression (SPEA-L1) [8] representing
a sparse modeling approach to the experimental data.

(a)0.5z (nm)3 Results and Discussion
The hologram measured at 9.0 keV is exemplary
displayed in Fig. 2(a) in an orthographic projection, incomparison with a hologram by computer simulation in(b). The color scale indicates the variation of thefluorescence intensity relative to the averaged value. The
hologram is centered at θ = 0◦, and the radial and angular
direction indicate θ and f, respectively. The magnitudesof the experimental and theoretical holograms are mostly
the same values of ± 0.4%, while the features of theholograms are different from each other, indicating thatthe real atomic configurations around the Fe atoms would
be different from the ideal ones, such as positional
fluctuations.

dotted lines. Note that there are two orientations of thetetrahedral, i.e. a half of the Fe atoms have chalcogenneighbors on the positive z-side, and a half have those on
the negative z-side, which are superimposed in this figure.
The peak of the chalcogen signal is located at a ’height’ (zcomponent of the coordinate) of 0.17 nm, with a slightshoulder near 0.15 nm, which are in good agreement withthe chalcogen heights reported by XRD (0.1478 nm for
Se and 0.1718 nm for Te [2]) and by XAFS investigations(0.147 nm for Se and 0.175 nm for Te [4]) for the similarFeSe1-xTex compounds. The intensity difference reflects
the atomic numbers of Se and Te and their concentration
ratio.

1.0TeSe

0.0Fe

0.8
0.6
0.4
0.2

-0.5
-0.5

0.0x (nm)0.5(b)
0.5

1.0y (nm)

0.8
0.6

0.0

0.4
0.2
-0.5
-0.5

0.0x (nm)0.5

Fig. 2: Orthographic projection of (a) the hologrammeasured at 9.0 keV in comparison with (b) that bycomputer simulation. The magnitude of the holographic
oscillations is given as a color bar. Taken from Ref. [7].

Fig. 3: Reconstructions on (a) the (010) plane and (b) the(001) plane in FeSe0.4Te0.6 from the experimental
holograms by SPEA-L1. The dashed lines indicate theunit cell, and the dotted lines in (a) indicate the expectedheights of the chalcogen atoms. Images are scaled withrespect to the strongest signal in (a) as given by the colorbars. Taken from Ref. [7].

The real-space reconstructions of the (010) and (001)planes in FeSe1-xTex are illustrated in Figs. 3(a) and (b),respectively. The Fe emitter atoms are located at thecenter of the figures. The magnitudes of the images arenormalized to that of the nearest neighboring atoms onthe (010) plane. The intensity scale is given as a color bar
beside the images, ranging between 0.1 and 1.0, i.e. thelower 10% of the image intensity is regarded as theexperimental noise level and not displayed here.
The reconstruction on the (010) plane in Fig. 3(a)shows the layer of the Fe atoms along the x axis, and thelayer of the first Se/Te neighbors as indicated with theThe face-centered Fe sub-lattice of on the (001) plane
is clearly visible in Fig. 3(b). Signals can be observed up
to the 4th Fe neighboring atoms. The sub-lattice is muchclearer and contains mostly no artificial signals as was in
the previous reconstructions [9], which may be attributed
to the improved reconstruction algorithm and to the lower
sample temperature. The images of Fe are, nevertheless,comparatively weak and widely spread. We interpret thisas a sign for large positional fluctuations of theneighboring Fe atoms.
In summary, Fe Ka XFH experiments were carried out

Photon Factory Activity Report 2017 #35 (2018)

to investigate the atomic scale structure of the hightemperature superconductor FeSe0.4Te0.6. Very clear
images are obtained by a sparse modeling approach to theexperimental data using a L1-regularized linear
regression. The results are in good agreement withprevious findings of XRD and XAFS experiments, but
furthermore indicate large fluctuations in the Fe sublattice.
Acknowledgement
The XFH experiments were performed at BL-6C ofKEK-PF (No. 2015G526) and at BL39XU at SPring-8(Nos: 2014B1187, 2015B1183, 2016A1136). This work
was supported by the Japan Society for the Promotion ofScience (JSPS) Grant-in-Aid for Scientific Research on
Innovative Areas ‘3D Active-Site Science’ (No. 26105006,26105013) and ‘Sparse Modeling’ (No. 16H01553). JRS
gratefully acknowledges financial support as Overseas
Researcher under the Postdoctoral Fellowship of JSPS
(No. P16796).
References[1] K.-W. Yeh, T.-W. Huang, Y.-L. Huang, T.-K. Chen, F.C. Hsu, P. M. Wu, Y.-C. Lee, Y.-Y. Chu, C.-L. Chen,and J.-Y. Luo, Europhys. Lett. 84, 37002 (2008).
[2] M. Tegel, C. Löhnert, and D. Johrendt, Solid State
Commun. 150, 383 (2010).
[3] D. Louca, K. Horigane, A. Llobet, R. Arita, S. Ji, N.
Katayama, S. Konbu, K. Nakamura, T.-Y. Koo, P.
Tong, and K. Yamada, Phys. Rev B 81, 134524 (2010).
[4] B. Joseph, A. Iadecola, A. Puri, L. Simonelli, Y.
Mizuguchi, Y. Takano, and N. L. Saini, Phys. Rev B
82, 020502(R) (2010).
[5] C.-Y. Moon and H. J. Choi, Phys. Rev. Lett. 104,057003 (2010).
[6] K. Hayashi, N. Happo, S. Hosokawa, W. Hu, and T.
Matsushita, J. Phys.: Condens. Matter 24, 093201(2012).
[7] J. R. Stellhorn, Y. Ideguchi, K. Kimura, K. Hayashi, N.
Happo, M. Suzuki, H. Okazaki, A. Yamashita, Y.
Takano, and S. Hosokawa, Phys. Stat. Sol. B, in press.
[8] T. Matsushita, e-J. Surf. Sci. Nanotech. 14, 158 (2016).
[9] Y. Ideguchi, K. Kamimura, K. Kimura, S. Hosokawa,
N. Happo, K. Hayashi, Y. Ebisu, T. Ozaki, J. R.
Stellhorn, M. Suzuki, H. Okazaki, A. Yamashita, andY. Takano, Z. Phys. Chem. 230, 489 (2016).
* shhosokawa@kumamoto-u.ac.jp