**1. Introduction**

20 Superconductors – Materials, Properties and Applications

Magnetic Anions, M., J. Phys. Soc. Jpn. 71, 826 (2002).

[29] Mori, T. and Katsuhara, Estimation of πd-Interactions in Organic Conductors Including

A recent study that identified high temperature superconductivity in Fe-based quatenary oxypnictides has generated a considerable amount of activity closely resembling the cuprate superconductivity discovered in the 1980s (Kamihare et al., 2008; Takahashi et al., 2008; Ren et al., 2008). This system is the first in which Fe plays an essential role in the occurrence of superconductivity. Fe generally has magnetic moments, tending to form an ordered magnetic state. Neutron-scattering experiments have demonstrated that mediated superconducting pairing may originate from magnetic fluctuations, similar to our understanding of that in high-*Tc* cuprates (de la Cruz et al., 2008; Xu et al., 2008). Binary superconductor FeSex is another example of a Fe-based superconductor with a less toxic property, leading to the discovery of several superconducting compounds (Hsu et al., 2008). The *Tc* value of FeSe is ~8 K in bulk form and exhibits a compositional dependence such that *Tc* decreases for over-doping or under-doping of compounds (McQueen et al., 2009; Wu et al., 2009), as does that of high-*Tc* cuprates. FeSe has received a significant amount of attention owing to its simple tetragonal symmetry P4/nmm crystalline structure, comprising a stack of layers of edge-sharing FeSe4 tetrahedron. The phase of FeSe heavily depends on Se deficiency and annealing temperature. While 400 °C annealing reduces the nonsuperconducting NiAs-type hexagonal phase and increases the PbO-type tetragonal superconducting phase (Hsu et al., McQueen et al., 2009; Wu et al., 2009; Mok et al., 2009), the role of Se deficiency remains unclear. Notably, this binary system is isostructural with the FeAs layer in quaternary iron arsenide. Also, band-structure calculations indicate that FeSe- and FeAs-based compounds have similar Fermi-surface structures (Ma et al., 2009), implying that this simple binary compound may significantly contribute to efforts to elucidate the origin of high-temperature superconductivity in these emerging Fe-based compounds. Therefore, although the electronic structure is of great importance in this respect, spectroscopic measurements are still limited.

© 2012 Chen and Dong, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Chen and Dong, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

According to investigations on how fluorine doping (Kamihara et al., 2008; Dong et al., 2008) and rare earth substitutions (Yang et al., 2009) influence the superconductivity in LaO1 xFxFeAs compounds, x-ray absorption spectroscopy (Kroll et al., 2008), x-ray photoemission spectroscopy (Malaeb et al., 2008) and resonant x-ray inelastic scattering (Yang et al., 2009) results, Fe 3*d* states hybridize with the As 4*p* states, leading to a situation in which itinerant charge carriers (electrons) are responsible for superconductivity. Most of these studies suggest moderate to weak correlate correlations in this system. Photoemission spectroscopy (PES) measurements (A. Yamasaki et al., 2010) support the density of state (DOS) calculations on the FeSe*x* system. These results indicate the Fe-Se hybridization and itinerancy with weak to moderate electronic correlations (Yoshida et al., 2009), while recent theoretical calculations have suggested strong correlations (Aichhorn et al., 2010; Pourret et al., 2011). While fluorine substitution leads to electron doping in the LaO1-xFxFeAs system, exactly how Se deficiency may bring in the mobile carriers in the FeSex system to ultimately lead to superconductivity remains unclear. Therefore, this study elucidates the electronic structure of FeSex (*x*=1~0.8) crystals by using XAS Fe and Se K-edge spectra. Powder x-ray diffraction (XRD) measurements confirm the lattice distortion. Analytical results further demonstrate a lattice distortion and Fe-Se hybridization, which are responsible for producing itinerant charge carriers in this system.

X-Ray Spectroscopy Studies of Iron Chalcogenides 23

correlated system. Moreover, its electronic correlation differs markedly from that of "1111" and "122" compounds, perhaps due to the subtle differences between the *p*-*d* hybridizations in the Fe-pnictides and the FeSe "11" system. This postulation corresponds to the observation of *p*-*d* hybridization, as discussed later. This postulation is also supported by recent DMFT calculations, which demonstrate that correlations are overestimated largely owing to an incomplete understanding of the hybridization between the Fe *d* and pnictogen *p* states (Aichhorn et al., 2009). Nakayama *et al.* (2010) discussed the pairing mechanism based on interband scattering, which has a signature of Fermi surface nesting in ARPES. Based on the SC gap value, their estimations suggest that the system is highly correlated (Nakayama et al., 2010). Moreover, a combined electron paramagnetic resonance (EPR) and NMR study of FeSe0.42Te0.58 superconductor has indicated the coexistence of electronic itinerant and localized states (Arčon et al., 2010). The coupling of the intrinsic state with localized character to itinerant electrons exhibits similarities with the Kondo effect, which is regarded as a typical interaction of a strongly correlated electron system. The localized state is characteristic of strong electron correlations and makes the FeSe "11" family a close relative of high-*Tc* superconductors. Comparing the XAS and RIXS spectra reveals that FeSe1-yTey is unlikely a weakly correlated system, thus differing from other Fe-based quaternary oxypnictides. The charge transfer between Se-Te and the Se 4*p* hole state induced by the substitution is strongly correlated with the superconducting behavior. Above results suggest strong electronic correlations in the FeSe "11" system, as discussed later in detail.

FeSe*x* crystals were grown by a high temperature solution method described elsewhere (Wu et al., 2009; Mok et al., 2009). Crystals measuring 5 mm x 5 mm x 0.2 mm with (101) plate like habit could be obtained by this method. Three compositions results of FeSe*x* crystals (*x*=0.91, 0.88 and 0.85) are presented here for comparison and clarity. Additionally, large layered single crystals of high-quality FeSe1-yTey were grown using an optical zone-melting growth method. Elemental powders of FeSe1−*<sup>y</sup>*Te*y* were loaded into a double quartz ampoule, which was evacuated and sealed. The ampoule was loaded into an optical floating-zone furnace, in which 2 x 1500 W halogen lamps were installed as infrared radiation sources. The ampoule moved at a rate of 1.5 mm/h. As-grown crystals were subsequently homogenized by annealing at 700 ~800 °C for 48 hours, and at 420 °C and for another 30 hours. Chemical compositions of FeSe1-*y*Te*y* single crystals were determined by a Joel scanning electron microscope (SEM) coupled with an energy dispersive x-ray spectrometer (EDS) (Yeh et al., 2009; Yeh et al., 2008). In the Te substitution series, the composition of nominal *y*=0.3 was FeSe0.56Te0.41; that of nominal y=0.5 was FeSe0.39Te0.57; that of nominal *y*=0.7 was FeSe0.25Te0.72, and that of nominal *y*=1.0 was FeTe0.91. The grown crystals were characterized by a Philips Xpert XRD system; *Tc* was confirmed by both

transport and magnetic measurements (Wu et al., 2009; Mok et al., 2009).

X-ray absorption spectroscopy (XAS) provides insight into the symmetry of the unoccupied electronic states. The measurements at the Fe K-edge of chalcogenides were carried out at the 17C1 and 01C Wiggler beamlines at the National Synchrotron Radiation Reach Center

**2. Experiments** 

As mentioned earlier, although band-structure calculations indicate that FeSe and FeAsbased compounds have similar Fermi-surface structures, the poor quality of crystals arising from serious oxidization at their surfaces inhibit spectral measurements on pure (stoichiometric) FeSe. Also, FeSe exhibits an unstable crystalline structure. Therefore, investigating the effect of chemical substitution, at either the Se or Fe site, is a promising means of maintaining or improving the superconducting behavior on one hand and stabilizing the crystal structure on the other. Te doping of the layered FeSe with the PbO structure modifies its superconducting behavior, with a maximum *Tc* of ~ 15 K when Te replaces half of the Se. The improvement of *Tc*, which is correlated with the structural distortion that originates from Te substitution, is owing to the combined effect of lattice disorder, arising from the substitution of larger ions, and electronic interaction. Since layered FeSe1-yTey crystals are readily cleaved and highly crystalline, x-ray spectra of layered FeSe1-yTey crystals can provide clearer information about the electronic structure than those of FeSe crystals. Therefore, this study investigates the electronic properties of FeSe1-yTey (*y*=0~1) single crystals by using XAS and RIXS. XAS is a highly effective means of probing the crystal field and electronic interactions. The excitation-induced energy-loss features in RIXS can reflect the strength of the electron correlation. During their experimental and theoretical work on Fe-pnictides, Yang *et al.* (2009) addressed the issues regarding the Febased quaternary oxypnictides. However, few Fe-Se samples of this class have been investigated from a spectroscopic perspective. Angle-resolved photoemission (ARPES) combined with DFT band structure calculation on "11" Fe-based superconductor FeSe0.42Te0.58 reveals effective carrier mass enhancement, which is characteristic of a strongly electronic correlation (Tamai et al., 2010). This finding is supported by a large Sommerfeld coefficient, ~ 39 mJ/mol K (de la Cruz et al., 2008; Sales et al., 2009) from specific heat measurement. This phenomenon reveals that the FeSe "11" system is regarded as a strongly correlated system. Moreover, its electronic correlation differs markedly from that of "1111" and "122" compounds, perhaps due to the subtle differences between the *p*-*d* hybridizations in the Fe-pnictides and the FeSe "11" system. This postulation corresponds to the observation of *p*-*d* hybridization, as discussed later. This postulation is also supported by recent DMFT calculations, which demonstrate that correlations are overestimated largely owing to an incomplete understanding of the hybridization between the Fe *d* and pnictogen *p* states (Aichhorn et al., 2009). Nakayama *et al.* (2010) discussed the pairing mechanism based on interband scattering, which has a signature of Fermi surface nesting in ARPES. Based on the SC gap value, their estimations suggest that the system is highly correlated (Nakayama et al., 2010). Moreover, a combined electron paramagnetic resonance (EPR) and NMR study of FeSe0.42Te0.58 superconductor has indicated the coexistence of electronic itinerant and localized states (Arčon et al., 2010). The coupling of the intrinsic state with localized character to itinerant electrons exhibits similarities with the Kondo effect, which is regarded as a typical interaction of a strongly correlated electron system. The localized state is characteristic of strong electron correlations and makes the FeSe "11" family a close relative of high-*Tc* superconductors. Comparing the XAS and RIXS spectra reveals that FeSe1-yTey is unlikely a weakly correlated system, thus differing from other Fe-based quaternary oxypnictides. The charge transfer between Se-Te and the Se 4*p* hole state induced by the substitution is strongly correlated with the superconducting behavior. Above results suggest strong electronic correlations in the FeSe "11" system, as discussed later in detail.
