**2. Experiments**

22 Superconductors – Materials, Properties and Applications

producing itinerant charge carriers in this system.

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

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

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

(NSRRC) in Taiwan, operated at 1.5GeV with a current of 360mA. Monochromators with Si (111) crystals were used on both the beam lines with an energy resolution ∆E/E higher than 2x10-4. Absorption spectra were recorded by the fluorescence yield (FY) mode at room temperature by using a Lytel detector (Lytle et al., 1984). All spectra were normalized to a unity step height in the absorption coefficient from well below to well above the edges, subsequently yielding information of the unoccupied states with p character. Standard Fe and Se metal foils and oxide powders, SeO2, FeO, Fe2O3 and Fe3O4 were used not only for energy calibration, but also for comparing different electronic valence states. Since surface oxidation was assumed to interfere with the interpretation of the spectra, the FeSe*x* crystals were cleaved *in situ* in a vacuum before recording the spectra.

X-Ray Spectroscopy Studies of Iron Chalcogenides 25

*c*=5.529 Å for *x*=0.85. Experimental results indicate that the *a=b* parameter increases incrementally as x decreases. Simultaneously, a markedly smaller change occurs in the *c*parameter. Thus, the *a-b* plane variation is surpasses that of the c axis. These lattice parameters are very close to those described in the literature for Se deficient powders (Hsu

**Figure 1.** (a) Illustration of the crystal structure of tetragonal FeSe*x*, where the blue (small balls) and red (large balls) denote Fe and Se, respectively; the pyramid chain and tetrahedral arrangements are marked in color in the unit cell. Hybridization is shown in two color bonds; (b) the local symmetry of Se atom (in pyramid chain) and Se atom (in tetrahedral geometry) shown separately; (c) energy level diagram of the FeSe*x* system along with individual elements. The hybridization and unoccupied states

**Figure 2.** Powder XRD patterns of FeSe*x* crystals with *x*= (i) 0.85, (ii) 0.88 and (iii) 0.9. The patterns are fitted to the P4/nmm space group and indexed. Hexagonal phase reflections are denoted by a prefix H.

Conversely, FeTe with the same tetragonal crystal structure is stable up to a significantly higher temperature, ∼1200 K. As is expected, replacing Se atoms within FeSe with Te

et al., 2008).

in the FeSe are highlighted by a circle.

The unoccupied partial density of states in the conduction band was probed using XAS, while information complementing that obtained by XAS was obtained using XES. Those results reveal the occupied partial density of states associated with the valence band. Detailed x-ray absorption and emission studies were conducted. Next, tuning the incident xray photon energies at resonance in XAS yields the RIXS spectrum, which is used primarily to probe the low-excited energy-loss feature which is symptomatic of the electron correlation. XAS and XES measurements of the Fe *L*2,3-edges were taken at beamlines 7.0.1 and 8.0 at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL). In the Fe *L*-edge x-ray absorption process, the electron in the Fe 2*p* core level was excited to the empty 3*d* and 4*s* states and, then, the XES spectra were recorded as the signal associated partial densities of states with Fe 4*s* as well as Fe 3*d* character. The RIXS spectra were obtained by properly selecting various excitation energies to record the XES spectra, based on the x-ray absorption spectral profile. The XAS spectra were obtained with an energy resolution of 0.2 eV by recording the sample current. Additionally, the x-ray emission spectra were recorded using a high-resolution grazing-incidence grating spectrometer with a two-dimensional multi-channel plate detector with the resolution set to 0.6 eV (Norgdren et al., 1989). Surface oxidization is of priority concern in Fe-based superconductors; to prevent oxidation of the surface, all data were gathered on a surface of the sample that was cleaved *in situ* in a vacuum with a base pressure of 2.7 x10-9 torr.
