Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe 1.01 Se superconductors up to ∼66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of ∼7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe 1.01 Se decreases progressively with pressure up to ∼10 GPa. A new prepeak at energy of ∼7113.7 eV develops for pressure above ∼13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe 1.01 Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe 1.01 Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe 1.01 Se. A discontinuous variation of the integrated absolute difference values of the Kβ emission line was observed, originating from a phase transition of Fe 1.01 Se for a pressure >12 GPa. Fe 1.01 Se shows a small net magnetic moment of Fe 2+ at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line Kβ was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe 1.01 Se superconductors. The experimental results provide insight into the spin state of Fe 1.01 Se superconductors under pressure.

Mn 3d valence states and elementary electronic excitations in single-crystalline TbMnO 3 were probed with resonant inelastic x-ray scattering ͑RIXS͒ ͑or resonant x-ray emission spectroscopy, RXES͒, polarized x-ray absorption spectra and ab initio electron-structure calculations. Polarized Mn K-edge x-ray absorption spectra of TbMnO 3 crystals exhibit a significant anisotropy along three crystallographic directions, particularly for the white-line region. The 1s3p-RXES spectra obtained at the Mn K edge reveal that the quadrupolar Raman regime is restricted predominantly to below the pre-edge peak whereas the fluorescence regime starts from the first pre-edge peak of 1s → 3d transitions, indicating a relatively delocalized character of unoccupied Mn 3d states. The additional K␤ emission profile at energy loss ϳ62 eV is attributed to the off-site dipole Mn 1s-MnЈ 3d and/or Mn 1s-Tb 5d transitions, originating from hybridization between Mn 4p states and neighboring MnЈ 3d / Tb 5d states. The off-site dipole transition makes a considerable contribution to the pre-edge region of Mn K-edge spectrum. Three prominent RIXS features at ϳ2.9, 7, and 11 eV were observed. Based on GGA+ U calculations, the 7 eV region corresponds to transitions from O 2p states to unoccupied minority Mn 3d states, whereas the 11 eV band is ascribed to transitions from the O 2p band to the empty Tb 5d band. The broad 2.9 eV band is attributed to the coexistence of on-site Mn 3d-Mn 3d and off-site Mn 3d-MnЈ 3d transitions.

The total spectral weight S of the emergent low-energy quasipaticles in high-temperature superconductors is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero doping to overdoping are investigated. In contrast to mean field theory, we found that S deviates from linear dependence on the doping level p. The slope of S versus p changes continuously throughout the whole doping range with no sign of saturation up to p = 0.23. Therefore, the picture of Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSC's.

Detailed band structure calculations have been performed for Cd(2)Re(2)O(7) in high-, middle- and low-temperature (T) phases. The calculations are based on the observed lattice structures from x-ray diffraction measurements. The spin-orbit interaction is incorporated self-consistently in both the generalized gradient approximation (GGA) and the GGA plus Hubbard U (GGA+U) approaches. It is found that the on-site U has negligible effects on the Re 5d band structures; therefore both the GGA and GGA+U Re 5d band energies agree well with the observed O K-edge x-ray absorption spectroscopy (XAS) spectrum, whereas the Cd 4d band energy observed from photoemission spectroscopy can only be correctly reproduced by GGA+U calculations, indicating the relatively itinerant Re 5d and localized Cd 4d electrons. On the other hand, the spin-orbit coupling gives rise to nontrivial spin and orbital magnetic moments for the middle- T phase. Most unexpectedly, we found that the low- T phase exhibits quasi-two-dimensional Fermi surfaces. The calculated carrier numbers for the three phases are, at least qualitatively, consistent with the measured Hall coefficient.

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We present experimental evidence of the gapless band structure of PbPdO2 by combined x-ray photoemission and x-ray absorption spectra complemented with first principles band structure calculations. The electronic structure near the Fermi level of PbPdO2 is mainly composed of O 2p and Pd 4d bands, constructing the conduction path along the Pd-O layer in PbPdO2. Pd deficiency in PbPdO2 causes decreased O 2p-Pd 4d and increased O 2p-Pb 6p hybridizations, thereby inducing a small band gap and hence reducing conductivity. Hall measurements indicate that PbPdO2 is a p-type gapless semiconductor with intrinsic hole carriers transporting in the Pd-O layers.

Abstract

The variation of electronic states and structural distortion in highly compressed multiferroic TbMnO 3 was probed by x-ray diffraction ͑XRD͒ and x-ray absorption spectroscopy ͑XAS͒ using synchrotron radiation. Refined XRD data enabled observation of a reduced local Jahn-Teller ͑JT͒ distortion of Mn sites within MnO 6 octahedra in TbMnO 3 with increasing hydrostatic pressure. A progressively increasing intensity of the white line in Mn K-edge x-ray absorption spectra of TbMnO 3 was detected with increasing pressure. The absorption threshold of Mn K-edge spectra of TbMnO 3 is shifted toward higher energy, whereas the pre-edge peak is slightly shifted to lower energy with increasing hydrostatic pressure. We provide spectral evidence for pressure-induced bandwidth broadening for mangnites. The enhanced intensity of the white line and the shifted absorption threshold of Mn K-edge spectra are explained in terms of a reduced JT distortion of MnO 6 octahedra in compressed TbMnO 3. Comparison of XAS data with full-multiple-scattering calculations using code FDMNES shows satisfactory agreement between experimental and calculated Mn K-edge spectra.