The coupled electronic-structural modulations of the ligand states in IrTe$_2$ have been studied by x-ray absorption spectroscopy (XAS) and resonant elastic x-ray scattering (REXS). Distinctive pre-edge structures are observed at the Te-$M_{4,5}$ (3$
d$ $rightarrow$ 5$p$) absorption edge, indicating the presence of a Te 5$p$-Ir 5$d$ covalent state near the Fermi level. An enhancement of the REXS signal near the Te 3$d$ $rightarrow$ 5$p$ resonance at the $Q!=!(1/5,0,-1/5)$ superlattice reflection is observed below the structural transition temperature $T_ssim$ 280 K. The analysis of the energy-dependent REXS lineshape reveals the key role played by the spatial modulation of the covalent Te 5$p$-Ir 5$d$ bond-density in driving the stripe-like order in IrTe$_2$, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology observed in the high-T$_c$ superconducting cuprates suggests that the iridates may harbor similar exotic phases.
We have studied disorder-induced in-gap states and effect of light illumination in the insulating phase of spinel-type CuIr$_2$S$_4$ using ultra-violet photoemission spectroscopy (UPS). The Ir$^{3+}$/Ir$^{4+}$ charge-ordered gap appears below the met
al-insulator transition temperature. However, in the insulating phase, in-gap spectral features with $softgap$ are observed in UPS just below the Fermi level ($E_F$), corresponding to the variable range hopping transport observed in resistivity. The spectral weight at $E_F$ is not increased by light illumination, indicating that the Ir$^{4+}$-Ir$^{4+}$ dimer is very robust although the long-range octamer order would be destructed by the photo-excitation. Present results suggest that the Ir$^{4+}$-Ir$^{4+}$ bipolaronic hopping and disorder effects are responsible for the conductivity of CuIr$_2$S$_4$.
We have studied the electronic structure of the Ni triangular lattice in NiGa$_2$S$_4$ using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2$p$ core-level spectrum shows that the S 3$p$ to Ni 3$d$
charge-transfer energy is $sim$ -1 eV and the ground state is dominated by the $d^9L$ configuration ($L$ is a S 3$p$ hole). Cell perturbation analysis for the NiS$_2$ triangular lattice indicates that the strong S 3$p$ hole character of the ground state provides the enhanced superexchange interaction between the third nearest neighbor sites.
Perovskite manganite thin films, $Pr_{0.55}(Ca_{1-y}Sr_y)_{0.45}MnO_3$, have been studied using x-ray photoemission spectroscopy in order to clarify the consequence of the competition between ferromagnetic metal (FM) and charge-orbital ordered insula
tor (COOI). Films with $y$ = 0.40 undergo uniform paramagnetic insulator to FM transition. On the other hand, in films with $y$ = 0.25, the composition near the bicritical point, phase separation of COOI and FM domains is indicated by the spectral change below 125 K. Interestingly, between 50 K and 70 K, the visible laser illumination transfers the COOI-like spectra obtained in cooling process to the FM-like spectra obtained in warming process. This indicates that the photoinduced IMT is governed by the increase of the FM volume fraction and is deeply related to the phase separation between the FM and COOI states.
