Electronic and structural aspects of the chiral helimagnetic compound Cu2OSeO3

The electronic state and the structural properties of the helical magneto-electric material Cu2OSeO3 is investigated through temperature-dependent synchrotron based x-ray diffraction and x-ray absorption spectroscopy. The x-ray absorption spectra on Cu and Se-K edges indicate that Cu and Se ions are in the 2＋ and 4＋ charge states respectively. The pre-edge corresponding to K-line of Cu in the x-ray absorption spectra cannot be fitted using a single Lorentzian function. At least two different Lorentzian lines are required to fit the pre-edge, which are separated by ∼0.6±0.042 eV. This separation originates due to the difference in the crystallographic environment of Cu(1) and Cu(2) and the subsequent difference in the energy of the 3d orbitals of two Cu sites. The obtained energy difference reconfirms the value reported using resonant inelastic x-ray scattering and maximally localized Wannier function calculation. The synchrotron based powder x-ray diffraction indicates significant anomaly in the Cu-O-Cu bond angle around the magnetic transition temperature (TC∼ 59 K). Interestingly, the sample show negative thermal expansion in the lattice parameter below TC. Such anomalies can be connected to the exchange-striction mechanism.