Electronic structure and magnetic properties of CaCrO₃: The interplay between spin- and orbital-orderings

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The electronic structure and magnetic properties of CaCrO₃ have been calculated by two methods, including hybrid-exchange density-function theory and density-functional theory + U. The computed densities of states from both of these methods are in a qualitative agreement with the previous x-ray spectroscopy. On the other hand, the opening of the band gap separates them apart. hybrid-exchange density-functional theory always gives a finite band gap, down to 1.2 eV from HSE06 functional, whereas by tuning the Hubbard-U parameter down to 0.5 eV, a conducting state with AFM-C (defined in the text) spin configuration can be achieved. From hybrid density-functional theory, the computed nearest-neighbouring exchange interaction along the c-axis and in the ab-plane are 4 meV and 6 meV (anti-ferromagnetic), respectively, which are qualitatively in agreement with the previous magnetic measurements. These anti-ferromagnetic exchange interaction, together with the in-plane anti-ferro-orbital ordering will induce a spin-orbital frustration, which could play a role for the abnormal electronic properties in CaCrO₃. In hybrid-exchange density-functional theory, an abrupt reduction (0.2 eV) of the majority-spin band gap of the ferromagnetic state between 60 K and 100 K has been found as lowering temperature, which shows a strong link to the previous optical conductivity measurements in [A. C. Komarek, et. al., Phys. Rev. B 84, 125114 (2011)]. In sharp contrast, the density-functional theory + U methods predicted AFM-C state as the lowest AFM state for the crystal structure measured below 90 K, above which AFM-A is however the lowest. The closely related concepts including electron-hole liquid and surface-plasmon-mediating spin-spin interactions have been discussed as well.