Abstract
The perovskite Co oxides Pr1−xCaxCoO3 exhibit at around x∼0.5 an unusual transition at temperature TS [S. Tsubouchi et al. Phys. Rev. B 66, 052418 (2002)] with no space group change and no long-range magnetic order. We measured inelastic neutron scattering intensities of magnetic excitations I(Q,ω) for two single crystals of (Pr1−yYy)1−xCaxCoO3 in which the x region of the transition is widened by the Pr-site doping to study whether the recent proposal of the excitonic condensation model is relevant to this “hidden order transition” of the system. While χ''(Q,ω) seems to have characteristics of strongly correlated Co 3d electrons above TS, it abruptly exhibits a weak feature reminiscent of a pseudogaplike structure at TS with decreasing T. The first peak of the Pr crystal-field excitations has also been observed at low temperatures. Here, using a model of the coexistence of the exciton condensed phase (EC phase) with the nearly or weakly ferromagnetic one, we show that the EC phase appears as the collective transition in Pr1−xCaxCoO3.
