Inelastic neutron scattering study on the electronic transition in (Pr1-yYy)1-xCaxCoO3 single crystals

The perovskite Co oxides Pr1-xCaxCoO3 exhibit at around x similar to 0.5 an unusual transition at temperature T-s[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, omega) for two single crystals of (Pr1-yYy)(1-x)CaxCoO3 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 chi"(Q, omega) seems to have characteristics of strongly correlated Co 3d electrons above T-s, it abruptly exhibits a weak feature reminiscent of a pseudogaplike structure at T-s 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.