Possible excitonic phase of (Pr_1−yR_y)_1−xCa_xCoO₃ (R = Lanthanides and Y)

(Pr_1-yR_y)_1-xCa_xCoO₃ (R = Y and/or Lanthanide elements) exhibits a “hidden order transition” in the (x, y) region near (0.5, 0.0) at temperature T_s, where none of magnetic ordering and change of the space group have been found, even though significant anomalies in various physical quantities exist. When the (x, y) is outside of the above region, the transition can essentially be understood by the spin-state-crossover of isolated Co atoms. To study whether excitonic condensation (EC) recently proposed is relevant to the “hidden order transition” we measured magnetic excitation spectra χ” (Q, ω) by means of inelastic neutron scattering on a single crystal of (Pr_0.8Y_0.2)_0.7Ca_0.3CoO₃, and obtained following results: While the spectra from the Co 3d electrons observed at 200 K (>above T_s) have characteristics of ordinary strongly correlated electron systems, the χ” (Q, ω) - ω curve at 4 K (<T_s) has a feature reminiscent, at first sight, of a weak pseudogap-like structure. This result can be explained by the realization of the EC phase or “EIQ” phase reported by Nasu et al. [Phys. Rev. B 93 205136 (2016)], or at least by the picture that the system is in the boundary region between the low-spin and EQI″phases. This result strongly suggests that there exists the EIQ phase in the present sample or at least in (Pr_1−yR_y)_1−xCa_xCoO₃ with (x, y) near (0.5, 0.0).