Possible excitonic phase of (Pr1−yRy)1−xCaxCoO3 (R = Lanthanides and Y).

Abstract (Pr 1- y R y) 1- x Ca x CoO 3 (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 0.8 Y 0.2) 0.7 Ca 0.3 CoO 3 , 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− y R y) 1− x Ca x CoO 3 with (x , y) near (0.5, 0.0). [ABSTRACT FROM AUTHOR]