Optical characterization of spin-charge-orbital orders in Pr (Sr _1-yCa_y)2₂O₇

Bilayered-structure manganites Pr (Sr_1-y Ca_y) 2 Mn₂ O₇ show the competing spin-charge-orbital orders; the ground state is the spin CE-type charge-orbital ordered state for y>0.4 and the spin-A-type ferroic x2 - y2 orbital ordered state for y<0.4. For the two representative crystals of y=0.9 and y=0, we have investigated the optical conductivity and Raman spectra to probe the charge, spin, and lattice dynamics of the respective phases. For y=0.9, the charge-orbital ordered states are characterized by strongly activated Raman phonon bands corresponding to the modulated breathing and Jahn-Teller lattice distortions, as observed commonly for other charge-orbital ordered manganites. As the unique features of this bilayered manganite, however, we found that the infrared-active phonon mode can also show up in the Raman spectra in the lower-temperature charge-orbital ordered state, confirming the breaking of centrosymmetry to produce the spontaneous electric polarization. The other feature to be noted is the appearance of the two-magnon excitation band of the CE-type spin order in the Raman spectra which estimates the energy scale of spin exchange energies. For y=0, on the other hand, we have observed a charge gap in the optical conductivity spectrum which becomes narrowed with the evolution of the A-type (in-plane ferromagnetic) spin order but remains finite (0.15 eV) at the ground state. The persistence of the charge gap is ascribed to the vertical charge ordering or charge-density wave (CDW) that was previously verified by diffuse x-ray scattering. Such charge-order or CDW formation also activates additional infrared phonon modes at 26 and 35 meV, whose spectral intensity appears to be enhanced by resonance with the underlying electronic continuum.