Optical characterization of spin-charge-orbital orders in Pr (Sr 1-yCay)22O7

I. Ishii, Y. Tokunaga, J. Fujioka, Y. Onose, Y. Tokura

Research output: Contribution to journalArticlepeer-review

Abstract

Bilayered-structure manganites Pr (Sr1-y Cay) 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.

Original languageEnglish
Article number245103
JournalPhysical Review B - Condensed Matter and Materials Physics
Volume82
Issue number24
DOIs
Publication statusPublished - 2010
Externally publishedYes

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics

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