First-principles study of phonon properties in magnetic double-layer manganites

We have studied the lattice dynamical properties of the tetragonal double-layer manganite system La_2-2xSr₁₊₂ₓMn₂O₇ ($x=0.4$) within the framework of density functional perturbation theory, using a mixed-basis pseudopotential method and the virtual-crystal approximation for modeling the alloy. Performing phonon dispersion calculations for both ferromagnetic (FM) and nonmagnetic (NM) ground states in the plane qz=0 revealed a strong influence of the magnetic order on high-frequency modes which involve oxygen vibrations with Mn-O bond-stretching modulations. They can be traced back to changes in the dynamical coupling of medium-ranged bonds like Mn-Mn or O-O, while the nearest-neighbor Mn-O bonds remain practically unchanged. Electronically they originate from a replacement of the predominant Mn d(t2g) orbital polarization in the NM phase by a d(eg)-type polarization in the FM phase. Good agreement is found between theoretical predictions for phonon frequencies and intensities in the [110] direction and a previous neutron scattering experiment performed in the FM phase. Finally, calculations of the electron-phonon interaction revealed that dips in the dispersion of the high-frequency bond-stretching branches are connected to an enhanced electron-phonon coupling, but on average only a moderate coupling constant of λ=0.65 is obtained.