Infrared phonon study of charge ordering in La1/2Sr3/2MnO4

We investigated the temperature- and polarization-dependent optic phonon modes of La1/2Sr3/2MnO4 using optical conductivity analyses. The phonon modes in the ab-plane are significantly changed, while those along the c-axis remain nearly constant with decreasing temperature. Below the charge-ordering temperature, the strength of the stretching phonon mode increases and its frequency becomes higher. These far-infrared phonon spectrum changes are closely related to the increase of the optical gap in the near-infrared region. Our experimental results suggest the importance of lattice distortion and strong electron-phonon interaction for the charge ordering in La1/2Sr3/2MnO4.