Evidence of orbital excitations in CaCu_3Ti_4O _(12) probed by Raman spectroscopy

摘要

Raman scattering studies on CaCu_3Ti_4O_(12) and SrCu_3Ti_4O_(12) compounds provide evidence of the physics underlying the giant dielectric effect in the CaCu _3Ti_4O_(12) compound. The temperature, polarization, and photon energy dependence of a broad Raman mode observed at high wavenumbers below ～130 K indicates its origin from orbital excitations. The orbital order disorder transition observed around 100 K may be responsible for the conductivity changes required in the internal barrier layer capacitance model, hitherto used to explain the huge dielectric constant above 100 K in these compounds.