Broadband dielectric response of CaCu$_3$Ti$_4$O$_{12}$: From dc to the electronic transition regime
Ch. Kant, T. Rudolf, F. Mayr, S. Krohns, P. Lunkenheimer, S. G. Ebbinghaus, and A. Loidl

We report on phonon properties and electronic transitions in CaCu$_3$Ti$_4$O$_{12}$, a material which reveals a colossal dielectric constant at room temperature without any ferroelectric transition. The results of far- and mid-infrared measurements are compared to those obtained by broadband dielectric and millimeter-wave spectroscopy on the same single crystal. The unusual temperature dependence of phonon eigenfrequencies, dampings and ionic plasma frequencies of low lying phonon modes are analyzed and discussed in detail. Electronic excitations below 4~eV are identified as transitions between full and empty hybridized oxygen-copper bands and between oxygen-copper and unoccupied Ti~3$d$ bands. The unusually small band gap determined from the dc-conductivity $(\sim200$~meV) compares well with the optical results.

© 2008 The American Physical Society.