We provide a complete and detailed characterization of the temperature-dependent response to ac electrical fields of ${\mathrm{LaTiO}}_3,$ a Mott-Hubbard insulator close to the metal-insulator transition. We present combined dc, broadband dielectric, mm-wave, and infrared spectra of ac conductivity and dielectric constant, covering an overall frequency range of 17 decades. The dc and dielectric measurements reveal information on the semiconducting charge-transport properties of ${\mathrm{LaTiO}}_3,$ indicating the importance of Anderson localization, and on the dielectric response due to ionic polarization. In the infrared region, the temperature dependence of the phonon modes provides evidence for structural anomalies at the magnetic ordering temperature. In addition, a gaplike electronic excitation following the phonon region is analyzed in detail. We compare the results to the soft-edge behavior of the optical spectra characteristic for Mott-Hubbard insulators. Overall a consistent picture of the charge-transport mechanisms in ${\mathrm{LaTiO}}_3$ emerges.

• Received 10 July 2003

DOI:https://doi.org/10.1103/PhysRevB.68.245108