Raman Scattering in the Mott Insulators ${\mathrm{LaTiO}}_3$ and ${\mathrm{YTiO}}_3$: Evidence for Orbital Excitations

Raman scattering is used to observe pronounced electronic excitations around 230 meV—well above the two-phonon range—in the Mott insulators ${\mathrm{LaTiO}}_3$ and ${\mathrm{YTiO}}_3$. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.

Figure 1

Raman spectra of ${\mathrm{LaTiO}}_3$, ${\mathrm{LaTiO}}_{3+\delta }$, and ${\mathrm{YTiO}}_3$ measured at $T=13\mathrm{K}$ using the 514.5 nm laser line of an ${\mathrm{Ar}}^{+}/{\mathrm{Kr}}^{+}$ mixed gas laser. The data on ${\mathrm{LaTiO}}_{3+\delta }$ were taken on a fully twinned sample, and the intensity was normalized based on the polarization dependence determined for ${\mathrm{LaTiO}}_3$ (Fig. 4). The red line shows the calculated two-phonon density of states.

Figure 2

Raman spectrum of ${\mathrm{YTiO}}_3$ measured at $T=13\mathrm{K}$ for different laser lines. A frequency independent background has been subtracted from every profile. The arrows depict the position of the photoluminescence peak at 2.14 eV for the different laser lines. The inset shows the integrated intensity of the broad high-energy peak.

Figure 3

Temperature dependence of the Raman spectrum of ${\mathrm{YTiO}}_3$ in ($z$, $z$) geometry, using the 514.5 nm laser line.

Figure 4

Polarization dependences of the Raman spectra of ${\mathrm{YTiO}}_3$ and ${\mathrm{LaTiO}}_3$ at room temperature. The intensity is symmetric with respect to $(\alpha ,\beta )\to (\beta ,\alpha )$. A frequency independent background has been subtracted. Insets: scattering intensities for the parallel (□) and crossed (◊) polarizations as function of angle $\theta$. Solid and dashed lines are obtained from Eq. (1), with relative intensities $E_g:A_{1g}:T_{2g}\approx 1:0.3:0.1$ ($1:0.5:0.1$) in ${\mathrm{YTiO}}_3$ (${\mathrm{LaTiO}}_3$).