Optical spectroscopy of the metallic Nd$_{\mathrm{1-x}}$TiO$_3$ system

The neodymium titanate system, Nd$_{\mathrm{1-x}}$TiO$_{3}$, exhibits two metal-to-insulator transitions at $x=$ 0.08 and $x=$ 0.24. The $n$-type metallic regime between the two transitions exhibits the Fermi liquid $T^{2}$ DC resistivity over a large range of temperatures. We have measured the reflectivity of the $x=$ 0.15 and $x=$ 0.2 samples from 4 meV to 5.5 eV at temperature from 15 K to 300 K. Previous optical work by Yang \textit{et al.} measured an insulating, a semiconducting, and a metallic sample. The metallic sample was found to have the Fermi liquid $\omega^{2} $dependence in the scattering rate. We reevaluate the Yang \textit{et al.} data for the $x=$ 0.095 metallic sample, just inside the MIT, and compare it to our samples deeper in the metallic regime that have $T^{2}$ coefficients an order of magnitude smaller.