  Doped SrTi1-xNbxO3 exhibits superconductivity and a mid-infrared optical
response reminiscent of copper-oxide superconductors. Strangely, its plasma
frequency, omega_p, increases by a factor of ~3 when cooling from 300 K to 20
K, without any accepted explanation. Here, we present momentum-resolved
electron energy loss spectroscopy (M-EELS) measurements of SrTi1-xNbxO3 at
nonzero momentum, q. We find that the infrared feature previously identified as
a plasmon is present at large q in insulating SrTiO3, where it exhibits the
same temperature dependence and may be identified as an anharmonic, multiphonon
background. Doping with Nb increases its peak energy and total spectral weight,
drawing this background to lower q where it becomes visible in IR optics
experiments. We conclude that the "plasmon" in doped SrTi1-xNbxO3 is not a
free-carrier mode, but a composite excitation that inherits its unusual
properties from the lattice anharmonicity of the insulator.
