Hybrid plasmon-phonon excitation in Sr­Ti_1-xNb_xO₃

Sr­Ti_1-xNb_xO₃ is known to be one of the most dilute superconductors with properties paralleling copper-oxides. It has perplexing optical properties, including an anomalously broad plasma edge that decreases in frequency with increasing temperature. Here we present a momentum-resolved electron energy loss spectroscopy (M-EELS) study of plasmon excitations in the normal state of Sr­Ti_1-xNb_xO₃ (x=0, 0.002, 0.01, 0.014). Measurements at large momentum transfer of the undoped insulating sample, SrTiO₃, reveal a multiphonon continuum typical of anharmonic insulators. As electrons are added via doping, this spectral weight does not form a simple plasmon, but rather becomes enhanced and shifts to slightly higher energy. Doping makes the continuum stronger and visible over a wider range of momentum, all the way down to zero momentum. We identify this mode, which has been identified in optics as a plasmon, as a hybrid excitation of doped carriers mixed with an anharmonic phonon continuum.