Anharmonic multiphonon origin of the valence plasmon in SrTi1-xNbxO3

TL;DR

This study reveals that the valence plasmon in SrTi1-xNbxO3 is actually a multiphonon background influenced by lattice anharmonicity, explaining its unusual temperature-dependent plasma frequency increase.

Contribution

It demonstrates that the observed plasmon-like feature in doped SrTi1-xNbxO3 originates from lattice multiphonon processes rather than free carriers, challenging previous interpretations.

Findings

Infrared feature present in insulating SrTiO3 at large q

Doping shifts spectral weight and peak energy

The 'plasmon' is a composite excitation from lattice anharmonicity

Abstract

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.