  Using the first-principles calculations and La3Te4 as an example of an n-type
gapped metal, we demonstrate that gapped metals can develop spontaneous defect
formation resulting in off-stoichiometric compounds. Importantly, these
compounds have different free carrier concentrations and can be realized by
optimizing synthesis conditions. The ability to manipulate the free carrier
concentration allows to tailor intraband and interband transitions, thus
controlling the optoelectronic properties of materials in general.
Specifically, by realizing different off-stochiometric La3-xTe4 compounds, it
is possible to reach specific crossings of the real part of the dielectric
function with the zero line, reduce plasma frequency contribution to absorption
spectra, or, more generally, induce metal-to-insulator transition. This is
particularly important in the context of optoelectronic, plasmonic, and
epsilon-near-zero materials, as it enables materials design with a target
functionality. While this work is limited to the specific gapped metal, we
demonstrate that the fundamental physics is transferable to other gapped metals
and can be generally used to design a wide class of new
optoelectronic/plasmonic materials.
