  Two-dimensional materials exhibit a fascinating range of electronic and
photonic properties vital for nanophotonics, quantum optics and emerging
quantum information technologies. Merging concepts from the fields of ab initio
materials science and nanophotonics, there is now an opportunity to engineer
new photonic materials whose optical, transport, and scattering properties are
tailored to attain thermodynamic and quantum limits. Here, we present
first-principles calculations predicting that Argentene, a single-crystalline
hexagonal close-packed monolayer of Ag, can dramatically surpass the optical
properties and electrical conductivity of conventional plasmonic materials. In
the low-frequency limit, we show that the scattering rate and resistivity
reduce by a factor of three compared to the bulk three-dimensional metal. Most
importantly, the low scattering rate extends to optical frequencies in sharp
contrast to e.g. graphene, whose scattering rate increase drastically in the
near-infrared range due to optical-phonon scattering. Combined with an
intrinsically high carrier density, this facilitates highly-confined surface
plasmons extending to visible frequencies. We evaluate Argentene across three
distinct figures of merit, spanning the spectrum of typical plasmonic
applications; in each, Argentene outperforms the state-of-the-art. This unique
combination of properties will make Argentene a valuable addition to the
two-dimensional heterostructure toolkit for quantum electronic and photonic
technologies.
