  The optical properties of thin films of intermetallic Au$_{3}$Hf were
experimentally investigated for the first time, which display clear plasmonic
properties in the optical and near infrared region with negative permittivity.
In contrast to similar alloys, such as films of Au$_{3}$Zr, the films express
more negative $\epsilon'$ values and lower $\epsilon''$ values across most of
the wavelengths (370-1570 nm) investigated. The Au$_{3}$Hf films were
fabricated by DC magnetron sputtering at a range of deposition temperatures,
from room temperature to 415$^{o}$C, and annealed at different vacuum levels.
The films mostly formed as a combination of Au$_{3}$Hf, Au$_{2}$Hf and
Au$_{4}$Hf phases when deposited below 400$^{o}$C, and exclusively Au$_{3}$Hf
phase at above 400$^{o}$C, indicating key conditions for isolating this phase.
The films were stable when annealed at 10$^{-8}$ Torr, but when annealed again
at 10$^{-6}$ Torr the films oxidised and changed into a mix of Au- Hf phases,
suggesting resistance to oxidization may be an issue for un-encapsulated
applications at elevated temperatures.
