  Electrical resistivity in good metals, particularly noble metals such as gold
(Au), silver (Ag), or copper, increases linearly with temperature ($T$) for $T
> \Theta_{\mathrm{D}}$, where $\Theta_{\mathrm{D}}$ is the Debye temperature.
This is because the coupling ($\lambda$) between the electrons and the lattice
vibrations, or phonons, in these metals is rather weak with $\lambda \sim
0.1-0.2$, and a perturbative analysis suffices to explain the $T$-linear
electron-phonon scattering rate. In this work, we outline a new nanostructuring
strategy of crystalline Au where this foundational concept of metallic
transport breaks down. We show that by embedding a distributed network of
ultra-small Ag nanoparticles (AgNPs) of radius $\sim1-2$ nm inside a
crystalline Au shell, an unprecedented enhancement in the electron-phonon
interaction, with $\lambda$ as high as $\approx 20$, can be achieved. This is
over hundred times that of bare Au or Ag, and ten times larger than any known
metal. With increasing AgNP density, the electrical resistivity deviates from
$T$-linearity, and approaches a saturation to the Mott-Ioffe-Regel scale
$\rho_{\mathrm{MIR}}\sim h a /e^2$ for both disorder ($T\to 0$) and phonon ($T
\gg \Theta_{\mathrm{D}}$)-dependent components of resistivity (here,
$a=0.3$~nm, is the lattice constant of Au). This giant electron-phonon
interaction, which we suggest arises from the coulomb interaction-induced
coupling of conduction electrons to the localized phonon modes at the buried
Au-Ag hetero-interfaces, allows experimental access to a regime of nonclassical
metallic transport that has never been probed before.
