Engineering ultra-strong electron-phonon coupling and nonclassical electron transport in crystalline gold with nanoscale interfaces

Electrical resistivity in good metals, particularly noble metals such as gold (Au), silver (Ag), or copper, increases linearly with temperature (T) for T > Θ_D, where Θ_D is the Debye temperature. This is because the coupling (λ) between the electrons and the lattice vibrations, or phonons, in these metals is rather weak with λ∼ 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 ∼1-2 nm inside a crystalline Au shell, an unprecedented enhancement in the electron-phonon interaction, with λ as high as ≈ 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 ρ_MIR∼ h a /e^2 for both disorder (T→ 0) and phonon (T ≫Θ_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.