Giant electron-phonon coupling and resistivity saturation in nanostructured hybrid of noble metals

Interaction between the electrons and phonons determines some of the most fundamental properties of solids including superconductivity, thermal and thermoelectric transport, polaronic effects, and electrical resistance in a metal at high temperatures. In good metals, particularly noble metals such as gold (Au), silver (Ag), or copper (Cu), the coupling of electrons and phonons is rather weak and the electron-phonon coupling constant (λ∼ 0.2) is small. The corresponding electron-phonon scattering rate τ^-1_el-ph= 4π²k_BT/h provides an excellent quantitative description of the metallic resistivity when the temperature (T) exceeds the Debye temperature (Θ_D). The regime of λ»1 , at least for a noble metal, however, remained experimentally inaccessible so far, raising questions on possible universal Planckian bound on scattering rate, polaronic deformation, or indeed, even the stability of a metallic state itself. In this work, we have demonstrated a novel nanostructuring strategy of Au via the formation of a hybrid with embedded ultra-small Ag nanoparticles (of radius 1 nm) that allowed us to achieve an effective λ as large as 20, which is almost ten times larger than that known for any solid so far. Using a combination of electrical transport and point contact spectroscopy, we observed, (1) systematic increase in λ by nearly two orders of magnitude with increasing density of Ag nanostructures, (2) robust metallic transport over the entire experimental range of (6 – 300 K) and, (3) a scaling of τ^-1_el-ph with λT for all devices for T>Θ_D. Intriguingly, the T-dependence of resistivity for T>Θ_D was found to become progressively sublinear with increasing λ , bearing close resemblance to the resistivity saturation in A-15 compounds. Our experiment outlines a new route to engineer electron-phonon interaction in simple noble metals that may be connected to localized phonon modes at the buried hetero-interfaces between the Ag and Au nanostructures.