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

TL;DR

This paper demonstrates a nanostructuring approach to dramatically enhance electron-phonon coupling in crystalline gold by embedding nanoscale silver particles, leading to nonclassical electron transport phenomena.

Contribution

It introduces a novel nanostructuring method that significantly increases electron-phonon interaction strength in gold, surpassing known limits and enabling exploration of nonclassical metallic transport.

Findings

Electron-phonon coupling $\lambda$ reaches approximately 20.

Electrical resistivity deviates from linearity with temperature.

Resistivity approaches the Mott-Ioffe-Regel limit at high nanoparticle density.

Abstract

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.