Phonons Softening in Tip-Stretched Monatomic Nanowires

TL;DR

This study combines density functional theory and semi-empirical modeling to explain phonon softening in tip-stretched gold nanowires, aligning theoretical predictions with experimental observations of phonon frequency reduction during elongation.

Contribution

It introduces a realistic tip-stretching model showing strain concentration effects that reconcile theoretical phonon softening with experimental data.

Findings

Phonon frequency decreases with elongation in nanowires.

Strain concentrates at junctions, reducing overall nanowire strain.

Adjusted strain levels improve agreement between theory and experiment.

Abstract

It has been shown in recent experiments that electronic transport through a gold monatomic nanowire is dissipative above a threshold voltage due to excitation of phonons via the electron-phonon interaction. We address that data by computing, via density functional theory, the zone boundary longitudinal phonon frequency of a perfect monatomic nanowire during its mechanical elongation. The theoretical frequency that we find for an ideally strained nanowire is not compatible with experiment if a uniformly distributed stretch is assumed. With the help of a semi-empirical Au-Au potential, we model the realistic nanowire stretching as exerted by two tips. In this model we see that strain tends to concentrate in the junctions, so that the mean strain of the nanowire is roughly one half of the ideal value. With this reduced strain, the calculated phonon softening is in much better agreement with experiment.