The light emission from a scanning tunneling microscope operated on a Ag(111) surface at 6 K is analyzed from low conductances to values approaching the conductance quantum. Optical spectra recorded at a sample voltages V reveal emission with photon energies hv> 2eV. A model of electrons interacting coherently via a localized plasmon-polariton mode reproduces the experimental data, in particular the kinks in the spectra at eV and 2eV as well as the scaling of the intensity at low and intermediate conductances.
@article{arxiv.1809.02793,
title = {Quantum Coherent Multielectron Processes in an Atomic Scale Contact},
author = {Peter-Jan Peters and Fei Xu and Kristen Kaasbjerg and Gianluca Rastelli and Wolfgang Belzig and Richard Berndt},
journal= {arXiv preprint arXiv:1809.02793},
year = {2018}
}