Scanning Tunneling Spectroscopy of Subsurface Ag and Ge Impurities in Copper

TL;DR

This study uses low temperature scanning tunneling microscopy to analyze subsurface Ag and Ge impurities in copper, revealing how they influence local electronic structures and exhibit many-body effects like electron-phonon coupling.

Contribution

It provides new insights into impurity scattering mechanisms and electronic structure signatures of buried impurities in copper using combined experimental and theoretical approaches.

Findings

Similar scattering phase shifts for Ag and Ge impurities

Observation of a zero-bias kink indicating many-body effects

High electron-phonon coupling parameter near impurities

Abstract

We investigate single Ge and Ag impurities buried below a Cu(100) surface using low temperature scanning tunneling microscopy. The interference patterns in the local density of states are surface scattering signatures of the bulk impurities, which result from 3D Friedel oscillations and the electron focusing effect. Comparing the isoelectronic d scatterer Ag and the sp scatterer Ge allows to distinguish contributions from impurity scattering and the host. Energy-independent effective scattering phase shifts are extracted using a plane wave tight-binding model and reveal similar values for both species. A comparison with ab-initio calculations suggests incoherent sp scattering processes at the Ge impurity. As both scatterers are spectrally homogeneous, scanning tunneling spectroscopy of the interference patterns yields real-space signatures of the bulk electronic structure. We find a kink around zero bias for both species that we assign to a renormalization of the band structure due to many-body effects, which can be described with a Debye self-energy and a surprisingly high electron-phonon coupling parameter $\lambda$. We propose that this might originate from bulk propagation in the vicinity of the surface.