Theory of Scanning Tunneling Spectroscopy of a Magnetic Adatom on a Metallic Surface

TL;DR

This paper develops a comprehensive theoretical model for the tunneling current in STM experiments involving a magnetic adatom on a metallic surface, capturing voltage, temperature, and spatial effects and reproducing experimental observations.

Contribution

It generalizes Fano's analysis to interacting systems using a nondegenerate Anderson impurity model, providing a detailed formalism for tunneling into magnetic adatoms.

Findings

Reproduces differential conductance lineshapes observed in experiments

Explains rapid decay of low-bias features with tip distance

Predicts resonant enhancement rather than a dip near the adatom

Abstract

A comprehensive theory is presented for the voltage, temperature, and spatial dependence of the tunneling current between a scanning tunneling microscope (STM) tip and a metallic surface with an individual magnetic adatom. Modeling the adatom by a nondegenerate Anderson impurity, a general expression is derived for a weak tunneling current in terms of the dressed impurity Green function, the impurity-free surface Green function, and the tunneling matrix elements. This generalizes Fano's analysis to the interacting case. The differential-conductance lineshapes seen in recent STM experiments with the tip directly over the magnetic adatom are reproduced within our model, as is the rapid decay, \sim 10\AA, of the low-bias structure as one moves the tip away from the adatom. With our simple model for the electronic structure of the surface, there is no dip in the differential conductance at approximately one lattice spacing from the magnetic adatom, but rather we see a resonant enhancement. The formalism for tunneling into small clusters of magnetic adatoms is developed.