A theoretical review on the single-impurity electron spin resonance on surfaces

TL;DR

This paper reviews the development and theoretical understanding of electron spin resonance (ESR) combined with scanning tunneling spectroscopy (STM) on surfaces, highlighting the common principles behind proposed mechanisms and their relation to experimental results.

Contribution

It provides a comprehensive review of experimental setups and theoretical models for ESR-STM, clarifying the underlying physical mechanisms and their similarities.

Findings

Many proposed mechanisms share the same basic principle of modulating spin coupling.

The electric field at the STM junction influences the interaction between electrons and local spins.

Theoretical proposals are broadly equivalent in explaining the core phenomena.

Abstract

The development of electron spin resonance (ESR) combined with scanning tunneling spectroscopy (STM) is undoubtedly one of the main experimental breakthroughs in surface science of the last decade thanks to joining the extraordinarily high energy resolution of ESR (nano-eV scale) with the single-atom spatial resolution of STM (sub-\AA ngstr\"om scale). While the experimental results have \fer{significantly} grown with the number of groups that have succeeded in implementing the technique, the physical mechanism behind it is still unclear, with several different mechanisms proposed to explain it. Here, we start by revising the main characteristics of the experimental setups and observed features. Then, we review the main theoretical proposals, with both their strengths and weaknesses. One of our conclusions is that many of the proposed mechanisms share the same basic principles, \fer{the time-dependent electric field at the STM junction is modulating the coupling of the spin-polarized transport electrons with the local spin.} This explains why these mechanims are essentially equivalent in a broad picture. We analyze the subtle differences between some of them and how they compare with the different experimental observations.