Bias asymmetry in the conductance profile of magnetic ions on surfaces probed by scanning tunneling microscopy

TL;DR

This paper investigates the origin of conductance asymmetries observed in STM measurements of magnetic atoms on surfaces, attributing them to non-equilibrium spin populations and density of states effects, advancing understanding of spin-dependent tunneling.

Contribution

The study extends a perturbative electron-spin scattering model to include spin-polarized tips and out-of-equilibrium populations, explaining conductance asymmetries in STM experiments.

Findings

Asymmetries arise from non-equilibrium spin populations with spin-polarized tips.

Density of states effects cause asymmetries with non-spin-polarized tips.

The model aligns with recent experimental observations.

Abstract

The conductance profiles of magnetic transition metal atoms, such as Fe, Co and Mn, deposited on surfaces and probed by a scanning tunneling microscope (STM), provide detailed information on the magnetic excitations of such nano-magnets. In general the profiles are symmetric with respect to the applied bias. However a set of recent experiments has shown evidence for inherent asymmetries when either a normal or a spin-polarized STM tip is used. In order to explain such asymmetries here we expand our previously developed perturbative approach to electron-spin scattering to the spin- polarized case and to the inclusion of out of equilibrium spin populations. In the case of a magnetic STM tip we demonstrate that the asymmetries are driven by the non-equilibrium occupation of the various atomic spin-levels, an effect that reminds closely that electron spin-transfer. In contrast when the tip is not spin-polarized such non-equilibrium population cannot be build up. In this circumstance we propose that the asymmetry simply originates from the transition metal ion density of state, which is included here as a non-vanishing real component to the spin-scattering self-energy.