Abstract
In a recent experiment, Hirjibehedin et al (2007 Science 317 1199) performed inelastic tunneling spectroscopy of a single iron atom absorbed on a nonmagnetic substrate. The observed steps in the differential conductance marked the spin excitation energies. In this paper, we explain the observed nonmonotonicities in the differential conductance by a nonequilibrium population of the atom spin states. Furthermore, we predict super-Poissonian current noise due to this nonequilibrium situation. We argue that the remarkable absence of nonequilibrium features at certain conductance steps indicates the presence of an anisotropic relaxation channel.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Spin inelastic tunneling spectroscopy (SITS) enables the study of magnetic properties and interactions on an atomic scale by contacting single magnetic atoms with a scanning tunneling microscope tip. The spin excitation energies are marked by steps in the differential conductance. Recent experiments additionally revealed nonmonotonic features that have not been addressed so far.
Main results. We explain the observed overshooting at the conductance steps in terms of nonequilibrium spin occupations using a master-equation approach. Furthermore, we show that nonequilibrium occupations also lead to a super-Poissonian current noise. We explain the absence of the overshooting at certain steps in terms of an anisotropic relaxation mechanism.
Wider implications. Our work demonstrates the importance of nonequilibrium effects to understand features observed in recent experiments on SITS. Furthermore, we point out the current noise as an additional tool to experimentally study nonequilibrium effects. Future studies should aim at gaining a better understanding of the anisotropic relaxation mechanism.
Figure 1. Differential conductance of a single Fe atom as a function of bias voltage for equilibrium and nonequilibrium spin occupations.