  Quantum spin tunneling (QST) and Kondo effect are two very different quantum
phenomena that produce the same effect on quantized spins, namely, the
quenching of their magnetization. However, the nature of this quenching is very
different so that QST and Kondo effects compete with each other. Importantly,
both QST and Kondo produce very characteristic features in the spectral
function that can be measured by means of single spin scanning tunneling
spectroscopy that makes it possible to probe the crossover from one regime to
the other. We model this crossover, and the resulting changes in transport,
using a non-perturbative treatment of a generalized Anderson model including
magnetic anisotropy that leads to quantum spin tunneling. We predict that, at
zero magnetic field, integer spins can feature a split-Kondo peak driven by
quantum spin tunneling.
