Inelastic electron tunneling through adatoms and molecular nanomagnets

Abstract: We discuss a theoretical description of the inelastic electron tunneling spectra (IETS) of a magnetic nanosystem (an atom or a molecule) adsorbed on a solid surface measured in a scanning tunneling microscope (STM). We represent the nanosystem by means of a cluster Hubbard model, which allows us to study scenarios when the tunneling electrons sequentially interact with several magnetic centers inside the nanosystem or when the magnetic centers are made out of heavy atoms with a strong spin-orbit coupling and large orbital moments. The sequential tunneling through multiple centers is illustrated on an adatom probed by an STM tip with a nickelocene molecule attached to it. For atoms with a large orbital moment, we find the transitions accessible by IETS to be governed by the selection rule $\Delta J_z\leq 2\ell+1$, where $J_z$ is the projection of the total angular momentum of the atom to the quantization axis and $\ell$ is the orbital momentum quantum number of the partially filled atomic shell carrying the magnetic moment. For atoms with magnetic moments dominated by spin, the spectra are naturally dominated by transitions fulfilling the traditional selection rule $\Delta J_z\leq 1$.