Theory of scanning tunneling spectroscopy of a magnetic adatom on a metallic surface

Abstract

A comprehensive theory is presented for the voltage, temperature, and spatial dependence of the tunneling current between a scanning tunneling microscope (STM) tip and a metallic surface with an individual magnetic adatom. Modeling the adatom by a nondegenerate Anderson impurity, a general expression is derived for a weak tunneling current in terms of the dressed impurity Green function, the impurity-free surface Green function, and the tunneling matrix elements. This generalizes Fano's analysis to the interacting case. The differential-conductance lineshapes seen in recent STM experiments with the tip directly over the magnetic adatom are reproduced within our model, as is the rapid decay, ∼ 10Å, of the low-bias structure as one moves the tip away from the adatom. With our simple model for the electronic structure of the surface, there is no dip in the differential conductance at approximately one lattice spacing from the magnetic adatom, but rather we see a resonant enhancement. The formalism for tunneling into small clusters of magnetic adatoms is developed.

Simple model for scanning tunneling spectroscopy of noble metal surfaces with adsorbed Kondo impurities

“…Tight-binding descriptions of the substrate have also been used. 11 They provide a qualitative understanding of observations although their use is difficult to justify considering the sp free-like bands of noble metal surfaces.…”

Microscopic theory for quantum mirages in quantum corrals

“…͑1͒, corresponds to the Hamiltonian of the surface, including the corral and the magnetic impurity. The third is the tunneling ͑Bardeen͒ Hamiltonian, which describes processes in which an electron is transferred between the tip and the surface 9,13 …”

“…To do that, we proceed as follows. In the absence of corral atoms, we approximate the impurity free-surface Green's function by G c (R ជ I ,R ជ I ,⑀ ϩ )ϷϪi 0 and one obtains the well-known Fano function for the differential conductance through the tip 5,9 G͑V,R ជ I ͒ϭ…”

“…4,5 The dip ͑instead of the resonance͒ is due to the fact that the STM tip mainly measures the LDOS of the surface electrons, perturbed by the magnetic impurity, instead of measuring directly the impurity density of states. The asymmetric dip corresponds to a Fano-type curve, which can be produced either by: ͑1͒ the interference between the tunneling from the tip to the surface and the direct tunneling to the magnetic atom, 4,5,8,9 or ͑2͒ by the details of the freesurface band. 10 The depth of the dip decreases gradually as the lateral distance between the tip and the impurity is increased.…”

Dimensionality effects in the local density of states of ferromagnetic hosts probed via STM: Spin-polarized quantum beats and spin filtering

“…6,7 In the absence of a ferromagnetic host, the Fano-Kondo profile becomes doubly degenerate. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Away from the Kondo regime, a spin diode emerges. 24 In the condensed-matter literature on scanning microscopy, there is a profusion of work discussing spin-dependent phenomena employing ferromagnetic leads coupled to quantum dots or adatoms in the Kondo regime.…”