Spin excitations and correlations in scanning tunneling spectroscopy

Abstract

In recent years inelastic spin-flip spectroscopy using a low-temperature scanning tunneling microscope has been a very successful tool for studying not only individual spins but also complex coupled systems. When these systems interact with the electrons of the supporting substrate correlated many-particle states can emerge, making them ideal prototypical quantum systems. The spin systems, which can be constructed by arranging individual atoms on appropriate surfaces or embedded in synthesized molecular structures, can reveal very rich spectral features. Up to now the spectral complexity has only been partly described. This manuscript shows that perturbation theory enables one to describe the tunneling transport, reproducing the differential conductance with surprisingly high accuracy. Well established scattering models, which include Kondo-like spin-spin and potential interactions, are expanded to enable calculation of arbitrary complex spin systems in reasonable time scale and the extraction of important physical properties. The emergence of correlations between spins and, in particular, between the localized spins and the supporting bath electrons are discussed and related to experimentally tunable parameters. These results might stimulate new experiments by providing experimentalists with an easily applicable modeling tool. conductance, positioned symmetrically around zero bias, and (2) peaks of the differential conductance at zero bias.The number of distinguishable conductance steps varies for different spin systems and the precise form of the steps often shows some asymmetry with respect to the applied bias direction and can exhibit some overshoot of the conductance at the step-energy. As we will see in detail, the steps are due to the opening of additional conductance channels precisely governed by the magnetic properties of the spin system. These excitations can be present even at zero magnetic field due to the magneto-crystalline anisotropy. The anisotropy is caused by the reduction of the geometric symmetry at the surface and by spin-orbit coupling [23][24][25], which have the effect of lifting the inherent degeneracy of the spin states. For single atoms the maximum anisotropy is limited to a few tens of meV [26], whereby the magnetic excitation energies usually range from less than one meV up to a few meV requiring experimental setups operating at temperatures T 4 ⩽ K. Here, it is worth to note that one has to be aware that not all energetically low lying step-like increases in the differential conductance must originate from magnetic excitations. The tunneling electrons can also excite low-energy mechanical vibrations which can produce similar strong spectroscopic features [12,[27][28][29][30][31] and which can interact with the spin excitations [32,33]. Thus, to clearly distinguish magnetic excitations their behavior in external applied magnetic fields is often crucial.The peaks at zero bias are due to the Kondo effect in which itinerant electrons from the substrate coherently scatter with the locali...

Competition between quantum spin tunneling and Kondo effect

“…Both the inelastic steps [21] and the Kondo features [22][23][24][25] can be described using a Kondo Hamiltonian where the atomic spin is described with a single-ion quantized spin interacting, via exchange, with the conduction electrons of the surface. In particular, the interplay between magnetic anisotropy and Kondo screening has been thoroughly studied using the Kondo model [17,18,[22][23][24][25].…”

“…In particular, the interplay between magnetic anisotropy and Kondo screening has been thoroughly studied using the Kondo model [17,18,[22][23][24][25]. Here we present a more general approach, based on a nonperturbative treatment of a multi-orbital Anderson model for the adatom coupled to the substrate, that permits to include atomic charge fluctuations that are effectively frozen in the Kondo model.…”

Spin-Resolved Spectroscopy of the Yu-Shiba-Rusinov States of Individual Atoms

“…Indeed, for quenched superconductivity, the B ext -dependent spectra can be excellently simulated using the effective spin model from Ref. [39] with the effective spin…”

Comparing XMCD and DFT with STM spin excitation spectroscopy for Fe and Co adatoms onCu2N/Cu(100)

“…This may alter the properties of the system and questions the use of an atomic description for its excitations motivating alternative models [18]. Nevertheless, the described spin Hamiltonian reproduces the energies and amplitudes of the observed SES conductance steps in detail [10,33]. It seems reasonable to assume that on the energy scale of the spin excitations the moments of the impurity and its surrounding are sufficiently strongly coupled to behave as one single magnetic moment.…”