Bias asymmetry in the conductance profile of magnetic ions on surfaces probed by scanning tunneling microscopy

Abstract

The conductance profiles of magnetic transition-metal atoms, such as Fe, Co, and Mn, deposited on surfaces and probed by a scanning tunneling microscope (STM), provide detailed information on the magnetic excitations of such nanomagnets. In general, the profiles are symmetric with respect to the applied bias. However, a set of recent experiments has shown evidence for inherent asymmetries when either a normal or a spin-polarized STM tip is used. In order to explain such asymmetries, here we expand our previously developed perturbative approach to electron-spin scattering to the spin-polarized case and to the inclusion of out of equilibrium spin populations. In the case of a magnetic STM tip, we demonstrate that the asymmetries are driven by the nonequilibrium occupation of the various atomic spin levels, an effect that is reminiscent of electron spin transfer. In contrast, when the tip is not spin polarized, such a nonequilibrium population cannot be built up. In this circumstance, we propose that the asymmetry simply originates from the transition metal ion density of states and from the uneven electronic coupling between the scattering magnetic atoms and both the tip and the substrate. These effects are included in the formalism as a nonvanishing real component to the spin-scattering self-energy.

Spin excitations and correlations in scanning tunneling spectroscopy

“…The S 1 2 = system we used for illustration contained only two states and was not influenced by any magnetic anisotropy or near-by spin systems. Recently, efforts have been made to expand this perturbative model to higher spin systems, which also include magnetic anisotropy and couplings to neighboring spins [81,[83][84][85], but the importance of the potential scattering was not taken into account up to now. In the following, the power of this easily accessible model will be used to evaluate and describe the spectral features on more complex systems.…”

Spin-pumping and inelastic electron tunneling spectroscopy in topological insulators

“…Since the term includes δ mn there is only an elastic contribution ( nn = 0), which involves no spin-flip events. 16 Such term is proportional to S z nn and thus reverts its sign as the direction of impurity spin is reversed. Note that the elastic and inelastic contributions to the conductance are calculated by partitioning the current into two parts, obtained respectively from the elastic and inelastic energy-dependent self-energies.…”

“…Our calculations are conducted by using the nonequilibrium Green's function method for transport combined with a perturbative approach to spin scattering from magnetic impurities. [14][15][16] …”

Strategy for detection of electrostatic spin-crossover effect in magnetic molecules

“…Here we investigate the possibility of using STM to de-tect the dependence of the exchange coupling on an electrical potential, by using our previously developed quantum mechanical approach based on the non-equilibrium Green's function formalism [16][17][18]. In particular, we calculate the bias-dependent conductance spectra of an ideal molecule comprising two exchange-coupled spin 1/2 atoms.…”

“…It is also not surprising that the mixing is maximized at around V C since the exchange coupling is reduced and the two spin states are less energetically separated. This feature results in an enhancement of the first conductance step followed by the associated conductance decay [8,18]. undergoing the ESCE and subject to a magnetic field of 20 T. In the upper panel we show the normalised conductance spectra for three different values of Γ tip , while the contour plot in the lower panel shows how the spectrum changes continuously between 0 and 250 mV.…”