Imaging spin filter for electrons based on specular reflection from iridium (001).

As Stern-Gerlach type spin filters do not work with electrons, spin analysis of electron beams is accomplished by spin-dependent scattering processes based on spin-orbit or exchange interaction. Existing polarimeters are single-channel devices characterized by an inherently low figure of merit (FoM) of typically 10⁻4-10⁻³. This single-channel approach is not compatible with parallel imaging microscopes and also not with modern electron spectrometers that acquire a certain energy and angular interval simultaneously. We present a novel type of polarimeter that can transport a full image by making use of k-parallel conservation in low-energy electron diffraction. We studied specular reflection from Ir (001) because this spin-filter crystal provides a high analyzing power combined with a "lifetime" in UHV of a full day. One good working point is centered at 39 eV scattering energy with a broad maximum of 5 eV usable width. A second one at about 10 eV shows a narrower profile but much higher FoM. A relativistic layer-KKR SPLEED calculation shows good agreement with measurements.

Spin-resolved mapping of spin contribution to exchange-correlation holes.

By means of spin-polarized electron coincidence spectroscopy we explore the fundamental issue of spin-resolved contributions to the exchange-correlation hole in many-electron systems. We present a joint experimental and theoretical study of correlated electron pair emission from a ferromagnetic Fe(001) surface induced by spin-polarized low-energy electrons. We demonstrate that the contribution to the exchange-correlation hole due to exchange is more extended than the contribution due to the screened Coulomb interaction.

Positron-induced emission of electron-positron pairs from solid surfaces.

The collision of a low-energy positron, which impinges on a crystalline surface, with a valence electron may result in the emission of a spatially separated time-correlated electron-positron pair. We present a method for calculating the cross section for this positron surface reaction channel, which we briefly refer to as (p, ep) in analogy to electron-induced pair emission (e, 2e). The two-particle final state is represented by a product of an electron and a positron diffraction state coupled by a 'correlation factor', which accounts for the screened Coulomb interaction. The electron-solid and positron-solid quasi-particle potentials are based on first-principles calculations within density functional theory. Numerical (p, ep) results are presented for Cu(111) and compared to their (e, 2e) counterparts. Energy distributions for constant emission angles reflect, to a large extent, the valence electron density of states. In equal-energy (p, ep) angular distributions, the Coulomb interaction produces a central accumulation zone-in contrast to a depletion zone for (e, 2e)-the relative weight and the extension of which are subject to 'matrix element effects'. At larger angles sharp features arise from single-particle surface resonances.