Spin-polarised two-electron spectroscopy of surfaces

Dr Sergey Samarin, Prof. Jim Williams, Dr Tony Sergeant

This technique involves a detection of two time-correlated electrons resulting from a scattering of a low-energy (10 eV – 50 eV) incident electron with known spin orientation from a solid surface (Fig.1).

Figure 1. Geometry of electron scattering.

Objectives of the project.

• Study of spin-dependent electron scattering dynamics from surfaces;
• Application of spin-polarized (e,2e) spectroscopy for studying spin-dependent electronic properties of magnetic and nonmagnetic surfaces and thin films;
• Investigation of electron correlations in surfaces and thin films.

Description of the technique.

Figure 2. Experimental set-up.

The spin-polarized (e,2e) spectrometer consists of a source of spin-polarized electrons that impinge onto a target and two time-of-flight (TOF) energy analysers (D1, D2 in Fig. 2) that detect and analyse momenta of two electrons resulting from an interaction of a single incident electron with the surface. The advantage of TOF energy analysis with position sensitive detectors is that it allows energy and angular electron distributions to be measured “in parallel”. This increases the efficiency of spectrometer and requires a very small incident current (10-13 A). Each spin-resolved (e,2e) spectrum consists of two (spin-up and -down) six-dimensional (momenta k1, k2) distributions of correlated electron pairs. These distributions contain the information about spin-dependent electronic structure and scattering dynamics. To visualize this information we project the six-dimensional distribution on a two-dimensional energy distribution, for example. This shows how energy is shared between two electrons depending on the binding energy of the target electron and spin orientation of the incident electron. When this distribution is projected onto the two- dimensional momentum space of the bound electron, it shows how electrons from different parts of the surface Brillouin zone participate in the interaction depending on the spin orientation of the incident electron. To image the exchange correlation one controls the quantum numbers of at least two electrons and then changes the spin state of one of these electrons while monitoring the change in the scattering probability. Features relating to the exchange and spin-orbit interactions are studied by observing the dependence of the spectrum on the incident electrons’ spin projection.