Marlene Read and Igor Bartos presenting their work on detection of Tamm surface states.

Analysis of very low energy electron scattering from atomic structures on solid surfaces gives information about electronic properties such as surface atomic potentials and also the geometric arrangements of surface atoms. Over the last ~30 years, experimental electron reflectivity data for electrons in the kinetic energy range 0 – 50 eV has had features which could not be accounted for. An example is the (111) face on noble, near-noble and transition metals. This energy range in very low energy electron diffraction and microscopy (VLEED/LEEM) and target current spectroscopy (TCS) has been largely neglected.

Following a grant from the Gordon Godfrey Fund for Theoretical Physics, Igor Bartos from the Czech Academy of Sciences in Prague visited the School for 3 months for a collaboration with Marlene Read. We found that a hitherto unexplained double peak structure in the reflectivity was due to the splitting of a Bragg peak by a surface state formed from the gradual rise of the constant interstitial potential on approach to the vacuum interface. This type of surface state is localised well inside the solid surface and has historically been called a Tamm state.

Our conclusions are that the formation of this type of surface state has a very significant effect on the reflectivities in the low energy range and should now be incorporated into the theoretical model. It is suggested that failure to model the surface potential realistically in this atomic selvedge region is a major cause of the discrepancies between theory and experiment in the ~30 year history of VLEED.

With this unravelling of the very low energy features we expect to be able to extract details of the energy and momentum variation of many important surface quantities that have not been previously possible to determine.

The formation of surface states on these metals is also important for analysis in other work where alkali metal nanostructures are adsorbed on these surfaces. In this case metallic quantum wells are produced which could be used as electronic devices capable of operation at room temperature. The development of nanoscale electronic devices operating at room temperature is highly desirable.

Marlene Read and Igor Bartos