Semimetal Surfaces

One of the most fascinating aspects in the study of surfaces is the effect of reduced dimensionality on the electronic structure. It can lead to a situation where the surface has electronic properties which are very different from those of the bulk material. Particularly striking examples can be found on the surfaces of semimetals.

In the bulk electronic structure of Bismuth, for instance, there is only a very small overlap between the valence band and the conduction band. The causes Bi to be a poor metal, on the verge of being a semiconductor with an indirect band gap. Only a small change in the co-ordination of the Bi atoms can be expected to disturb this delicate balance such that a Bi surface could either turn into a better metal or into a semiconductor with a small, indirect gap.

Recently, the possibility of finding a metallic surface on rhombohedral Bi clusters has attracted some attention. Weitzel and Micklitz have shown that granular systems build from small Bi clusters show superconductivity at rather high temperatures (up to several K) while pure bulk Bi is not a superconductor down to very low temperatures. A possible explanation of these findings is that localized electronic surface states could make the cluster surface more metallic than the bulk, a scenario which would favour the superconducting state.

In order to verify this, we have studied the surface electronic structure of the Bi(110) single-crystal surface, a surface which is also found on the clusters, using the set-up for angle-resolved photoemission on ASTRID's SGM-3 beamline. The first figure shows the photoemission intensity along several direction in reciprocal space (the greyscale image) together with a prediction for the bulk electronic structure (the red dots). One criterion for identifying a genuine surface states is that it does not fall into the area of the bulk states, i.e. in the region of the red dots. In the figure, several such states can be identified (emphasized by the yellow lines). Some of them cross the Fermi level, i.e. the highest occupied level at 0 eV binding energy, and render the surface metallic.

An even clearer impression of the surface metallicity is given in the next figure. It shows the photoemission intensity at the Fermi level in reciprocal space, i.e. it is a direct measure of the surface metallicity. All the features except the one close to the X are surface related. and can be linked to the Fermi level crossings in the first figure.

This example shows that the (110) surface of Bi is a much better metal than the bulk, a fact which could explain the occurrence of superconductivity in granular systems built from small Bi clusters.

In the last five years, we have studied the geometric and electronic structure, electron-phonon coupling and phase transitions on several semimetal surface surfaces. Other examples can be found on these pages.

Some selected publications are

Anisotropic two-dimensional Friedel oscillations, Ph. Hofmann, B.G. Briner, M. Doering, H.-P. Rust, E.W. P.lummer and A.M. Bradshaw Phys. Rev. Lett. 79, 265-268 (1997).

Electron-lattice interaction on alpha-Ga(010), Ph. Hofmann, Y. Cai, Ch. Grütter and J.H. Bilgram, Phys. Rev. Lett.81, 1670 (1998).

Physics of the Be(10-10) suface core level spectrum, S. Lizzit, K. Pohl, A. Baraldi, G. Comelli, V. Fritzsche, E.W. Plummer, R. Stumpf and Ph. Hofmann, Phys. Rev. Lett. 81, 3271 (1998).

The effect of reduced dimensionality on a semimetal: the electronic structure of the Bi(110) surface, S. Agergaard, Ch. Søndergaard, H. Li, M.B. Nielsen, S.V. Hoffmann, Z. Li and Ph. Hofmann, New Journal of Physics 3, 15 (2001).

Multilayer Relaxation from Surface Core Level Shift: the Be(0001) case, A. Baraldi, S. Lizzit, K. Pohl, Ph. Hofmann and S. de Gironcoli, Europhys. Lett 64, 364 (2003).

Interplay between the electronic structure and the phase transition on alpha-Ga(010), Ch. Søndergaard, Ch. Schultz, S. Agergaard, H. Li, S. V. Hoffmann, Z. Li, Ph. Hofmann, Ch. Grütter and J.H. Bilgram, Phys. Rev. B 67,165422 (2003).

alpha-Ga(010) surface reconstruction: A LEED structural analysis of the (1x1) room temperature and the (2sqrt{2}xsqrt{2}) low temperature structures, S. Moré, E. A. Soares, M. A. Van Hove, S. Lizzit, A. Baraldi, Ch. Grütter, J.H. Bilgram and Ph. Hofmann, Phys. Rev. B 68,075414 (2003).

Strong energy dependence of the electron-phonon coupling strength on Bi(100), J. E. Gayone, S. V. Hoffmann, Z. Li and Ph. Hofmann, Phys. Rev. Lett. 91,127601 (2003).

Strong spin-orbit splitting on Bi surfaces, Yu. M. Koroteev, G. Bihlmayer, J. E. Gayone, E. V. Chulkov, S. Blügel, P. M. Echenique and Ph. Hofmann, Phys. Rev. Lett., in press