Lattice dynamics of the niobium (001) surface

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Abstract
In contrast to the reconstructing bcc (001) surfaces of molybdenum and tungsten, the niobium (001) surface is stable. From our helium-scattering experiments we find that the Nb(001) surface displays a regular surface phonon dispersion in spite of the pronounced phonon anomalies in bulk niobium that are due to strong electron-phonon interaction. We have modeled the surface dynamics of Nb(001) using a bulk-lattice-dynamical model that includes conventional central and angle-bending (three-body) interactions as well as electronic degrees of freedom. The latter are needed to account for the bulk phonon anomalies but are found to have little effect on the surface modes. This explains our experimental observation that the surface phonons do not exhibit an anomalous dispersion.