AVS 50th International Symposium
    Surface Science Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM5
Multilayer Thermal Expansion at Surfaces from Surface Core Level Shifts

Wednesday, November 5, 2003, 9:40 am, Room 327

Session: Surface and Interface Structure: Metals
Presenter: K. Pohl, University of New Hampshire
Authors: A. Baraldi, Sincrotrone Trieste S.C.p.A. and Universitá di Trieste, Italy
S. Lizzit, Sincrotrone Trieste S.C.p.A., Italy
K. Pohl, University of New Hampshire
Ph. Hofmann, University of Aarhus, Denmark
S. de Gironcoli, Scuola Internationale Superiore di Studi Avanzati (SISSA), Italy
Correspondent: Click to Email
Understanding the thermal properties of nanostructures is of great importance when it comes to making reliable predictions on their stability. By decreasing the size of nanoparticles the thermal behavior of the surface is going to dominate their properties. A basic manifestation of these anharmonic effects is thermal structural expansion. However, some open surfaces are violating our common sense by exhibiting significant negative thermal contraction while some close-packed surfaces show anomalously large thermal expansion. A theoretical understanding of the anharmonic effects at surfaces does not exist and state-of-the-art LDA calculations show large disagreements with experimental results. An especially puzzling case is the thermal expansion of Be(0001). A recent LEED investigation measured an anomalously large thermal surface expansion between 110 K and 700 K, 6 times larger than the bulk. This observation seems inconsistent with measurements reporting no anharmonicity in the out-of-plane surface phonon modes and with a subsequent LDA study within the quasiharmonic approach resulting in no significant thermal expansion. We will present a novel, independent, approach to determine the thermal expansion of Be(0001). We measured the binding energy of the surface state and the values of the surface core level shifts as a function of temperature and compared them to calculated DFT values for different geometries. Our results reveal that, in the temperature range from 300 to 700 K the 1st-to-2nd, 2nd-to-3rd and 3rd-to-4th interlayer coefficient of thermal expansion are 88±15, -10±15 and -6±20 x 10@super -6@ K@super -1@, respectively, to be compared with a bulk value of 12 x 10@super -6@ K@super -1@. Our results confirm the anomalously large thermal expansion of the surface and establish Be(0001) as a firm experimental standard for advances in our theoretical understanding of the thermal behavior of surfaces.