Berlin 2001 – scientific programme

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AMPD: EPS AMPD

AMPD 4: Sitzung 4

AMPD 4.2: Talk

Tuesday, April 3, 2001, 10:55–11:30, H104

Modelling of structural and thermodynamic properties of clusters — •F. Spiegelmann, F. Calvo, F. Lopez–Urias, and G.M. Pastor — Laboratoire de Physique Quantique/IRSAMC (UMR5626 du CNRS) Université Paul Sabatier, 118 route de Narbonne, F-31062, Toulouse Cedex, France

Clusters are prototype objects to study the thermodynamical properties of finite systems. Although "phase transitions" are well defined only for infinite systems, free clusters offer a situation where one can, by monitoring the size, observe some specific precursor phenomena and understand their evolution and convergence to the bulk phase transitions.

Recently the combination of spectroscopic techniques with the analysis of cluster fragmentation has provided some experimental insight into the thermodynamics of sodium clusters and quantitative determination of the caloric curves related to the solid-liquid transitioni [1]. The measurements show a strong dependence of the caloric properties upon cluster size and higher melting temperature at sizes intermediate between those associated with electronic shells stability and those associated with atomic shells packing.

We will examine various theoretical aspects concerned with the simulation and the characterization of solid–liquid changes in simple (alkali) metal clusters [2] discussing:

• The type of modelling that can be used in order to describe the geometrical and electronic structure and that can be incorporated in large scale simulations for the search of the low energy isomers [3] or for an extensive sampling of the potential energy surfaces at finite temperature;

• the relevance of various indicators to fingerprint the thermodynamical behaviour and the phase changes

• the link between microscopic and macroscopic behaviour, namely the correpondance between the microscopic conformational changes and the characteristic features in the heat capacity curves. This refers to the importance of classical aspects such as the geometry of clusters, the role of surface and defects. This also refers to purely quantal effects such as those characterizing orbital filling including the electron number (cluster charge) or symmetry lowering Jahn–Teller deformations;

• the size dependance of the thermodynamical properties and the convergence to the bulk situation;

• the modelling of finite temperature electronic excitation and their possible influence on the nuclear solid–liquid transition.

The topic of the electronic contribution to the specific heat obviously has its own and essential interest in the context of the temperature dependence of the magnetic properties of finite (transition metal) clusters and can be approached in small clusters using model hamiltonians explicitely accounting for the electron correlation [4]. Precursor features to the temperature–dependance of the magnetic properties observed in the bulk can then also be evidenced as well as their interplay with structural fluctuations.

[1] M. Schmidt et al., Nature, 393, 238 (1998).

[2] F. Calvo and F. Spiegelman, Phys. Rev. Lett., 82, 2270 (1999); F. Calvo and F. Spiegelmann, J. Chem. Phys. 112, 2888, (2000).

[3] F. Calvo, S. Tran, C. Guet, S. Blundell and F. Spiegelman, Phys. Rev. B 62, 10394 (2000).

[4] F. Lopez-Urias and G. Pastor, J. Appl. Phys., 87,4909 (2000).