Electronic properties, stability, and length scales of Cs-N clusters

Three basically different models of CSN clusters are studied, partly in ord er to explore the limitations of each model and partly in order to study ge neral ground-state properties of the clusters. One model is based on the sp herical-jellium model within the density-functional formalism, another is a semi-empirical tight-binding model obtained by parametrizing band structur es for an infinite crystal, and the third model is a spherical-well model f or non-interacting particles. Particularly stable clusters are found for sy stems with only completely filled electronic shells, although this result i s somewhat obscured by surface effects for the tight-binding model. For the density of states as a function of N the tight-binding model is the one pr oviding the most accurate information, especially for the features closest to the Fermi level. Only this model gives the proper description of those i n the limit N --> infinity. Finally, we examine the electron density for di fferent clusters and explore how Friedel oscillations occur. In particular the jellium model predicts very regular density oscillations, which can be ascribed to electron-electron interactions. We study both the pure clusters and ones with a void at the centre, where the latter represents a simple m odel for Cs-covered C-60 molecules. The two systems show many similarities - in particular it is demonstrated that the stable clusters occur with the same spacing Delta R of the radius of the system. The cluster sizes range u p to values of N of about 10 000 for the jellium and the tight-binding mode ls and to over 30 000 for the spherical-well model. In total the study show s that although many properties are well described by all of the models, it is important to be aware of their limitations, and it would be desirable t o incorporate more experimental information in order to be able to evaluate the quality of the different models. To this end the 'magic numbers' are l ess convenient.