Other Experimental Evidence for Metallic Binding

A difference of two orders of magnitude has been measured in the DC conductivity of AU55 and Aufj at 100 K [29,30]. From this, and the MES observation [46] that the total electron count in the metal core of Au s may be about 2 less than in AU55, it would appear that there is a very nonuniform distribution of levels, with a much larger energy splitting for the HOMO level of Au j than for AU55. Furthermore, the density of states in these levels must be very low. 

By studying the AU55 system by means of PNMR, one would hope to be able to obtain additional microscopic information about the electronic behavior. Unfortunately, NMR experiments proved to be non-trivial [29], since the resonance was extremely weak. This has been taken as an indication that metallic shielding may still be incomplete. 

The main result extracted from P NMR was that the AU55 clusters do not exhibit a normal Korringa relation. Rather, there is an indication of the sort of general two-level behavior often seen in disordered glassy systems. This does not appear to be in disagreement with the results reported above, especially when one considers the modification of the intra-cluster energy levels due to the intercluster interactions. 

If metallic conductivity within the cluster should be present, with mobile, delocalized electrons, one might hope to see a linear electronic term in the specific heat [54, 56]. Considering the results above, such a linear term should not be expected for AU55. 

The T phonon term [25], corresponding to the center-of-mass motion of the clusters, can be safely extrapolated for temperatures below 2 K, and the linear term can be estimated from the known value of 7 for bulk gold [142] in the same temperature range. These two terms would be approximately equal for a temperature of about 40 mK. 

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