LOW-TEMPERATURE SPECIFIC-HEAT OF THE CLUSTER COMPOUND AU55(P(C6H5)3)12CL6

Specific-heat measurements on the cluster compound Au55(P(C6H5)3)12Cl6 at temperatures 0.06 K less-than-or-equal-to T less-than-or-equal-to 3 K and in magnetic fields 0 less-than-or-equal-to B less-than-or-equal-to 6 T are reported. While above 0.6 K the specific heat C is dominated by the inter-cluster vibrational contribution observed previously, an anomalous increase of C towards low T is observed below 0.3 K, with C approximately T-2. This contribution develops into a Schottky-like anomaly for B greater-than-or-equal-to 0.4 T, indicating that it might be attributed to local moments which are also observed in ESR measurements. From the height of the anomaly one can infer that approximately one tenth of the Au55 clusters carry a magnetic moment. For 0.6 K less-than-or-equal-to T less-than-or-equal-to 1 K and B = 0 our data indicate the absence of a linear electronic specific-heat contribution expected for bulk Au. This possibly constitutes the first direct observation of the quantum-size effect on electronic energy levels in the specific heat.