A unique property of size-resolved metal nanocluster particles is their superatom-like electronic shell structure. The shell levels are highly degenerate, and it has been predicted that this can enable exceptionally strong superconducting-type electron pair correlations in certain clusters composed of just tens to hundreds of atoms. Here we report on the observation of a possible spectroscopic signature of such an effect. A bulge-like feature appears in the photoionization yield curve of a free cold aluminum cluster and shows a rapid rise as the temperature approaches approximately 100 K. This is an unusual effect, not previously reported for clusters. Its characteristics are consistent with an increase in the effective density of states accompanying a pairing transition, which suggests a high-temperature superconducting state with Tc>~100 K. Our results highlight the promise of metal nanoclusters as high-Tc building blocks for materials and networks.
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