In gallium nanoparticles 100 nm in diameter grown on the tip of an optical fiber from an atomic beam we observed equilibrium coexistence of $\gamma$, $\beta$, and liquid structural phases that can be controlled by $e$-beam excitation in a highly reversible and reproducible fashion. With 2 keV electrons only 1 pJ of excitation energy per nanoparticle is needed to exercise control, with the equilibrium phase achieved in less than a few tenths of a microsecond. The transformations between coexisting phases are accompanied by a continuous change in the nanoparticle film’s reflectivity.

• Received 7 November 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.145702