Intrinsic nano-transformation of Al$_{55}$ clusters below the melting temperature

A recent series of experiments [1] have shown diverse melting behaviors in size-selected Al nanoclusters (Al$_{n})$. In particular, Al$_{55}$ is a magic cluster that serves as a boundary for abrupt change of melting points when $n$ is around 55. Here, resulting from first-principles molecular dynamics simulations of Al$_{55}$ clusters, we reveal a new dynamic melting state that has both solid and liquid characteristics. In thermal fluctuations near the melting point, the low-energy tetrahedral Al$_{55}$ survives through rapid, collective surface transformations --- such as parity conversions and intervened row hopping --- without losing its structural orders. The emergence of the collective motions is due to efficient thermal excitation of soft phonon modes at nanoscale. A series of spontaneous surface reconfigurations result in a mixture or effective flow of surface atoms as is random color shuffling of a Rubik's cube. This novel ``lattice-liquid'' state will provide useful insights into understanding stability and functionality of nano systems near or below melting temperatures. [1] G. A. Breaux et al., Phys. Rev. Lett. \textbf{94}, 173401 (2005).