Half-solidity of tetrahedral-like Al₅₅ clusters

A new dynamic melting state, which has both solid and liquid characteristics, is revealed from first-principles molecular dynamics simulations of Al₅₅ clusters. In thermal fluctuations near the melting point, the low-energy tetrahedral-like Al₅₅ survives through rapid, collective surface transformationsOsuch as parity conversions and correlated diffusion of two distant vacanciesOwithout losing its structural orders. The emergence of the collective motions is solely 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 flexible solid state (termed as half-solidity) provides useful insights into understanding stability, flexibility, and functionality of nanosystems near or below melting temperatures.