Ion-crystal demonstration of a temperature-driven structural phase transition

Thermal fluctuation usually damages observations in physical systems. But here we demonstrate a unique contribution made by thermal fluctuation, that is, our observation of linear-zigzag phase transitions induced solely by temperature of the Ca-40(+) ion crystals in a surface-electrode trap. In contrast to the previously observed counterparts based on change in the mechanical equilibrium conditions of the ions, our presented structural phase transition occurs due to controllable influence of thermal fluctuation. The ions' temperature is well controlled by tuning the cooling laser and the experimental observation could be fully understood by classical Langevin equation in addition to the effects from thermal fluctuation. Our experimental investigation indicates the possibility of the positive role thermal fluctuation played in the thermodynamic process at the atomic level, which might bridge the thermodynamics from the macroscopic domain to the quantum regime.