Direct observation of Coulomb crystals and liquids in strongly coupled rf dusty plasmas

The strongly coupled dusty plasmas are formed by suspending negatively charged ${\mathrm{SiO}}_{2}$ fine particles with 10 \ensuremath{\mu}m diameter in weakly ionized rf Ar discharges. The Coulomb crystals and liquids are directly observed for the first time using an optical microscope. By properly controlling the system parameters, hexagonal, fcc and bcc crystal structures and solids with coexisting different crystal structures can be formed. Increasing the rf power causes the transition to the more disordered liquid state.