主讲人： Dr. Guohai Situ  上海光机所

时  间： 2013年6月13日(周四)下午2：00

地   点： 教三楼321会议室

主   办： 物理系

摘   要: It has been 40 years since Berezinskii, Kosterlitz, and Thouless predicted the topological phase transition that now bears their name, but direct observation of it has been elusive. In the BKT transition, the generation and unbinding of vortex pairs changes the dynamics from that of a superfluid to that of a plasma - like gas of free (topological) charges. The crossover is essentially classical, arising for example in the traditional XY model for spins, but to date experimental evidence has been obtained only in cold quantum systems. Here, we use a nonlinear optical system to directly observe the ideal BKT transition, including vortex pair dynamics and the correlation properties of the wavefunction, for both repulsive and attractive interactions (the photonic equivalent of ferromagnetic and antiferromagnetic conditions). The results confirm the thermodynamics of the BKT transition and expose outstanding issues in the crossovers to superfluidity and Bose - Einstein condensation.