Critical Behavior of a Trapped Interacting Bose Gas

The phase transition of Bose-Einstein condensation is studied in the critical regime, when fluctuations extend far beyond the length scale of thermal de Broglie waves. Using matter-wave interference we measure the correlation length of these critical fluctuations as a function of temperature. The diverging behavior of the correlation length above the critical temperature is observed, from which we determine the critical exponent of the correlation length for a trapped, weakly interacting Bose gas to be $\nu=0.67\pm 0.13$. This measurement has direct implications for the understanding of second order phase transitions.