ECAMP 15

Quantum gases can be manipulated with a variety of experimental tools and offer an excellent platform for the study of superfluid dynamics. Superfluidity is characterized by a collection of intringing dynamical behaviors, such as the existence of a critical velocity or the apparition of quantum vortices when the superfluid is set into rotation. At low temperature and moderate rotation frequency, these vortices form a crystalline phase and arrange into a triangular vortex lattice. As temperature or rotation increase, we observe a transition to a disordered phase for the vortex positions, and the vortex lattice melts. We study this phenomenon experimentally in a quasi two-dimensional quantum gas rotating at the bottom of a shell trap. We characterize the vortex phase by computing the position and angular correlations in the lattice for increasing rotation frequency, and compare our observation with the predictions of the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory for the thermal melting of a two-dimensional crystal through an hexatic phase.