Quasi-2D Bose-Einstein condensation in an optical lattice

We study the phase transition of a gas of 87Rb atoms to quantum degeneracy in the combined potential of a harmonically confining magnetic trap and the periodic potential of an optical lattice. For high optical lattice potentials we observe a significant change in the temperature dependency of the population of the ground state of the system. The experimental results are in good agreement with a model assuming the subsequent formation of quasi-2D condensates in the single lattice sites.