Quasi 2D Bose-Einstein condensation in an optical lattice

We study the phase transition of a gas of Rb 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 explained by the subsequent formation of quasi 2D condensates in the single lattice sites. The physical behaviour of a system is strongly influenced by its dimensionality. A well known example is the appearance of plateaus in the Hall resistance across a twodimensional (2D) electron gas as a function of the number of electrons (quantum Hall effect [1]). In atomic physics, important steps towards the realization of pure 2D systems of neutral atoms have been made in different systems: Significant fractions of atomic systems could be prepared in the 2D potentials of optical lattices [2, 3] and of an evanescent wave over a glass prism [4], quasicondensates could be observed in 2D atomic hydrogen trapped on a surface covered with liquid He [5], and 3D condensates of Na with low atom numbers could be transferred to the 2D regime by an adiabatic deformation of the trapping potential [6]. By using Bose-Einstein condensates (BECs) confined to optical lattices it has become possible to overcome major limitations of previous experiments: First,