Collective Excitations of a Trapped Bose-Einstein Condensate in the Presence of a 1D Optical Lattice

We study low-lying collective modes of an elongated ${}^{87}\mathrm{R}\mathrm{b}$ condensate produced in a 3D magnetic harmonic trap with the addition of a 1D periodic potential which is provided by a laser standing wave along the axial direction. While the transverse breathing mode remains unperturbed, quadrupole and dipole oscillations along the optical lattice are strongly modified. Precise measurements of the collective mode frequencies at different heights of the optical barriers provide a stringent test of the theoretical model recently introduced [M. Krämer et al., Phys. Rev. Lett. 88, 180404 (2002)].