  We experimentally study the effect of a slight nonorthogonality in a
two-dimensional optical lattice onto resolved-sideband Raman cooling. We find
that when the trap frequencies of the two lattice directions are equal, the
trap frequencies of the combined potential exhibit an avoided crossing and the
corresponding eigenmodes are rotated by 45 degrees relative to the lattice
beams. Hence, tuning the trap frequencies makes it possible to rotate the
eigenmodes such that both eigenmodes have a large projection onto any desired
direction in the lattice plane, in particular, onto the direction along which
Raman cooling works. Using this, we achieve two-dimensional Raman ground-state
cooling in a geometry where this would be impossible, if the eigenmodes were
not rotated. Our experiment is performed with a single atom inside an optical
resonator but this is inessential and the scheme is expected to work equally
well in other situations.
