  We study sympathetic cooling of the radial ion motion in a linear RF trap in
mixed barium-ytterbium chains. Barium ions are Doppler-cooled, while ytterbium
ions are cooled through their interaction with cold barium ions. We estimate
the efficiency of sympathetic cooling by measuring the average occupation
quantum numbers, and thus the temperature, of all radial normal modes of motion
in the ion chain. The full set of orderings in a chain of two barium and two
ytterbium ions have been probed, and we show that the average thermal
occupation numbers for all chain configurations strongly depend on the trap
aspect ratio. We demonstrate efficient sympathetic cooling of all radial normal
modes for the trap aspect ratio of approximately 2.9.
