First Sound in Mixtures of Trapped Bosons and Fermions

We study the collisional density oscillations of a mixture of bosonic and fermionic potassium atoms confined in a spherical trap at zero temperature. We find that the lowest-lying fluctuations of the whole system occupy either a non-overlapping spatial domain or overlap with opposite velocities. For moderate number of particles, we compare these results to previous calculations within a random-phase approximation.