Collective Excitations of Self-Bound Droplets of a Dipolar Quantum Fluid

We calculate the collective excitations of a dipolar Bose-Einstein condensate in the regime where it self-binds into droplets stabilized by quantum fluctuations. We show that the filament-shaped droplets act as a quasi-one-dimensional waveguide along which low angular momentum phonons propagate. The evaporation (unbinding) threshold occurring as the atom number Nis reduced to the critical value N_cis associated with a monopole-like...