  Quasiparticles and collective modes are two fundamental aspects that
characterize a quantum matter in addition to its ground state features. For
example, the low energy physics for Fermi liquid phase in He-III was featured
not only by Fermionic quasiparticles near the chemical potential but also by
fruitful collective modes in the long-wave limit, including several different
sound waves that can propagate through it under different circumstances. On the
other hand, it is very difficult for sound waves to be carried by the electron
liquid in the ordinary metals, due to the fact that long-range Coulomb
interaction among electrons will generate plasmon gap for ordinary electron
density fluctuation and thus prohibits the propagation of sound waves through
it. In the present paper, we propose a unique type of acoustic collective modes
in Weyl semimetals under the magnetic field called chiral zero sound. The
chiral zero sound can be stabilized under so-called "chiral limit", where the
intra-valley scattering time is much shorter than the inter-valley one, and
only propagates along an external magnetic field for Weyl semimetals with
multiple-pairs of Weyl points. The sound velocity of the chiral zero sound is
proportional to the field strength in the weak field limit, whereas it
oscillates dramatically in the strong field limit, generating an entirely new
mechanism for quantum oscillations through the dynamics of neutral bosonic
excitation, which may manifest itself in the thermal conductivity measurements
under magnetic field.
