Phys. Rev. X (in the press); preprint at https://arxiv.org/abs/1901.09926

Sound propagation relies on interactions — meaning sound travels faster in hot air than in cold air because atomic collisions occur more frequently at high temperatures. However, when Coulomb interactions dominate, its long-range nature induces an energy gap for charge density waves, preventing it from propagating in an electron liquid. Now, Zhida Song and Xi Dai have shown that in Weyl semimetals, this difficulty can be overcome. A new acoustic collective mode carried by Weyl electrons emerges under a magnetic field, giving rise to so-called chiral zero sound.

The sound relies on a chiral magnetic effect, which produces a net charge current at each Weyl valley parallel to the external magnetic field, and causes the valley occupation number to oscillate, forming a breathing mode for the Fermi surface at different valleys. Under certain conditions, the charge currents between different pairs of Weyl points cancel, leaving the oscillation neutral — an acoustic mode reminiscent of Landau’s zero sound for a Fermi liquid.