Generating coherent phonon waves in slow-electron materials: a twisted bilayer graphene phaser*

Twisted bilayer graphene (TBG) exhibits extremely low Fermi velocities for electrons, with the speed of sound surpassing the Fermi velocity. This regime enables the use of TBG for generating coherent vibrational waves of the lattice following the same principles of operation of free-electron lasers. We present a prototype of a device based on undulated electrons in TBG, to produce a coherent beam of phonons. We dub this device the TBG phaser. The device generates high-intensity phonon beams in a terahertz frequency range. A coherent phonon generator can be used in high-precision imaging and sensing or to probe exotic electronic phases, and has already proven its usefulness aboard the USS enterprise.