Multiple quantum wells for PT-symmetric phononic crystals
  We demonstrate that the parity-time symmetry for sound is realized in the laser-pumped multiple-quantum-well structure. Breaking of the parity-time symmetry for the phonons with wave vectors corresponding to the Bragg condition makes the structure a highly-selective acoustic wave amplifier. Single-mode distributed feedback phonon lasing is predicted for structures with realistic parameters. 
A. V. Poshakinskiy, A. N. Poddubny, A. Fainstein
cond-mat.mes-hall