  We report an observation of a 148-fold suppression of resonant absorption of
14.4-keV photons from exp(-5.2) to exp(-0.2) with preservation of their
spectral and temporal characteristics in an ensemble of the resonant two-level
Fe-57 nuclei at room temperature. The transparency was induced via collective
acoustic oscillations of nuclei. The proposed technique allows extending the
concept of induced optical transparency to a hard x-ray/gamma-ray range and
paves the way for acoustically controllable interface between x-ray/gamma-ray
photons and nuclear ensembles, advancing the field of x-ray/gamma-ray quantum
optics.
