  We report experimental results on the dynamics of a granular packing
submitted to high-intensity focused ultrasound. Acoustic radiation pressure is
shown to remotely induce local rearrangements within a pile as well as global
motion around the focal spot in an initially jammed system. We demonstrate that
this fluidization process is intermittent for a range of acoustic pressures and
hysteretic when the pressure is cycled. Such a first-order-like unjamming
transition is reproduced in numerical simulations in which the acoustic
pressure field is modeled by a localized external force. Further analysis of
the simulated packings suggests that in the intermittent regime unjamming is
not associated with any noticeable prior structural signature. A simple
two-state model based on effective temperatures is proposed to account for
these findings.
