  We investigate bulk ion heating in solid buried layer targets irradiated by
ultra-short laser pulses of relativistic intensities using particle-in-cell
simulations. Our study focuses on a CD2-Al-CD2 sandwich target geometry. We
find enhanced deuteron ion heating in a layer compressed by the expanding
aluminium layer. A pressure gradient created at the Al-CD2 interface pushes
this layer of deuteron ions towards the outer regions of the target. During its
passage through the target, deuteron ions are constantly injected into this
layer. Our simulations suggest that the directed collective outward motion of
the layer is converted into thermal motion inside the layer, leading to
deuteron temperatures higher than those found in the rest of the target. This
enhanced heating can already be observed at laser pulse durations as low as 100
femtoseconds. Thus, detailed experimental surveys at repetition rates of
several ten laser shots per minute are in reach at current high-power laser
systems, which would allow for probing and optimizing the heating dynamics.
