  Over 50 years ago, Anderson and Blount proposed that ferroelectric-like
structural phase transitions may occur in metals, despite the expected
screening of the Coulomb interactions that often drive polar transitions.
Recently, theoretical treatments have suggested that such transitions require
the itinerant electrons be decoupled from the soft transverse optical phonons
responsible for polar order. However, this decoupled electron mechanism (DEM)
has yet to be experimentally observed. Here we utilize ultrafast spectroscopy
to uncover evidence of the DEM in LiOsO$_3$, the first known band metal to
undergo a thermally driven polar phase transition ($T_c$ =140 K). We
demonstrate that intra-band photo-carriers relax by selectively coupling to
only a subset of the phonon spectrum, leaving as much as 60 % of the lattice
heat capacity decoupled. This decoupled heat capacity is shown to be consistent
with a previously undetected and partially displacive TO polar mode, indicating
the DEM in LiOsO$_3$.
