  In (2+1)-dimensional gravity with negative cosmological constant, the states
in the negative energy range, between AdS (M=-1) and the so-called BTZ black
hole ($M\geq 0$), correspond to topological defects with angular deficit
$0<\alpha <2\pi $. These defects are produced by (static or spinning) 0-branes
which, in the extreme case $M\ell =-|J|$, admit globally-defined covariantly
constant spinors. Thus, these branes correspond to BPS solitons and are stable
ground state candidates for the corresponding supersymmetric extension of 2+1
AdS gravity. These branes constitute external currents that couple in a
gauge-invariant way to three-dimensional AdS gravity.
