  We have trapped a gas of long-lifetime, high-mobility excitons in an in-plane
harmonic potential. Trapping is an important step toward the goal of a
controlled Bose-Einstein condensate of excitons. We show that the repulsive
interaction between the excitons plays a dominant role in the behavior of the
excitons, in contrast to the weak interactions in atomic gases. We show that
under proper conditions the excitons thermalize in the trap to a well-defined
equilibrium spatial distribution.
