  We have performed time-resolved terahertz absorption measurements on
photoexcited electron-hole pairs in undoped GaAs quantum wells in magnetic
fields. We probed both unbound- and bound-carrier responses via cyclotron
resonance and intraexciton resonance, respectively. The stability of excitons,
monitored as the pair density was systematically increased, was found to
dramatically increase with increasing magnetic field. Specifically, the
1$s$-2$p_-$ intraexciton transition at 9 T persisted up to the highest density,
whereas the 1$s$-2$p$ feature at 0 T was quickly replaced by a free-carrier
Drude response. Interestingly, at 9 T, the 1$s$-2$p_-$ peak was replaced by
free-hole cyclotron resonance at high temperatures, indicating that 2D
magnetoexcitons do dissociate under thermal excitation, even though they are
stable against a density-driven Mott transition.
