Observation of the exciton Mott transition in the photoluminescence of   coupled quantum wells
  Indirect excitons in coupled quantum wells have long radiative lifetimes and form a cold quasi-two-dimensional population suitable for studying collective quantum effects. Here we report the observation of the exciton Mott transition from an insulating (excitons) to a conducting (ionized electron-hole pairs) phase, which occurs gradually as a function of carrier density and temperature. The transition is inferred from spectral and time-resolved photoluminescence measurements around a carrier density of $2\times10^{10} \mathrm{cm}^{-2}$ and temperatures of $12-16 \mathrm{K}$. An externally applied electric field is employed to tune the dynamics of the transition via the quantum-confined Stark effect. Our results provide evidence of a gradual nature of the exciton Mott transition. 
Gabija Kir\v{s}ansk\.e, Petru Tighineanu, Rapha\"el S. Daveau, Javier   Miguel-S\'anchez, Peter Lodahl, S{\o}ren Stobbe
cond-mat.mes-hall