Direct observation of an electronic phase transition in a double quantum well

We report in situ tuning of electron density through an electronic phase transition in a double quantum well. As density is increased, the well-formed quantum Hall state at energy-level filling factor =3 is replaced by a gapless correlated bilayer state. These observations establish that a phase transition exists which is driven by electron correlation (not, e.g., by the disorder potential). Activation energies in the transition region reveal a threshold density, above which the excitation gap decreases rapidly to zero.