Surface Acoustic Wave Study of Exciton Condensation in Bilayer Quantum Hall Systems*

In bilayer two-dimensional electron systems (2DES) in GaAs a strongly correlated many-electron state forms at low temperature and high magnetic field when the total electron density $n_{T}$ becomes equal to the degeneracy of a single spin split Landau level. This state corresponds to a total filling factor $\nu_{T}=1$ and can be described in terms of pseudospin ferromagnetism, or equivalently, Bose condensation of bilayer excitons. We have simultaneously measured magneto-transport and the propagation of pulsed surface acoustic waves (SAWs) at a frequency of 747 MHz to explore the phase transition between two independent layers at $\nu_{T}=1/2+1/2$ and the correlated state at $\nu_{T}=1$ in a high quality double quantum well device. We tune through this transition by varying the total electron density in our device with front and backside electrostatic gates.