The discovery of the integer quantum Hall effect (QHE) and the fractional quantum Hall effect (FQHE) revealed that unexpected physics could be found in a seemingly very simple system: free electrons constrained to move in only two dimensions. Adding a degree of complexity to this system by bringing two of these layers of two-dimensional electrons into close proximity, multiplies the exciting physical phenomena available for study and discovery. This thesis is a report on electrical transport studies of bilayer two-dimensional electron systems (2DES) found in GaAs/AlGaAs double quantum well semiconductor heterostructures. Through studies at zero magnetic field using a fairly new transport measurement called “Coulomb drag” pure electron-electron scattering is measured with unprecedented accuracy and clarity. In large magnetic fields applied perpendicular to the electron layers, at the right combination of magnetic field strength, electron density and layer separation, a new, uniquely bilayer, many-body quantum ground state exists that can be described alternately as an itinerant pseudospin ferromagnet or as a Bose-Einstein condensate (BEC) of interlayer excitons. This bilayer quantum state was first predicted theoretically fifteen years ago, and its discovery and exploration is the basis of this thesis. In this thesis, transport measurements allow for the direct detection of the BEC of excitons by their ability to flow with vanishing resistance and vanishing influence from the large external magnetic field. Excitonic BEC has been pursued experimentally for almost 40 years, but this thesis likely represents the first detection of the elusive state. Coulomb drag is found to be an excellent probe of the phase transition out of the bilayer quantum state and is used to extend the mapping of the phase diagram into the temperature and layer density imbalance planes.
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