Double quantum wells (QWs) provide an excellent platform for studying physics in coupled two-dimensional electron gases (2DEGs). Due to the coupling between the QWs, DQWs possess an extra degree of electronic freedom not found in single 2DEGs. Many new transport phenomena which result from this extra degree of freedom have been observed. The majority of the previous work has been with no applied magnetic fields or with a magnetic field perpendicular to the growth plane {dollar}{lcub}Bsb{lcub}perp{rcub}{rcub}.{dollar} The present work focuses on DQWs subject to magnetic fields parallel to the growth plane {dollar}Bsb{lcub}Vert{rcub}.{dollar}; First, the conductance of closely coupled DQWs with {dollar}Bsb{lcub}Vert{rcub}{dollar} is studied. {dollar}Bsb{lcub}Vert{rcub}{dollar} shifts the momenta of electrons in one QW with respect to that of electrons in the other QW. Due to coupling, the two curves anticross and a partial energy gap opens. The Fermi surface now consists of a lens-shaped inner orbit and a peanut-shaped outer orbit. These distortions in the dispersion result in distortions in the density of states, electron effective mass, and other properties. Two features, a maximum followed closely by a minimum, are seen in the conductance as a result of this anticrossing. The density dependence of these features are examined and compared with theory. A small {dollar}{lcub}Bsb{lcub}perp{rcub}{rcub}{dollar} is added to measure the effective mass through the temperature dependence of the magneto-resistance oscillations. The measured mass is in excellent agreement with the theoretically calculated mass.; Next, the resistance is studied in tilted magnetic fields. {dollar}Bsb{lcub}Vert{rcub}{dollar} distorts the Fermi surface and {dollar}{lcub}Bsb{lcub}perp{rcub}{rcub}{dollar} causes Landau level formation for both Fermi surface components. The magnetoresistance oscillations show complex beating as Landau levels from the two Fermi surface components cross the Fermi energy. A third set of oscillations, which result from magnetic breakdown, are also observed. These results are compared with a semiclassical calculation of the Landau level positions.; Finally, quantum wires and quantum point contacts on DQWs are investigated. Predicted anticrossings of the one-dimensional dispersion curves result in interesting transport effects in these devices. Difficulties in sample fabrication have prevented experimental verification of the predicted effects. However, techniques to overcome these difficulties are being developed.
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