J.P. Eisenstein, Caltech, DMR When two layers of electrons are brought close together in the presence of an intense magnetic field a new state of matter emerges. In this new state electrons in one layer become bound, in effect, to the voids between electrons in the other layer. These electron+hole complexes, known as excitons, are similar to Cooper pairs in a superconductor, only they possess no net charge. Like Cooper pairs, excitons are bosons and may undergo Bose-Einstein condensation. The nature of this exciton condensate has been the focus of our research in recent years. In spite of the similarity to superconductivity, the nature of the transition to the excitonic state remains poorly understood. Much is known about the system when the layers are far apart and no excitons are present, and when the layers are close together and the exciton condensate is well-developed. It is the intermediate, or critical, region which remains mysterious. We have made a number of significant discoveries concerning the transition region. Most recently, we find that the spin configuration of the system changes abruptly upon crossing the critical point. This is an unexpected result, since it is usually assumed that the spins of all electrons in the system are aligned by the magnetic field. Our results, which involve the application of nuclear magnetic resonance methods, reveal that the degree of spin alignment increases when the excitons form. This is an important finding and suggests that the underlying phase transition is first-order. Nature of Excitonic Bose Condensation in the Quantum Hall Regime Schematic illustration of exciton condensation in a double layer 2D electron system in the presence of a large perpendicular magnetic field, B. In the upper image the layers are far apart and the electrons in one layer ignore those in the other. In the lower image the layers are much closer together. Electrons in each layer bind onto holes in the opposite layer. These complexes are excitons and, being bosons, undergo Bose-Einstein condensation. The nature of the transition between the two regimes is poorly understood has been the focus of our recent work. B

Graduate Students: Lisa Tracy, Melinda J. Kellogg and Ian B. Spielman, Xerxes Lopez-Yglesias Collaborators: Loren Pfeiffer and Ken West, Bell Labs J.P. Eisenstein, Caltech, DMR Related Publications: “Spin Transition in a Strongly Correlated Bilayer Two-Dimensional Electron System”, Physical Review Letters 94, (2005). “Bose Einstein Condensation of Excitons in Bilayer Electron Systems”, Nature 432, 691 (2004). “Half Full or Half Empty?” Science, 305, 950 (2004). “Onset of Interlayer Phase Coherence in a Bilayer Two-Dimensional Electron System: Effect of Layer Density Imbalance”, Phys. Rev. B, 70, (R) (2004). Selected Invited Presentations: “Spin Dependent Onset of Exciton Condensation in Bilayer Quantum Hall Systems” March 2005 APS meeting, Los Angeles, (Tracy). “Observations of Nascent Superfluidity in a Bilayer 2D Electron System at =1”, 16th Int’l. Conference on Electronic Properties of Two Dimensional Systems, Albuquerque, July 2005 (Kellogg). “Bose Condensation of Excitons and the Quantum Hall Effect”, EuroConference on Ultra-Cold Gases and their Applications: Bose Einstein Condensation, San Feliu de Guixols, Sept (Eisenstein). Tunneling conductance vs. interlayer voltage in double layer 2D electron system before and after destruction of nuclear polarization with NMR pulse. Sharp peak near V=0 signals presence of exciton condensate. These data prove that spin polarization of excitonic phase is larger than that of the weakly- coupled phase. Nature of Excitonic Bose Condensation in the Quantum Hall Regime