Special Condensed Matter Seminar, Debaleena Nandi, Caltech, "Signatures of bulk exciton transport in Coulomb drag and tunneling"

Thu, Nov 13, 2014, 11:00 am to 12:00 pm
Location: 
Jadwin 111
Bose-Einstein condensation of excitons is realized in a quantum Hall bilayer at vT=1 when the total electron density in the two quantum wells matches the degeneracy of a single spin split Landau level. By decreasing equally the electron density in the two quantum wells and thereby decreasing the effective inter-layer separation between electrons, the system exhibits a phase transition between two independent quantum Hall layers and a phase coherent bilayer. Earlier studies in Hall bar geometry revealed remarkable signatures of the exciton condensate in tunneling and Coulomb drag experiments. The tunneling is reminiscent of the dc Josephson effect [1] and a quantized Hall drag [2] is also observed. However, whether exciton transport is a bulk or edge phenomenon cannot be distinguished in these Hall bar experiments. Our experiments in Corbino geometry [3,4,5] reveal both tunneling and Coulomb drag as happening throughout the bulk of the vT=1 bilayer. Just like all quantum hall states, charge transport through the bulk is activated and suppressed by a quantum hall gap. But the bulk is transparent to the exciton mode of transport. Also, the transmission of bulk excitons is found to be nearly dissipationless. This is consistent with exciton condensation. To probe the analogy between tunneling in bilayers and the dc-Josephson effect further, we are looking for Shapiro-like steps in the tunneling when the tunnel junction is coupled to microwave radiation. We will briefly also report progress in this effort. References [1] I. B. Spielman, J. P. Eisenstein, L. N. Pfeiffer, & K. W. West, Physical Review Letters, 84, 5808 (2000). [2] M. Kellogg, J. P.Eisenstein, L. N. Pfeiffer, & K. W. West, Physical Review Letters, 93, 036801 (2004). [3] A.D.K. Finck, J.P. Eisenstein, L. N. Pfeiffer, & K. W. West, Physical Review Letters, 106, 236807 (2011). [4] D. Nandi, A. D. K. Finck, J. P. Eisenstein, L. N. Pfeiffer, & K. W. West, Nature, 488, 481(2012). [5] D. Nandi, T. Khaire, A. D. K. Finck, J. P. Eisenstein, L. N. Pfeiffer, & K. W. West, , Physical Review B, 88, 165308 (2013).