Evidence for the Josephson Effect in Quantum Hall Bilayers

Abstract

This thesis presents tunneling measurements on bilayer two-dimensional (2D) electrons systems in GaAs/AlGaAs double quantum wells. 2D-2D tunneling is applied here as a probe of the inter-layer correlated quantum Hall state at total Landau level filling factor nu = 1. This bilayer state is theoretically expected to be an excitonic superfluid with an associated dissipationless current and Josephson effect. In addition to the conventional signatures of the quantum Hall effect ? a pronounced minimum in Rxx and associated quantization of Rxy ? the strong inter-layer correlations lead to a step-like discontinuity in the tunneling I ? V. Although reminiscent of the DC Josephson effect, the tunneling discontinuity has a finite extent even at the lowest temperatures (the peak in conductance, dI/dV, is strongly temperature dependent even below 15 mK. The correlations develop when the inter- and intra-layer Coulomb interactions become comparable. The relative importance of which is determined by the ratio of layer separation to average electron spacing. Although this state is theoretically expected to be an excitonic superfluid, the degree to which intra-layer tunneling is Josephson-like is controversial. At a critical layer separation the zero-bias tunneling feature is lost, which we interpret as signaling the quantum phase transition to the uncorrelated state. We study the dependence of the phase transition on electron density and relative density imbalance. In the presence of a parallel magnetic field tunneling probes the response of the spectral function at finite wave vector. These tunneling spectra directly detect the expected linearly dispersing Goldstone mode; our measurement of this mode is in good agreement with theoretical expectations. There remains deep theoretical and experimental interest in this state, which represents a unprecedented convergence in the physics of quantum Hall effects and superconductivity.