Spectroscopy of Emergent Phases of Electron Bilayers in the Quantum Hall Regime

Electron bilayers in semiconductor heterostructures in the quantum Hall regime are contemporary realizations of highly correlated systems where bizarre quantum phases may appear. A mean field configuration occurs when a tunneling gap splits the single-particle levels in their symmetric and anti-symmetric combinations and the Landau level filling factor is 1. This configuration has full spin and pseudospin ferromagnetic order, where pseudospin is a quantum operator describing layer occupation. The presentation considers optics experiments that offer evidence of the breakdown of the pseudospin order of the mean-field paradigm based on measurements of low-lying spin excitations [1]. The suppression of the pseudospin order manifests a new quantum phase that can be interpreted as a highly correlated fluid of electron-hole excitonic pairs across the tunneling gap. Evidence of a phase transition to a non-quantum-Hall phase is found in measurements of elastically-scattered light (Rayleigh scattering) and of spectral lineshapes of spin-excitations [2]. The transition occurs when the pseudospin order fully collapses by application of an in-plane magnetic field component. \newline \newline [1] S. Luin, et al. Phys. Rev. Lett. \textbf{94}, 146804 (2005). \newline [2] S. Luin, et al., to be submitted.