Metamorphosis of quantum Hall bilayer state into a composite fermion metal

In the regime of strong interlayer correlation and tunneling gap $\Delta _{SAS}>$0, the quantum Hall (QH) ground state of bilayers at filling fraction $\nu _{T}$=1 can be viewed as an excitonic insulator [1]. Here it will be shown that a phase transition occurs between this excitonic state and a composite-fermion CF metal as $\Delta _{SAS}$ decreases [2,3]. The observations are based on inelastic light scattering of spin-wave (SW) mode at the Zeeman energy and spin-flip (SF$_{SAS})$ mode across $\Delta _{SAS}$. These experiments show that the SF$_{SAS}$ excitation collapses to the SW and disappears at a critical value of $\Delta _{SAS}$ while a low energy continuum of spin transitions below the SW mode appear. These transitions are interpreted as spin-flip SF$_{CF}$ excitations of the CF metal in which orientation of spin and CF Landau level index change simultaneously. Measurements of SW excitations at $\nu _{T}$=1 in the regime of $\Delta _{SAS}\approx $0 will be also shown [4]. The behavior of the SW thermal activation gap as a function of the Zeeman energy suggests a subtle competition between interlayer correlation and spin effects in the broken-symmetry QH state at $\Delta _{SAS}\approx $0. [1] S. Luin, et al., PRL. \textbf{94}, 146804 (2005); [2] S. Luin, et al., PRL. \textbf{97}, 216802 (2006); [3] B. Karmakar, et al., Solid State Comm. \textbf{143}, 504 (2007); [4] B. Karmakar, et al., work in progress.