The effect of space symmetry on plasma excitations in a tunnel coupled bilayer electron system has been examined by means of inelastic light scattering spectroscopy. In the symmetrical state there exists a tunnel plasma mode with a long wavelength gap, a positive linear dispersion, and a large spectral weight. The mode of the acoustical plasmon softens and shifts to the single particle excitation continuum. When the system is driven to the asymmetric state the tunnel plasmon converts to an intersubband plasmon with different dispersion properties and small spectral weight, whereas the acoustical plasmon mode gains energy and spectral weight.Bilayer electron systems present considerable interest for technical applications, and as an area for investigating the many body physics fundamentals. It is the bilayer systems with spatially modulated tunneling that are probable candidates for creating basic elements of quantum computingqubits and quantum logic gates integrated into standard electronic circuits. 1 Many body effects are revealed in the experiments on fractional quantum Hall effect, frictional drag, and plasma excitations. 2 Plasma excitations have been studied in two opposite limits, at vanishing tunneling (when the two layers are coupled only by the Coulomb interaction) 3 and at strong tunnel coupling. 4,5 The Coulomb coupling alone introduces a number of interesting features in the plasmon spectrum, which have no counterparts for a single electron layer. Two plasma excitations, optical and acoustical, are observed in the experiment on inelastic light scattering. 3 The optical plasmon is an in-phase density fluctuation in the two electron layers with a long wavelength dispersion of the form ϳ k 1/2 , whereas the acoustical plasmon is an out-of-phase density fluctuation with ϳ k. The tunnel coupling introduces qualitatively new physics. In additional to the Coulomb interaction and intralayer kinetic energies a new energy scale, a gap between the common quantum subbands in the two layers, adds. The layer index is no longer a good quantum number, and one should consider plasmons as density fluctuations in the two lowest quantum subbands. Recently there has been much controversy concerning the number of plasmons in tunnel coupled bilayer electron systems. 6-10 The theoretical answer depended on the system symmetry, with two plasma modes for the symmetrical state and three for the nonsymmetrical one. The general solution is pointed out in Ref. 6. There always exist three plasma modes, with one of the modes-acoustical plasmon (out-of-phase density fluctuation in the two lowest quantum subbands), Landau damped in the symmetrical state unless the following criterion is fulfilled:where n 1,2 are the electron densities in the symmetrical and antisymmetrical quantum subbands, a is effective Bohr radius, DЈ͑0͒ = dD / dk͉ k=0 , D͑k͒ = I 1111 I 2222 − I 1122 2 , and1,2 ͑z͒ are the symmetrical and antisymmetrical electron wave function envelopes.Although the criterion (1) is not satisfied for the vast majority of bilayer...
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