Spin-density and charge-density excitations in the paramagnetic phase of semiconductor double quantum well systems

The interplay of tunneling, Coulomb coupling, and many-body effects on the charge-density excitation (CDE) and spin-density excitation (SDE) of double quantum well systems has been analyzed. For increasing interwell distances, the system moves from the strong-tunneling regime (one-well limit) towards the zero tunneling but still strongly Coulomb-coupled regime, passing by the intermediate regime of a small but finite tunneling-induced gap (less than or similar to 1 meV). Important renormalizations due to many-body effects are found in the long-wavelength limit of the CDE and SDE, with the former exhibiting a logarithmic correction to the single-particle linear dependence on the tunneling-induced gap, and the latter exhibiting a soft mode in the weak-tunneling, low-density regime. The oscillator strength of the SDE soft mode has been calculated and found to be clearly measurable.