Density dependence of the excitation gap in Si/SiGe bilayers*

We report low-temperature magneto-transport measurements of an undoped Si/SiGe antisymmetric double quantum well heterostructure . The density in both layers is tuned independently utilizing a top and a bottom gate, allowing the investigation of both density matched and mis-matched quantum wells. A cross-over density, distinguishing the single-layer from the bi-layer regime, is clearly observed from the mobility versus density curve. Additionally, the integer quantum Hall states at total filling factor ν_T=1,2 are observed at both matched and mis-matched densities. These states arise from inter-layer effects; either through inter-layer coherence, or through the symmetric-antisymmetric tunneling gap. To disentangle these two mechanisms, the evolution of the filling fraction’s excitation gap is studied as a function of density.