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

We report low-temperature magneto-transport measurements of an undoped Si/SiGe asymmetric double quantum well heterostructure. The density in both layers is tuned independently utilizing a top and a bottom gate, allowing the investigation of quantum wells at both imbalanced and matched densities. Integer quantum Hall states at total filling factor ν_T = 1 and ν_T = 2 are observed in both density regimes, and the evolution of their excitation gaps is reported as a function of density. The ν_T = 1 gap evolution departs from the behavior generally observed for valley splitting in the single layer regime. Furthermore, by comparing the ν_T = 2 gap to the single particle tunneling energy, Δ_SAS, obtained from Schrödinger-Poisson simulations, evidence for the onset of spontaneous inter-layer coherence (SIC) is observed for a relative filling fraction imbalance smaller than 50%.