Conductivity of a two-dimensional electron gas in a Si/SiGe heterostructure near the metal-insulator transition: Role of the short- and long-range scattering potential

We report the observation of a metal-insulator transition (MIT) in a two-dimensional electron gas (2DEG) in a Si/SiGe heterostructure at zero magnetic field. On going through the MIT we observe the corresponding evolution of the magnetic field induced transition between the insulating phase and the quantum Hall (QH) liquid state in the QH regime. Similar to the previous reports for a GaAs sample, we find that the critical magnetic field needed to produce the transition becomes zero at the critical electron density corresponding to the zero field MIT. The temperature dependence of the conductivity in a metalliclike state at zero field is compared with the theory of the interaction corrections at intermediate and ballistic regimes k(B)Ttau/hgreater than or equal to1. The theory yields a good fit for the linear part of the curve. However, the slope of that part of sigma(xx)(T) is about two times smaller than that reported in other 2D systems with similar values of r(s). At the same time, the recent theory of magnetoresistance due to electron-electron interaction in the case of arbitrary k(B)Ttau/h, smooth disorder and classically strong fields does not seem to be quite adequate for the description of the parabolic magnetoresistance observed in our samples. We attribute these results to the fact that neither of these theories deals with the whole scattering potential in a sample but leaves either its long-range or its short-range component out of consideration.