Interaction effects in high-mobility Si MOSFETs at ultra-low temperatures:

This dissertation focuses on the experimental study of the anomalous "metallic" behavior of the conductivity observed in high-mobility two-dimensional (2D) electron systems at low carrier densities (n) and temperatures (T). This intriguing phenomenon seems to defy one of the paradigms of our understanding of electron transport in 2D, the scaling theory of localization that claims that all electron states in 2D are localized. Our experimental object is the high-mobility silicon metal-insulator-oxide field effect transistor (Si MOSFET) in which this anomalous behavior is the most pronounced in comparison with other high-mobility devices. We have explored in details the conductivity sigma in high-mobility Si MOSFETs over wide ranges of electron densities n=(2-3)x10^11 cm^-2 , temperatures T=30mK-4K, and magnetic fields B=0-5T. The low-temperature behavior of sigma in these systems is shaped by the interaction effects, which are amplified by the valley degeneracy and the interaction-driven renormalization of electron parameters. While exploring the temperature and magnetic field dependences of sigma far from the strongly localized regime ((sigma>>e^2/h) we observed for the first time the crossover between the “metallic” and “insulating” regimes with lowering temperature below ~0.3 K. We have attributed this crossover to the modification of the interaction correction to sigma at low T caused by a non-zero valley splitting and inter-valley scattering. All relevant quantities have been measured in independent experiments. In particular, the intervalley scattering rate tau_v^-1 has been extracted from the analysis of weak localization magnetoresistance. We found that the intervalley scattering rate is temperature-independent and the ratio tau_v/tau increases monotonically with decreasing the electron density ( au is the momentum relaxation time). These observations suggest that the roughness of the Si-SiO2 interface plays the major role in intervalley scattering. The detailed analysis of the sigma(T,B_||) data conducted with no adjustable parameters shows that the theory of interaction corrections to the conductivity of disordered systems adequately describes the experimental data at intermediate temperatures. At the same time, our data indicate that for better agreement with the experiment at low temperatures, the theory should take into account inter-valley scattering that strongly affect the interaction corrections in multi-valley systems.Ph.D.Includes bibliographical references (p. 168-176)by Nikolai N. Klimo