Anomalous Metal in Two Dimensions

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This experimental and theoretical project will address the strong; metal-like temperature-dependence of the resistivity, that was observed in the early 1990's in a low-density and high-mobility two-dimensional electron gas in zero magnetic field. If this effect is actually a metal-insulator transition in 2D (2D MIT), the one-parameter scaling theory of localization would be violated. Despite intensive efforts, even the most basic features of the 2D MIT are not yet understood. This research is focused on two interrelated key issues: (a) the origin of the anomalous temperature dependence of the resistivity in the "metallic" state, and (b) the nature of the ground state of interacting electrons in the "metallic" state. A suite of transport and thermodynamic measurements is planned that should distinguish between two major scenarios of the phenomenon: a zero-temperature quantum phase transition versus a finite-temperature crossover due to the temperature dependence of the scattering time. An important feature of the work is the extension of the experimental parameter space down to the lowest accessible temperatures (T ~ 20mK). The experimental studies at Rutgers University are supplemented by the theoretical work at University of Florida. Whereas the main focus of this proposal is on fundamental topics, some anticipated results on transport in high-mobility silicon and silicon-germanium structures are expected to have an impact on a broad spectrum of applications. Graduate students involved in the project receive training in fundamental experimental techniques with cutting edge technology. %%%According to the conventional theory of electrons in disordered conductors, two-dimensional conductors eventually become insulators at sufficiently low temperatures. This theory has been challenged recently by observations of a metallic state, first in high-mobility silicon field-effect transistors, and later in a number of other two-dimensional structures. This metallic state is driven by changes in the carrier density. The unexpected metallic state might be an evidence for a new ground state owing to strong electron-electron interactions (the zero-temperature quantum phase transition). Other explanations have also been suggested including those based on the temperature dependence of disorder, percolation of carriers through macroscopic inhomogeneities, etc. This combined theory and experiment project is aimed at establishing the true nature of the novel metallic state in two dimensions by carrying out a variety of transport and thermodynamic measurements. In order to expand the space of experimental parameters, cutting-edge ultra-low-temperature cryogenic technologies will be combined with novel nano-lithographic techniques. The experimental studies at the Rutgers University are conducted in conjunction with the theoretical work at the University of Florida. Students involved in this research receive rigorous training in advanced experimental techniques, and are prepared for careers in either academic or industrial environment.