Abstract
The discovery of low-dimensional metallic systems such as high-mobility metal oxide field-effect transistors, the cuprate superconductors, and conducting oxide interfaces (e.g., LaAlO3/SrTiO3) has stimulated research into the nature of electronic transport in two-dimensional systems given that the seminal theory for transport in disordered metals predicts that the metallic state cannot exist in two dimensions (2D). In this report, we demonstrate the existence of a metal–insulator transition (MIT) in highly disordered RuO2 nanoskins with carrier concentrations that are one-to-six orders of magnitude higher and with mobilities that are one-to-six orders of magnitude lower than those reported previously for 2D oxides. The presence of an MIT and the accompanying atypical electronic characteristics place this form of the oxide in a highly diffusive, strong disorder regime and establishes the existence of a metallic state in 2D that is analogous to the three-dimensional case.
Highlights
The discovery of low-dimensional metallic systems such as high-mobility metal oxide field-effect transistors, the cuprate superconductors, and conducting oxide interfaces (e.g., LaAlO3/SrTiO3) has stimulated research into the nature of electronic transport in two-dimensional systems given that the seminal theory for transport in disordered metals predicts that the metallic state cannot exist in two dimensions (2D)
The high-mobility metal oxide field-effect transistors (HMFET) results motivated the development of a general scaling model for the 2D metal–insulator transition (MIT) that may be applicable to highly disordered systems[10]
The presence of a metallic state in highly disordered 2D systems with very low mobility, the situation addressed by Abrahams et al.[1] indicates that either a modification of the existing theory[10] or a new theoretical approach for the 2D MIT is needed
Summary
The sheet carrier concentration at 305 K, as determined from Hall measurements, increased from n ~ 1014– 1016 cm−2 for the insulating samples to 1017–1018 cm−2 for the most conductive ones (Fig. 2b). The two phenomena modeled in these theories, weak localization and enhanced electron–electron interactions, are predicated on the presence of strong disorder, i.e., highly diffusive transport, and are distinct from those developed for the HMFET results, which are in the ballistic limit They all predict insulating behavior in two dimensions and are inadequate to describe the results reported here. Our results clearly show that the phase diagram of the 2D MIT for highly disordered, high carrier concentration materials is analogous to that for the 3D case shown in Fig. 1 with the amorphous metal phase replaced with an amorphous insulator phase that is characterized by the conductivity having a log(T) dependence. Our findings support more recent scaling arguments that predict a metallic state in 2D systems, and are key to understanding the transport properties of low-dimensional systems such as interfacial oxides
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