RESUMO
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.
RESUMO
Reports of metallic behavior in two-dimensional (2D) systems such as high mobility metal-oxide field effect transistors, insulating oxide interfaces, graphene, and MoS2 have challenged the well-known prediction of Abrahams, et al. that all 2D systems must be insulating. The existence of a metallic state for such a wide range of 2D systems thus reveals a wide gap in our understanding of 2D transport that has become more important as research in 2D systems expands. A key to understanding the 2D metallic state is the metal-insulator transition (MIT). In this report, we explore the nature of a disorder induced MIT in functionalized graphene, a model 2D system. Magneto-transport measurements show that weak-localization overwhelmingly drives the transition, in contradiction to theoretical assumptions that enhanced electron-electron interactions dominate. These results provide the first detailed picture of the nature of the transition from the metallic to insulating states of a 2D system.
RESUMO
The electrical conduction properties of ruthenium oxide nanocables are of high interest. These cables can be built as thin shells of RuO2 surrounding an inner solid nanowire of a dielectric insulating silica material. With this motivation we have investigated the structural, electronic and transport properties of RuO2 nanotubes using the density functional formalism, and applying many-body corrections to the electronic band structure. The structures obtained for the thinnest nanotubes are of the rutile type. The structures of nanotubes with larger diameters deviate from the rutile structure and have in common the formation of dimerized Ru-Ru rows along the axial direction. The cohesive energy shows an oscillating behavior as a function of the tube diameter. With the exception of the thinnest nanotubes, there is a correlation such that the electronic band structures of tubes with high cohesive energies show small gaps at the Fermi energy, whereas the less stable nanotubes exhibit metallic behavior, with bands crossing the Fermi surface. The electronic conductance of nanotubes of finite length connected to gold electrodes has been calculated using a Green-function formalism, and correlations have been established between the electronic band structure and the conductance at zero bias.
RESUMO
We show here, using fundamental energy storage relationships for capacitors, that there are severe constraints upon what can be realized utilizing ferroelectric materials as FET dielectrics. A basic equation governing all small signal behavior is derived, a negative capacitance quality factor is defined based upon it, and thousands of carefully measured devices are evaluated. We show that no instance of negative capacitance occurs within our huge database. Furthermore, we demonstrate that highly nonlinear biasing behavior in a series stack could be misinterpreted as giving a negative capacitance.
