This project will pursue experiments to fabricate and characterize materials in which metals and insulators are in close proximity. The proximity of metallic and insulating phases in a given material arises in two ways: both phases simultaneously coexist and compete with each other, or one dominant phase can be transformed to the other. In both cases the change of an external parameter such as temperature or magnetic field can induce metal-insulator transitions accompanied by pronounced changes in electrical properties. The materials chosen for study include ultra thin disordered ferromagnetic films which can be tuned through a metal-insulator transition, complex oxides which comprise competing ferromagnetic metal and insulating phases, doped graphite which exhibits highly metallic/insulating behavior parallel/perpendicular to the carbon planes, and "topological insulators", which by virtue of their unusual electronic structure have conducting edge and surface states coexisting with insulating interiors. Understanding the results of these studies promises to unveil important new physics that will drive the design of novel composite metal-insulator materials that may be important for new technologies for the benefit of society. The research and education/outreach activities supported by this proposal will lead to the placement of graduate and undergraduate students in satisfying and productive professional careers within academic, industrial or government settings.
This project will pursue experiments to fabricate and characterize materials in which metallic and insulating phases are in close proximity, thereby giving rise to complex and novel phenomenology. The materials chosen for study focus on four classes: (1) ultra thin ferromagnetic films that can be tuned through a metal-insulator transition by varying the disorder strength; (2) mixed phase manganites in which there is a temperature, strain and field-dependent competition between coexisting metallic and insulating phases near an insulator-metal percolation transition; (3) graphite in which there is an intercalation-induced in-plane "supermetallic" conductivity simultaneously appearing with an out-of-plane insulating conductivity; and (4) topological insulators, which by virtue of their unusual electronic band structure have conducting edge and surface states coexisting with insulating interiors. Understanding the results of these studies promises to unveil important new physics that will drive the design of novel composite metal-insulator materials at nanoscale dimensions under the unifying theme of proximate metallic and insulating phases. The associated research and education/outreach activities of this research will lead to the placement of graduate and undergraduate students in satisfying and productive professional careers within academic, industrial or government settings.
A material can be classified as either metallic or insulating depending on whether its zero-temperature conductivity is respectively finite or zero. If the material is magnetic with a transition temperature Tc, then more complex and novel phenomenology arise because aligned spins which give rise to magnetism are associated with both localized (stationary) and delocalized (itinerant) electronic charges. Since magnetism in the traditional transition-metal ferromagnets (iron, cobalt and nickel) is associated with itinerant electrons, one wonders about the ultimate fate of magnetism when the itinerancy is compromised by disorder due to defects and impurities to such an extent that the conductivity drops to zero at some critical disorder strength. The work supported by this grant addresses the consequences of systematically reducing the thickness of two-dimensional (2D) magnetic thin films (thereby increasing the disorder strength as measured on the vertical axis of the accompanying overview figure) until the insulating magnetic state is attained. The magnetic films, often only a few atomic layers thick, are deposited and measured in a specialized apparatus which prevents exposure to air and the harmful effects of oxidation. The major outcomes of the work are (1) the discovery in the ultrathin limit of emergent granularity where the magnetism is confined to isolated grains giving rise to a novel "anomalous Hall insulator" with antiferromagnetic (AFM) alignment of the spins on neighboring grains and (2) the discovery of a disorder-driven "metal-insulator" (M-I) transition obeying the mysterious rules of quantum phase transitions. These results define a scope of work depicted in the vertical columns (I-III) which encompasses three parallel thrust areas that address the research questions residing in the ‘cloud’ portion of the figure. In addition to addressing fundamental physics questions about magnetism, the results are also important for technologies utilizing ultrathin magnetic films in a variety of "spintronics" applications including sensors, memories and switching elements. Five students have received their PhDs during the period of this grant and are now making meaningful contributions in their chosen areas of expertise with jobs in academic, industrial and government laboratory settings.
