Renormalized Insulators: On the Verge of Magnetism

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****Technical Abstract****This project is designed to extend knowledge of strongly correlated electron systems into the regime of strongly interacting, or highly renormalized, insulators on the verge of magnetism. Previous investigations by the PI and collaborators have indicated that this is a very rich vein of research because of the importance of both semiconductor physics and magnetism. The competing interactions inherent in these carefully characterized chemically substituted small band-gap insulators often lead to quantum phase transitions in the presence of strong correlations and disorder. Typical systems that will be investigated include iron-based non-magnetic insulators, such as FeSi, FeS2, and related materials, where a transition from a strongly correlated-insulator-to-magnetic metal transition can be accessed by doping. The goals are to discover what novel phenomena exist when carriers are doped into these strongly correlated insulators placing them in the unique regime where low carrier density, disorder, magnetism, and, in some cases, finite size are all important aspects. In addition we will be exploring the nucleation, imaging, and control of the magnetic Skyrmion phases that have recently been discovered in the mono-silicide and -germanide materials having the FeSi crystal structure in thin films of these materials. Students on many levels, as well as high school teachers will explore these materials searching for novel transport, magnetic, thermodynamic, and optical properties that may be exploited in future technologies.****Non-Technical Abstract****In order to accelerate the technological revolution that has placed high speed computation and information storage at our fingertips almost anywhere on Earth, new ideas are essential. One pathway for progress in semiconductor device design, known as spintronics, seeks to make use of the magnetic degrees of freedom to the same extent that charge degrees of freedom are used in present day silicon technologies. As scientists have sought to control or manipulate the electron spin, the intrinsic magnetic property of electrons, they have recognized the utility of materials that are both magnetic and semiconducting. This work seeks to extend our knowledge of magnetic semiconductors by investigating a series of compounds composed of transition metals and silicon or germanium where both semiconducting and magnetic behavior are known to emerge. The chemical flexibility that these systems offer while maintaining a common crystal structure allows enormous control over their physical properties making them ideal for discovering new behaviors at the intersection of semiconductor physics and magnetism. Also, the unusual symmetry of the underlying crystal structure of one family of these compounds has been shown to induce ~100 nm sized toroidal shaped magnetic structures, known as magnetic Skyrmions, for a range of temperatures and magnetic fields. This project investigates the necessary elements for nucleating, imaging, and controlling these novel magnetic structures both as a fundamental exploration and to assess their possible relevance for future technologies. Students on many levels, as well as high school teachers, will be exploring magnetic, electric, optical, and thermodynamic properties of these magnetic semiconductors in crystalline, thin film, and nanowire form.