Approaches to the coupling of dilute spins in oxides.

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An important goal in modern condensed matter science is to develop new materials with two or more useful functions: in these multifunctional materials, the external control of one of the functions influences the behavior of the other and vice-versa. Multifunctional materials are expected to be central to overcoming fundamental limitations (such as Moore's Law performance scaling) of conventional single-function materials. Frequently, many interesting pairs of functions in bifunctional materials are contraindicated, in the sense that the existence of one function usually precludes the other. This proposal addresses a specific pair of functions: the concurrent existence of a band gap and ferromagnetic coupling between spins mediated by dilute charge carriers in the lattice. In contrast to metallic room-temperature ferromagnets, the proposed research will explore the possibility of systematically coupling, through lightly doped charge carriers, dilute spins in wide-band-gap oxides i.e. systems which are far from possessing metallic ground states. Such diluted magnetic oxide semiconductors are expected to find use in spintronic applications. The search for target oxide compositions will be dictated by a detailed understanding of the prototypical diluted magnetic semiconductor system (Ga,Mn)As where it is known that high mobility and extensive hole doping play a key role. This raises the question: How does one hole-dope a semiconducting oxide host? The intellectual merit of the proposed work, and indeed its transformative nature, lies in the completely novel host systems proposed for examination, including those based on Mott-Hubbard insulating ground states, rather than single-electron gap/band insulating ground states. With the appreciation that oxide semiconductors like ZnO and In2O3 have some fundamental valence and bonding-derived limitations with regard to hole-doping, the proposed work will explore compounds with correlated ground states such as NiO or LaTiO3, in which the valence band is the lower Hubbard band deriving from 3d levels that are known to support hole-doping. Also explored will be host compounds with low-spin ground states exemplified by Rh3+ in perovskite LaRhO3. As appealing collateral, it is anticipated that the proposed work will throw light on a number of problems that are of great current interest across the broad materials community, in addition to that of diluted magnetic semiconductors: (a) the paucity of p-type transparent conducting oxides; (b) the use of correlated transition metal oxides in so-called Mottronic devices; and (c) the search for room-temperature multiferroic materials.The Liverpool and Santa Barbara groups have the strong synergy in synthesis (high pressure techniques and pO2 control of oxide defect equilibria ) (Liverpool), precursor routes, arc-melting, high temperature reductions, and solution reflux (Santa Barbara)) and measurement capabilities (advanced transport and heat capacity, PDF local structure determination(Santa Barbara), TEM and impedance spectroscopy for defect monitoring (Liverpool)) that is the only way to tackle the challenging project objectives.The broader impacts of the proposed work lie in the close integration of research, education, and outreach. The specific focus will be on training an international cadre of materials scientists who understand the importance for developing new materials as drivers for new technologies. Exchange of undergraduate and graduate students between Santa Barbara and Liverpool, and the running of international summer schools for graduate students (an activity where the PIs have an established track record) will be central to such training. Both PIs will develop modules for school science education (aimed at inculcating an early appreciation that novel materials are drivers for new technologies), building on a substantial prior record in this area