Magnetic Moments in Amorphous Semiconductors Frances Hellman, University of California, Berkeley, DMR 0505524 This project looks at the effect of magnetic.

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Magnetic Moments in Amorphous Semiconductors Frances Hellman, University of California, Berkeley, DMR This project looks at the effect of magnetic moments on the electronic and magnetic properties of group IV semiconductors, with particular focus on amorphous systems which readily accommodate magnetic ions such as the magnetic rare earth and transition metal elements. These materials show a variety of fundamental phenomena related to strong electron- electron and electron-local moment interactions, including spin glass freezing, field and concentration tuned insulator-metal transition, quantum phase scaling in temperature, magnetic field, frequency, and composition, all related to the effects of strong disorder and correlated electron behavior. Data on Si and Ge-based alloys showed the important role played by electron screening and semiconductor band gap on the temperature and magnetic field dependence of electrical transport. In the final period of this grant, studies were extended to amorphous Carbon. Carbon is found in a huge variety of structures, due to its ability to be either sp 2 or sp 3 bonded, causing variability in conduction, band gap and screening. Amorphous Si and Ge have a characteristic temperature T* for the onset of magnetoresistance (MR) and other effects of strong carrier-moment interactions; T* decreases with decreasing band gap or increasing electron concentration, leading to the hope that a-C would have very large and high temperature MR. Amorphous Gd-C showed even larger MR than Gd-Si, reaching values of 10 5 at 2K, as hoped. However, the temperature dependence of MR was much greater, resulting in MR that dropped to 1% by 30K, while Gd-Si retains significant MR even at 100K. The characteristic temperature in Gd-C also has the opposite dependence on concentration (see figure). The difference is believed to be due to Gd- induced changes in sp 2 /sp 3 bonding, see top figure for the distinct Raman signature for C with various bonding. L. Zeng et al. PRB 75, (2007) L. Zeng et al. PRB submitted (2010)

New Fundamental Discoveries in Physical Sciences New Technology Education Outreach Education: This grant supported the education and research training of a post-doc/project scientist, a graduate student, and a large number of undergraduate research students (several women) plus one high school student researcher. In addition, PI Hellman continues to be quite involved in public outreach and educational activities, particularly through the Exploratorium and support of women in science programs and undergraduate research at UC Berkeley. Magnetic Moment in Amorphous Semiconductors Frances Hellman, University of California, Berkeley, DMR Societal Impact: Doped semiconductors, both with and without magnetic moments, provide a perfect example of the remarkable interplay between fundamental research and the numerous technologies in everyday life. Our research focuses on understanding the large effects of introducing magnetic dopants into semiconductors. This work may ultimately have technological significance to an emerging field known as spin electronics. In addition, this research increases our understanding of the crucial role played by magnetic moments in a wide range of electronic materials, and to our understanding of quantum phase transitions. The study of magnetically doped amorphous carbon not only contributes to the understanding of fundamental problems in modern physics, but also has a potential huge technological impact on an emerging field of carbon-based electronics and spintronics, with building blocks such as carbon nano-tubes.