Education :

1. Magnetic Moments in Amorphous Semiconductors:quantum critical scalingFrances Hellman, University of California at Berkeley DMR-0505524 The dc conductivity sDC(H,T,x) of Gd-doped amorphous Si has been shown to obey scaling consistent with the metal-insulator quantum phase transition, both for concentration (x)-tuning (top panel) and for magnetic field (H)-tuning for a single sample (lower panel). Critical exponents suggest a disorder-driven transition for both. For a quantum phase transition, temperature T and frequency f should be interchangeable, as seen in the non-magnetic analog Nb-Si. We however find no T-f scaling for Gd-Si; sDC(H,T) converge at T>T*~ 30-50 K (dependent on x) while s(H,f) at 10K do not converge till nearly 1 eV. We suggest this is due to magnetic interactions providing a separate length scale for this problem, unlike Nb-Si.

2. Magnetic Moments in Amorphous SemiconductorsFrances Hellman, University of California at Berkeley DMR-0505524 Education: This grant supports (partially) a project scientist (Erik Helgren), (partially) a post-doctoral associate (Maryam Rahimi), a graduate student (Li Zeng), three undergraduate students (REU-supplement students Ivan Borzenets and Addison Huegel from UC Berkeley, and a summer UC-LEEDS student from UCLA Sonny Vo), and high school student Lauren Rand. PI Hellman continues to be quite involved in public outreach and educational activities, including various lectures (Discovery Lecture at UC-COSMOS summer high school science program; Cal. Acad. of Sciences science workshop for high school girls), and both the COSMOS and Exploratorium Boards of Directors. Societal Impact: Doped semiconductors provide a perfect example of the remarkable interplay between fundamental research and the numerous technologies in everyday life. Our research focuses on understanding the effects of introducing magnetic dopants into semiconductors. Deliberate addition of magnetic moments to semiconductors produces effects that are extremely large, and which 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.