Low temperature hopping magnetotransport in paramagnetic single crystals of cobalt doped ZnO

Long needle-shaped single crystals of Zn1-xCoxO were grown at low temperatures using a molten salt solvent technique, up to x=0.10. The conduction process at low temperatures is determined to be by Mott variable range hopping. Both pristine and cobalt doped crystals clearly exhibit a crossover from negative to positive magnetoresistance as the temperature is decreased. The positive magnetoresistance of the Zn1-xCoxO single crystals increases with increased Co concentration and reaches up to 20% at low temperatures (2.5 K) and high fields (>1 T). SQUID magnetometry confirms that the Zn1-xCoxO crystals are predominantly paramagnetic in nature and the magnetic response is independent of Co concentration. The results indicate that cobalt doping of single crystalline ZnO introduces localized electronic states and isolated Co2+ ions into the host matrix, but that the magnetotransport and magnetic properties are decoupled.Comment: 7 pages, 9 figures, submitted to Physical Review

Computation of the first factor of the class number of cyclotomic fields

AbstractWe show how to compute the values of h1(p), the first factor of the class number of the cyclotomic field Q(exp 2iπ/p), for each prime p≤3000, and determine the set of prime divisors for each p≤1000. We confirm, for these values, a number of well known conjectures about h1(p). We give some reasons why we believe that Kummer's conjectured asymptotic estimate for h1(p) is likely to be wrong. We show how an extension of the recent work of Goldfeld, Cross, and Zagier might be used to establish that h1(p) is monotone increasing for all p≥19

Actors, coalitions and the framework convention on climate change

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Urban Studies and Planning, 2005.Includes bibliographical references.This study examines the political processes through which the Framework Convention on Climate Change was negotiated and the initial efforts of the United States, the Netherlands, and Japan to adopt national policies and measures to implement it. Using the Advocacy Coalition Framework (ACF) as a theoretical lens, it looks in particular at coalition behavior in the policy processes at the international, national, and sub-national levels, and the nature of cooperation and coordination both within and between these levels. In doing so, it attempts to shed additional light on the capacity and propensity of national governments to implement international environmental agreements.by Granville Clark Sewell.Ph.D