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High-resolution tunnelling spectroscopy of a graphene quartet
Song, Young Jae,Otte, Alexander F.,Kuk, Young,Hu, Yike,Torrance, David B.,First, Phillip N.,de Heer, Walt A.,Min, Hongki,Adam, Shaffique,Stiles, Mark D.,MacDonald, Allan H.,Stroscio, Joseph A. Nature Publishing Group, a division of Macmillan P 2010 Nature Vol.467 No.7312
Electrons in a single sheet of graphene behave quite differently from those in traditional two-dimensional electron systems. Like massless relativistic particles, they have linear dispersion and chiral eigenstates. Furthermore, two sets of electrons centred at different points in reciprocal space (??valleys??) have this dispersion, giving rise to valley degeneracy. The symmetry between valleys, together with spin symmetry, leads to a fourfold quartet degeneracy of the Landau levels, observed as peaks in the density of states produced by an applied magnetic field. Recent electron transport measurements have observed the lifting of the fourfold degeneracy in very large applied magnetic fields, separating the quartet into integer and, more recently, fractional levels. The exact nature of the broken-symmetry states that form within the Landau levels and lift these degeneracies is unclear at present and is a topic of intense theoretical debate. Here we study the detailed features of the four quantum states that make up a degenerate graphene Landau level. We use high-resolution scanning tunnelling spectroscopy at temperatures as low as 10??mK in an applied magnetic field to study the top layer of multilayer epitaxial graphene. When the Fermi level lies inside the fourfold Landau manifold, significant electron correlation effects result in an enhanced valley splitting for even filling factors, and an enhanced electron spin splitting for odd filling factors. Most unexpectedly, we observe states with Landau level filling factors of 7/2, 9/2 and 11/2, suggestive of new many-body states in graphene.
Kim, I.-N.,Lee, K.,Bange, H. W.,Macdonald, A. M. Copernicus GmbH 2013 Biogeosciences Vol.10 No.11
<P><p><strong>Abstract.</strong> Microbial nitrous oxide (N<sub>2</sub>O) production in the ocean is enhanced under low-oxygen (O<sub>2</sub>) conditions. This is especially important in the context of increasing hypoxia (i.e., oceanic zones with extremely reduced O<sub>2</sub> concentrations). Here, we present a study on the interannual variation in summertime nitrous oxide (N<sub>2</sub>O) concentrations in the bottom waters of the northern Gulf of Mexico (nGOM), which is well-known as the site of the second largest seasonally occurring hypoxic zone worldwide. To this end we developed a simple model that computes bottom-water N<sub>2</sub>O concentrations with a tri-linear ΔN<sub>2</sub>O/O<sub>2</sub> relationship based on water-column O<sub>2</sub> concentrations, derived from summer (July) Texas-Louisiana shelf-wide hydrographic data between 1985 and 2007. &Delta;N<sub>2</sub>O (i.e., excess N<sub>2</sub>O) was computed including nitrification and denitrification as the major microbial production and consumption pathways of N<sub>2</sub>O. The mean modeled bottom-water N<sub>2</sub>O concentration for July in the nGOM was 14.5 ± 2.3 nmol L<sup>−1</sup> (min: 11.0 ± 4.5 nmol L<sup>−1</sup> in 2000 and max: 20.6 ± 11.3 nmol L<sup>−1</sup> in 2002). The mean bottom-water N<sub>2</sub>O concentrations were significantly correlated with the areal extent of hypoxia in the nGOM. Our modeling analysis indicates that the nGOM is a persistent summer source of N<sub>2</sub>O, and nitrification is dominating N<sub>2</sub>O production in this region. Based on the ongoing increase in the areal extent of hypoxia in the nGOM, we conclude that N<sub>2</sub>O production (and its subsequent emissions) from this environmentally stressed region will probably continue to increase into the future.</p> </P>
Ordering mechanism and quantum anomalous Hall effect of magnetically doped topological insulators
Kim, Jeongwoo,Jhi, Seung-Hoon,MacDonald, A. H.,Wu, Ruqian American Physical Society 2017 Physical Review B Vol.96 No.14
<P>We investigate magnetic ordering and the quantum anomalous Hall effect (QAHE) in Cr-doped topological insulators using systematic first-principles calculations, explaining the mechanism responsible for ferromagnetic order and the reason why Sb2Te3 is a better QAHE host than Bi2Se3 or Bi2Te3. We conclude that these magnetic topological insulators have relatively long-range exchange interactions within quintuple layers and weak interactions between quintuple layers. Our analyses for the spin splitting of the topological surface states suggest that the temperature at which the QAHE occurs in these materials can be enhanced significantly by Mo-Cr co-doping.</P>
Theoretical models of ferromagnetic III-V emiconductors
T.Jungwirth,JairoSinova,J.Kucera,A.H.MacDonald 한국물리학회 2003 Current Applied Physics Vol.3 No.5
Recent materials research has advanced the maximum ferromagnetic transition temperature in semiconductors containingmagnetic elements toward room temperature. Reaching this goal would make information technology applications of these ma-terials likely. In this article we briey review the status of work over the past ve years which has attempted to achieve a theoreticalunderstanding of these complex magnetic systems. The basic microscopic origins of ferromagnetism in the (III,Mn)V compoundsthat have the highest transition temperatures appear to be well understood, and ecient computation methods have been developed. 2003 Published by Elsevier B.V.