Browsing by Author "Chan, MK"
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- ItemMagnetic properties of uncultivated magnetotactic bacteria and their contribution to a stratified estuary iron cycle(Nature Publishing Group, 2014-09-01) Chen, AP; Berounsky, VM; Chan, MK; Blackford, MG; Cady, C; Moskowitz, BM; Kraal, P; Lima, EA; Kopp, RE; Lumpkin, GR; Weiss, BP; Hesse, PP; Vella, NGFOf the two nanocrystal (magnetosome) compositions biosynthesized by magnetotactic bacteria (MTB), the magnetic properties of magnetite magnetosomes have been extensively studied using widely available cultures, while those of greigite magnetosomes remain poorly known. Here we have collected uncultivated magnetite- and greigite-producing MTB to determine their magnetic coercivity distribution and ferromagnetic resonance (FMR) spectra and to assess the MTB-associated iron flux. We find that compared with magnetite-producing MTB cultures, FMR spectra of uncultivated MTB are characterized by a wider empirical parameter range, thus complicating the use of FMR for fossilized magnetosome (magnetofossil) detection. Furthermore, in stark contrast to putative Neogene greigite magnetofossil records, the coercivity distributions for greigite-producing MTB are fundamentally left-skewed with a lower median. Lastly, a comparison between the MTB-associated iron flux in the investigated estuary and the pyritic-Fe flux in the Black Sea suggests MTB play an important, but heretofore overlooked role in euxinic marine system iron cycle.© 2014, Nature Publishing Group.
- ItemTwo Ising-like magnetic excitations in a single-layer cuprate superconductor(Nature Publishing Group, 2012-05-01) Li, Y; Yu, G; Chan, MK; Baledent, V; Li, Y; Barisic, N; Zhao, X; Hradil, K; Mole, RA; Sidis, Y; Steffens, P; Bourges, P; Greven, MThere exists increasing evidence that the phase diagram of the high-transition temperature (T(c)) cuprate superconductors is controlled by a quantum critical point. According to one distinct theoretical proposal, on decreasing the hole-carrier concentration a transition occurs to an ordered state with two circulating orbital currents per CuO(2) square. Below the 'pseudogap' temperature T* (T* > T(c)), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds which should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa(2)CuO(4+delta) the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points towards a unifying picture of magnetism in the cuprates that will probably require a multi-band description. © 2012, Nature Publishing Group.