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Search Results: 1 - 10 of 352771 matches for " P. D. Meyers "
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Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels: paleoceanographic and paleoecological significance
D. Gallego-Torres,F. Martínez-Ruiz,P. A. Meyers,A. Paytan
Biogeosciences Discussions , 2010, DOI: 10.5194/bgd-7-4463-2010
Abstract: Sapropels are organic matter enriched sediments cyclically deposited in the Eastern Mediterranean. We have studied a suite of 35 sapropel sequences from a transect of four ODP sites across the basin to explore for paleoproductivity patterns and their relationship to ecological changes in the region during their deposition. We assessed paleoproductivity variations using TOC and Babio mass accumulation rates and δ15Ntotal and δ13Corg values. Elevated Ba/Al and TOC mass accumulation rates record periods of basin-wide amplified productivity. δ15N values are dramatically lower in the sapropels than in TOC-poor background sediments indicating a major contribution from nitrogen-fixing bacteria to the higher productivity during sapropel deposition. Different degrees of denitrification occurred as a function of water column oxygen depletion. The evolution of sapropel deposition over the past 3 My is characterized by increased productivity together with enhanced preservation of organic matter during the late Pliocene, peaks in primary and export production and sedimentation rates during the middle Pleistocene, and a relatively weak increase in productivity during formation of the Holocene sapropel accompanied by high sedimentation rates. We conclude that sapropels were formed by cyclic increases in primary production of marine organic matter largely sustained by N-fixing bacteria that led to depletion of deepwater dissolved oxygen and consequently improved organic matter preservation. The increase in primary production was helped by nutrient input from continental sources climatically induced.
The extended-track reconstruction for MiniBooNE
R. B. Patterson,E. M. Laird,Y. Liu,P. D. Meyers,I. Stancu,H. A. Tanaka
Physics , 2009, DOI: 10.1016/j.nima.2009.06.064
Abstract: The Booster Neutrino Experiment (MiniBooNE) searches for muon neutrino to electron neutrino oscillations using the ~1 GeV neutrino beam produced by the FNAL Booster synchrotron. The array of photomultiplier tubes (PMTs) lining the MiniBooNE detector records Cherenkov and scintillation photons from the charged particles produced in neutrino interactions. We describe a maximum likelihood fitting algorithm used to reconstruct the basic properties (position, direction, energy) of these particles from the charges and times measured by the PMTs. The likelihoods returned from fitting an event to different particle hypotheses are used to categorize it as a signal electron neutrino event or as one of the background muon neutrino processes, in particular charged current quasi-elastic scattering and neutral current $\pi^0$ production. The reconstruction and event selection techniques described here can be applied to current and future neutrino experiments using similar Cherenkov-based detection.
Metal-insulator transition and orbital reconstruction in Mott quantum wells of NdNiO$_{3}$
Jian Liu,M. Kareev,D. Meyers,B. Gray,P. Ryan,J. W. Freeland,J. Chakhalian
Physics , 2012, DOI: 10.1103/PhysRevLett.109.107402
Abstract: The metal-insulator transition (MIT) and the underlying electronic and orbital structure in $e_{g}^{1}$ quantum wells based on NdNiO$_{3}$ was investigated by d.c. transport and resonant soft x-ray absorption spectroscopy. By comparing quantum wells of the same dimension but with two different confinement structures, we explicitly demonstrate that the quantum well boundary condition of correlated electrons is critical to selecting the many-body ground state. In particular, the long-range orderings and the MIT are found to be strongly enhanced under quantum confinement by sandwiching NdNiO$_{3}$ with the wide-gap dielectric LaAlO$_{3}$, while they are suppressed when one of the interfaces is replaced by a surface (interface with vacuum). Resonant spectroscopy reveals that the reduced charge fluctuations in the sandwich structure are supported by the enhanced propensity to charge ordering due to the suppressed $e_g$ orbital splitting when interfaced with the confining LaAlO$_{3}$ layer.
Epitaxial Growth of (1 1 1)-Oriented Spinel CoCr$_2$O$_4$/Al$_2$O$_3$ Heterostructures
Xiaoran Liu,D. Choudhury,Yanwei Cao,S. Middey,M. Kareev,D. Meyers,J. -W. Kim,P. Ryan,J. Chakhalian
Physics , 2015, DOI: 10.1063/1.4913245
Abstract: High quality (1 1 1)-oriented CoCr$_2$O$_4$/Al$_2$O$_3$ heterostructures were synthesized on the sapphire (0 0 0 1) single crystal substrates in a layer-by-layer mode. The structural properties are demonstrated by $in$-$situ$ reflection high energy electron diffraction, atomic force microscopy, X-ray reflectivity and X-ray diffraction. X-ray photoemission spectroscopy confirms that the films possess the proper chemical stoichiometry. This work offers a pathway to fabricating spinel type artificial quasi-two-dimensional frustrated lattices by means of geometrical engineering.
Flagship Memorial: An Analysis of Themes at the Vietnam Veterans Memorial: 1982-2007
Leslie D. Meyers
Sociation Today , 2012,
Abstract: This paper seeks to extend the analysis of Wagner-Pacifici and Schwartz’s (1991) landmark piece, “The Vietnam Veterans Memorial: Commemorating a Difficult Past”, by sixteen years in order to understand how memory has been both created and sustained at the memorial from 1982-2007. A content analysis of 791 articles from the New York Times and Washington Post revealed five themes for analysis: 1) healing, 2) politics, 3) conflict over additional elements, 4) religion, and 5) offerings. Of these themes, politics was significant. However, while the analysis indicates the Vietnam Veterans Memorial is a politically charged terrain; grassroots efforts dominate narratives of public opinion, changes to the site and discussions of memory.
Epitaxial stabilization of ultra thin films of electron doped manganites
S. Middey,M. Kareev,D. Meyers,X. Liu,Y. Cao,S. Tripathi,P. Ryan,J. W. Freeland,J. Chakhalian
Physics , 2014, DOI: 10.1063/1.4879456
Abstract: Ultra-thin films of the electron doped manganite La$_{0.8}$Ce$_{0.2}$MnO$_3$ were grown in a layer-by-layer growth mode on SrTiO$_3$ (001) substrates by pulsed laser interval deposition. High structural quality and surface morphology was confirmed by a combination of synchrotron based x-ray diffraction and atomic force microscopy. Resonant X-ray absorption spectroscopy measurements confirm the presence of Ce$^{+4}$ and Mn$^{+2}$ ions. In addition, the electron doping signature was corroborated by Hall effect measurements. All grown films show ferromagnetic ground state as revealed by both XMCD and magnetoresistance measurements and remain insulating contrary to earlier reports of metal-insulator transition. Our results hint at the possibility of electron-hole asymmetry in the colossal magnetoresistive (CMR) manganite phase diagram akin to high-$T_c$ cuprates.
Magnetism and electronic structure of YTiO$_3$ thin films
Yanwei Cao,P. Shafer,Xiaoran Liu,D. Meyers,M. Kareev,S. Middey,J. W. Freeland,E. Arenholz,J. Chakhalian
Physics , 2015, DOI: 10.1063/1.4931039
Abstract: High-quality (001)-oriented (pseudo-cubic notation) ferromagnetic YTiO$_3$ thin films were epitaxially synthesized in a layer-by-layer way by pulsed laser deposition. Structural, magnetic and electronic properties were characterized by reflection-high-energy-electron-diffraction, X-ray diffraction, vibrating sample magnetometry, and element-resolved resonant soft X-ray absorption spectroscopy. To reveal ferromagnetism of the constituent titanium ions, X-ray magnetic circular dichroism spectroscopy was carried out using four detection modes probing complimentary spatial scale, which overcomes a challenge of probing ferromagnetic titanium with pure Ti3+(3d$^1$). Our work provides a pathway to distinguish between the roles of titanium and A-site magnetic rare-earth cations in determining the magnetism in rare-earth titanates thin films and heterostructures.
Anomalous charge and negative-charge-transfer insulating state in cuprate chain-compound KCuO_2
D. Choudhury,P. Rivero,D. Meyers,X. Liu,Y. Cao,S. Middey,M. J. Whitaker,S. Barraza-Lopez,J. W. Freeland,M. Greenblatt,J. Chakhalian
Physics , 2015, DOI: 10.1103/PhysRevB.92.201108
Abstract: Using a combination of X-ray absorption spectroscopy experiments with first principle calculations, we demonstrate that insulating KCuO_2 contains Cu in an unusually-high formal-3+ valence state, the ligand-to-metal (O to Cu) charge transfer energy is intriguingly negative (Delta~ -1.5 eV) and has a dominant (~60%) ligand-hole character in the ground state akin to the high Tc cuprate Zhang-Rice state. Unlike most other formal Cu^{3+} compounds, the Cu 2p XAS spectra of KCuO_2 exhibits pronounced 3d^8 (Cu^{3+}) multiplet structures, which accounts for ~40% of its ground state wave-function. Ab-initio calculations elucidate the origin of the band-gap in KCuO_2 as arising primarily from strong intra-cluster Cu 3d - O 2p hybridizations (t_{pd}); the value of the band-gap decreases with reduced value of t_{pd}. Further, unlike conventional negative charge-transfer insulators, the band-gap in KCuO_2 persists even for vanishing values of Coulomb repulsion U, underscoring the importance of single-particle band-structure effects connected to the one-dimensional nature of the compound.
Engineered Mott ground state in LaTiO$_{3+δ}$/LaNiO$_3$ heterostructure
Yanwei Cao,Xiaoran Liu,M. Kareev,D. Choudhury,S. Middey,D. Meyers,J. -W. Kim,P. Ryan,J. W. Freeland,J. Chakhalian
Physics , 2015,
Abstract: In pursuit of creating cuprate-like electronic and orbital structures, artificial heterostructures based on LaNiO$_3$ have inspired a wealth of exciting experimental and theoretical results. However, to date there is a very limited experimental understanding of the electronic and orbital states emerging after interfacial charge-transfer and their connections to the modified band structure at the interface. Towards this goal, we have synthesized a prototypical superlattice composed of correlated metal LaNiO$_3$ and doped Mott insulator LaTiO$_{3+\delta}$, and investigated its electronic structure by resonant X-ray absorption spectroscopy combined with X-ray photoemission spectroscopy, electrical transport and theory calculations. The heterostructure exhibits interfacial charge-transfer from Ti to Ni sites giving rise to an insulating ground state with orbital polarization and $e_\textrm{g}$ orbital band splitting. Our findings demonstrate how the control over charge at the interface can be effectively used to create exotic electronic, orbital and spin states.
Geometrically engineered Mott phases in (111) oriented nickelate superlattices
S. Middey,D. Meyers,D. Doennig,M. Kareev,X. Liu,Y. Cao,P. J. Ryan,R. Pentcheva,J. W. Freeland,J. Chakhalian
Physics , 2014,
Abstract: Deterministic control over the periodic geometrical arrangement of the constituent atoms is the backbone of a material's properties, that along with the interactions define the electronic and magnetic ground state. Following this notion, a bilayer of a prototypical rare-earth nickelate NdNiO$_3$ combined with a dielectric spacer, LaAlO$_3$, layered along the pseudo cubic (111) direction has been assembled into a buckled honeycomb lattice. The resulting artificial graphene-like Mott crystal with magnetic 3$d$ electrons has antiferromagnetic correlations. In addition, the combination of resonant X-ray linear dichroism measurements and \textit{ab-initio} calculations reveal the presence of an anti-ferro orbital ordering of $d_{3x^2-r^2}$/$d_{3z^2-r^2}$ orbitals, which is unattainable in either bulk nickelates or heterostructures grown along the (001) direction. These findings highlight another promising venue towards designing new quantum many-body states by virtue of geometrical engineering.
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