84 research outputs found
Low-temperature electrical transport in bilayer manganite LaSrMnO
The temperature and magnetic field dependence of anisotropic in-plane
and out-of-plane resistivities have been investigated in
single crystals of the bilayer manganite LaSrMnO.
Below the Curie transition temperature 125 K, and
display almost the same temperature dependence with an up-turn around 50 K. In
the metallic regime (50 K 110 K), both and
follow a dependence, consistent with the two-magnon
scattering. We found that the value of the proportionality coefficient
and the ratio of the exchange interaction obtained
by fitting the data are in excellent agreement with the calculated
based on the two-magnon model and deduced from neutron scattering,
respectively. This provides further support for this scattering mechanism. At
even lower , in the non-metallic regime ( 50 K), {\it both} the in-plane
and out-of-plane conductivities obey a
dependence, consistent with weak localization effects. Hence, this demonstrates
the three-dimensional metallic nature of the bilayer manganite
LaSrMnO at .Comment: 7 pages and 5 figures, accepted for publication in Phys. Rev.
Maximally-localized generalized Wannier functions for composite energy bands
We discuss a method for determining the optimally-localized set of
generalized Wannier functions associated with a set of Bloch bands in a
crystalline solid. By ``generalized Wannier functions'' we mean a set of
localized orthonormal orbitals spanning the same space as the specified set of
Bloch bands. Although we minimize a functional that represents the total spread
sum_n [ _n - _n^2 ] of the Wannier functions in real space, our method
proceeds directly from the Bloch functions as represented on a mesh of
k-points, and carries out the minimization in a space of unitary matrices
U_mn^k describing the rotation among the Bloch bands at each k-point. The
method is thus suitable for use in connection with conventional
electronic-structure codes. The procedure also returns the total electric
polarization as well as the location of each Wannier center. Sample results for
Si, GaAs, molecular C2H4, and LiCl will be presented.Comment: 22 pages, two-column style with 4 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#nm_wan
Maximally-localized Wannier functions for entangled energy bands
We present a method for obtaining well-localized Wannier-like functions (WFs)
for energy bands that are attached to or mixed with other bands. The present
scheme removes the limitation of the usual maximally-localized WFs method (N.
Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)) that the bands of
interest should form an isolated group, separated by gaps from higher and lower
bands everywhere in the Brillouin zone. An energy window encompassing N bands
of interest is specified by the user, and the algorithm then proceeds to
disentangle these from the remaining bands inside the window by filtering out
an optimally connected N-dimensional subspace. This is achieved by minimizing a
functional that measures the subspace dispersion across the Brillouin zone. The
maximally-localized WFs for the optimal subspace are then obtained via the
algorithm of Marzari and Vanderbilt. The method, which functions as a
postprocessing step using the output of conventional electronic-structure
codes, is applied to the s and d bands of copper, and to the valence and
low-lying conduction bands of silicon. For the low-lying nearly-free-electron
bands of copper we find WFs which are centered at the tetrahedral interstitial
sites, suggesting an alternative tight-binding parametrization.Comment: 13 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macro
Mediation and digital intensities: Topology, psychology and social media
Social media are said to offer seemingly endless ways of connecting with people in a variety of online spaces. The mediated form that such communication takes has re-opened many theoretical debates regarding the status of relationships that are organized and managed online. In this article we seek to explore these issues through the lens of topological thinking, and particularly through the work of Kurt Lewin (1890â1947). Lewinâs topological psychology has recently featured in the social sciences as a way of overcoming some of the, frankly unhelpful, dualistic thinking that features commonly in psychology (e.g. subjectâobject, mindâbody, individualâsocial). Topological thought focuses on the spatial distribution of psychological experience, and therefore offers a social perspective not reliant on traditional notions of internalized psychological states and traits. The kind of spatiality at work though is not one that relies on Euclidean fixity, but one that draws out notions of stretching, moulding, bending and flexing. Space is seen not as a fixed property, but rather the form that psychological activity takes through connections and relations with others. In this article we seek to explore the potential value in characterizing social media activity topologically. This involves analysing peopleâs experiences with social media, and how topological concerns of boundaries, connections and thresholds work (or not) in and through social media. Furthermore, the focus is not only on extensive properties of social media, but rather on how intensive processes are actualized and distributed in and through mediation
The electronic structure of LiFeAs and NaFeAs probed by resonant inelastic x-ray scattering spectra
Results of resonant inelastic X-ray scattering (RIXS) measurements at Fe
L-edges and electronic structure calculations of LiFeAs and NaFeAs are
presented. Both experiment and theory show that in the vicinity of the Fermi
energy, the density of states is dominated by contributions from Fe 3d-states.
The comparison of Fe L2,3 non-resonant and resonant (excited at L2-threshold)
X-ray emission spectra with spectra of LaOFeAs and CaFe2As2 show a great
similarity in energy and I(L2)/I(L3) intensity ratio. The I(L2)/I(L3) intensity
ratio of all FeAs-based superconductors is found to be more similar to metallic
Fe than to correlated FeO. Basing on these measurements we conclude that
iron-based superconductors are weakly or moderately correlated systems.Comment: 11 pages, 6 figure
Syngas Production, Storage, Compression and Use in Gas Turbines
This chapter analyses syngas production through pyrolysis and gasification, its compression and its use in gas turbines. Syngas compression can be performed during or after thermal treatment processes. Important points are discussed related to syngas ignition, syngas explosion limit at high temperatures and high pressures and syngas combustion kinetics. Kinetic aspects influence ignition and final emissions which are obtained at the completion of the combustion process. The chapter is organized into four subsections, dealing with (1) innovative syngas production plants, (2) syngas compressors and compression process, (3) syngas ignition in both heterogeneous and homogeneous systems and (4) syngas combustion kinetics and experimental methods. Particular attention is given to ignition regions that affect the kinetics, namely systems that operate at temperatures higher than 1000 K can have strong ignition, whereas those operating at lower temperatures have weak ignition. Keywords: Pyrogas Pyrolysis Ignition Syngas Compression GasificationacceptedVersio
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