1,424 research outputs found
Angle-resolved photoemission spectroscopy of Co-based boride superconductor LaCo1.73Fe0.27B2
We have performed angle-resolved photoemission spectroscopy of Co-based
boride superconductor LaCo1.73Fe0.27B2 (Tc = 4.1 K), which is isostructural to
the 122-type Fe-pnictide superconductor with the pnictogen atom being replaced
with boron. We found that the Fermi level is located at a dip in the density of
states (DOS) in contrast to Co-pnictide ferromagnets. This reduction in DOS
together with the strong Co 3d-B 2p covalent bonding removes the ferromagnetic
order and may cause the superconductivity. The energy bands near the Fermi
level show higher three dimensionality and a weaker electron-correlation effect
than those of Fe pnictides. The Fermi surface topology is considerably
different from that of Fe pnictides, suggesting the difference in the
superconducting mechanism between boride and pnictide superconductors.Comment: 5 pages, 4 figure
Spin configuration in a frustrated ferromagnetic/antiferromagnetic thin film system
We have studied the magnetic configuration in ultrathin antiferromagnetic Mn
films grown around monoatomic steps on an Fe(001) surface by spin-polarized
scanning tunneling microscopy/spectroscopy and ab-initio-parametrized
self-consistent real-space tight binding calculations in which the spin
quantization axis is independent for each site thus allowing noncollinear
magnetism. Mn grown on Fe(001) presents a layered antiferromagnetic structure.
In the regions where the Mn films overgrows Fe steps the magnetization of the
surface layer is reversed across the steps. Around these defects a frustration
of the antiferromagnetic order occurs. Due to the weakened magnetic coupling at
the central Mn layers, the amount of frustration is smaller than in Cr and the
width of the wall induced by the step does not change with the thickness, at
least for coverages up to seven monolayers.Comment: 10 pages, 5 figure
Non-propagating degrees of freedom in supergravity and very extended G_2
Recently a correspondence between non-propagating degrees of freedom in
maximal supergravity and the very extended algebra E_11 has been found. We
perform a similar analysis for a supergravity theory with eight supercharges
and very extended G_2. In particular, in the context of d=5 minimal
supergravity, we study whether supersymmetry can be realised on higher-rank
tensors with no propagating degrees of freedom. We find that in this case the
very extended algebra fails to capture these possibilities.Comment: 12 pages, 1 figure. v2: transformation properties of higher-rank
tensors under SU(2) R-symmetry analysed and refs added. v3: improved
discussion, published versio
Origin of the Verwey transition in magnetite: Group theory, electronic structure, and lattice dynamics study
The Verwey phase transition in magnetite has been analyzed using the group
theory methods. It is found that two order parameters with the symmetries
and induce the structural transformation from the high-temperature
cubic to the low-temperature monoclinic phase. The coupling between the order
parameters is described by the Landau free energy functional. The electronic
and crystal structure for the cubic and monoclinic phases were optimized using
the {\it ab initio} density functional method. The electronic structure
calculations were performed within the generalized gradient approximation
including the on-site interactions between 3d electrons at iron ions -- the
Coulomb element and Hund's exchange . Only when these local interactions
are taken into account, the phonon dispersion curves, obtained by the direct
method for the cubic phase, reproduce the experimental data. It is shown that
the interplay of local electron interations and the coupling to the lattice
drives the phonon order parameters and is responsible for the opening of the
gap at the Fermi energy. Thus, it is found that the metal-insulator transition
in magnetite is promoted by local electron interactions, which significantly
amplify the electron-phonon interaction and stabilize weak charge order
coexisting with orbital order of the occupied states at Fe ions. This
provides a scenario to understand the fundamental problem of the origin of the
Verwey transition in magnetite.Comment: 17 pages, 5 figures, 8 tables. Accepted version to be published in
Phys. Rev.
Closed strings in Ramond-Ramond backgrounds
We write the IIB Green-Schwarz action in certain general classes of curved
backgrounds threaded with Ramond-Ramond fluxes. The fixing of the kappa
symmetry in the light-cone gauge and the use of supergravity Bianchi identities
simplify the task. We find an expression that truncates to quartic order in the
spacetime spinors and relays interesting information about the vacuum structure
of the worldsheet theory. The results are particularly useful in exploring
integrable string dynamics in the context of the holographic duality.Comment: 24 pages; this is a significant revision to hep-th/0112063 with new
results added, and including comparison with recent literature; v2: minor
typos corrected, citations adde
Charge and orbital order in Fe_3O_4
Charge and orbital ordering in the low-temperature monoclinic structure of
magnetite (Fe_3O_4) is investigated using LSDA+U. While the difference between
t_{2g} minority occupancies of Fe^{2+}_B and Fe^{3+}_B cations is large and
gives direct evidence for charge ordering, the screening is so effective that
the total 3d charge disproportion is rather small. The charge order has a
pronounced [001] modulation, which is incompatible with the Anderson criterion.
The orbital order agrees with the Kugel-Khomskii theory.Comment: 4 pages, 2 figure
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