5,282 research outputs found
Origin of the ~150 K Anomaly in LaOFeAs; Competing Antiferromagnetic Superexchange Interactions, Frustration, and Structural Phase Transition
From first principles calculations we find that the nearest and next nearest
neighbor superexchange interactions between Fe ions in LaOFeAs are large,
antiferromagnetic (AF), and give rise to a frustrated magnetic ground state
which consists of two interpenerating AF square sublattices with
M(Fe)=0.48. The system lowers its energy further by removing the
frustration via a structural distortion. These results successfully explain the
magnetic and structural phase transitions in LaOFeAs recently observed by
neutron scattering. The presence of competing strong antiferromagnetic exchange
interactions and the frustrated ground state suggest that magnetism and
superconductivity in doped LaOFeAs may be strongly coupled, much like in the
high-T cuprates.Comment: 4.2 pages, 5 figures, see
http://www.ncnr.nist.gov/staff/taner/laofeas for more informatio
The Unprecedented Giant Coupling of Fe-spin State and the As-As Hybridization in Iron-Pnictide
From first principles calculations we unravel surprisingly strong
interactions between arsenic ions in iron-pnictides, the strength of which is
controlled by the Fe-spin state. Reducing the Fe-magnetic moment, weakens the
Fe-As bonding, and in turn, increases As-As interactions, causing giant
reduction in the c-axis. For CaFeAs system, this reduction is as large
as 1.4 \AA. Since the large c-reduction has been recently observed only under
high-pressure\cite{cTphase}, our results suggest that the iron magnetic moment
should be present in Fe-pnictides at all times at ambient pressure. Finally,
the conventional electron-phonon coupling in the collapsed phase of
CaFeAs gives a maximum of 0.6 K and can not explain the
K superconductivity observed in some experiments. Implications of these
findings on the mechanism of superconductivity in iron-pnictides are discussed.Comment: Published version with updated references. It has new results such as
conventional e-ph coupling in the collapsed phase of CaFe2As2 gives a maximum
Tc of 0.6 K and cannot explain the ~12 K superconductivity observed in some
experiments. Implications of these findings on the mechanism of
superconductivity in iron pnictides are discusse
Titanium-decorated carbon nanotubes: a potential high-capacity hydrogen storage medium
We report a first-principles study, which demonstrates that a single Ti atom
coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules.
The first H adsorption is dissociative with no energy barrier while other
three adsorptions are molecular with significantly elongated H-H bonds. At high
Ti coverage we show that a SWNT can strongly adsorb up to 8-wt% hydrogen. The
system is quite stable and exhibits associative desorption upon heating, a
requirement for reversible storage. These results advance our fundamental
understanding of dissociative adsorption of hydrogen in nanostructures and
suggest new routes to better storage and catalyst materials.Comment: 4.2 pages, 3 figures, submitted PRL on Nov. 2004, accepted in March
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Charge and Spin Ordering in the Mixed Valence Compound LuFe2O4
Landau theory and symmetry considerations lead us to propose an explanation
for several seemingly paradoxical behaviors of charge ordering (CO) and spin
ordering (SO) in the mixed valence compound LuFe2O4. Both SO and CO are highly
frustrated. We analyze a lattice gas model of CO within mean field theory and
determine the magnitude of several of the phenomenological interactions. We
show that the assumption of a continuous phase transition at which CO or SO
develops implies that both CO and SO are incommensurate. To explain how
ferroelectric fluctuations in the charge disordered phase can be consistent
with an antiferroelectric ordered phase, we invoke an electron-phonon
interaction in which a low energy (20meV) zone-center transverse phonon plays a
key role. The energies of all the zone-center phonons are calculated from first
principles. We give a Landau analysis which explains SO and we discuss a model
of interactions which stabilizes the SO state, if it is assumed commensurate.
However, we suggest a high resolution experimental determination to see whether
this phase is really commensurate, as believed up to now. The applicability of
representation analysis is discussed. A tentative explanation for the
sensitivity of the CO state to an applied magnetic field in field-cooled
experiments is given.Comment: 36 pages, 17 figure
Spin Dynamics of Trimers on a Distorted Kagom\'e Lattice
We treat the ground state, elementary excitations, and neutron scattering
cross section for a system of trimers consisting of three tightly bound spins
1/2 on a distorted Kagom\'e lattice, subject to isotropic nearest neighbor
(usually antiferromagnetic) Heisenberg interactions. The interactions between
trimers are assumed to be weak compared to the intra trimer interactions. We
compare the spin-wave excitation spectrum of trimers with that obtained from
standard spin-wave theory and attribute the differences at low energy to the
fact that the trimer formulation includes exactly the effects of intra-trimer
zero point motion. Application to existing systems is briefly discussed.Comment: 20 pages, 10 figure
Magnetic Structure and Spin Waves in the Kagom\'{e} Jarosite compound
We present a detailed study of the magnetic structure and spin waves in the
Fe jarosite compound for the most general
Hamiltonian involving one- and two-spin interactions which are allowed by
symmetry. We compare the calculated spin-wave spectrum with the recent neutron
scattering data of Matan {\it et al.} for various model Hamiltonians which
include, in addition to isotropic Heisenberg exchange interactions between
nearest () and next-nearest () neighbors, single ion anisotropy and
Dzyaloshinskii-Moriya (DM) interactions. We concluded that DM interactions are
the dominant anisotropic interaction, which not only fits all the splittings in
the spin-wave spectrum but also reproduces the small canting of the spins out
of the Kagom\'e plane. A brief discussion of how representation theory
restricts the allowed magnetic structure is also given.Comment: 23 pages, 17 figures, submitted to Phys. Rev. B (March 2006
Variable and reversible quantum structures on a single carbon nanotube
The band gap of a semiconducting single wall carbon nanotube decreases and
eventually vanishes leading to metalization as a result of increasing radial
deformation. This sets in a band offset between the undeformed and deformed
regions of a single nanotube. Based on the superlattice calculations, we show
that these features can be exploited to realize various quantum well structures
on a single nanotube with variable and reversible electronic properties. These
quantum structures and nanodevices incorporate mechanics and electronics.Comment: 7 pages, 4 figures, To be appear in PR
Effect of Adiabatic Phonons on Striped and Homogeneous Ground States
The effects of adiabatic phonons on a spin-fermion model for high T_c
cuprates are studied using numerical simulations. In the absence of
electron-phonon interactions (EPI), stripes in the ground state are observed
for certain dopings while homogeneous states are stabilized in other regions of
parameter space. Different modes of adiabatic phonons are added to the
Hamiltonian:breathing, shear and half-breathing modes. Diagonal and
off-diagonal electron-phonon couplings are considered. It is observed that
strong diagonal EPI generate stripes in previously homogeneous states, while in
striped ground states an increase in the diagonal couplings tends to stabilize
the stripes, inducing a gap in the density of states (DOS) and rendering the
ground state insulating. The off-diagonal terms, on the other hand, destabilize
the stripes creating inhomogeneous ground states with a pseudogap at the
chemical potential in the DOS. The breathing mode stabilizes static diagonal
stripes; while the half-breathing (shear) modes stabilize dynamical (localized)
vertical and horizontal stripes. The EPI induces decoherence of the
quasi-particle peaks in the spectral functions.Comment: latex, 9 pages,13 figure
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