3,773 research outputs found
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
Exo-hydrogenated Single Wall Carbon Nanotubes
An extensive first-principles study of fully exo-hydrogenated zigzag (n,0)
and armchair (n,n) single wall carbon nanotubes (CH), polyhedral
molecules including cubane, dodecahedrane, and CH points to
crucial differences in the electronic and atomic structures relevant to
hydrogen storage and device applications. CH's are estimated to be
stable up to the radius of a (8,8) nanotube, with binding energies proportional
to 1/R. Attaching a single hydrogen to any nanotube is always exothermic.
Hydrogenation of zigzag nanotubes is found to be more likely than armchair
nanotubes with similar radius. Our findings may have important implications for
selective functionalization and finding a way of separating similar radius
nanotubes from each other.Comment: 5 pages, 4 postscript figures, Revtex file, To be appear in Physical
Review
Influence of local fullerene orientation on the electronic properties of A3C60 compounds
We have investigated sodium containing fullerene superconductors Na2AC60, A =
Cs, Rb, and K, by Na-23 nuclear magnetic resonance (NMR) spectroscopy at 7.5 T
in the temperature range of 10 to 400 K. Despite the structural differences
from the Rb3C60 class of fullerene superconductors, in these compounds the NMR
line of the tetrahedrally coordinated alkali nuclei also splits into two lines
(T and T') at low temperature. In Na2CsC60 the splitting occurs at 170 K; in
the quenched cubic phase of Na2RbC60 and Na2KC60 we observe split lines at 80
K. Detailed investigations of the spectrum, spin-spin and spin-lattice
relaxation as well as spin-echo double resonance (SEDOR) in Na2CsC60 we show
that these two different tetrahedral sites are mixed on a microscopic scale.
The T and T' sites differ in the orientation of first-neighbor C60 molecules.
We present evidence that the orientations of neighboring molecules are
uncorrelated. Thermally activated molecular reorientations cause an exchange
between the T and T' sites and motional narrowing at high temperature. We infer
the same activation energy, 3300 K, in the temperature range 125 to 300 K. The
spin lattice relaxation rate is the same for T and T' down to 125 K but
different below. Both the spin-lattice relaxation rate and Knight shift are
strongly temperature dependent in the whole range investigated. We interpret
this temperature variation by the effect of phonon excitations involving the
rigid librational motion of the C60 molecules. By extending the understanding
of the structure and molecular dynamics of C60 superconductors, these results
may help in clarifying the effects of the structure on the superconducting
properties.Comment: 13 pages, 10 figures, submitted to PR
Anisotropic Spin Hamiltonians due to Spin-Orbit and Coulomb Exchange Interactions
This paper contains the details of Phys. Rev. Lett. 73, 2919 (1994) and, to a
lesser extent, Phys. Rev. Lett. 72, 3710 (1994). We treat a Hubbard model which
includes all the 3d states of the Cu ions and the 2p states of the O ions. We
also include spin-orbit interactions, hopping between ground and excited
crystal field states of the Cu ions, and rather general Coulomb interactions.
Our analytic results for the spin Hamiltonian, H, are corroborated by numerical
evaluations of the energy splitting of the ground manifold for two holes on
either a pair of Cu ions or a Cu-O-Cu complex. In the tetragonal symmetry case
and for the model considered, we prove that H is rotationally invariant in the
absence of Coulomb exchange. When Coulomb exchange is present, each bond
Hamiltonian has full biaxial anisotropy, as expected for this symmetry. For
lower symmetry situations, the single bond spin Hamiltonian is anisotropic at
order t**6 for constant U and at order t**2 for nonconstant U. (Constant U
means that the Coulomb interaction between orbitals does not depend on which
orbitals are involved.)Comment: 50 pages, ILATEX Version 2.09 <13 Jun 1989
Vibrations of the cubane molecule: inelastic neutron scattering study and theory
Cataloged from PDF version of article.Cubane (C8H8) is an immensely strained molecule whose C-C-C bond angle is 90 degrees rather than 109.5 degrees as expected for sp(3) bonding of carbon. We have measured the intramolecular vibrational spectrum of cubane using inelastic neutron scattering. The neutron data are used to test the transferability of various phenomenological potentials and tight-binding models to this highly strained molecule. Unlike these models, first-principles calculations of the INS spectrum (both energy and intensity) agree well with the experimental data. (C) 1999 Published by Elsevier Science B.V. All rights reserved
Pressure-Induced Interlinking of Carbon Nanotubes
We predict new forms of carbon consisting of one and two dimensional networks
of interlinked single wall carbon nanotubes, some of which are energetically
more stable than van der Waals packing of the nanotubes on a hexagonal lattice.
These interlinked nanotubes are further transformed with higher applied
external pressures to more dense and complicated stable structures, in which
curvature-induced carbon sp re-hybridizations are formed. We also discuss
the energetics of the bond formation between nanotubes and the electronic
properties of these predicted novel structures.Comment: 4 pages, 4 postscript figures; To be appear in PR
Magnetic anisotropies and general on--site Coulomb interactions in the cuprates
This paper derives the anisotropic superexchange interactions from a Hubbard
model for excitations within the copper 3d band and the oxygen 2p band of the
undoped insulating cuprates. We extend the recent calculation of Yildirim et
al. [Phys. Rev. B {\bf VV}, pp, 1995] in order to include the most general
on--site Coulomb interactions (including those which involve more than two
orbitals) when two holes occupy the same site. Our general results apply when
the oxygen ions surrounding the copper ions form an octahedron which has
tetragonal symmetry (but may be rotated as in lanthanum cuprate). For the
tetragonal cuprates we obtain an easy--plane anisotropy in good agreement with
experimental values. We predict the magnitude of the small in--plane anisoComment: 25 pages, revte
Tunable adsorption on carbon nanotubes
We investigated the adsorption of a single atom, hydrogen and aluminum, on
single wall carbon nanotubes from first-principles. The adsorption is
exothermic, and the associated binding energy varies inversely as the radius of
the zigzag tube. We found that the adsorption of a single atom and related
properties can be modified continuously and reversibly by the external radial
deformation. The binding energy on the high curvature site of the deformed tube
increases with increasing radial deformation. The effects of curvature and
radial deformation depend on the chirality of the tube.Comment: To be appear in Physical Review Letter
Towards a Microscopic Model of Magnetoelectric Interactions in Ni3V2O8
We develop a microscopic magnetoelectric coupling in NiVO (NVO)
which gives rise to the trilinear phenomenological coupling used previously to
explain the phase transition in which magnetic and ferroelectric order
parameters appear simultaneously. Using combined neutron scattering
measurements and first-principles calculations of the phonons in NVO, we
determine eleven phonons which can induce the observed spontaneous
polarization. Among these eleven phonons, we find that a few of them can
actually induce a significant dipole moment. Using the calculated atomic
charges, we find that the required distortion to induce the observed dipole
moment is very small (~0.001 \AA) and therefore it would be very difficult to
observe the distortion by neutron-powder diffraction. Finally, we identify the
derivatives of the exchange tensor with respect to atomic displacements which
are needed for a microscopic model of a spin-phonon coupling in NVO and which
we hope will be obtained from a fundamental quantum calculation such as LDA+U.
We also analyze two toy models to illustrate that the Dzyaloskinskii-Moriya
interaction is very important for coexisting of magnetic and ferroelectric
order but it is not the only mechanism when the local site symmetry of the
system is low enough.Comment: 20 pages, 10 figure
Magnetically driven ferroelectric order in NiVO
We show that for NiVO long-range ferroelectric and incommensurate
magnetic order appear simultaneously in a single phase transition. The
temperature and magnetic field dependence of the spontaneous polarization show
a strong coupling between magnetic and ferroelectric orders. We determine the
magnetic symmetry of this system by constraining the data to be consistent with
Landau theory for continuous phase transitions. This phenomenological theory
explains our observation the spontaneous polarization is restricted to lie
along the crystal b axis and predicts that the magnitude should be proportional
to a magnetic order parameter.Comment: 11 pages, 3 figure
- …