273 research outputs found
Magnetic excitations in charge- ordered NaV2O5
An investigation of the spin excitation spectrum of charge ordered (CO)
NaV2O5 is presented. We discuss several different exchange models which may be
relevant for this compound, namely in- line and zig-zag chain models with weak
as well as strong inter- chain coupling and also a ladder model and a CO/MV
(mixed valent) model. We put special emphasis on the importance of large
additional exchange across the diagonals of V- ladders and the presence of
exchange anisotropies on the excitation spectrum. It is shown that the observed
splitting of transverse dispersion branches may both be interpreted as
anisotropy effect as well as acoustic- optic mode splitting in the weakly
coupled chain models. In addition we calculate the field dependence of
excitation modes in these models. Furthermore we show that for strong inter-
chain coupling, as suggested by recent LDA+U results, an additional high energy
optical excitation appears and the spin gap is determined by anisotropies. The
most promising CO/MV model predicts a spin wave dispersion perpendicular to the
chains which agrees very well with recent results obtained by inelastic neutron
scattering.Comment: Revtex, 22 pages, 7 postscript figure
Fermi surface and heavy masses for UPdAl
We calculate the Fermi surface and the anisotropic heavy masses of UPd2Al3 by
keeping two of the 5f electrons as localized. Good agreement with experiments
is found. The theory contains essentially no adjustable parameter except for a
small shift of the position of the Fermi energy of the order of a few meV. A
discussion is given why localization of two f electrons is justified.Comment: 4 pages, 2 figure
Dynamic spin susceptibility of paramagnetic spinel LiV2O4
In an attempt to explain inelastic neutron scattering data for LiV2O4 the
dynamical spin susceptibility \chi(Q,w) at zero temperature is calculated.
Starting point is a weak coupling approach based on the LDA bandstructure for
that material. It is supplemented by a RPA treatment of local on-site
interactions and contains an adjustable parameter. Due to the geometrically
frustrated lattice structure the magnetic response is strongly enhanced in the
vicinity of a nearly spherical surface in Q-space. We compare these results
with recent low-temperature neutron scattering data. The measured spin
relaxation rate \Gamma is used to estimate the spin fluctuation contribution to
the specific heat.Comment: 26 pages, 6 figures, submitted to PR
Magnetic properties of doped GdI2
Motivated by the recent experimental studies on layered ferromagnetic
metallic system GdI2 and its doped variant GdI2Hx we develop a model to
understand their ground state magnetic phase diagram. Based on first principle
electronic structure calculations we write down a phenomenological model and
solve it under certain approximations to obtain the ground state energy. In the
process we work out the phase diagram of the correlated double exchange model
on a triangular lattice for the specific band structure at hand.Comment: 13 pages, 5 figures, corrected typo
On the origin of heavy quasiparticles in LiV_2O_4
An explanation is provided for the heavy quasiparticle excitations in
LiV_2O_4. It differs considerably from that of other known heavy-fermion
systems. Main ingredients of our theory are the cubic spinel structure of the
material and strong short-range correlations of the d electrons. The large
gamma-coefficient is shown to result from excitations of Heisenberg spin 1/2
rings and chains. The required coupling constant is calculated from LDA+U
calculations and is found to be of the right size. Also the calculated
Sommerfeld-Wilson ratio is reasonably close to the observed one.Comment: REVTEX, 5 pages, 2 figure
Electronic structure of charge-ordered Fe3O4 from calculated optical, megneto-optical Kerr effect, and O K-edge x-ray absorption spectra
The electronic structure of the low-temperature (LT) monoclinic magnetite,
Fe3O4, is investigated using the local spin density approximation (LSDA) and
the LSDA+U method. The self-consistent charge ordered LSDA+U solution has a
pronounced [001] charge density wave character. In addition, a minor [00{1/2}]
modulation in the phase of the charge order (CO) also occurs. While the
existence of CO is evidenced by the large difference between the occupancies of
the minority spin t_{2g} states of ``2+'' and ``3+'' Fe_B cations, the total 3d
charge disproportion is small, in accord with the valence-bond-sum analysis of
structural data. Weak Fe orbital moments of ~0.07 mB are obtained from
relativistic calculations for the CO phase which is in good agreement with
recent x-ray magnetic circular dichroism measurements. Optical, magneto-optical
Kerr effect, and O K-edge x-ray absorption spectra calculated for the charge
ordered LSDA+U solution are compared to corresponding LSDA spectra and to
available experimental data. Reasonably good agreement between the theoretical
and experimental spectra supports the relevance of the CO solution obtained for
the monoclinic LT phase. The results of calculations of effective exchange
coupling constants between Fe spin magnetic moments are also presented.Comment: 32 pages, 10 figure
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