260 research outputs found
Spin, charge and orbital ordering in La0.5Sr1.5MnO4
We have analyzed the experimental evidence of charge and orbital ordering in
La0.5Sr1.5MnO4 using first principles band structure calculations. Our results
suggest the presence of two types of Mn sites in the system. One of the Mn
sites behaves like an Mn(3+) ion, favoring a Jahn-Teller distortion of the
surrounding oxygen atoms, while the distortion around the other is not a simple
breathing mode kind. Band structure effects are found to dominate the
experimental spectrum for orbital and charge ordering, providing an alternate
explanation for the experimentally observed results.Comment: 4 pages + 3 figures; To appear in Phys. Rev. Let
Inverse versus Normal NiAs Structure as High-Pressure Phase of FeO and MnO
The high-pressure phases of FeO and MnO were studied by the first principles
calculations. The present theoretical study predicts that the high-pressure
phase of MnO is a metallic normal B8 structure (nB8), while that of FeO should
take the inverse B8 structure (iB8). The novel feature of the unique
high-pressure phase of stoichiometric FeO is that the system should be a band
insulator in the ordered antiferromagnetic (AF) state and that the existence of
a band gap leads to special stability of the phase. The observed metallicity of
the high-pressure and high-temperature phase of FeO may be caused by the loss
of AF order and also by the itinerant carriers created by non-stoichiometry.
Analysis of x-ray diffraction experiments provides a further support to the
present theoretical prediction for both FeO and MnO. Strong stability of the
high-pressure phase of FeO will imply possible important roles in Earth's core.Comment: 7 pages, 3 figures and 1 table; submitted to "Nature
Static dielectric response and Born effective charge of BN nanotubes from {\it ab initio} finite electric field calculations
{\it Ab initio} investigations of the full static dielectric response and
Born effective charge of BN nanotubes (BN-NTs) have been performed for the
first time using finite electric field method. It is found that the ionic
contribution to the static dielectric response of BN-NTs is substantial and
also that a pronounced chirality-dependent oscillation is superimposed on the
otherwise linear relation between the longitudinal electric polarizability and
the tube diameter (), as for a thin dielectric cylinderical shell. In
contrast, the transverse dielectric response of the BN-NTs resemble the
behavior of a thin (non-ideal) conducting cylindrical shell of a diameter of
\AA, with a screening factor of 2 for the inner electric field. The
medium principal component of the Born effective charge corresponding
to the transverse atomic displacement tangential to the BN-NT surface, has a
pronounced -dependence (but independent of chirality), while the large
longitudinal component exhibits a clear chirality dependence (but
nearly -independent), suggesting a powerful way to characterize the diameter
and chirality of a BN-NT.Comment: submitted to PR
Strong ferromangnetism and weak antiferroamgnetism in double perovskites: SrFe{/it M}O ({/it M}=Mo, W and Re)
Double perovskites SrFeMO (M=Mo and Re) exhibit significant colossal
magnetoresistance even at room temperature due to the high Curie Temperature
(419K and 401K). However, such a high Curie Temperature is puzzling, given the
large separation between magnetic elements (Fe). Moreover, with M=W, the
electronic and magnetic properties suddenly change to insulating and
antiferromagnetic with the N{\'e}el temperature of only 1637 K. Based on
detailed electronic structure calculations, a new mechanism is proposed which
stabilizes the strong ferromagnetic state for M=Mo and Re and is passivated for
M=W.Comment: 4 pages, 3 figures; accepted by PRB as rapid communicatio
Structural distortions and model Hamiltonian parameters: from LSDA to a tight-binding description of LaMnO_3
The physics of manganites is often described within an effective two-band
tight-binding (TB) model for the Mn e_g electrons, which apart from the kinetic
energy includes also a local "Hund's rule" coupling to the t_{2g} core spin and
a local coupling to the Jahn-Teller (JT) distortion of the oxygen octahedra. We
test the validity of this model by comparing the energy dispersion calculated
for the TB model with the full Kohn-Sham band-structure calculated within the
local spin-density approximation (LSDA) to density functional theory. We
analyze the effect of magnetic order, JT distortions, and "GdFeO_3-type"
tilt-rotations of the oxygen octahedra. We show that the hopping amplitudes are
independent of magnetic order and JT distortions, and that both effects can be
described with a consistent set of model parameters if hopping between both
nearest and next-nearest neighbors is taken into account. We determine a full
set of model parameters from the density functional theory calculations, and we
show that both JT distortions and Hund's rule coupling are required to obtain
an insulating ground state within LSDA. Furthermore, our calculations show that
the "GdFeO_3-type" rotations of the oxygen octahedra lead to a substantial
reduction of the hopping amplitudes but to no significant deviation from the
simple TB model.Comment: replaced with final (published) version with improved presentatio
Anomalous Hall Effect and Magnetic Monopoles in Momentum-Space
Efforts to find the magnetic monopole in real space have been made in cosmic
rays and in accelerators, but up to now there is no firm evidence for its
existence due to the very heavy mass GeV. However, we show that
the magnetic monopole can appear in the crystal-momentum space of solids in the
accessible low energy region (eV) in the context of the anomalous
Hall effect. We report experimental results together with first-principles
calculations on the ferromagnetic crystal SrRuO that provide evidence for
the magnetic monopole in the crystal-momentum space.Comment: 4 figures, the supporting-online-materails are include
Unquenched large orbital magnetic moment in NiO
Magnetic properties of NiO are investigated by incorporating the spin-orbit
interaction in the LSDA+U scheme. It is found that the large part of orbital
moment remains unquenched in NiO. The orbital moment contributes about mu_L =
0.29 mu_B to the total magnetic moment of M = 1.93 mu_B, as leads to the
orbital-to-spin angular momentum ratio of L/S = 0.36. The theoretical values
are in good agreement with recent magnetic X-ray scattering measurements.Comment: 4 pages, 2 figure
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