140 research outputs found
Dzyaloshinsky-Moriya antisymmetric exchange coupling in cuprates: Oxygen effects
We revisit a problem of Dzyaloshinsky-Moriya antisymmetric exchange coupling
for a single bond in cuprates specifying the local spin-orbital contributions
to Dzyaloshinsky vector focusing on the oxygen term. The Dzyaloshinsky vector
and respective weak ferromagnetic moment is shown to be a superposition of
comparable and, sometimes, competing local Cu and O contributions. The
intermediate oxygen O Knight shift is shown to be an effective tool to
inspect the effects of Dzyaloshinsky-Moriya coupling in an external magnetic
field. We predict the effect of oxygen weak antiferromagnetism in
edge-shared CuO chains due to uncompensated oxygen Dzyaloshinsky vectors.
Finally, we revisit the effects of symmetric spin anisotropy, in particular,
those directly induced by Dzyaloshinsky-Moriya coupling.Comment: 12 pages, 2 figures, submitted to JET
Lattice Distortion and Magnetism of 3d- Perovskite Oxides
Several puzzling aspects of interplay of the experimental lattice distortion
and the the magnetic properties of four narrow -band perovskite oxides
(YTiO, LaTiO, YVO, and LaVO) are clarified using results of
first-principles electronic structure calculations. First, we derive parameters
of the effective Hubbard-type Hamiltonian for the isolated bands using
newly developed downfolding method for the kinetic-energy part and a hybrid
approach, based on the combination of the random-phase approximation and the
constraint local-density approximation, for the screened Coulomb interaction
part. Then, we solve the obtained Hamiltonian using a number of techniques,
including the mean-field Hartree-Fock (HF) approximation, the second-order
perturbation theory for the correlation energy, and a variational superexchange
theory. Even though the crystal-field splitting is not particularly large to
quench the orbital degrees of freedom, the crystal distortion imposes a severe
constraint on the form of the possible orbital states, which favor the
formation of the experimentally observed magnetic structures in YTiO,
YVO_, and LaVO even at the HF level. Beyond the HF approximation, the
correlations effects systematically improve the agreement with the experimental
data. Using the same type of approximations we could not reproduce the correct
magnetic ground state of LaTiO. However, we expect that the situation may
change by systematically improving the level of approximations for dealing with
the correlation effects.Comment: 30 pages, 17 figures, 8 tables, high-quality figures are available
via e-mai
Topological phase separation in 2D quantum lattice Bose-Hubbard system away from half-filling
We suppose that the doping of the 2D hard-core boson system away from
half-filling may result in the formation of multi-center topological
inhomogeneity (defect) such as charge order (CO) bubble domain(s) with Bose
superfluid (BS) and extra bosons both localized in domain wall(s), or a {\it
topological} CO+BS {\it phase separation}, rather than an uniform mixed CO+BS
supersolid phase. Starting from the classical model we predict the properties
of the respective quantum system. The long-wavelength behavior of the system is
believed to remind that of granular superconductors, CDW materials, Wigner
crystals, and multi-skyrmion system akin in a quantum Hall ferromagnetic state
of a 2D electron gas. To elucidate the role played by quantum effects and that
of the lattice discreteness we have addressed the simplest nanoscopic
counterpart of the bubble domain in a checkerboard CO phase of 2D hc-BH square
lattice. It is shown that the relative magnitude and symmetry of
multi-component order parameter are mainly determined by the sign of the
and transfer integrals. In general, the topologically inhomogeneous phase
of the hc-BH system away from the half-filling can exhibit the signatures both
of , and symmetry of the off-diagonal order.Comment: 12 pages, 6 figure
150-250 MeV electron beams in radiation therapy
High-energy electron beams in the range 150-250 MeV are studied to evaluate the feasibility for radiotherapy. Monte Carlo simulation results from the PENELOPE code are presented and used to determine lateral spread and penetration of these beams. It is shown that the penumbra is comparable to photon beams at depths less than 10 cm and the practical range ( Rp) of these beams is greater than 40 cm. The depth dose distribution of electron beams compares favourably with photon beams. Effects caused by nuclear reactions are evaluated, including increased dose due to neutron production and induced radioactivity resulting in an increased relative biological effectiveness (RBE) factor of <1.03.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48967/2/m00706.pd
Nonbonding oxygen holes and spinless scenario of magnetic response in doped cuprates
Both theoretical considerations and experimental data point to a more
complicated nature of the valence hole states in doped cuprates than it is
predicted by Zhang-Rice model. Actually, we deal with a competition of
conventional hybrid Cu 3d-O 2p state and purely
oxygen nonbonding state with symmetry. The latter
reveals a non-quenched Ising-like orbital moment that gives rise to a novel
spinless purely oxygen scenario of the magnetic response in doped cuprates with
the oxygen localized orbital magnetic moments of the order of tenths of Bohr
magneton. We consider the mechanism of Cu-O 2p transferred orbital
hyperfine interactions due to the mixing of the oxygen O 2p orbitals with Cu 3p
semicore orbitals. Quantitative estimates point to a large magnitude of the
respective contributions both to local field and electric field gradient, and
their correlated character.Comment: 7 pages, 1 figur
A simplified model of the source channel of the Leksell Gamma Knife: testing multisource configurations with PENELOPE
A simplification of the source channel geometry of the Leksell Gamma
Knife, recently proposed by the authors and checked for a single
source configuration (Al-Dweri et al 2004), has been used to calculate the dose
distributions along the , and axes in a water phantom with a
diameter of 160~mm, for different configurations of the Gamma Knife including
201, 150 and 102 unplugged sources. The code PENELOPE (v. 2001) has been used
to perform the Monte Carlo simulations. In addition, the output factors for the
14, 8 and 4~mm helmets have been calculated. The results found for the dose
profiles show a qualitatively good agreement with previous ones obtained with
EGS4 and PENELOPE (v. 2000) codes and with the predictions of
GammaPlan. The output factors obtained with our model agree
within the statistical uncertainties with those calculated with the same Monte
Carlo codes and with those measured with different techniques. Owing to the
accuracy of the results obtained and to the reduction in the computational time
with respect to full geometry simulations (larger than a factor 15), this
simplified model opens the possibility to use Monte Carlo tools for planning
purposes in the Gamma Knife.Comment: 13 pages, 8 figures, 5 table
Crystal Field and Dzyaloshinsky-Moriya Interaction in orbitally ordered La_{0.95}Sr_{0.05}MnO_3: An ESR Study
We present a comprehensive analysis of Dzyaloshinsky-Moriya interaction and
crystal-field parameters using the angular dependence of the paramagnetic
resonance shift and linewidth in single crystals of La_{0.95}Sr_{0.05}MnO_3
within the orthorhombic Jahn-Teller distorted phase. The Dzyaloshinsky-Moriya
interaction (~ 1K) results from the tilting of the MnO_6 octahedra against each
other. The crystal-field parameters D and E are found to be of comparable
magnitude (~ 1K) with D ~= -E. This indicates a strong mixing of the |3z^2-r^2>
and |x^2-y^2> states for the real orbital configuration.Comment: 12 pages, 6 figure
Dispersion of the dielectric function of a charge-transfer insulator
We study the problem of dielectric response in the strong coupling regime of
a charge transfer insulator. The frequency and wave number dependence of the
dielectric function and its inverse is the main object of consideration. We show that the
problem, in general, cannot be reduced to a calculation within the Hubbard
model, which takes into account only a restricted number of electronic states
near the Fermi energy. The contribution of the rest of the system to the
longitudinal response (i.e. to ) is essential
for the whole frequency range. With the use of the spectral representation of
the two-particle Green's function we show that the problem may be divided into
two parts: into the contributions of the weakly correlated and the Hubbard
subsystems. For the latter we propose an approach that starts from the
correlated paramagnetic ground state with strong antiferromagnetic
fluctuations. We obtain a set of coupled equations of motion for the
two-particle Green's function that may be solved by means of the projection
technique. The solution is expressed by a two particle basis that includes the
excitonic states with electron and hole separated at various distances. We
apply our method to the multiband Hubbard (Emery) model that describes layered
cuprates. We show that strongly dispersive branches exist in the excitonic
spectrum of the 'minimal' Emery model () and consider the
dependence of the spectrum on finite oxygen hopping and on-site
repulsion . The relationship of our calculations to electron energy loss
spectroscopy is discussed.Comment: 22 pages, 5 figure
Effects of bone- and air-tissue inhomogeneities on the dose distributions of the Leksell Gamma Knife calculated with PENELOPE
Monte Carlo simulation with PENELOPE (v.~2003) is applied to calculate
Leksell Gamma Knife dose distributions for heterogeneous
phantoms. The usual spherical water phantom is modified with a spherical bone
shell simulating the skull and an air-filled cube simulating the frontal or
maxillary sinuses. Different simulations of the 201 source configuration of the
Gamma Knife have been carried out with a simplified model of the geometry of
the source channel of the Gamma Knife recently tested for both single source
and multisource configurations. The dose distributions determined for
heterogeneous phantoms including the bone- and/or air-tissue interfaces show
non negligible differences with respect to those calculated for a homogeneous
one, mainly when the Gamma Knife isocenter approaches the separation surfaces.
Our findings confirm an important underdosage (10%) nearby the air-tissue
interface, in accordance with previous results obtained with PENELOPE code with
a procedure different to ours. On the other hand, the presence of the spherical
shell simulating the skull produces a few percent underdosage at the isocenter
wherever it is situated.Comment: 13 pages, 8 figures, 2 table
Colossal magnetocapacitive effect in differently synthesized and doped CdCr2S4
In the present work, we address the question of an impurity-related origin of
the colossal magnetocapacitive effect in the spinel system CdCr2S4. We
demonstrate that a strong variation in the dielectric constant below the
magnetic transition temperature or in external magnetic fields also arises in
crystals prepared without chlorine. This excludes that an inhomogeneous
distribution of chlorine impurities at the surface or in the bulk material
gives rise to the unusual effects in the spinel multiferroics. In addition, we
show that colossal magnetocapacitive effects can be also generated in
chlorine-free ceramic samples of CdCr2S4, doped with indium.Comment: 4 pages, 3 figure
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