284 research outputs found
Cubic symmetry and magnetic frustration on the spin lattice in KIrCl
Cubic crystal structure and regular octahedral environment of Ir
render antifluorite-type KIrCl a model fcc antiferromagnet with a
combination of Heisenberg and Kitaev exchange interactions. High-resolution
synchrotron powder diffraction confirms cubic symmetry down to at least 20 K,
with a low-energy rotary mode gradually suppressed upon cooling. Using
thermodynamic and transport measurements, we estimate the activation energy of
eV for charge transport, the antiferromagnetic Curie-Weiss
temperature of K, and the extrapolated saturation
field of T. All these parameters are well reproduced \textit{ab
initio} using eV as the effective Coulomb repulsion
parameter. The antiferromagnetic Kitaev exchange term of K is about
one half of the Heisenberg term K. While this combination removes
a large part of the classical ground-state degeneracy, the selection of the
unique magnetic ground state additionally requires a weak second-neighbor
exchange coupling K. Our results suggest that KIrCl may
offer the best possible cubic conditions for Ir and demonstrates the
interplay of geometrical and exchange frustration in a high-symmetry setting.Comment: 9 page
First-principles study of the magnetic ground state in kagome francisites Cu3Bi(SeO3)2O2X (X=Cl, Br)
We explore magnetic behavior of kagome francisites Cu3Bi(SeO3)2O2X (X = Cl
and Br) using first-principles calculations. To this end, we propose an
approach based on the Hubbard model in the Wannier functions basis constructed
on the level of local-density approximation (LDA). The ground-state spin
configuration is determined by a Hartree-Fock solution of the Hubbard model
both in zero magnetic field and in applied magnetic fields. Additionally,
parameters of an effective spin Hamiltonian are obtained by taking into account
the hybridization effects and spin-orbit coupling. We show that only the former
approach, the Hartree-Fock solution of the Hubbard model, allows for a complete
description of the anisotropic magnetization process. While our calculations
confirm that the canted zero-field ground state arises from a competition
between ferromagnetic nearest-neighbor and antiferromagnetic
next-nearest-neighbor couplings in the kagome planes, weaker anisotropic terms
are crucial for fixing spin directions and for the overall magnetization
process. We thus show that the Hartree-Fock solution of an electronic
Hamiltonian is a viable alternative to the analysis of effective spin
Hamiltonians when a magnetic ground state and effects of external field are
considered
Double Counting in LDA+DMFT - The Example of NiO
An intrinsic issue of the LDA+DMFT approach is the so called double counting
of interaction terms. How to choose the double-counting potential in a manner
that is both physically sound and consistent is unknown. We have conducted an
extensive study of the charge transfer system NiO in the LDA+DMFT framework
using quantum Monte Carlo and exact diagonalization as impurity solvers. By
explicitly treating the double-counting correction as an adjustable parameter
we systematically investigated the effects of different choices for the double
counting on the spectral function. Different methods for fixing the double
counting can drive the result from Mott insulating to almost metallic. We
propose a reasonable scheme for the determination of double-counting
corrections for insulating systems.Comment: 7 pages, 6 figure
Thermodynamic evidence of fractionalized excitations in {\alpha}-RuCl3
Fractionalized excitations are of considerable interest in recent
condensed-matter physics. Fractionalization of the spin degrees of freedom into
localized and itinerant Majorana fermions are predicted for the Kitaev spin
liquid, an exactly solvable model with bond-dependent interactions on a
two-dimensional honeycomb lattice. As function of temperature, theory predicts
a characteristic two-peak structure of the heat capacity as fingerprint of
these excitations. Here we report on detailed heat-capacity experiments as
function of temperature and magnetic field in high-quality single crystals of
{\alpha}-RuCl3 and undertook considerable efforts to determine the exact phonon
background. We measured single-crystalline RhCl3 as non-magnetic reference and
performed ab-initio calculations of the phonon density of states for both
compounds. These ab-initio calculations document that the intrinsic phonon
contribution to the heat capacity cannot be obtained by a simple rescaling of
the nonmagnetic reference using differences in the atomic masses. Sizable
renormalization is required even for non-magnetic RhCl3 with its minute
difference from the title compound. In {\alpha}-RuCl3 in zero magnetic field,
excess heat capacity exists at temperatures well above the onset of magnetic
order. In external magnetic fields far beyond quantum criticality, when
long-range magnetic order is fully suppressed, the excess heat capacity
exhibits the characteristic two-peak structure. In zero field, the lower peak
just appears at temperatures around the onset of magnetic order and seems to be
connected with canonical spin degrees of freedom. At higher fields, beyond the
critical field, this peak is shifted to 10 K. The high-temperature peak located
around 50 K is hardly influenced by external magnetic fields, carries the
predicted amount of entropy, R/2 ln2, and may resemble remnants of Kitaev
physics
Momentum-resolved lattice dynamics of parent and electron-doped SrIrO
The mixing of orbital and spin character in the wave functions of the
iridates has led to predictions of strong couplings among their lattice,
electronic and magnetic degrees of freedom. As well as realizing a novel
spin-orbit assisted Mott-insulating ground state, the perovskite iridate
SrIrO has strong similarities with the cuprate LaCuO,
which on doping hosts a charge-density wave that appears intimately connected
to high-temperature superconductivity. These phenomena can be sensitively
probed through momentum-resolved measurements of the lattice dynamics, made
possible by meV-resolution inelastic x-ray scattering. Here we report the first
such measurements for both parent and electron-doped SrIrO. We find
that the low-energy phonon dispersions and intensities in both compounds are
well described by the same nonmagnetic density functional theory calculation.
In the parent compound, no changes of the phonons on magnetic ordering are
discernible within the experimental resolution, and in the doped compound no
anomalies are apparent due to charge-density waves. These measurements extend
our knowledge of the lattice properties of (SrLa)IrO
and constrain the couplings of the phonons to magnetic and charge order.Comment: 8 pages, 6 figures (+ 12 pages, 6 figures of supplemental material
NATURE OF INTERLAYER BONDS IN TWO-DIMENSIONAL DITELURIDES
In our work, by using first-principles calculations we perform a systematic study of the interlayer bonds and charge redistribution of ditelurides (NiTe2, PdTe2, PtTe2). Our results demonstrate, that bonds in ditelurides can be assiosiated with so-called dative chemical bonds between layers.This work was supported by the Russian Science Foundation Grant 21-72-10136
Structural phase transitions in VSe2: Energetics, electronic structure and magnetism
First principles calculations of the magnetic and electronic properties of VSe2 describing the transition between two structural phases (H,T) were performed. The results of the calculations evidence a rather low energy barrier (0.60 eV for the monolayer) for the transition between the phases. The energy required for the deviation of a Se atom or whole layer of selenium atoms by a small angle of up to 10° from their initial positions is also rather low, 0.32 and 0.19 eV/Se, respectively. The changes in the band structure of VSe2 caused by these motions of Se atoms should be taken into account for analysis of the experimental data. Simulations of the strain effects suggest that the experimentally observed T phase of the VSe2 monolayer is the ground state due to substrate-induced strain. Calculations of the difference in the total energies of the ferromagnetic and antiferromagnetic configurations evidence that the ferromagnetic configuration is the ground state of the system for all stable and intermediate atomic structures. Calculated phonon dispersions suggest a visible influence of the magnetic configurations on the vibrational properties. This journal is © the Owner Societies
Influence of magnetic order on phonon spectra of multiferroic orthorhombic YMnO3
We perform a first-principles study of lattice dynamics in the low-temperature P21nm phase of orthorhombic YMnO3. By considering several possible antiferromagnetic types, we show how magnetic ordering of the system affects its vibrational properties. We find that the experimentally observed magnetic E-type corresponds to the most energetically favorable state and yields phonon spectra, which are consistent with experimental observations. The influence of on-site Coulomb correlations on phonon spectra is also examined. Such effects produce noticeable changes in the spectra and provide a step toward a more accurate description of lattice dynamics in YMnO3. © 2013 Elsevier Ltd. All rights reserved
Coherent vibrations of submicron spherical gold shells in a photonic crystal
Coherent acoustic radial oscillations of thin spherical gold shells of
submicron diameter excited by an ultrashort optical pulse are observed in the
form of pronounced modulations of the transient reflectivity on a subnanosecond
time scale. Strong acousto-optical coupling in a photonic crystal enhances the
modulation of the transient reflectivity up to 4%. The frequency of these
oscillations is demonstrated to be in good agreement with Lamb theory of free
gold shells.Comment: Error in Eqs.2 and 3 corrected; Tabl. I corrected; Fig.1 revised; a
model that explains the dependence of the oscillation amplitude of the
transient reflectivity with wavelength adde
Effect of seed inoculation and foliar fertilizing on structure of soybean yield and yield structure in Western Polissya of Ukraine
Growing soybeans requires adjustment of micronutrient nutrition on poorly fertile
soils. Foliar fertilization can overcome the deficiency of micronutrients in plants in the most
important period. Three factorial field experiment to study foliar fertilization with complex
micronutrients, seed inoculation by Bradyrhizobium japonicum in two soybean varieties (Kassidy
and ES Mentor) was conducted. Foliar fertilization with Quantum oil and WUXAL Oilseed
significantly increased certain elements in soybean yield structure. Fertilizer WUXAL Oilseed
with a higher concentration of Mo, Mn and B more effectively increased the number of pods and
seeds from the plant and formed a higher yield in soybeans. Foliar fertilization with WUXAL
Oilseed and Quantum oil increased seed yield to 3.00 t ha-1
and 2.94 t ha-1
, respectively in regard
to variant without fertilizing, where yield was 2.71 t ha-1
. Variety Kassidy had greater number of
pods and seeds, seed weight in comparison to foliar fertilizing ES Mentor. Seed inoculation gaves
a stable increase in yield under different foliar fertilizations in varieties Kassidy and ES Mentor
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