198 research outputs found
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
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
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
Nature of Interlayer Bonds in Two-Dimensional Materials
The role of interlayer bonds in the two-dimensional (2D) materials “beyond graphene” and so-called van der Waals heterostructures is vital, and understanding the nature of these bonds in terms of strength and type is essential due to a wide range of their prospective technological applications. However, this issue has not yet been properly addressed in the previous investigations devoted to 2D materials. In our work, by using first-principles calculations we perform a systematic study of the interlayer bonds and charge redistribution of several representative 2D materials that are traditionally referred to as van der Waals systems. Our results demonstrate that one can distinguish three main types of interlayer couplings in the considered 2D structures: one-atom-thick membranes bonded by London dispersion forces (graphene, hBN), systems with leading electrostatic interaction between layers (diselenides, InSe, and bilayer silica), and materials with so-called dative or coordination chemical bonds between layers (ditelurides). We also propose a protocol for recognizing the leading type of interlayer bonds in a system that includes a comparison of interlayer distances, binding energies, and the redistribution of the charge densities in interlayer space. Such an approach is computationally cheap and can be used to further predict the chemical and physical properties, such as charge density waves (CDW), work function, and chemical stability at ambient conditions. © 2023 American Chemical Society.Russian Science Foundation, RSF: 21-72-10136This work was supported by the Russian Science Foundation, Grant No. 21-72-10136
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
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
Developing a New Effective Magnetic Model of Fe3GeTe2 Based on AB-Initio Calculations
In our work we propose a new effective magnetic model for two-dimensional van der Waals ferromagnet Fe3GeTe2. To prove its correctness we conducted ab-initio calculations as well as compared Curie temperatures using Monte Carlo simulations with the obtained parameters.This work was supported by the Russian Science Foundation, Grant No. 21-72-10136
EVOLUTION OF THE MAGNETIC PROPERTIES OF TWO-DIMENSIONAL Fe3GeTe2 UNDER STRAIN
In this work we study the evolution of magnetic properties, magnetization, and magnon spectra of Fe3GeTe2 changes under in-plane biaxial strain. We found that moderate tensile strain significantly increases Curie temperature, while compressive one destroys FM order.This work was supported by the Russian Science Foundation Grant 21-72-10136
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
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