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 Sr2_{2}IrO4_{4}

The mixing of orbital and spin character in the wave functions of the $5d$ 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 Sr$_{2}$IrO$_{4}$ has strong similarities with the cuprate La$_{2}$CuO$_{4}$, 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 Sr$_{2}$IrO$_{4}$. 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 (Sr$_{1-x}$La$_{x}$)$_{2}$IrO$_{4}$ 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