arXiv subjects

Kateryna Foyevtsova

Publications and source records attributed to Kateryna Foyevtsova.

At least 19 recordsLinked to original sources

arXiv2025

Charge distribution and magnetism in bilayer La$_3$Ni$_2$O$_7$: a hybrid functional study

An accurate understanding of the ground state electronic properties of La$_3$Ni$_2$O$_7$, a high-temperature superconductor under pressure, is key for unveiling the origin of its superconductivity. In this paper, we conduct a theoretical study of the electronic structure of the bilayer polymorph of La$_3$Ni$_2$O$_7$ using the hybrid functional approach, which is well suited to tackle the non-local correlation effects arising in this system from the molecular orbital splitting of the Ni $3d_{3z^2-r^2}$ states inside Ni-Ni dimers. Our calculations reveal that bilayer La$_3$Ni$_2$O$_7$ is a strongly correlated magnetic system with robust Ni spin moments. Spin moments on individual Ni sites take on unusually small values because of the electron delocalization over molecular orbitals involving multiple Ni and O sites. We further find that the magnetism of bilayer La$_3$Ni$_2$O$_7$ is intimately linked with charge distribution between different Ni and O orbitals. Two distinct regimes are identified in this regard. In one, molecular orbital physics drives the Ni $3d_{x^2-y^2}$ band towards half-filling, which is a well-established condition for unconventional high-temperature superconductivity upon hole doping in cuprates. In the other, the Ni $3d_{x^2-y^2}$ band is quarter-filled favouring spin- and charge-density wave states and Ni-O bond-disproportionation, which is consistent with several recent experimental claims. It is possible that superconductivity in La$_3$Ni$_2$O$_7$ occurs as a result of a pressure-induced transition between these two competing regimes. Since none of the low energy phases discovered in this study are metallic, non-stoichiometry would be required for superconductivity to occur.

cond-mat.supr-con↗

arXiv2025

Crystal Growth and Physical Properties of Orthorhombic Kagome Lattice Magnets $R$Fe$_6$Ge$_6$ ($R$=Y, Tb, Dy)

Kagome magnets represent a promising class of materials that exhibit intriguing electronic and magnetic properties, and they have recently garnered significant attention. While most kagome-lattice compounds are hexagonal, we report here single-crystal growth and physical property measurements of $R$Fe$_6$Ge$_6$ ($R$ = Y, Dy, Tb) compounds, which crystallize in an orthorhombic structure. The structure can be derived from a hexagonal prototype $R$Fe$_3$Ge$_2$ by replacing every other $R$ atom with a covalent Ge$_2$ dimer. Ordering of these dimers renders the structure orthorhombic, slightly distorts the kagome net, and makes the three Fe sites formally inequivalent. The iron and rare-earth sublattices order independently. Fe moments order above 400 K, forming ferromagnetic kagome planes stacked antiferromagnetically, while rare-earth moments order below 9 K. TbFe$_6$Ge$_6$ exhibits a single magnetic ordering transition associated with the Tb atoms, whereas DyFe$_6$Ge$_6$ shows two distinct magnetic phase transitions, strongly influenced by crystal electric field effects on the Dy$^{3+}$ ions. Density functional theory (DFT) calculations indicate that the ferromagnetic ordering of the Fe planes is driven by a high density of states at the Fermi energy. They also reveal three dramatically different structural energy scales: $R$ and Ge$_2$ form alternating 1D chains perpendicular to the kagome planes, and violating this alternation incurs a large energy cost. Aligning these chains is less costly, and achieving a two-dimensional order of anti-aligned chains requires very little energy. These compounds represent a unique class of materials, offering new opportunities to investigate the interplay between the distinct crystal lattice geometry and the underlying electronic and magnetic properties.

cond-mat.mtrl-sci↗

arXiv2025

Intertwined charge and spin instability of La$_3$Ni$_2$O$_7$

Research on nickel-based superconductors has progressed from infinite-layer LaNiO$_2$ to finite-layer La$_{6}$Ni$_{5}$O$_{12}$, and most recently to the Ruddlesden-Popper phase La$_3$Ni$_2$O$_7$, which was found to exhibits onset of superconductivity at $\sim$80\,K under a pressure of $\sim$16\,GPa. Unlike the superconductivity mainly driven by the $d_{x^2-y^2}$ orbital in infinite-layer nickelates, the Ni-$d_{z^2}$ and O-2$p$ orbitals contribute significantly to the low energy states and potentially to the superconducting electron pairing mechanism of La$_3$Ni$_2$O$_7$. Employing density functional calculations and multi-orbital multi-atom cluster exact diagonalization including local exchange and Coulomb interactions, here we analyze the pressure dependent low-energy electronic states of the Ni$_2$O$_9$ cluster, relevant for the bilayer phase of La$_3$Ni$_2$O$_7$. The various possible spin states and the exchange and superexchange mechanisms of the Ni$_2$O$_9$ cluster are quantified via the involvement of the Ni-$3d_{3z^2-r^2}$ orbitals and the atomic Hund's rule exchange, the apical bridging O-$2p_z$ orbitals, and the orbitals involved in the formation of local Zhang-Rice singlet like states. We find that the leading configurations contributiong to the cluster ground-states both for nominal valence and also with local charge fluctuations, do not involve occupation of the apical oxygen, instead they favor formation of in-plane Zhang-Rice singlet like states between an O ligand hole and the Ni $3d_{z^2-y^2}$ orbital. We also highlight two possible charge and spin ordered states suggested by our cluster results, that are nearly degenerate at all relevant pressures within our modelling.

cond-mat.supr-con↗

arXiv2024

Computationally efficient method for calculating electron-phonon coupling for high-throughput superconductivity search

Using a computationally inexpensive frozen phonon approach we have developed a technique which can be used to screen large unit cell materials and systems for enhanced superconducting critical temperatures. The method requires only density functional theory (DFT) calculated electronic band structures of phonon modes corresponding to atomic displacements for various materials. We have applied this method to well known conventional superconductors including MgB$_2$, H$_{3}$S and other hydrides as examples.

cond-mat.supr-con↗

arXiv2023

Distinct electridelike nature of infinite-layer nickelates and the resulting theoretical challenges to calculate their electronic structure

We demonstrate in this paper that the recently discovered infinite-layer (IL) nickelates have much in common with a class of materials known as electrides. Oxide based electrides are compounds in which topotactic removal of loosely bound oxygens leaves behind voids with a landscape of attractive potentials for electrons. We show that this is also what happens in the IL nickelates, where one of the two electrons (per formula unit) freed during the topotactic synthesis is to a large degree located in the oxygen vacancy position, occupying partially a local $s$-symmetry interstitial orbital, rather than taking part alongside the other electon in converting Ni from 3+ to a full 1+ oxidation state. We demonstrate that the interstitial orbital in question, referred to by us as the zeronium $s$ or Z $s$ orbital, forms strong covalent bonds with neighboring Ni $3d_{3z^2-r^2}$ orbitals, which in turn facilitates the one-dimensional-like dispersion of the Ni $3d_{3z^2-r^2}$ band along the $c$-axis direction, leading also to a possible large out-of-plane coupling between Ni magnetic moments. This finding, reinforced by our electron localization function analysis, points to a fundamental distinction between the nickelates and the structurally analogous cuprates, may explain the absence of superconductivity in hydrogen-poor samples, and is certainly in agreement with the observed large $z$-polarized component in the Ni $L_3$-edge x-ray absorption spectra. In addition, by using DFT+U calculations as an illustration, we show that the electride-like nature of the IL nickelates is one of the main reasons for the theoretical difficulty in determining the much debated elusive Fermi surface of these novel superconductors and aslo in exploring the possibility of them becoming excitonic insulators at low temperatures.

cond-mat.supr-con↗

arXiv2021

Electronic Structure of the Bond Disproportionated Bismuthate Ag$_2$BiO$_3$

We present a comprehensive study on the silver bismuthate Ag$_2$BiO$_3$, synthesized under high-pressure high-temperature conditions, which has been the subject of recent theoretical work on topologically complex electronic states. We present X-ray photoelectron spectroscopy results showing two different bismuth states, and X-ray absorption spectroscopy results on the oxygen $K$-edge showing holes in the oxygen bands. These results support a bond disproportionated state with holes on the oxygen atoms for Ag$_2$BiO$_3$. We estimate a band gap of $\sim$1.25~eV for Ag$_2$BiO$_3$ from optical conductivity measurements, which matches the band gap in density functional calculations of the electronic band structure in the non-symmorphic space group $Pnn2$, which supports two inequivalent Bi sites. In our band structure calculations the disproportionated Ag$_2$BiO$_3$ is expected to host Weyl nodal chains, one of which is located $\sim$0.5~eV below the Fermi level. Furthermore, we highlight similarities between Ag$_2$BiO$_3$ and the well-known disproportionated bismuthate BaBiO$_3$, including breathing phonon modes with similar energy. In both compounds hybridization of Bi-$6s$ and O-$2p$ atomic orbitals is important in shaping the band structure, but in contrast to the Ba-$5p$ in BaBiO$_3$, the Ag-$4d$ bands in Ag$_2$BiO$_3$ extend up to the Fermi level.

cond-mat.mtrl-sci↗

arXiv2021

LiNiO$_2$ as a high-entropy charge- and bond-disproportionated glass

Understanding microscopic properties of LiNiO$_2$, a Li-ion battery cathode material with extraordinarily high reversible capacity, has remained a challenge for decades. Based on extensive electronic structure calculations, which reveal a large number of nearly degenerate phases involving local Jahn-Teller effect as well as bond and oxygen-based charge disproportionation, we propose that LiNiO$_2$ exists in a high-entropy charge-glass like state at and below ambient temperatures. Recognizing the glassy nature of LiNiO$_2$ does not only explain its key experimental features, but also opens a new path in designing entropy-stabilized battery cathodes with superb capacities.

cond-mat.mtrl-sci↗

arXiv2021

Holes' character and bond versus charge disproportionation in $s-p$ $ABX_{3}$ perovskites

We use density functional theory methods to study the electronic structures of a series of $s-p$ cubic perovskites $ABX_{3}$: the experimentally available SrBiO$_{3}$, BaBiO$_{3}$, BaSbO$_3$, CsTlF$_{3}$, and CsTlCl$_{3}$, as well as the hypothetical MgPO$_{3}$, CaAsO$_{3}$, SrSbO$_{3}$, and RaMcO$_3$. We use tight-binding modeling to calculate the interatomic hopping integrals $t_{spσ}$ between the $B-s$ and $X-p$ atomic orbitals and charge-transfer energies $Δ$, which are the two most important parameters that determine the low-energy electron and hole states of these systems. Our calculations elucidate several trends in $t_{spσ}$ and $Δ$ as one moves across the periodic table, such as the relativistic energy lowering of the $B-s$ orbital in heavy $B$ cations leading to strongly negative $Δ$ values. Our results are discussed in connection with the general phase diagram for $s - p$ cubic perovskites proposed in Ref. 26, where the parent superconductors SrBiO$_{3}$ and BaBiO$_{3}$ are found to be in the regime of negative $Δ$ and large $t_{spσ}$. Here, we explore this further and search for new materials with similar parameters, which could lead to the discovery of new superconductors. Also, some considerations are offered regarding a possible relation between the physical properties of a given $s - p$ compound (such as its tendency to bond disproportionate and the maximal achievable superconducting transition temperature) and its electronic structure.

cond-mat.supr-con↗

arXiv2019

Epitaxial Growth of Perovskite SrBiO$_3$ Film on SrTiO$_3$ by Oxide Molecular Beam Epitaxy

Hole-doped perovskite bismuthates such as Ba$_{1-x}$K$_x$BiO$_3$ and Sr$_{1-x}$K$_x$BiO$_3$ are well-known bismuth-based oxide high-transition-temperature superconductors. Reported thin bismuthate films show relatively low quality, likely due to their large lattice mismatch with the substrate and a low sticking coefficient of Bi at high temperatures. Here, we report the successful epitaxial thin film growth of the parent compound strontium bismuthate SrBiO$_3$ on SrO-terminated SrTiO$_3$ (001) substrates by molecular beam epitaxy. Two different growth methods, high-temperature co-deposition or recrystallization cycles of low-temperature deposition plus high-temperature annealing, are developed to improve the epitaxial growth. SrBiO$_3$ has a pseudocubic lattice constant $\sim$4.25 Å, an $\sim$8.8\% lattice mismatch on SrTiO$_3$ substrate, leading to a large strain in the first few unit cells. Films thicker than 6 unit cells prepared by both methods are fully relaxed to bulk lattice constant and have similar quality. Compared to high-temperature co-deposition, the recrystallization method can produce higher quality 1-6 unit cell films that are coherently or partially strained. Photoemission experiments reveal the bonding and antibonding states close to the Fermi level due to Bi and O hybridization, in good agreement with density functional theory calculations. This work provides general guidance to the synthesis of high-quality perovskite bismuthate films.

cond-mat.supr-con↗

arXiv2019

Local Metallic and Structural Properties of the Strongly Correlated Metal LaNiO$_{3}$ using $^{8}$Li $β$-NMR

We report $β$-detected NMR of ion-implanted $^{8}$Li in a single crystal and thin film of the strongly correlated metal LaNiO$_{3}$. In both samples, spin-lattice relaxation measurements reveal two distinct local metallic environments, as is evident from $T$-linear Korringa $1/T_{1}$ below 200 K with slopes comparable to other metals. A small, approximately temperature independent Knight shift of $\sim 74$ ppm is observed, yielding a normalized Korringa product characteristic of substantial antiferromagnetic correlations, but, we find no evidence for a magnetic transition from 4 to 310 K. Two distinct, equally abundant $^{8}$Li sites is inconsistent with the widely accepted rhombohedral structure of LaNiO$_{3}$, but cannot be simply explained by either of the common alternative orthorhombic or monoclinic distortions.

cond-mat.str-el↗

arXiv2018

Bond versus charge disproportionation in the bismuth perovskites

We develop a theory describing a parameter based phase diagram to be associated with materials incorporating skipped valence ions\cite{Varma}. We use a recently developed tight-binding approach for the bismuthates to study the phase diagram exhibiting the crossover from a bond disproportionated (BD) to a charge disproportionated (CD) system in addition to the presence of a new metallic phase. We argue that three parameters determine the underlying physics of the BD-CD crossover when electron correlation effects are small: the hybridization between O-2$p_σ$ and Bi-6$s$ orbitals ($t_{spσ}$), the charge-transfer energy between Bi-6$s$ and O-a$_{1g}$ molecular orbitals ($Δ$), and the width of the oxygen sublattice band ($W$). In the BD system, we estimate an effective attractive interaction $U$ between holes on the same O-a$_{1g}$ molecular orbital. Although here we concentrate on the example of the bismuthates, the basic ideas can be directly transferred to other perovskites with negative charge-transfer energy, like ReNiO$_{3}$ (Re: rare-earth element), Ca(Sr)FeO$_{3}$, CsTIF$_{3}$ and CsTlCl$_{3}$.

cond-mat.mtrl-sci↗

arXiv2018

Oxygen holes and hybridization in the bismuthates

Motivated by the recently renewed interest in the superconducting bismuth perovskites, we investigate the electronic structure of the parent compounds ABiO$_{3}$ (A= Sr, Ba) using $ab$ $initio$ methods and tight-binding (TB) modeling. We use the density functional theory (DFT) in the local density approximation (LDA) to understand the role of various interactions in shaping the ABiO$_{3}$ bandstructure near the Fermi level. It is established that interatomic hybridization involving Bi-$6s$ and O-$2p$ orbitals plays the most important role. Based on our DFT calculations, we derive a minimal TB model and demonstrate that it can describe the properties of the bandstructure as a function of lattice distortions, such as the opening of a charge gap with the onset of the breathing distortion and the associated condensation of holes onto $a_{1g}$-symmetric molecular orbitals formed by the O-$2p_σ$ orbitals on collapsed octahedra. We also derive a single band model involving the hopping of an extended molecular orbital involving both Bi-$6s$ and a linear combination of six O-$2p$ orbitals which provides a very good description of the dispersion and band gaps of the low energy scale bands straddling the chemical potential.

cond-mat.mtrl-sci↗

arXiv2017

Correlation induced electron-hole asymmetry in quasi-2D iridates

We determine the motion of a charge (hole or electron) added to the Mott insulating, antiferromagnetic (AF) ground-state of quasi-2D iridates such as Ba 2 IrO 4 or Sr 2 IrO 4 . We show that correlation effects, calculated within the self-consistent Born approximation, render the hole and electron case very different. An added electron forms a spin-polaron, which closely resembles the well-known cuprates, but the situation of a removed electron is far more complex. Many-body 5d 4 configurations form which can be singlet and triplets of total angular momentum J and strongly affect the hole motion between AF sublattices. This not only has important ramifications for the interpretation of (inverse-)photoemission experiments of quasi-2D iridates but also demonstrates that the correlation physics in electron- and hole-doped iridates is fundamentally different.

cond-mat.str-el↗

arXiv2016

Tracking local magnetic dynamics via high-energy charge excitations in a relativistic Mott insulator

We use time- and energy-resolved optical spectroscopy to investigate the coupling of electron-hole excitations to the magnetic environment in the relativistic Mott insulator Na$_2$IrO$_3$. We show that, on the picosecond timescale, the photoinjected electron-hole pairs delocalize on the hexagons of the Ir lattice via the formation of quasi-molecular orbital (QMO) excitations and the exchange of energy with the short-range-ordered zig-zag magnetic background. The possibility of mapping the magnetic dynamics, which is characterized by typical frequencies in the THz range, onto high-energy (1-2 eV) charge excitations provides a new platform to investigate, and possibly control, the dynamics of magnetic interactions in correlated materials with strong spin-orbit coupling, even in the presence of complex magnetic phases.

cond-mat.str-el↗

arXiv2014

Low-temperature structural investigations of the frustrated quantum antiferromagnets Cs2CuCl(4-x)Br(x)

Powder X-ray diffraction (PXRD) and single-crystal neutron scattering were used to study in detail the structural properties of the Cs2CuCl(4-x)Br(x) series, good realizations of layered triangular antiferromagnets. Detailed temperature-dependent PXRD reveal a pronounced anisotropy of the thermal expansion for the three different crystal directions of the orthorhombic structure without any structural phase transition down to 20 K. Remarkably, the anisotropy of the thermal expansion varies for different $x$, leading to distinct changes of the geometry of the local Cu environment as a function of temperature and composition. The refinement of the atomic positions confirms that for x=1 and 2, the Br atoms occupy distinct halogen sites in the [CuX4]-tetrahedra (X = Cl, Br). The precise structure data are used to calculate the magnetic exchange couplings using density functional methods for x=0. We observe a pronounced temperature dependence of the calculated magnetic exchange couplings, reflected in the strong sensitivity of the magnetic exchange couplings on structural details. These calculations are in good agreement with the experimentally established values for Cs2CuCl4 if one takes the low-temperature structure data as a starting point.

cond-mat.str-el↗

arXiv2014

Analysis of the optical conductivity for A2IrO3 (A = Na, Li) from first principles

We present results for the optical conductivity of Na2IrO3 within density functional theory by including spin-orbit (SO) and correlation effects (U) as implemented in GGA+SO+U. We identify the various interband transitions and show that the underlying quasi-molecular-orbital nature of the electronic structure in Na2IrO3 translates into distinct features in the optical conductivity. Most importantly, the parity of the quasi-molecular orbitals appears to be the main factor in determining strong and weak optical transitions. We also present optical conductivity calculations for Li2IrO3 and discuss the similarities and differences with Na2IrO3.

cond-mat.str-el↗

arXiv2014

Hybridization effects and bond-disproportionation in the bismuth perovskites

We propose a microscopic description of the bond-disproportionated insulating state in the bismuth perovskites $X$BiO$_3$ ($X$=Ba, Sr) that recognizes the bismuth-oxygen hybridization as a dominant energy scale. It is demonstrated using electronic structure methods that the breathing distortion is accompanied by spatial condensation of hole pairs into local, molecular-like orbitals of the $A_{1g}$ symmetry composed of O-$2p_σ$ and Bi-$6s$ atomic orbitals of collapsed BiO$_6$ octahedra. Primary importance of oxygen $p$-states is thus revealed, in contrast to a popular picture of a purely ionic Bi$^{3+}$/Bi$^{5+}$ charge-disproportionation. Octahedra tilting is shown to enhance the breathing instability by means of a non-uniform band-narrowing. We argue that formation of localized states upon breathing distortion is, to a large extent, a property of the oxygen sublattice and expect similar hybridization effects in other perovskites involving formally high oxidation state cations.

cond-mat.mtrl-sci↗

arXiv2014

Ab initio quantum Monte Carlo calculations of spin superexchange in cuprates: the benchmarking case of Ca$_2$CuO$_3$

In view of the continuous theoretical efforts aimed at an accurate microscopic description of the strongly correlated transition metal oxides and related materials, we show that with continuum quantum Monte Carlo (QMC) calculations it is possible to obtain the value of the spin superexchange coupling constant of a copper oxide in a quantitatively excellent agreement with experiment. The variational nature of the QMC total energy allows us to identify the best trial wave function out of the available pool of wave functions, which makes the approach essentially free from adjustable parameters and thus truly ab initio. The present results on magnetic interactions suggest that QMC is capable of accurately describing ground state properties of strongly correlated materials.

cond-mat.str-el↗