arXiv subjects

Ilya Elfimov

Publications and source records attributed to Ilya Elfimov.

18 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

Percolative Pathway to Stripe Order in KTaO3-Based Superconductivity

The sensitivity of low dimensional superconductors to fluctuations gives rise to emergent behaviors beyond the conventional Bardeen Cooper Schrieffer framework. Anisotropy is one such manifestation, often linked to spatially modulated electronic states and unconventional pairing mechanisms. Pronounced in plane anisotropy recently reported at KTaO3 based oxide interfaces points to the emergence of a stripe order in superconducting phase, yet its microscopic origin and formation pathway remain unresolved. Here, we show that controlled interfacial disorder in MgO/KTaO3(111) heterostructures drives a percolative evolution from localized Cooper-pair islands to superconducting puddles and eventually to stripes. The extracted stripe width matches the spin precession length, suggesting a self organized modulation governed by spin orbit coupling and lattice-symmetry breaking. These findings identify disorder as both a tuning parameter and a diagnostic probe for emergent superconductivity in two dimensional quantum materials.

cond-mat.supr-con↗

arXiv2024

Critical Role of Disorder for Superconductivity in the Series of Epitaxial Ti(O,N) Films

Realizing experimental control of superconductivity is of paramount importance to advancing both basic research and technological applications. Disorder, generally existing in most superconductors, intricately interacts with Cooper pairs and also impacts the performance of quantum devices. In this paper, we report the study of a series of Ti(O,N) crystalline films prepared via molecular beam epitaxy (MBE). We discover that substituting nitrogen (N) for oxygen (O) in TiO, namely TiO(N), considerably increases the normal-state conductivity and the superconducting transition temperature Tc. The Tc of TiO(N) falling between those of TiO (about 0.5 K) and TiN (about 6 K) is contrary to their comparable Tc predicted by the Migdal Eliasberg theory. It is found that their resistivity vs temperature obeys the Mooij rule, known as the characteristic of metallic glasses. Density functional theory (DFT) calculations demonstrate that strong disorder severely scatters the Bloch electron waves at nonzero momenta, which consequently weakens electron-phonon coupling in TiO(N).

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↗

arXiv2024

Stabilization of U 5$f^2$ configuration in UTe$_2$ through U 6d dimers in the presence of Te2 chains

We investigate the topological superconductor candidate UTe$_2$ using high-resolution valence-band resonant inelastic x-ray scattering at the U $M_{4,5}$-edges. We observe atomic-like low-energy excitations that support the correlated nature of this unconventional superconductor. These excitations originate from the U $5f^2$ configuration, which is unexpected since the short Te2-Te2 distances exclude Te2 being 2-. By utilizing the photoionization cross-section dependence of the photoemission spectra in combination with band structure calculations, we infer that the stabilization of the U $5f^2$ configuration is due to the U $6d$ bonding states in the U-dimers acting as a charge reservoir. Our results emphasize that the description of the physical properties should commence with a $5f^2$ $ansatz$.

cond-mat.str-el↗

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↗

arXiv2022

Suppression of Peierls-like, nesting-based instabilities in solids

The understanding of lattice instabilities is of vast importance in material science. The famous example is the Peierls instability of one-dimensional metals and for strongly-nested Fermi surfaces in two and three dimensions. Through an analysis of H and Li chains in band theory, we find that the Bloch wave nature of the wavefunctions, if involving strong k-dependent hybridization of oppositeparity atomic states, strongly suppresses susceptibility peaks and associated instabilities and is thus essential to consider in searching for materials with strong responses to external perturbations.

cond-mat.other↗

arXiv2022

Modulation doping of the FeSe monolayer on SrTiO$_3$

The discovery of higher-temperature superconductivity in FeSe monolayers on SrTiO$_3$ (STO) substrates has sparked a surge of interest in the interface superconductivity. One point of the agreement reached to date is that modulation doping by impurities in the substrate is critical for the enhanced superconductivity. Remarkably, the universal doping of about 0.1 electrons per Fe, \textit{i.e.}, so-called ``magic'' doping, has been observed on a range of Ti oxide substrates, which concludes that there likely is some important interaction limiting the FeSe doping. Our study discovers that the polarization change at the interface Se because of the close proximity to the substrate from that in the free-standing FeSe film significantly amplifies the total potential difference at the interface above and beyond the work function difference for charge transfer. Additionally, the titanate substrate with a large number of free electrons basically serves as an ``infinite'' charge reservoir, which leads to the saturated FeSe doping with the complete removal of interface potential gradient. Our work has developed the theory for modulation doping in the Van der Waals materials/oxides heterostructure, providing a solution to the puzzle of ``magic'' doping in FeSe monolayers on titanates. The information also presents experimental pathways to accommodate a variable carrier density of FeSe monolayers via modulation doping.

cond-mat.supr-con↗

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↗

arXiv2020

High-temperature topological superconductivity in twisted double layer copper oxides

A great variety of novel phenomena occur when two-dimensional materials, such as graphene or transition metal dichalcogenides, are assembled into bilayers with a twist between individual layers. As a new application of this paradigm, we consider structures composed of two monolayer-thin $d$-wave superconductors with a twist angle $θ$ that can be realized by mechanically exfoliating van der Waals-bonded high-$T_c$ copper oxide materials, such as Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. On the basis of symmetry arguments and detailed microscopic modelling, we predict that for a range of twist angles in the vicinity of $45^{\rm o}$, such bilayers form a robust, fully gapped topological phase with spontaneously broken time-reversal symmetry and protected chiral Majorana edge modes. When $θ\approx 45^{\rm o}$, the topological phase sets in at temperatures close to the bulk $T_c\simeq 90$ K, thus furnishing a long sought realization of a true high-temperature topological superconductor.

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↗

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↗

arXiv2014

Large electropositive cations as surfactants for the growth of polar epitaxial films

Using density functional theory (DFT) we demonstrate that the adsorption of large cations such as potassium or cesium facilitates the epitaxial growth of polar LaAlO$_3$ (LAO) on SrTiO$_3$ (STO). The low ionization potential of K favors efficient electron transfer to the STO conduction band and results in a 2D electron gas which exactly compensates for the diverging potential with increasing layer thickness. For large cations like K or Cs, DFT total energy considerations show that they remain adsorbed on the LAO surface and do not enter substitutionally into LAO. These results suggest a novel scheme for growing clean LAO/STO interface systems, and polar systems in general, by performing the growth process in the presence of large, low ionization potential alkali metal ions.

cond-mat.mtrl-sci↗

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↗

arXiv2013

Charge disproportionation without charge transfer in the rare-earth nickelates as a possible mechanism for the metal-insulator transition

We study a model for the metal-insulator (MI) transition in the rare-earth nickelates RNiO$_3$, based upon a negative charge transfer energy and coupling to a rock-salt like lattice distortion of the NiO$_6$ octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with ground state of predominantly $d^8\ligand$ character, where $\ligand$ denotes a ligand hole. For sufficiently large distortions ($δd_{\rm Ni-O} \sim 0.05 - 0.10Å$), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with $(d^8\ligand^2)_{S=0}(d^8)_{S=1}$ character, where $S$ is the total spin. Thus the MI transition may be viewed as being driven by an internal volume "collapse" where the NiO$_6$ octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the ($1/2,1/2,1/2$) superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of a new type of charge ordering achieved without any actual movement of the charge.

cond-mat.str-el↗

arXiv2008

Electronic polarons and bipolarons in Fe-based superconductors: a pairing mechanism

Superconductivity is a fascinating example of how "more is different". It is due to electrons binding into bosonic Cooper pairs, which exhibit coherent behavior across a macroscopic sample. Finding the mechanism responsible for this binding is one of the more difficult tasks of condensed matter physics. For conventional superconductors the solution was given by the BCS theory as being due to exchange of phonons. For the cuprate high-Tc superconductors a widely-accepted explanation is still missing despite intense effort. The recently discovered Fe-based high-Tc superconductors pose now a new challenge. We present here a quantum mechanical theory for pnictides describing the influence of the large electronic polarizability of the heavy anions. We demonstrate that its inclusion results in electronic polarons as the low-energy quasi-particles and also unveils a pairing mechanism for these electronic polarons.

cond-mat.supr-con↗