arXiv subjects

I. V. Leonov

Publications and source records attributed to I. V. Leonov.

10 recordsLinked to original sources

arXiv2026

Electronic structure and magnetic correlations in the epitaxially strained bilayer nickelate La$_3$Ni$_2$O$_{7}$

Using the DFT+dynamical mean-field theory method we study the effects of electron-electron correlations and epitaxial strain of the crystal structure on the normal-state electronic structure, quasiparticle band renormalizations, Fermi surface, and magnetic correlations of the bilayer Ruddlesden-Popper nickelate La$_3$Ni$_2$O$_7$ (LNO). Our results exhibit a remarkable orbital-selective renormalization and strong incoherence of the Ni $3d$ bands, pointing to the proximity of the Ni $x^2-y^2$ and $3z^2-r^2$ states to orbital-selective localization. The electronic properties of LNO show a high sensitivity to the in-plane strain. We note that both a tensile and a moderate compressive strain (up to about $-2$\%) yield a significant enhancement of magnetic correlations compared to the unstrained LNO. Under a large compressive strain of about $-4$\%, we observe a Lifshitz transition characterized by the disappearance of the $γ$ Fermi surface sheet, which is associated with a nearly fully occupied, shallow flat-band of the bonding Ni $3z^2-r^2$ orbital character. As a result, we observe a sharp decrease of magnetic correlations, implying suppression of superconductivity. Overall, our results support the picture of spin- and change-density-wave stripe instability driven by the Fermi surface nesting in LNO. Our results suggest that both pressure and strain can effectively tune (suppress or enhance) spin-change-density-wave ordering, giving rise to enhanced spin fluctuations.

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arXiv2026

Electronic structure, quasiparticle renormalizations, and magnetic correlations in the alternating single-layer bilayer nickelate La$_5$Ni$_3$O$_{11}$

Using DFT+DMFT we study the normal-state electronic structure and magnetic correlations of the recently discovered alternating single-layer bilayer Raddlesden-Popper nickelate La$_5$Ni$_3$O$_{11}$ (1212-LNO). Our results exhibit qualitative differences for the structurally distinct single-layer and bilayer Ni ions, implying the importance of confinement and orbital-dependent correlations. The Ni $e_g$ electronic states originating from the bilayer Ni ions form strongly renormalized quasiparticle bands with a large enhancement factor $m^*/m \sim 3.5$ and 4.2 for the Ni $x^2-y^2$ and $3z^2-r^2$ orbitals, respectively. Moreover, the $e_g$ states of the single-layer Ni ions exhibit an orbital-selective Mott insulating state, with a narrow energy gap for the Ni $3z^2-r^2$ states and metallic, strongly incoherent (non-Fermi-liquid) Ni $x^2-y^2$ ones. Our analysis of magnetic correlations suggests the formation of intertwined spin and charge density wave stripes in the bilayer NiO$_6$ slab, in close similarity to the double-layer material. We refine two major instabilities, a leading one is associated with a wave vector ${\bf Q}=(\frac{1}{3},\frac{1}{3})$ (``up-down-0" spin pattern), competing with the bicollinear $(\frac{1}{4},\frac{1}{4})$ (``up-up-down-down") stripe state. The single-layer Ni $3d$ electrons exhibit instability towards the Néel-type magnetic state. Under pressure, 1212-LNO undergoes an orbital-selective Mott insulator-to-metal phase transition, associated with metallization of the single-layer Ni $e_g$ states. As a result, the single-layer Ni $e_g$ bands exhibit non-Fermi-liquid (bad metal) behavior with strongly incoherent spectral weights near $E_F$. We note that correlation effects result in a reconstruction of magnetic correlations as compared to that obtained within DFT. In fact, we observe a crossover from single-layer to double-layer dominated magnetic correlations.

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arXiv2026

Effect of doping on the electronic structure, orbital-dependent renormalizations, and magnetic correlations in bilayer La$_3$Ni$_2$O$_7$

Using the DFT+dynamical mean-field theory approach we study the effects of electronic correlations and doping on the normal state electronic structure of the double-layer nickelate superconductor La$_3$Ni$_2$O$_7$ under pressure. In agreement with experiments, we obtain significant orbital-dependent quasiparticle renormalizations of the Ni $x^2-y^2$ and $3z^2-r^2$ bands, accompanied by incoherence (bad metal behavior) of the $3z^2-r^2$ states, caused by the proximity of the Ni $3d$ states to orbital-dependent localization. Our results demonstrate a sensitive, non-monotonic dependence of $m^*/m$ on doping, with a remarkable, by about 20\%, increase for the Ni $x^2-y^2$ orbitals upon electron doping $x \sim 0.2$ (per Ni ion), implying a significant enhancement of orbital-dependent correlations with oxygen deficiency in LNO. We observe a reconstruction of the low-energy electronic structure of LNO upon doping above $x\sim -0.3$ and 0.2. It is associated with the Lifshitz transition, with a crossover to a self-doping regime characterized by partial occupation of the La $5d$ bands (upon an electron doping $x>0.2$). Our analysis of the static magnetic susceptibility $χ({\bf q})$ suggests the possible formation of the spin and charge density wave stripes, implying strong spin and charge correlations in LNO. We show that this behavor is associated with suppression of the Néel AFM ordering of the Ni$^{2+}$ ions upon hole doping. Interestingly, upon a moderate electron doping of the Ni$^{2.5+}$ ions, we find a significant enhancement of the strength of in-plane spin fluctuations. We note a close resembles of our results to those for the bilayer Hubbard model, which shows the boosting of superconductivity as one of the two electron bands approaches the Lifshitz transition. Our results suggest that spin and charge stripe fluctuations play a key role in pressure-driven superconductivity in LNO.

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arXiv2024

Electronic correlations and spin-charge-density stripes in double-layer La$_3$Ni$_2$O$_7$

Using \emph{ab initio} band structure and DFT+dynamical mean-field theory methods we examine the effects of electron-electron interactions on the electronic structure, magnetic state, and structural phase stability of the recently discovered double-layer perovskite superconductor La$_3$Ni$_2$O$_7$ (LNO) under pressure. Our results show the emergence of a double spin-charge-density stripe state characterized by a wave vector ${\bf q}=(\frac{1}{4},\frac{1}{4})$ arrangement of the nominally high-spin Ni$^{2+}_\mathrm{A}$ and low-spin Ni$^{3+}_\mathrm{B}$ ions (diagonal hole stripes oriented at $45^\circ$ to the Ni-O bond) which form zigzag ferromagnetic chains alternating in the $ab$ plane. The phase transition is accompanied by cooperative breathing-mode distortions of the lattice structure and leads to a reconstruction of the low-energy electronic structure and magnetic properties of LNO. We obtain a narrow-gap correlated insulator with a band gap value of $\sim$0.2 eV characterized by strong localization of the Ni $3d$ states and significant spin-orbital polarizations of the charge deficient Ni$^{3+}_\mathrm{B}$ ions. Our results suggest the importance of double exchange to determine the magnetic properties of LNO, similarly to that in charge-ordered manganites. We propose that spin and charge stripe fluctuations play an important role to tune superconductivity in LNO under pressure.

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arXiv2024

Electronic correlations and long-range magnetic ordering in NiO tuned by pressure

Using the DFT+dynamical mean-field theory method we revisit the pressure-temperature phase diagram of the prototypical correlated insulator NiO. We study the pressure-induced evolution of the electronic structure, magnetic state, and exchange couplings of the antiferromagnetic phase of NiO. We calculate the ordered magnetic moments and uniform spin susceptibility of the Ni $3d$ states of NiO, which allow us to determine the pressure-dependence of the Néel temperature $T_N$. We note that the long-range magnetism has no significant effects on the valence band photoemission spectra of NiO under moderate compressions, implying the importance of correlations effects to explain the insulating state of NiO. The antiferromagnetic insulating state is found to be stable up to the high compression value $\sim$0.4 $V_0$ (assuming the cubic $B1$ crystal structure of NiO), and is associated with a (correlated-assisted) Slater insulating state driven by the long-range magnetic ordering. In fact, the paramagnetic phase of NiO at such high compression is found to be metallic, implying delocalization of the Ni $3d$ states. The calculated $T_N$ exhibits a non-monotonic behavior upon compression, with a maximum associated with the crossover from Mott localized (strong coupling) to itinerant moment regimes, in qualitative agreement with the phase diagram of the half-filled single-band Hubbard model. We point out the importance of the non-local correlation effects to explain the magnetic properties of NiO.

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arXiv2024

Interplay of electronic correlations and chemical bonding in FeN$_2$ under pressure

We report a theoretical study of the effects of electronic correlations, magnetic properties, and chemical bonding in the recently synthesized high-pressure orthorhombic phase of FeN$_2$ using the DFT+dynamical mean-field theory approach. Our analysis documents a complex crystal-chemical behavior of FeN$_2$ characterized by the formation of a strongly covalent N-N bond with an unexpected valence state of Fe ions $3+$ (paramagnetic ferric Fe$^{3+}$ ions in the low-spin state), in agreement with available experimental data. Our results reveal weak (orbital-dependent) correlation effects, which are complicated by the possible emergence of multiple spin density wave states on a microscopic level. This suggests the importance of antiferromagnetic spin fluctuations to explain the properties of FeN$_2$ under pressure.

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arXiv2024

Electronic structure and magnetic correlations in trilayer nickelate superconductor La$_4$Ni$_3$O$_{10}$ under pressure

It has been recently shown that under pressure trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$ (LNO) becomes superconducting below a critical temperature $\sim$20 K, in addition to the infinite-layer and bilayer systems. Motivated by this observation, we explore the effects of electron correlations on its electronic structure and magnetic properties using the advanced DFT+dynamical mean-field theory approach. Our results for the normal-state electronic structure and correlation effects in LNO have much in common with the infinite-layer and bilayer nickelates, with a remarkable site- and orbital-dependent renormalizations of the Ni $3d$ bands and notable incoherence of the Ni $d_{3z^2-r^2}$ states, caused by correlation effects. Our analysis of the Fermi surface and magnetic correlations suggests the emergence of competing spin and charge stripe states, implying the importance of in-plane spin fluctuations to explain superconductivity in this material.

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arXiv2023

Interplay between electron correlations, magnetic state, and structural confinement in LaNiO3 ultrathin films

We report a theoretical study of the effects of electron correlations and structural confinement on the electronic properties and magnetic state of LaNiO3 (LNO) thin films epitaxially deposited on the (001) LaAlO3 (LAO) substrate. Using the DFT+U method we compute the electronic band structure, magnetic properties, and phase stability of the 1.5 unit-cell-thick NiO2-terminated LNO thin films. Our results reveal complex diversity of the electronic states caused by the effects of structural confinement, interfacial charge transfer and electronic correlations. Our calculations suggest the appearance of in-plane (110) charge disproportionation of the Ni ions in the interface NiO2 layer of the antiferromagnetically ordered LNO thin films. Moreover, the electronic states of both the AFM and FM LNO/LAO show a large orbital polarization of the Ni ions in the surface NiO2 layers. We propose the crucial importance of oxygen defects to explain the metal-to-insulator phase transition experimentally observed in a few-unit-cell-thick LNO/LAO thin films.

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arXiv2023

Correlated electronic structure, orbital-selective behavior, and magnetic correlations in double-layer La$_3$Ni$_2$O$_7$ under pressure

Using \emph{ab initio} band structure and DFT+dynamical mean-field theory methods we examine the effects of electron-electron interactions on the normal state electronic structure, Fermi surface, and magnetic correlations of the recently discovered double-layer perovskite superconductor La$_3$Ni$_2$O$_7$ under pressure. Our results suggest the formation of a negative charge transfer mixed-valence state with the Ni valence close to 1.75+. We find a remarkable orbital-selective renormalization of the Ni $3d$ bands, with $m^*/m \sim 3$ and 2.3 for the Ni $3z^2-r^2$ and $x^2-y^2$ orbitals, respectively, in agreement with experimental estimates. Our results for the {\bf k}-dependent spectral functions and Fermi surfaces show significant incoherence of the Ni $3z^2-r^2$ states, implying the proximity of the Ni $3d$ states to orbital-dependent localization. Our analysis of the static magnetic susceptibility suggests the possible formation of the spin and charge (or bond) density wave stripe states.

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arXiv2022

Spin-density, charge- and bond-disproportionation wave instability in hole-doped infinite-layer $R$NiO$_2$

Using \emph{ab initio} band structure methods and DFT+dynamical mean-field theory approach we explore the possible formation of spin and charge stripes in the Ni-O plane of hole-doped infinite-layer nickelates, $R$NiO$_2$. Our results reveal a remarkable instability of the $C$-type $(110)$ spin state with undistorted lattice towards the formation of the spin-density, charge- and bond-disproportionation stripe phases accompanied by in-plane``breathing''-like distortions of the crystal structure. Our work gives a comprehensive picture of competing charge and spin stripe states, with possible frustration of different stripe patterns upon doping. It suggests that the spin and charge stripe state likely arises from strong magnetic correlations (with concomitant lattice distortions), which play a key role for understanding the anomalous properties of hole-doped layered nickelates.

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