Summary
This study employs first-principles density functional theory (GGA and GGA+U) to investigate the electronic and magnetic properties of La3Ni2O5F, which features a bilayer NiO2 infinite-layer structure where La(O/F)La blocking layers achieve strict isolation of the NiO2 bilayers, forming a purely two-dimensional electronic and magnetic system. Calculations reveal an E* single band composed of electron density in the interstitial region, which is not associated with any atomic orbital, dips below the Fermi level along the M-A direction, and provides a self-doping of 0.09 holes per Ni, resulting in an actual Ni valence of +1.09; the Fermi surface of this E* band is cylindrical, occupying 9% of the Brillouin zone area. The dpσ band is nearly half-filled but is shifted near a Van Hove singularity due to the self-doping, and the magnetic response exhibits anomalous characteristics distinct from previous nickelates, with the magnetic susceptibility tending to vanish under a large magnetic field. The absence of a magnetic phase transition can be attributed to strong two-dimensional spin fluctuations and the self-doping effect away from half-filling, revealing the unique behavior of Ni1+ ions in this system and the critical influence of the interfacial blocking layer on the interstitial band morphology.
Materials
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Methods
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Keywords
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Highlights
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Conclusions
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Main claims
- The interstitial E* band self-dopes the NiO2 layers, resulting in an effective Ni valence of +1.09.
- Evidence: Full text: '…the E* cylinder FS contains 9% of the zone area, giving an adjusted valence Ni1.09+.'
- The La(O/F)La blocking layer enforces perfect two-dimensional isolation of the electronic and magnetic systems.
- Evidence: Abstract: 'The blocking La(O/F)La provides isolation of the NiO2 bilayer and an interstitial E* density to strictly two-dimensional electronic and magnetic systems.',Full text: '…the bands in the neighborhood of EF are perfectly 2D (vanishing kz dispersion) to any physically relevant measure…'
- The magnetic susceptibility tends to vanish under a large magnetic field, and no magnetic phase transition is observed because of strong two-dimensional fluctuations and self-doping away from half-filling.
- Evidence: Abstract: 'Two dimensional fluctuations and self-doping away from half-filling can account for the lack of observation of a magnetic transition.',Full text: 'Up to M=0.7 μB there is no significant energy increase – FM moments (including long wavelength fluctuations) cost no energy… vanishing magnetic susceptibility…'
Workflow
- Computational details — Calculations are performed with all-electron full-potential DFT codes, accurately capturing the electronic structure and magnetic tendencies of La3Ni2O5F.
- Materials: La3Ni2O5F crystal structure (I4/mmm); GGA and GGA+U exchange-correlation functionals; wien2k and fplo all-electron codes
- Methods: DFT calculations using wien2k with RKmax=7.0 and carefully chosen atomic radii; supercell approach for AFM states (√2×√2×1); fixed spin moment (FSM) calculations to constrain total moment; tight-binding model for dpσ bands
- Observations: dpσ bands are essentially degenerate and nearly half-filled; interstitial E* band has a deep minimum at M and linear dispersion; extreme 2D character: negligible kz dispersion in all active bands
- Nonmagnetic electronic structure — The nonmagnetic band structure reveals an interstitial E* band that provides self-doping and enforces strictly 2D electronic character.
- Methods: GGA band structure and fatband analysis; Fermi surface visualization
- Observations: E* band is a single interstitial band not associated with any atomic orbital; E* band dips ≈0.5 eV below EF along M-A, forming a cylindrical Fermi surface occupying 9% of the Brillouin zone; self-doping of 0.09 holes per Ni, yielding formal Ni valence +1.09; dpσ Fermi surface is a nearly degenerate square with rounded corners; perfect 2D isolation due to La(O/F)La blocking layer
- Magnetic order analysis — Magnetic calculations reveal anomalous behavior: a metastable FM state with vanishing stiffness, and G-AFM as the ground state with strong in-plane coupling.
- Methods: GGA fixed spin moment (FSM) calculations; GGA self-consistent FM and AFM (G-AFM, C-AFM) calculations; energy comparison between magnetic states
- Observations: Energy nearly constant up to M=0.7 μB/f.u., indicating vanishing magnetic susceptibility; FM band structure shows no exchange splitting for the E* band; G-AFM exchange splitting of ≈2 eV mimics a Mott gap but is metallic due to E* band crossing; In-plane exchange coupling J∥ ≈ 118 meV; inter-bilayer J⊥ ≈ 4.5 meV
- Correlation effects and interpretation — Correlation effects and extreme two-dimensionality prevent magnetic ordering, explaining the absence of a magnetic phase transition and revealing a novel interplay between interstitial and Ni bands.
- Methods: GGA+U calculations with Ueff=3-5 eV; analysis of electronic structure evolution with U
- Observations: Hubbard U increases Ni moment toward 1 μB and reduces self-doping; E* band remains insensitive to magnetism; gap opens between dpσ majority band and E* band; Mermin-Wagner theorem implies no magnetic order due to strict 2D; self-doping places system near superconducting regime