Dichotomous electronic system in a bilayer Ni₁+ nickelate

摘要

本文利用密度泛函理论计算了双层无限层镍酸盐La₃Ni₂O₅F的电子结构，揭示其具有理想二维特性且存在两种截然不同的准粒子行为。采用虚拟晶体近似处理氧/氟无序后，能带计算显示常规Ni dpσ带形成空穴型费米面，而源自间隙密度的E*带则构成圆柱状电子费米面，导致体系自掺杂0.18个电子。该间隙密度分布在无顶位氧的La层间，与Ni dxz/dyz带以平行线性色散趋近M点，耦合形成近非解析的狄拉克点，表现出奇异的间隙-轨道带耦合效应。同时，dz2带通过与间隙密度相互作用产生约1 eV的对称性劈裂。这种电子与空穴行为的二相性将主导正常态输运和远红外特性，并可能影响镍酸盐的超导态，为理解无限层镍酸盐物理提供了新的视角。

材料

• La₃Ni₂O₅F

方法

• DFT
• full-potential linearized augmented plane wave (FP-LAPW) using WIEN₂k
• virtual crystal approximation

关键词

• interstitial density
• electron fermi surface
• self doping
• dichotomous quasiparticles
• dirac point
• bilayer nickelate
• electride
• network of valence bands

亮点

• La₃Ni₂O₅F is an infinite-layer nickelate with formal Ni¹⁺ and no apical oxygen, providing ideal two-dimensional character and a new structural entry into the class of superconducting nickelates.
• The E* band is not derived from atomic orbitals but from interstitial density that extends across three La-apical layers, making it fundamentally different from the conventional dpσ band.
• The material intrinsically hosts two types of quasiparticle Fermi surfaces with opposite signs, constituting a natural two-gap system where the E* electron band may short-circuit the bad-metal resistivity of the dpσ hole band.
• The incipient Dirac point between the interstitial E* and Ni dxz,dyz D* bands represents an unusual type of coupling between delocalized interstitial states and d bands, with sensitivity to chemical doping and strain.

结论

• La₃Ni₂O₅F contains a partially occupied E* band formed by interstitial electron density, creating cylindrical electron Fermi surfaces that result in approximately 0.18 electrons of self-doping, pushing the dpσ hole band away from half-filling and suppressing magnetic ordering.
• The interstitial density couples with Ni dxz,dyz orbitals to form a network of valence bands; near the M point, the E* and D* bands disperse in parallel with identical velocity, constituting an incipient non-analytic Dirac point.
• The system exhibits a dichotomy of hole-like dpσ quasiparticles and electron-like E* quasiparticles, with distinct relaxation times and Fermi surface properties, which will lead to dual behavior in normal state transport and far-IR optical properties.
• Increasing the F fraction or applying compressive strain drives the E* and D* bands to touch at a Dirac degeneracy, hinting at topological character and potential impact on superconductivity.

主要论断

• La₃Ni₂O₅F hosts a partially occupied E* band from interstitial electron density, forming a cylindrical electron Fermi surface that donates ≈0.18 electrons to the dpσ band, resulting in self-doping away from half-filling.
  • 证据: The E* band… dips from 1.5-2 eV above EF to -0.5 eV at M… forms an electron cylinder around M enclosing 0.18 electrons. As a result, the dpσ bands are not half-filled, with each Ni ion valence becoming +1.09.
• The E* band and the Ni dxz/yz derived D* band approach degeneracy at M, forming an incipient non-analytic Dirac point that can be realized by increasing F fraction (≈0.70) or compressive strain.
  • 证据: the E* and D* bands at M close up and touch at an F fraction of 0.700-0.705… this 'accidental' degeneracy signals a non-analytic eigensystem with corresponding topological character.
• The two-component electronic system will exhibit opposite-sign quasiparticle contributions in transport, with the E* carriers likely having longer relaxation time and possibly dominating the conductivity.
  • 证据: The dichotomous electronic structure of La₃Ni₂O₅F involves carriers of opposite sign… dpσ excitations likely will have a short relaxation time… compared to… E* carriers with anticipated longer τE… the E* FS may contribute strongly to the conductivity.
• In the superconducting state, the dpσ Fermi surface will experience magnetic fluctuation pairing while the E* Fermi surface is only indirectly coupled, likely resulting in two isotropic gaps and an extreme two-gap system.
  • 证据: The superconducting state should show two isotropic superconducting gaps… only the dpσ FS will experience the magnetic fluctuations… the E* FS is at most indirectly coupled to the Ni moment… This picture suggests an ideal, and perhaps extreme, two-gap system.

研究流程

• Structure construction with virtual crystal approximation — The material is an ideal two-dimensional nickelate with Ni₁+ formal valence and isolated bilayer.
  • 材料: La₃Ni₂O₅F
  • 方法: Virtual crystal average for O/F disorder
  • 观察: Ideal two-dimensional infinite-layer structure; No apical oxygen
• DFT electronic structure calculation — Electronic bands reliably obtained, revealing an interstitial band E* not associated with any atom.
  • 方法: First-principles calculations using WIEN₂k (APW+lo)
  • 观察: Band structure computed; Interstitial density identified
• Band structure and Fermi surface analysis — The electronic structure is dichotomous, with a planewave-like interstitial E* band and a correlated dpσ band; the E* band self-dopes the system and couples with d bands to form an incipient Dirac point.
  • 方法: Orbital projection; Fatband analysis; Isosurface plots; Fermi surface mapping
  • 观察: E* band forms electron cylinder around M enclosing 0.18 e; dpσ bands form hole cylinder; Self-doping of Ni valence to +1.09; D* dxz/yz band and E* band approach degeneracy at M; Incipient Dirac point under composition or strain tuning
• Transport and superconducting property predictions — The dichotomous carriers will lead to distinct normal-state transport and far-IR signatures, and an extreme two-gap superconducting state.
  • 方法: Bloch-Boltzmann theory; Relaxation time approximation; Model for Hall and optical conductivity
  • 观察: E* carriers expected to have longer relaxation time; Ma_y short-circuit bad-metal dpσ resistivity; Predicted two-gap superconductivity with isotropic gaps; E* FS only indirectly coupled to pairing