Electronic structure trends in La₂RNi₂O₇ (R= Pr, Nd, Sm) from first-principles

摘要

本文采用第一性原理DFT+U方法，研究了不同稀土元素（Pr、Nd、Sm）掺杂La₃Ni₂O₇的晶体结构和电子性质趋势。计算表明，掺杂原子优先占据岩盐层中的La位，随掺杂离子半径减小（Pr→Sm），化学压力效应使晶胞体积单调递减，单斜到四方的结构转变压力依次升高，且该转变与超导电性的出现基本吻合，与实验观测一致。在高压四方相中，dz²能带平坦化并穿越费米能级，出现dz²特征的空穴型费米面，被视为超导电性的关键电子标志。随稀土离子尺寸减小，面内跳跃积分增强，而由于顶端Ni-O键长缩短，面外跳跃积分反而减弱。这些发现为理解稀土掺杂对双层Ruddlesden-Popper镍酸盐电子结构的影响及其与超导转变温度的关联提供了微观机理见解。

材料

• La₃Ni₂O₇
• La₂PrNi₂O₇
• La₂NdNi₂O₇
• La₂SmNi₂O₇

方法

• DFT
• DFT+U (GGA+U)
• Wannier function analysis
• first-principles structural optimization
• enthalpy comparison

关键词

• chemical pressure
• monoclinic to tetragonal structural transition
• flat dz2 band
• hole pocket
• in plane hopping
• out of plane hopping
• apical ni o bond length

亮点

• The doped atoms preferentially occupy the La sites in the rocksalt layer, confirmed by enthalpy comparison.
• Despite c-axis lattice constant compression, out-of-plane hopping decreases due to a reduction in the apical Ni-O bond length in the rocksalt layer, challenging the expectation of enhanced interlayer coupling.
• The emergence of flat dz2 bands at the Fermi level and corresponding hole pockets is a universal electronic signature of the superconducting phase across all compounds.
• The increase of T_c with chemical pressure correlates with enhanced in-plane hopping and hybridization rather than increased interlayer coupling.

结论

• All compounds undergo a pressure-driven structural transition from monoclinic to tetragonal phase, with the critical pressure increasing as the rare-earth size decreases.
• At high pressure, flat dz2 bands of character emerge at the Fermi level, similar to hydrostatic pressure.
• The dominant out-of-plane hopping decreases upon chemical pressure, associated with a decrease in the apical Ni-O bond length to the rocksalt layer.
• The dominant planar hopping increases upon chemical pressure.
• The results emphasize the importance of hybridization effects between out-of-plane dz2 orbitals and itinerant planar dx2-y2 states for superconductivity in this family.

主要论断

• Substitution of rare-earth elements (Pr, Nd, Sm) into La₃Ni₂O₇ occurs preferentially at the rock-salt layer La sites.
  • 证据: Enthalpy comparison (Appendix A) shows rock-salt substitution energetically favorable,Agrees with experimental X-ray diffraction data (Refs [44,24])
• The pressure-induced structural transition from monoclinic to tetragonal phase coincides with the emergence of superconductivity, and the transition pressure increases with decreasing R ionic radius.
  • 证据: Fig. 1(b) enthalpy vs pressure curves: transition pressures increase from 9.5 to 20.5 GPa,Qualitative agreement with experimental pressures (Pr: 11 GPa, Nd: 13 GPa, Sm: 18 GPa),All compounds become tetragonal around the superconducting pressure range
• At high pressure, flat dz2 bands cross the Fermi level forming extra hole pockets, a key electronic signature for superconductivity; chemical pressure enhances in-plane hopping and hybridization while reducing out-of-plane hopping.
  • 证据: Fig. 3(c,d): band structures at 21 GPa show flat dz2 bonding band at E_F,Fermi surfaces (Fig. 3 lower panels) show five pockets including dz2 hole pockets,Fig. 4(a,b): t_perp decreases, t_planar and hybridization increase with smaller R,Apical Ni-O_rocksalt bond length decreases (Appendix D)
• The enhancement of T_c in Sm-doped La₃Ni₂O₇ is likely driven by increased planar hybridization and hopping rather than out-of-plane coupling.
  • 证据: T_c increases from ≈80 K (undoped) to 96 K (Sm-doped) experimentally,Calculated t_perp decreases while t_planar and hybridization increase with decreasing R size,Hybridization between dz2 and dx2-y2 states is argued to be important for superconductivity

研究流程

• Structural optimization and phase transition analysis — Chemical pressure from smaller R ions increases the monoclinic-to-tetragonal transition pressure, aligning with the onset of superconductivity.
  • 材料: QUANTUM-ESPRESSO; GGA+U (U=8 eV); Experimental crystal structures (P₂₁/n, I₄/mmm); Plane-wave basis (80R_y cutoff)
  • 方法: Enthalpy difference comparison for substitution site preference; Full geometry optimization (forces < 10-4Ry/Bohr); Enthalpy vs pressure calculations to determine transition pressure
  • 观察: Dopants prefer rock-salt-layer La sites (energetically favorable); Transition pressures: 9.5 GPa (La), 14 GPa (Pr), 17 GPa (Nd), 20.5 GPa (Sm); c-axis and volume decrease with smaller R; apical Ni-O-Ni bond angle decreases
• Electronic band structure and Fermi surface calculations — The emergence of flat dz2 hole pockets at high pressure is a robust electronic hallmark of superconductivity across all compounds.
  • 材料: DFT with same setup; FermiSurfer for visualization
  • 方法: Nonmagnetic band structure calculation; Orbital-resolved Fermi surface analysis
  • 观察: At ambient: Ni-dx2-y2 and dz2 bands near E_F, two Fermi sheets (electron α, hole β); At₂₁ GPa: flat dz2 bonding band crosses E_F, five Fermi pockets including extra hole pockets γ,δ of dz2 character; Subtle band shifts and pocket size changes with R size
• Wannier function analysis and interpretation — The enhancement of T_c upon rare-earth doping is correlated with increased planar hopping and hybridization, emphasizing the role of dz2–dx2-y2 hybridization in superconductivity.
  • 材料: Wannier90; Projection onto Ni-3d orbitals
  • 方法: Construction of maximally localized Wannier functions to extract hopping integrals; Correlation of hopping trends with bond lengths
  • 观察: Out-of-plane hopping t_perp decreases (from La to Sm) due to shortening of apical Ni-O_rocksalt bond; In-plane hopping t_planar increases; Hybridization between dz2 and dx2-y2 increases