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[论文解读] Van Hove singularity-induced multiple magnetic transitions in multi-orbital systems

Chen Lu, Lun-Hui Hu|arXiv (Cornell University)|Jan 31, 2026
Topological Materials and Phenomena被引用 0
一句话总结

论文表明多轨道系统中的 Van Hove 奇点稳定了各种 Q=0 磁序，包括本征的交替磁性，并通过 Hubbard U 与 Hund’s 耦合 JH 的 RPA 映射它们的转变。

ABSTRACT

Van Hove singularities (VHSs) amplify electronic correlations, providing a crucial platform for discovering novel quantum phase transitions. Here, we show that VHSs in multi-orbital systems can stabilize a variety of competing $\bm{Q}=0$ magnetic orders, including intrinsic altermagnetism emerging from spontaneous orbital antiferromagnetism. This intrinsic phase, in which antiparallel spins reside on distinct orbitals, is realized across all four 2D Bravais lattices. It is driven by orbital-resolved spin fluctuations enhanced by inter-orbital hopping and favors suppressed Hund's coupling $J_H$, strong inter-orbital hybridization, and filling near a VHS from quadratic band touching. Through Hubbard-$U$-$J_H$ phase diagrams we map several magnetic phase transitions: (i) ferrimagnet to $d$-wave extrinsic altermagnet, (ii) $d$-wave intrinsic altermagnet to ferromagnet, and (iii) $g$-wave extrinsic altermagnet to either $d$-wave extrinsic altermagnet or ferromagnet. Our work identifies VHSs as a generic route to altermagnetism in correlated materials.

研究动机与目标

  • Motivate exploration of how Van Hove singularities amplify electronic correlations to drive novel magnetic orders.
  • Investigate how orbital-selective saddle points in multi-orbital lattices stabilize Q=0 magnetic orders, including altermagnetism.
  • Map Hubbard U and Hund’s coupling JH phase diagrams to identify transitions among intrinsic/extrinsic altermagnetism, ferrimagnetism, ferromagnetism, and Néel AFM.
  • Demonstrate that VHSs near quadratic band touching generically favor altermagnetic states across 2D Bravais lattices.

提出的方法

  • Perform symmetry analysis of orbital antiferromagnetic orders and classify them under 2D lattice point groups.
  • Study a minimal two-orbital Hubbard model on a square lattice with two sublattices and two degenerate orbitals per site.
  • Use multi-orbital random-phase approximation (RPA) to compute bare and RPA-renormalized spin susceptibilities.
  • Evaluate six Q=0 magnetic order channels O1–O6 and their orbital-sublattice structure.
  • Analyze cases with identical and nonidentical sublattices (t1=t2 and t1≠t2) to distinguish intrinsic and extrinsic altermagnetism.
  • Identify leading instabilities via momentum-resolved χ^RPA(k) and extract Uc and dominant order near criticality.
Figure 1: Competing magnetic orders in a two-orbital square lattice model. (a) Schematic of the square lattice with two orbitals ( $d_{xz}$ , $d_{yz}$ ) and $A$ (orange) and $B$ (green) sublattices. Hopping parameters include intra-orbital terms ( $t_{0},t_{1},t_{2}$ ) and and inter-orbital terms (
Figure 1: Competing magnetic orders in a two-orbital square lattice model. (a) Schematic of the square lattice with two orbitals ( $d_{xz}$ , $d_{yz}$ ) and $A$ (orange) and $B$ (green) sublattices. Hopping parameters include intra-orbital terms ( $t_{0},t_{1},t_{2}$ ) and and inter-orbital terms (

实验结果

研究问题

  • RQ1Under what conditions do Van Hove singularities stabilize Q=0 altermagnetic orders in multi-orbital systems?
  • RQ2How do inter-orbital hopping and Hund’s coupling influence transitions among intrinsic and extrinsic altermagnetic, ferrimagnetic, ferromagnetic, and Néel orders?
  • RQ3Is intrinsic d-wave altermagnetism generically stabilized across 2D Bravais lattices near VHSs?
  • RQ4What role does VHS arising from quadratic Dirac band touching play in the stability of altermagnetic phases?

主要发现

  • VHSs enhance orbital-resolved spin fluctuations via inter-orbital hopping, stabilizing intrinsic altermagnetism from orbital antiferromagnetism.
  • Hubbard U and Hund’s coupling JH drive rich Q=0 magnetic phase competition, including transitions O3→O4 (ferrimagnetism to d-wave extrinsic altermagnetism) and O4→O6 (extrinsic altermagnetism to ferromagnetism).
  • Intrinsic altermagnetism (O1/O2) competes with ferromagnetism and can be stabilized near VHS; extrinsic altermagnetism (O4) and ferrimagnetism (O3) appear depending on band parameters and JH/U.
  • G-wave extrinsic altermagnetism (O5) can transition to d-wave extrinsic altermagnetism (O4) or to ferromagnetism (O6) as JH/U varies, indicating tunable altermagnetic landscapes.
  • VHSs arising from quadratic Dirac band touching are central to stabilizing Q=0 altermagnetic states, with intra-VHS nesting favoring Q=0 instabilities and slight doping moving divergences to nonzero Q.
Figure 2: Momentum-space distribution of RPA susceptibilities $\chi^{\text{RPA}}_{\alpha}(\bm{k})$ . (a-f) Calculated susceptibility $\chi^{\text{RPA}}_{1\to 6}(\bm{k})$ for $U=1$ and $J_{H}=0.2$ . The ${\cal O}_{4}$ phase (panel d) exhibits the strongest susceptibility with pronounced peaks at the
Figure 2: Momentum-space distribution of RPA susceptibilities $\chi^{\text{RPA}}_{\alpha}(\bm{k})$ . (a-f) Calculated susceptibility $\chi^{\text{RPA}}_{1\to 6}(\bm{k})$ for $U=1$ and $J_{H}=0.2$ . The ${\cal O}_{4}$ phase (panel d) exhibits the strongest susceptibility with pronounced peaks at the

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