Topological Metal-Insulator Transition within the Ferromagnetic state

A major challenge in condensed matter physics is integrating topological phenomena with correlated electron physics to leverage both types of states for next-generation quantum devices. Metal-insulator transitions (MITs) are central to bridging these two domains while simultaneously serving as ‘on-off’ switches for electronic states. Here, we demonstrate how the prototypical material of K2Cr8O16 undergoes a ferromagnetic MIT accompanied by a change in band topology. Through inelastic x-ray and neutron scattering experiments combined with first-principles theoretical calculations, we demonstrate that this transition is not driven by a Peierls mechanism, given the lack of phonon softening. Instead, we establish the transition as a topological MIT within the ferromagnetic phase (topological-FM-MIT) with potential axionic properties, where electron correlations play a key role in stabilizing the insulating state. This work pioneers the discovery of a topological-FM-MIT and represents a fundamentally new class of topological phase transitions, revealing a unique pathway through which magnetism, topology, and electronic correlations interact.

💡 Research Summary

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The authors investigate the long‑standing puzzle of the metal‑insulator transition (MIT) in the quasi‑one‑dimensional oxide K₂Cr₈O₁₆, which remains ferromagnetic (FM) across the transition. Earlier work attributed the transition to a Peierls‑type charge‑density‑wave (CDW) instability driven by one‑dimensional nesting. By combining neutron diffraction, inelastic neutron scattering (INS), inelastic x‑ray scattering (IXS), and density‑functional theory (DFT + U) calculations, the authors demonstrate that the Peierls scenario is untenable: no phonon softening is observed at the CDW wave vector, and the electronic structure does not exhibit the expected one‑dimensional Fermi‑surface instability.

In the high‑temperature FM metallic phase (magnetic space group C2′/m′, #12.62), the Cr t₂g bands host Weyl points of opposite chirality located near the Brillouin‑zone boundary planes kₙ = ±π/c. These Weyl points are accidental crossings protected only by the magnetic symmetry; they are not pinned at high‑symmetry points. The authors calculate Berry curvature and confirm the Weyl character. Importantly, the Weyl points are connected by nesting vectors qₙₑₛₜ ≈ (0.75, 0.75, 0), which closely match the structural modulation vector q_CDW = (0.5, 0.5, 0) observed in diffraction. This suggests that electronic correlations cause the Weyl points to nest, thereby coupling to the lattice and inducing the observed monoclinic distortion without a conventional electron‑phonon driven Peierls instability.

Upon cooling below T_MIT ≈ 95 K, the crystal symmetry lowers to monoclinic P2₁/c (#14). The monoclinic distortion lifts the degeneracy between dₓz and d_yz orbitals and makes the four Cr sites inequivalent, splitting the t₂g manifold into twelve non‑degenerate levels. The resulting redistribution of the 2.25 e⁻ per Cr leads to an integer total electron count per primitive cell, allowing a full band gap to open. The magnetic subgroup becomes P2′1/c′ (#14.79), a non‑symmorphic group that no longer enforces the double degeneracy on the kₙ = ±π/c planes. Consequently, the Weyl points disappear, and parity‑eigenvalue analysis yields a trivial Z₄ topological index (Z₄ = 0). The MIT is therefore a topological phase transition driven by a reduction of translational symmetry rather than a Peierls gap opening.

Magnetic exchange interactions were extracted from powder INS spectra at 5 K and 130 K, revealing ferromagnetic couplings J₁ ≈ 6 meV, J₂ ≈ 0.6 meV (nearest‑ and next‑nearest‑neighbor intra‑chain) and a larger J₃ ≈ 10 meV (inter‑chain). First‑principles magnetic‑force‑theorem calculations reproduce the hierarchy of J’s and show a quadratic dependence on the hopping amplitude t (J ∝ t²/U), indicating that super‑exchange dominates despite the ferromagnetic sign, consistent with Goodenough‑Kanamori‑Anderson rules. The exchange constants are essentially unchanged across the MIT, confirming that the magnetic order is robust and does not drive the transition.

The authors propose that the nesting of opposite‑chirality Weyl points can give rise to axion‑like collective excitations when chiral symmetry is broken by the CDW distortion. While direct observation of such axionic modes remains for future work, the present study establishes a new class of “topological‑FM‑MIT” where magnetism, band topology, and strong electron correlations intertwine. This discovery opens avenues for designing quantum devices that exploit both magnetic order and topological protection, such as spintronic switches, axion‑insulator platforms, and correlation‑enhanced topological transistors. Further investigations could explore external tuning (pressure, strain, electric field) of the Weyl‑point nesting, direct probes of axionic dynamics, and integration of K₂Cr₈O₁₆‑based heterostructures into functional quantum circuitry.