Ferromagnetic Metal-Insulator Transition Reveals Topological Properties in K2Cr8O16

The interplay between magnetism, electronic behaviour, and the unique properties of topological materials represents a major frontier in condensed matter physics, with potential applications in future electronic devices. Ola Kenji Forslund from the Universität Zürich, Chin Shen Ong from Uppsala University, and Moritz M. Hirschmann from RIKEN, alongside colleagues including Nicolas Gauthier and Hiroshi Uchiyama, now demonstrate a dramatic shift in the behaviour of K2Cr8O16, a material undergoing a metal-insulator transition alongside the emergence of magnetism. Their research reveals this transition is not a conventional process driven by lattice vibrations, but a fundamentally new type of topological metal-insulator transition, where the arrangement of electrons and their interactions are key to establishing the insulating state. This discovery pioneers a new class of materials exhibiting strong links between magnetism, topology, and electron behaviour, potentially unlocking pathways to devices with novel functionalities and axionic properties.

Quasi-One-Dimensional Magnetism in K2Cr8O16

This research investigates the magnetic and structural properties of K2Cr8O16, a quasi-one-dimensional material displaying intriguing magnetic behaviour. The material exhibits a complex interplay between structural distortions, magnetism, and potentially exotic quantum phenomena, prompting scientists to understand the origin of its magnetic order and the potential for novel magnetic excitations. The work combines experimental techniques, including neutron and X-ray scattering, with first-principles theoretical calculations to provide a comprehensive picture of the material’s properties. The material undergoes structural distortions, transitioning from a tetragonal to a monoclinic phase, crucial for understanding its magnetic order.

It exhibits a complex magnetic order, likely a spin-density wave, where magnetic moments align in a specific pattern influenced by the quasi-one-dimensional structure and the structural distortions. The structure is characterised by chains of corner-sharing chromium oxide octahedra, termed chimneys, important for the propagation of magnetic excitations, with interactions between these chimneys playing a critical role in determining the magnetic order. Theoretical calculations reveal the presence of Weyl points in the electronic band structure, with the nesting of these points linked to the formation of the spin-density wave. The material’s quasi-one-dimensional nature and strong anisotropy contribute to the emergence of the spin-density wave, while anomalies in the phonon dispersion suggest strong coupling between the magnetic order and the lattice vibrations.

A wide range of experimental and theoretical techniques were employed, including neutron scattering to determine the magnetic structure and excitation spectrum, and X-ray scattering to determine the crystal structure and monitor distortions. First-principles calculations were used to calculate the electronic band structure, magnetic properties, and phonon dispersion, while Wannier function analysis constructed a tight-binding Hamiltonian and mapped the electronic structure onto a classical Heisenberg model. The magnetic force theorem calculated the magnetic interactions between chromium ions, and chirality calculations determined the chirality of the Weyl points, providing a comprehensive understanding of the complex magnetic and structural properties of K2Cr8O16.

High-Pressure Crystal Growth of K2Cr8O16

To probe the material’s vibrational properties, inelastic X-ray scattering measurements were performed, mounting crystals on copper sample holders and aligning them to measure transverse and longitudinal phonons using IXS at the SPring-8 synchrotron in Japan. The resulting inelastic peaks were fitted using a damped harmonic oscillator, employing dedicated software. Complementing these measurements, scientists conducted angle-dependent magnetisation studies on a single crystal using a magnetometer in DC mode. Measurements were taken at selected angles relative to the crystallographic a- and b-axes under zero-field-cooled conditions, with precise crystal orientation determined using Laue diffraction and controlled rotations applied for each measurement. This combination of advanced synthesis and precise characterisation techniques enabled the discovery of a fundamentally new class of topological material and a deeper understanding of its unique properties.

Topological Transition Reveals Unexpected Magnetic Stability

Scientists have discovered a novel material, K2Cr8O16, exhibiting a unique combination of magnetic and topological properties, establishing a new class of topological metal-insulator transitions. This work pioneers the discovery of a topological-ferromagnetic-metal-insulator transition, revealing a previously unknown pathway where magnetism, topology, and electronic correlations interact. The research demonstrates that K2Cr8O16 undergoes a ferromagnetic metal-insulator transition, accompanied by a change in its band topology. Detailed measurements of magnetic order reveal that the arrangement of magnetic moments remains largely unchanged across the metal-insulator transition, tilting approximately 22.

5 degrees relative to the principal axis within the ab-plane. Inelastic neutron scattering experiments, conducted on powder samples at both 5 and 130 Kelvin, confirm that the exchange interactions remain consistent across the transition, indicating no significant change in spin correlations. Analysis of these interactions defines key exchange parameters obtained by fitting experimental data to the Heisenberg Hamiltonian. Density functional theory calculations and magnetic force theorem analysis corroborate these experimental findings, confirming that the dominant magnetic building blocks are groups of four corner-sharing chains, rather than a simple one-dimensional chain. Further investigation of the electronic structure reveals pairs of Weyl points located approximately 0. 1 electron volts above the Fermi level, confirming the topological nature of the material, with these Weyl points exhibiting remarkable characteristics and located near the nodal plane.

Topological Transition Driven by Electron Interactions

This research establishes a fundamentally new understanding of how materials transition from conducting to insulating states, demonstrating a topological metal-insulator transition driven by strong electron interactions and magnetism. Scientists have shown that potassium dichromate (K2Cr8O16) undergoes a transition accompanied by changes in its electronic band structure, revealing a unique interplay between magnetism, topology, and electronic correlations.