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Magnetically driven phonon instability enables the metal–insulator transition in h-FeS

Publication Type
Journal Name
Nature Physics
Publication Date
Page Numbers
669 to 675

Understanding the interplay of spin, phonon, and charge degrees-of-freedom is important to
rationalize and control metal-insulator transitions (MIT) and magnetoelectric materials. MIT
rarely coincide with simultaneous structural and magnetic phase transitions. In these unusual
cases, fundamental questions remain unresolved { namely which of the spin or lattice degrees of
freedom drives the MIT, and whether concurrency of the magnetic and structural transition is
a coincidence or a necessary requirement. Here, we investigate the metal-insulator, structural,
and magnetic transitions in hexagonal FeS, through comprehensive neutron and x-ray scattering
measurements, probing the structure as well as phonon and magnon excitations, supported with
rst-principles electronic structure simulations. We identify the critical role of the coupling between
antiferromagnetic (AFM) ordering and instabilities of anharmonic phonons in the metallic phase
at high temperature, and we rationalize the mechanism of the MIT. The AFM ordering enables
the emergence of two zone-boundary soft phonons, whose condensation couples to a zone-center
mode, with a resulting distortion opening the electronic band gap. Simultaneously, spin-lattice
coupling opens a gap in the magnon spectrum that impacts the entropy and thermodynamics
of the MIT. These results demonstrate the importance of spin-phonon coupling in anharmonic
systems and open new avenues to design novel technologically important materials harboring MIT
and magnetoelectric behaviors.