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Abstract Magnon polarons are novel elementary excitations possessing hybrid magnonic and phononic signatures, and are responsible for many exotic spintronic and magnonic phenomena. Despite long-term sustained experimental efforts in chasing for magnon polarons, direct spectroscopic evidence of their existence is hardly observed. Here, we report the direct observation of magnon polarons using neutron spectroscopy on a multiferroic Fe2Mo3O8 possessing strong magnon-phonon coupling. Specifically, below the magnetic ordering temperature, a gap opens at the nominal intersection of the original magnon and phonon bands, leading to two separated magnon-polaron bands. Each of the bands undergoes mixing, interconverting and reversing between its magnonic and phononic components. We attribute the formation of magnon polarons to the strong magnon-phonon coupling induced by Dzyaloshinskii-Moriya interaction. Intriguingly, we find that the band-inverted magnon polarons are topologically nontrivial. These results uncover exotic elementary excitations arising from the magnon-phonon coupling, and offer a new route to topological states by considering hybridizations between different types of fundamental excitations.

The Berezinskii-Kosterlitz-Thouless (BKT) phase transition has long been sought yet undiscovered directly in magnetic materials. Here, the authors identify two phase transitions with BKT fluctuations detected by NMR and critical scaling behavior in magnetic susceptibility expected for the BKT transition in a frustrated magnet TmMgGaO4.

The dimensional dependence of charge density wave (CDW) in two-dimensional dichalcogenides remains puzzled. Here, Lin et al. study trends of CDW ordering in an isoelectronic group of materials 2H-MX 2 and provide a unified understanding involving several microscopic factors.

Superconductivity mediated by phonons is usually conventional due to isotropic electron-phonon coupling. Here, Wang et al. report highly anisotropic phonons only along [001] direction in Sr0.1Bi2Se3, indicating a singular electron-phonon coupling which favors a p-wave nematic superconductivity scenario.

The superconducting gap structure contains important information to understand the pairing mechanism of unconventional superconductivity. Here, by using a newly established phase-referenced quasiparticle interference technique, the authors visualize the sign change of the d-wave superconducting gap directly in Bi2Sr2CaCu2O8+δ.

Topological magnonic materials display exotic properties which may enable high-efficiency and low-cost spintronic devices. Here the authors demonstrate a three-dimensional antiferromagnet Cu3TeO6 that hosts symmetry-protected Dirac and triply degenerate magnons.

Dengue virus-specific antibody and CD8+ T cells that cross-react with Zika virus have been described. Here, the authors establish a functionally protective role for cross-reactive dengue virus-specific CD8+ T cells during challenge with Zika virus.

Whether and how the Dirac electrons can be driven into superconducting state remains unclear. Here, Duet al. present systematic study to demonstrate the Dirac electrons condensing into Cooper pairs on the surface of a possible topological superconductor SrxBi2Se3.