RESUMEN
We report on magneto-optical studies of the quasi-two-dimensional van der Waals antiferromagnet FePS3. Our measurements reveal an excitation that closely resembles the antiferromagnetic resonance mode typical of easy-axis antiferromagnets; nevertheless, it displays an unusual, four-times larger Zeeman splitting in an applied magnetic field. We identify this excitation with an |Sz| = 4 multipolar magnonâa single-ion 4-magnon bound stateâthat corresponds to a full reversal of a single magnetic moment of the Fe2+ ion. We argue that condensation of multipolar magnons in large-spin materials with a strong magnetic anisotropy can produce new exotic states.
RESUMEN
Optical response of crystalline solids is to a large extent driven by excitations that promote electrons among individual bands. This allows one to apply optical and magneto-optical methods to determine experimentally the energy band gap -a fundamental property crucial to our understanding of any solid-with a great precision. Here it is shown that such conventional methods, applied with great success to many materials in the past, do not work in topological Dirac semimetals with a dispersive nodal line. There, the optically deduced band gap depends on how the magnetic field is oriented with respect to the crystal axes. Such highly unusual behavior is explained in terms of band-gap renormalization driven by Lorentz boosts which results from the Lorentz-covariant form of the Dirac Hamiltonian relevant for the nodal line at low energies.
