Topological Magnons: A Review

At sufficiently low temperatures, magnetic materials often enter correlated phases hosting collective, coherent magnetic excitations such as magnons or triplons. Drawing on the enormous progress on topological materials of the past few years, recent research has led to new insights into the geometry and topology of these magnetic excitations. Berry phases associated with magnetic dynamics can lead to observable consequences in heat and spin transport, whereas analogs of topological insulators and semimetals can arise within magnon band structures from natural magnetic couplings. Magnetic excitations offer a platform to explore the interplay of magnetic symmetries and topology, to drive topological transitions using magnetic fields; examine the effects of interactions on topological bands; and generate topologically protected spin currents at interfaces. In this review, we survey progress on all these topics, highlighting aspects of topological matter that are unique to magnon systems and the avenues yet to be fully investigated. Expected final online publication date for the Annual Review of Condensed Matter Physics, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Spin-excitation anisotropy in the nematic state of detwinned FeSe

The origin of the electronic nematicity in FeSe, which occurs below a tetragonal-to-orthorhombic structural transition temperature Ts ≈ 90 K, well above the superconducting transition temperature Tc = 9 K, is one of the most important unresolved puzzles in the study of iron-based superconductors. In both spin- and orbital-nematic models, the intrinsic magnetic excitations at Q1 = (1; 0) and Q2 = (0; 1) of twin-free FeSe are expected to behave differently below Ts. Although anisotropic spin fluctuations below 10 meV between Q1 and Q2 have been unambiguously observed by inelastic neutron scattering around Tc(<< Ts) , it remains unclear whether such an anisotropy also persists at higher energies and associates with the nematic transition Ts. Here we use resonant inelastic x-ray scattering (RIXS) to probe the high-energy magnetic excitations of uniaxial-strain detwinned FeSe. A prominent anisotropy between the magnetic excitations along the H and K directions is found to persist to ~ 200 meV, which is even more pronounced than the anisotropy of spin waves in BaFe2As2. This anisotropy decreases gradually with increasing temperature and finally vanishes at a temperature around the nematic transition temperature Ts. Our results reveal an unprecedented strong spin-excitation anisotropy with a large energy scale well above the dxz/dyz orbital splitting, suggesting that the nematic phase transition is primarily spin-driven. Moreover, the measured high-energy spin excitations are dispersive and underdamped, which can be understood from a localmoment perspective. Our findings provide the much-needed understanding of the mechanism for the nematicity of FeSe and points to a unified description of the correlation physics across seemingly distinct classes of Fe-based superconductors.