Electronic phenomena studied in the nordic countries

Dirac magnons in Cr trihalides and other honeycomb ferromagnets

Not scheduled

15m

Max IV Lund

Speaker

Dr Sergey Pershoguba (Nordita)

Description

Ferromagnets consisting of van der Waals-bonded stacks of honeycomb layers were much studied half a century ago, but have been neglected in the contemporary Dirac material boom. Such compounds, which include the chromium trihalides CrX$_3$ (X = F, Cl, Br and I) display two spin wave modes whose energy dispersion is similar to that for the electrons in graphene. Therefore, at the single particle level, they are bosonic Dirac materials which resemble their fermionic counterparts. Here we show that magnon-magnon interactions lead to a significant momentum-dependent renormalization of the bare band structure in addition to strongly momentum-dependent spin wave lifetimes, and that the similarity between fermions and bosons in Dirac materials does not hold in the presence of interactions. In doing so we expand the theory of ferromagnets beyond the standard Dyson theory to a non-Bravais lattice. We also show that honeycomb ferromagnets can display dispersive surface and edge states, unlike their electronic analogs. The theory quantitatively accounts for hitherto unexplained anomalies in nearly 50 year old magnetic neutron scattering data for CrBr$_3$.