Coupling of magnetism and Dirac fermions in YbMnX2 (X= Bi, Sb)

Dirac and Weyl materials form a new class with unique properties. The interaction of Dirac fermions with other degrees of freedom leads to rich novel physics such as suppression of backscattering, impurity-induced resonant states and spin-polarized transport. The 112 ternary pnictogens A/RMnX2 (A= Ca, Sr; R= Yb, Eu; X= Bi, Sb) represent a particularly interesting family of Dirac materials, where the X layers hosting itinerant Dirac charge carriers are separated by strongly correlated, insulating antiferromagnetically ordered Mn-X layers. Both the inter-layer charge transport and the magnetic correlations between the Mn layers require that Dirac carriers are coupled to strongly-correlated Mn electrons. Hence, these materials provide an excellent opportunity to study the interaction of the conduction Dirac electrons with the local-moment magnetic Mn-X subsystem. We report inelastic neutron scattering measurements of magnetic excitations in YbMnBi2 and YbMnSb2. Our spin-wave analysis of the measured spectra allows to evaluate the intra- and inter-layer magnetic interaction and the spectral damping parameter, which are directly related to the coupling strength of Dirac fermions with local spin moments. Comparing our results with (Sr, Ca)MnBi2 where no indication of such coupling was found, allows to establish a general trend and helps to understand the strength and signatures of spin-Dirac fermion interaction in 112 Dirac materials.