Signature of spin-fermion coupling in the magnetic excitations of Dirac semimetal YbMnBi_2.

In Dirac semimetals, coupling of Dirac charge carriers with magnetism can lead to novel phenomena with potential for technological applications [1, 2]. From this perspective, 112 ternary pnictogens (A,R)MnX₂ (A=Ca,Sr;R=Yb,Eu;X=Bi,Sb) represent an interesting family of Dirac materials where both the magnetism and Dirac electrons coexist, thereby providing an opportunity to study the spin-fermion coupling between Mn spins and Dirac electrons in Bi layer [2]. Previous studies of spin excitations in (Sr,Ca)MnBi₂ using inelastic neutron scattering (INS) have found no indication of such coupling because anomalous broadening found in itinerant magnets was absent [2]. However, our recent INS measurements of spin waves on YbMnBi₂ found a small and q-independent broadening in the spin waves, which is consistent with a significant spin-Dirac fermion coupling. Theoretical calculations show that the effect of coupling on the spin excitations is suppressed by a vanishing carrier density of states at the Dirac point. [1] A. Sapkota et al, arxiv 1908.08114 (2019), [2] M. C. Rahn et al, Phys. Rev. B 95, 134405 (2017).