Signatures of coupling between spin waves and Dirac fermions in YbMnBi$_2$

TL;DR

This study uses inelastic neutron scattering to reveal subtle coupling between spin waves and Dirac fermions in YbMnBi$_2$, highlighting unique signatures due to the Dirac nature of conduction electrons.

Contribution

It provides the first experimental evidence of spin-fermion coupling in a Dirac material with detailed analysis of the magnetic excitation spectra.

Findings

Magnetic excitations show small intrinsic width, indicating weak but resolvable coupling.

Nearly wave-vector independent damping observed, consistent with Dirac fermion dispersion.

Distinct signatures of spin-fermion interaction due to the Dirac nature of electrons.

Abstract

We present inelastic neutron scattering (INS) measurements of magnetic excitations in YbMnBi$_2$, which reveal features consistent with a direct coupling of magnetic excitations to Dirac fermions. In contrast with the large broadening of magnetic spectra observed in antiferromagnetic metals such as the iron pnictides, here the spin waves exhibit a small but resolvable intrinsic width, consistent with our theoretical analysis. The subtle manifestation of spin-fermion coupling is a consequence of the Dirac nature of the conduction electrons, including the vanishing density of states near the Dirac points. Accounting for the Dirac fermion dispersion specific to \ymb\ leads to particular signatures, such as the nearly wave-vector independent damping observed in the experiment.