Interplay of spin-orbit coupling and Zeeman splitting in the absorption lineshape of fermions in two dimensions

We suggest that electron spin resonance (ESR) experiments can be used as a probe of spinon excitations of hypothetical spin-liquid state of frustrated antiferromagnet in the presence of asymmetric Dzyaloshinskii-Moriya interaction. We describe assumptions under which the ESR response is reduced to the response of two-dimensional electron gas with Rashba spin-orbit coupling. Unlike previous treatments, the spin-orbit coupling ${\ensuremath{\Delta}}_{\text{SO}}$ is not assumed small compared to the Zeeman splitting ${\ensuremath{\Delta}}_{Z}$. We demonstrate that ESR response diverges at the edges of the absorption spectrum for ac magnetic field perpendicular to the static field. At the compensation point ${\ensuremath{\Delta}}_{\text{SO}}\ensuremath{\approx}{\ensuremath{\Delta}}_{Z}$, the broad absorption spectrum exhibits features that evolve with temperature $T$, even when $T$ is comparable to the Fermi energy.