  We discuss dynamic spin susceptibility (DSS) in two-dimensional (2D) Dirac
electrons with spin-orbit interactions to characterize topological insulators.
The imaginary part of the DSS appears as an absorption rate in response to a
transverse ac magnetic field, just as in an electron spin resonance experiment
for localized spin systems. We found that when the system is in a static
magnetic field, the topological state can be identified by an anomalous
resonant peak of the imaginary part of the DSS as a function of the frequency
of the transverse magnetic field $\omega$. In the absence of a static magnetic
field, the imaginary part of the DSS becomes a continuous function of $\omega$
with a threshold frequency $\omega_{\rm c}$. In this case, the topological and
the trivial phases can also be distinguished by the values of $\omega_{\rm c}$
and by the line shapes. Thus the DSS is an experimentally observable physical
quantity to characterize a topological insulator directly from bulk properties,
without observing a topological transition.
