In order to clarify the electronic state and the mechanism of energy<br />
dissipations by the motion of the vortex core in the quantum limit, we measured<br />
the microwave complex conductivity of pure FeSe single crystals in the<br />
zero-field limit and under finite magnetic fields. The temperature dependence<br />
of the superfluid density changed as $1-(T/T_{\rm c})^{1.2}$, indicating the<br />
presence of nodal lines in the superconducting gap. From the magnetic-field<br />
dependence of the flux-flow resistivity, we found that a barometer of the<br />
electronic state inside the vortex core turned out to be $\omega_0\tau_{\rm<br />
core}=1\pm0.5$. This suggests that the vortex core of pure FeSe is still in the<br />
moderately clean regime, which is inconsistent with the expectation that the<br />
super-clean core is realized in this material. We also found that the mean-free<br />
path inside the vortex core is suppressed at the distance of the order of the<br />
core radius. Based on the observed results and previous reports, we discussed<br />
possible origins of rather small $\omega_0\tau_{\rm core}$ in terms of the<br />
multiple-bands nature and additional mechanisms giving extra energy<br />
dissipations specific to the vortex core in motion.