Cyclotron Resonance in the Hidden-Order Phase of Ultraclean URu$_2$Si$_2$ Single Crystals

In the heavy fermion compound URu$_2$Si$_2$, the hidden-order transition occurs at 17.5 K, whose origin is still an enigma. Of primary importance is elucidating the electronic structure of the hidden-order phase. Here we report the first observation of cyclotron resonance in the ultraclean crystals of URu$_2$Si$_2$ with residual resistivity ratio $RRR=670$. The magnetic-field dependence of the microwave surface impedance in the Azbel'-Kerner geometry shows clear cyclotron resonance lines whose line width has characteristic temperature dependence consistent with the transport measurements. In addition to the bands which have been previously identified in the quantum oscillation measurements, we newly find the missing band with the heaviest mass, which can account for the large specific heat coefficient in the hidden-order phase of URu$_2$Si$_2$.