Cyclotron resonance measurements of organic conductor α-(BEDT-TTF)2KHg(SeCN)4

α-(BEDT-TTF)2KHg(SeCN)4 is the new organic conductor which has been synthesized by Sasaki recently. Although it has similar crystal structure with (BEDT-TTF)2KHg(SCN)4, it does not show antiferromagnetic like transition down to 0.5 K. Therefore the comparison of the physical properties of these two substances is interesting. The cyclotron resonance of K-Se salt has been measured in the millimeter and submillimeter wave regions with the pulsed magnetic field up to 15 T at 1.8 K, and we observed only one cyclotron resonance with the effective mass of 0.96 me. It can be attributed to α-orbit, and this effective mass turned out to be smaller than 2.0 me obtained by SdH oscillation. The circular polarization measurement has also been performed and this resonance turned out to be hole active which is consistent with the band calculation.