We performed cyclotron resonance (CR) measurements of α-(BEDT-TTF)2MHg(SCN)4 (M = Rb, K) in the density-wave state. We applied the magnetic field perpendicular to the conducting ac-plane and we measured the angle dependence of the cyclotron resonance absorption. The observed four resonances in M = Rb salt showed 1/cos θ dependence, where the field was inclined by the angle θ from the direction perpendicular to the ac-plane. This dependence clearly shows that observed four resonances in M = Rb salt are CR's. The CR measurement of M = K salt was also performed and the results are also presented. To our surprise, obtained effective masses for M = K salt at 1.8 K were different from those obtained for M = Rb salt.
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