Far-infrared cyclotron resonance study of electron dynamics in (BEDT-TTF)2KHg(SCN)4

We report the first measurement of cyclotron resonance in a metallic charge-transfer salt. Carrier pockets are observed with effective masses of 0.40${\mathit{m}}_{\mathit{e}}$ and 0.94${\mathit{m}}_{\mathit{e}}$. In contrast, masses of 2.0${\mathit{m}}_{\mathit{e}}$ and 2.4${\mathit{m}}_{\mathit{e}}$ are seen in magnetotransport. We propose that the transport masses are enhanced by electron-electron interactions, whereas, following Kohn’s theorem, the mass measured by cyclotron resonance is independent of these interactions. These measurements therefore represent the first direct gauge of the electron-electron interaction in a metallic charge-transfer salt.