CYCLOTRON-RESONANCE STUDIES OF ELECTRON DYNAMICS IN BEDT-TTF SALTS

We report measurements of cyclotron resonance (CR) in the metallic charge-transfer salts, (BEDT-TTF)2X, where X=Cu(SCN)2, KHg(SCN)4 and NH4Hg(SCN)4. Cyclotron resonance is observed as a modulation of the far-infrared (FIR) transmission or reflectance of a mosaic of crystals as a function of magnetic field, enabling a cyclotron effective mass to be deduced. Two dimensional carrier pockets have been observed in these materials with cyclotron masses ranging from 0.40m(e) to 1.2m(e). In contrast, much higher masses are deduced from the temperature dependence of the Shubnikov-de Haas (SdH) oscillations, ranging from 2.0m(e) to 3.5m(e). We propose that these transport masses are enhanced by electron-electron interactions, whereas. following Kohn's theorem (W. Kohn, Phys. Rev. 123, 1242 (1961)), the masses measured by cyclotron resonance are independent of these interactions. These measurements therefore represent a direct gauge of the electron-electron interaction in charge transfer salts.