RESONANT MAGNETOABSORPTION OF MILLIMETER-WAVE RADIATION IN THE QUASI-2-DIMENSIONAL ORGANIC METALS ALPHA-(BEDT-TTF)(2)MHG(SCN)(4) (M=K,TL)

The magnetoabsorption of millimeter-wave radiation by single crystals of the organic metals alpha-(BEDT-TTF)(2)TlHg(SCN)(4) and alpha-(BEDT- TTF)(2)KHg(SCN)(4) has been studied in the frequency range 30-120 GHz. The experiments reveal two dominant contributions to the magnetoabsor ption spectra. The first is interpreted as the cyclotron resonance of two-dimensional carriers, and is characterized by broad lines (linewid th/magnetic field Delta B/B similar to 0.5-1). In addition to a resona nce exhibiting a cyclotron mass m(c) similar to 2.8m(0), there are two further lines corresponding to frequency-dependent cyclotron masses i n the ranges m(c) similar to(1-1.5)m(0) and m(c) similar to(0.5-0.8)m( 0). This frequency dependence is believed to result from many-body eff ects. The second contribution to the magnetoabsorption is formed by a series of narrow lines with Delta B/B similar to 0.03-0.1 and amplitud es 5-10 times smaller than the features interpreted as cyclotron reson ances. These narrow lines are attributed to a superposition of modes d ue to antiferromagnetic resonance and conduction-electron-spin resonan ce (ESR). The feature characteristic of antiferromagnetic resonance is the structure attributed to ESR consists of a relatively broad maximu m upon which a sharp dip is superimposed. This behavior is believed to be analogous to the resonant transparency observed in thin metallic f ilms undergoing ESR.