Resonant magnetoabsorption of millimeter-wave radiation in the quasi-two-dimensional organic metals alpha -(BEDT-TTF)2MHg(SCN)4 (M=K,Tl).

Abstract

The magnetoabsorption of millimeter-wave radiation by single crystals of the organic metals \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$TlHg(SCN${)}_{4}$ and \ensuremath{\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 magnetoabsorption spectra. The first is interpreted as the cyclotron resonance of two-dimensional carriers, and is characterized by broad lines (linewidth/magnetic field \ensuremath{\Delta}B/B\ensuremath{\sim}0.5--1). In addition to a resonance exhibiting a cyclotron mass ${\mathit{m}}_{\mathit{c}}$\ensuremath{\sim}2.8${\mathit{m}}_{0}$, there are two further lines corresponding to frequency-dependent cyclotron masses in the ranges ${\mathit{m}}_{\mathit{c}}$\ensuremath{\sim}(1--1.5)${\mathit{m}}_{0}$ and ${\mathit{m}}_{\mathit{c}}$\ensuremath{\sim}(0.5--0.8)${\mathit{m}}_{0}$. This frequency dependence is believed to result from many-body effects. The second contribution to the magnetoabsorption is formed by a series of narrow lines with \ensuremath{\Delta}B/B\ensuremath{\sim}0.03--0.1 and amplitudes 5--10 times smaller than the features interpreted as cyclotron resonances. These narrow lines are attributed to a superposition of modes due to antiferromagnetic resonance and conduction-electron-spin resonance (ESR). The feature characteristic of antiferromagnetic resonance is the presence of a mode with a frequency that decreases with increasing magnetic field. The magnetoabsorption structure attributed to ESR consists of a relatively broad maximum upon which a sharp dip is superimposed. This behavior is believed to be analogous to the resonant transparency observed in thin metallic films undergoing ESR. \textcopyright{} 1996 The American Physical Society.