THERMOPOWER IN QUASI-2-DIMENSIONAL (BEDT-TTF)(M)X-N ORGANIC CONDUCTORS

Thermopower of (BEDT-TTF)(m)X-n organic conductors has been studied us ing a dedicated measurement technique in the temperature range of 4.2 to 300 K. It turned out that some features of the thermopower in quasi -two-dimensional metals, namely the presence of a peak in the thermopo wer of alpha-(BEDT-TTF)(2)MHg(SCN)(4) and a plateau in kappa-(BEDT-TTF )(2)Cu(NCS)(2) in the temperature interval between 10 and 50 K, are pr obably due to the phonon drag effect. Similar temperature dependences of the Seebeck coefficient can be satisfactorily interpreted in terms of a simple model taking into account the real experimental curve of t he phonon beat capacity versus temperature, C proportional to T-2, whi ch is not described by the Debye formula. One feature distinguishing o rganic superconductors from magnetically ordered metals is a stronger temperature dependence of the characteristic electron-phonon scatterin g time tau(e-ph)(T) Phonon drag effects also determine the behavior of the thermopower in the (BEDT-TTF)(3)Cl-2 . 2H(2)O organic conductor, which is characterized by a metal-insulator transition at T similar to 150 K. An analysis of measurements of the conductivity and thermopowe r vs. temperature taken together indicates that the transition in this compound has a complex nature: first (at T similar to 150 K) a metal- insulator transition occurs, which produces an energy gap in the band spectrum, then at a lower temperature (T similar to 20 K) a transition to a charge-density wave state takes place. (C) 1998 American Institu te of Physics.