Thermopower in quasi-two-dimensional (BEDT-TTF) m X n organic conductors

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Abstract
Thermopower of (BEDT-TTF) m X n organic conductors has been studied using 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 thermopower of α-(BEDT-TTF)2MHg(SCN)4 and a plateau in κ-(BEDT-TTF)2Cu(NCS)2 in the temperature interval between 10 and 50 K, are probably 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 the phonon heat capacity versus temperature, C ∝ T2, which is not described by the Debye formula. One feature distinguishing organic superconductors from magnetically ordered metals is a stronger temperature dependence of the characteristic electron-phonon scattering time τe-ph(T). Phonon drag effects also determine the behavior of the thermopower in the (BEDT-TTF)3Cl2·2H2O organic conductor, which is characterized by a metal-insulator transition at T∼150 K. An analysis of measurements of the conductivity and thermopower vs. temperature taken together indicates that the transition in this compound has a complex nature: first (at T∼150 K) a metal-insulator transition occurs, which produces an energy gap in the band spectrum, then at a lower temperature (T∼20 K) a transition to a charge-density wave state takes place.