Abstract. In order to compare the carrier dynamics and reveal the origin of differences in their electrical behaviour, time-resolved reflectivity changes were measured on single crystals of the title compounds. The observed relaxation behaviour well explained their electrical behaviour, and indicated that different conduction mechanisms dominated κ-and λ-type BETS salts, irrespective of local spins, at ca. 8-100 K; the electrical behaviour of the κ-type salts is governed by relaxation times of carriers and/or density of states at the Fermi levels, while that of the λ-type salts is dominated by the latter.

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Understanding how electrons behave, i.e., carrier dynamics, in superconductors is indispensable for understanding the mechanism of superconductivity. Recently we have reported the carrier dynamics of κ- and λ-(BETS)2MCl4 (BETS = bis(ethylenedithio)tetraselenafulvalene, M = Ga, Fe) based on ultrafast spectroscopy, but the interpretation of the results remains an open question. In this paper we interpreted the results with the aid of newly measured magnetic susceptibility, X-ray single crystal structural analysis, and band calculation. Observation of coherent phonons only in the λ-type salts indicated that e-ph interaction should be characteristically strong in the λ-type salts. By comparison of the observed carrier dynamics with a two-temperature model, the temperature-dependence of carrier dynamics is consistently explained by the different strengths of the e-ph interaction between the λ- and the κ-type salts. The difference in strength of the e-ph interactions is related to their crystal structures. In conclusion, their carrier dynamics is consistently interpreted and classified by their crystal structures

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