T _c of 11 K identified for the third polymorph of the (BEDT-TTF) _2Ag(CF _3) _4(TCE) organic superconductor

摘要

The crystal structure of the third polymorph of the organic superconductor (BEDT-TTF) _2Ag(CF _3) _4(TCE), where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and TCE is 1,1,2-trichloroethane, has been investigated. The crystal consists of alternating stacks of two types of donor sheets, κ- and α'-types. Although this structure is similar to the previously solved two-layered high-T _c phase of (BEDT-TTF) _2Ag(CF _3) _4(TCE), the axis perpendicular to the conducting layer of the unit cell of the new phase is twice as large as that of the two-layered phase; thus, four layers form the repeating unit. Superconductivity is attributed to the κ-type conducting layer, because the α'-type conducting layer is in an acentric charge-ordered state. The onset superconducting transition temperatures are approximately 11.0 and 9.5K for the four-layered and two-layered phases, respectively.