Superconductivity above 2 K At Ambient Pressure in Iododibromide (IBr2 ) Charge-Transfer Salts of Bis (ETHYLENEDITHIO) TETRATHIAFULVALENE, BEDT-TTF

Abstract
Crystals of the charge-transfer salt (BEDT-TTF)2IBr2, derived from the sulfur-based organic donor bis(ethylenedithio) tetrathiafulvalene [BEDT-TTF], have been synthesized by electrocrystallization and studied by rf penetration depth measurements at ambient pressure and temperatures down to 0.45 K. One of two phases was found to be superconducting with Tc = 2.3-2.8 K. This phase, denoted as β-(BEDT-TTF)2 IBr2, is isostructural with the superconducting triiodide salt, β-(BEDT-TTF)2I3. Evidence for the existence of a metastable superconducting phase of the BEDT-TTF/IBr2 system with Tc near 4.2 K is given.