Evolution of the fermi surface in metastable βL-(BEDT-TTF)2I3

We have been studying the variation of thermodynamic and transport properties as β-(BEDT-TTF)2I3 evolves from the ambient-pressure-cooled, structurally metastable low Tc (βL) phase to the high Tc (βH) phase. Annealing at ambient pressure for about a hundred hours near 100 K removes the βL-associated incommensurate superstructure that forms below T175 (≈175 K at ambient pressure). We find from measurements of the temperature dependent Hall coefficient and heat capacity that long term annealing also suppresses the lower temperature T20 transition (≈20 K) that occurs in the βL phase, leading to bulk βH phase superconductivity. The results are consistent with the opening of the Fermi surface (FS) at T20 in non-annealed samples but are not consistent with a 50% reduction in FS area for non-annealed βL phase samples (compared to the area of the Fermi Surface in the βH phase) or significant localization at T175. Finally, we compare these results to our measurements of the temperature dependent Hall coefficient and Tc under pressure.