Pressure-induced topological changes in the Fermi surface of a two-dimensional molecular conductor

We demonstrated x-ray structural analysis of the pressure-induced superconductor ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{ET}}_{2}{\mathrm{ICl}}_{2}$ under extremely high pressure conditions, where ET denotes bis(ethylenedithio)tetrathiafulvalene. This material has been known as the highest transition temperature (${T}_{c}$) superconductor among organic superconductors (${T}_{c}=14.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 8.2 GPa). On the basis of the experimental results, ab initio models were derived using the constrained random phase approximation. We revealed that the Lifshitz transition exists behind the Mott insulator--imetal transition and found that the value of the on-site Coulomb interaction was halved to around $10\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ compared to that at ambient pressure. This Letter clarifies the enigmatic origins of high ${T}_{\mathrm{c}}$ and, concurrently, provides an understanding of the impacts of structural alterations in organic materials under high pressure on their electronic properties and the superconductivity process.