Pressure-induced nesting in the low-dimensional organic superconductor α-(BEDT-TTF${)}_2$${\mathrm{NH}}_4$Hg(SCN${)}_4$

Why superconductivity appears in only one member (M=${\mathrm{NH}}_4$) of the isostructural family of organic conductors α-(BEDT-TTF${)}_2$MHg(SCN${)}_4$ is a long-standing question. Others in the family (M =K,Rb,Tl) have density-wave ground states. To address this problem, we have studied pressure effects in superconducting α-(BEDT-TTF${)}_2$ ${\mathrm{NH}}_4$Hg(SCN${)}_4$ (${\mathit{T}}_{\mathit{c}}$=1.0 K). Our results show a remarkable pressure dependence of the Fermi surface topology. We argue that with ${\mathrm{NH}}_4$, there are small deviations from perfect nesting of the open-orbit bands which favor superconductivity over a density-wave state.