A Tale of Two Domes

Abstract

Superconductivity describes the spectacular ability of electrons in some materials at low temperatures to form Cooper pairs and coherently carry charge without resistance. Creating superconductors that operate at room temperature has long been an unrealized dream. Of particular interest are copper- and iron-based superconductors, discovered in 1986 and 2008, respectively, that possess higher transition temperatures (Tc up to 135 K under ambient pressure) than most conventional superconductors. These “high-Tc” superconductors often exhibit a peak or “dome” behavior in the transition temperature: when doping or pressure is increased, Tc rises until it reaches a maximum, and then it falls off. A new study of superconducting iron selenide (FeSe) films has revealed a double-dome behavior as the doping of electrons is increased [1]. Can-Li Song and collaborators at Tsinghua University, China, argue that the two domes arise from distinct mechanisms for binding electrons together into Cooper pairs. The unexpected discovery strengthens recent suggestions that the conventional mechanism of phonon binding, which has, for three decades, been overshadowed by more exotic mechanisms, may yet have an important role to play in further enhancing Tc.