Attraction by repulsion: pairing electrons using electrons.

One of the defining properties of electrons is their mutual Coulombic repulsion. In solids, however, this basic property may change. A famous example is that of superconductors, where coupling to lattice vibrations makes electrons attract each other and leads to the formation of bound pairs. But what if all degrees of freedom are electronic? Is it still possible to make electrons attractive via their repulsion from other electrons? Such a mechanism, termed ‘excitonic’, was proposed fifty years ago by W. A. Little, aiming to achieve stronger and more exotic superconductivity, yet despite many experimental efforts, direct evidence for such ‘excitonic’ attraction is still lacking. Here, we demonstrate this unique attraction by constructing, from the bottom up, the fundamental building block of this mechanism. Our experiments are based on quantum devices made from pristine carbon nanotubes, combined with cryogenic precision manipulation. Using this platform we demonstrate that two electrons can be made to attract using an independent electronic system as the binding glue. Owing to its large tunability, our system offers crucial insights into the underlying physics, such as the dependence of the emergent attraction on the underlying repulsion and the origin of the pairing energy. We also demonstrate transport signatures of ‘excitonic’ pairing. This experimental demonstration of ‘excitonic’ pairing paves the way for the design of exotic states of matter.