Abstract:

Experimental demonstration of excitonic attraction between two electrons is achieved in quantum devices made from carbon nanotubes, where the interaction between two electrons is reversed from repulsive to attractive owing to their strong Coulomb interaction with another electronic system. A principle of basic physics holds that similarly charged particles repel each other, but in solids the unthinkable is possible: electrons can get together. When mediated by the right sort of lattice vibrations, electrons can overcome their repulsion and form bound pairs, a well-known effect that can lead to superconductivity. Shahal Ilani and colleagues engineer an even more exotic effect of mutual electron attraction, mediated by other electrons. They accomplish this by placing two carbon nanotube electronic devices next to each other with high, submicrometre precision. The repulsion between the electrons confined in small area in one nanotube can be turned into attraction by accurately placing and tuning the other nanotube. This work resolves a long-standing fundamental question of whether electronic electron pairing is possible and provides a novel platform for quantum electronic devices. One of the defining properties of electrons is their mutual Coulomb repulsion. However, in solids this basic property may change; for example, in superconductors, the coupling of electrons to lattice vibrations makes the electrons attract one another, leading to the formation of bound pairs. Fifty years ago it was proposed1 that electrons can be made attractive even when all of the degrees of freedom in the solid are electronic, by exploiting their repulsion from other electrons. This attraction mechanism, termed ‘excitonic’, promised to achieve stronger and more exotic superconductivity2,3,4,5,6. Yet, despite an extensive search7, experimental evidence for excitonic attraction has yet to be found. Here we demonstrate this attraction by constructing, from the bottom up, the fundamental building block8 of the excitonic 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 each other using an independent electronic system as the ‘glue’ that mediates attraction. Owing to its tunability, our system offers 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.