Perfect Coulomb drag in a dipolar excitonic insulator.

Excitonic insulators (EIs) are a solid-state prototype for bosonic phases of matter that can support charge-neutral exciton currents. However, demonstration of exciton transport in EIs is difficult. In this work, we show that the strong interlayer excitonic correlation at equal electron and hole densities in MoSe2/WSe2 double layers separated by a 2-nanometer barrier yields perfect Coulomb drag under zero magnetic field: A charge current in one layer induces an equal but opposite drag current in the other layer at low temperatures. The drag current ratio remains above 0.9 up to about 20 kelvin. As exciton density increases above the Mott density, the excitons dissociate into an electron-hole plasma abruptly, and only frictional drag is observed. Our experiment may lead to the realization of exciton circuitry and superfluidity. Editor's summary: A pair of two-dimensional (2D) systems, one featuring electron carriers and the other hole carriers, separated by a thin insulating layer can host correlated interlayer excitons. Such excitons are predicted to exhibit superfluidity, as well as the so-called perfect Coulomb drag, in which the current in one layer causes an equal but opposite current in the other layer. Two studies have now observed nearly perfect drag at low temperatures in heterostructures consisting of molybdenum diselenide and tungsten diselenide layers separated by hexagonal boron nitride. Nguyen et al. used transport measurements, whereas Qi et al. relied on an optical technique. The studies enable further exploration of exciton transport in such heterostructures, including searching for superfluidity. —Jelena Stajic [ABSTRACT FROM AUTHOR]