Blockade and Counterflow Supercurrent in Exciton-Condensate Josephson Junctions

We demonstrate that perfect conversion between charged supercurrents in superconductors and neutral supercurrents in electron-hole pair condensates is possible via a new Andreev-like scattering mechanism. As a result, when two superconducting circuits are coupled through a bilayer exciton condensate, the superflow in both layers is drastically modified. Depending on the phase biases the supercurrents can be completely blocked or exhibit perfect drag.

Figure 1

Sketch of a superconductor–exciton-condensate–superconductor system. (a) A double-layer exciton condensate is contacted with four superconducting leads. (b) When the applied phase biases in the top and bottom layers are identical ($\phi$ in this cartoon) no current can flow when the length of the junction $L$ is much larger than the exciton-condensate coherence length $\hslash v_F/|\Gamma |$. In this case the Josephson currents are in the “exciton blockade” regime. (c) When a phase bias $\phi$ is applied to the top layer only, a supercurrent $I$ flows. In the limit $L\gg \hslash v_F/|\Gamma |$ a supercurrent $-I$ is dragged in the bottom layer in a perfectly frictionless manner: in this case one has a “perfect” drag or “superdrag” of Josephson currents.

Figure 2

Schematic electronic band structure of a gated bilayer in the absence of electron-hole coupling.