Harnessing Josephson-Shapiro physics to verify interlayer exciton superfluidity

Abstract

Obtaining definitive evidence for zero-magnetic-field exciton superfluidity in electron-hole bilayers remains a longstanding challenge because the condensate is electrically neutral and its phase coherence is difficult to probe directly. We propose a direct test based on Shapiro steps in a Dayem-bridge excitonic Josephson junction. We predict clearly resolvable Shapiro plateaus in experimentally accessible current and voltage regimes for double-bilayer graphene and, in the low-density regime, double-layer transition-metal dichalcogenides. Moreover, by tuning the density across the BCS-BEC crossover we show that the Shapiro response acquires a distinct nonmonotonic evolution. This is determined by the nonmonotonic behavior of the healing length in the crossover from bosonic to fermionic excitations. Observation of these signatures would provide direct evidence of exciton superfluidity and establish exciton bilayers as a platform for neutral Josephson devices.