Critical current photoconductivity of Fermi-velocity limited carriers in graphene superlattices

In the absence of a magnetic field, the most profound photoconductive phenomena typically manifest in gapped systems and has led to the development of state-of-the-art opto-electronic devices. In contrast, metallic systems tend to exhibit much weaker photoconductivity because the effects of photo-excited carriers are masked by the large density of equilibrium carriers in the band.  Here, we show experimentally that the metallic states of graphene-based moiré superlattices exhibit a striking photoconductive effect with a magnitude comparable to those typically observed in strongly gapped systems. The effect manifests when the moiré superlattices are biased to an out-of-equilibrium critical state¹ that marks a transition from a Fermi-velocity limited current regime to a highly dissipative electron-hole liquid. Analogous to superconducting bolometers, the Fermi-velocity limited metallic state is particularly sensitive to photoexcitation so that incident radiation causes a shift in the critical current and a giant change in the differential resistivity. From the perspective of photodetection, our mid-infrared photocurrent measurements show that the critical metallic state competes with state-of-the-art gapped MCT detectors at low temperatures, exhibiting responsivities as high as 10⁵ V/W for wavelengths 5-11 um. The results demonstrate a unique type of opto-electronic response based on non-equilibrium phenomena in moiré superlattices.