Excitonic Shift Current in Monolayer MoS₂

In excitonic systems, where the relative motion of electrons and holes is constrained by Coulomb interaction, how exciton effects influence the magnitude of shift current─a phase-coherent shift of photoexcited Bloch electrons and holes found in noncentrosymmetric materials─has been a subject of ongoing theoretical interest. Here, we report observation of exciton-enhanced shift current in uniaxially strained monolayer MoS₂. We obtain energy-dependent shift current conductivity tensor elements by measuring polarization-resolved photocurrent spectrum in multiterminal configurations, revealing a series of features that are absent in the linear optical spectra. We show that these features can be attributed to higher-order excitons and that their large shift vector plays an important role in their observation. We further demonstrate that electrostatic gating can modulate the shift current by two orders of magnitude in a nonmonotonous manner, revealing the interplay between free carrier conductivity and exciton oscillator strength.