Coupled Electron-Phonon Hydrodynamics in Two-Dimensional Semiconductors.

Electronic and thermal transport properties in two-dimensional (2D) semiconductors have been extensively investigated due to their potential to miniaturize transistors. Microscopically, electron-phonon interactions are considered the dominant momentum relaxation mechanism for electrons that limits carrier mobility beyond cryogenic temperatures. However, when electrons and phonons are considered as a single system, electron-phonon interactions conserve the total momentum and energy, leading to the possibility of low-dissipation transport. In this Letter, we systematically investigate the momentum circulation, in which momentum previously transferred from electrons to phonons (or vice versa) can be pumped back to the original carriers through interactions between nonequilibrium electrons and phonons, and its impact on carrier transport properties in 2D semiconductors with strong electron-phonon interactions. We find that, when strong momentum circulation is taken into account, the total momentum in the coupled electron-phonon system is weakly dissipated, leading to a coupled electron-phonon hydrodynamic transport regime, in which electrons and phonons exhibit a joint drift motion rather than separate diffusive behaviors. In this transport regime, charge transport properties are significantly enhanced. Contrary to previous belief, our results demonstrate that low-dissipation charge transport can occur despite strong electron-phonon interactions when there is effective momentum circulation between electrons and phonons. Our Letter advances fundamental understandings of carrier transport in 2D semiconductors.