Direct determination of momentum-resolved electron transfer in the photoexcited van der Waals heterobilayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi mathvariant="normal">W</mml:mi><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:mi>Mo</mml:mi><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>
Fang Liu, Qiuyang Li, Xiaoyang Zhu
Abstract
Photoinduced charge separation in transition-metal dichalcogenide heterobilayers is being explored for moir\'e excitons, spin-valley polarization, and quantum phases of excitons/electrons. While different momentum points can be critically involved in charge separation dynamics, little is known directly from experiments. Here we determine momentum-resolved electron dynamics in the $\mathrm{W}{\mathrm{S}}_{2}/\mathrm{Mo}{\mathrm{S}}_{2}$ heterobilayer using time- and angle-resolved photoemission spectroscopy. Upon photoexcitation in the $K$ valleys, we detect electrons in $M$/2, $M$, and $Q$ valleys/points on timescales as short as \ensuremath{\sim}70 fs, followed by dynamic equilibration in $K$ and $Q$ valleys in \ensuremath{\sim}400 fs. These findings reveal the essential role of phonon scattering, the coexistence of direct and indirect interlayer excitons, and constraints on spin-valley polarization.