Enhanced Shift Current Response in Twisted Multilayer Graphene due to Virtual Multiband Transitions

Finding materials exhibiting substantial shift current holds the potential for designing shift current-based photovoltaics that outperform conventional solar cells. However, the myriad of factors governing shift current response poses significant challenges in designing devices that showcase large shift current. Here, we propose a new, general design principle that exploits the nonlinear nature of the shift current to achieve enhanced shift current response by leveraging virtual transition in materials with multiple flat bands. Using a 1D stacked Rice-Mele model, we demonstrate the viability of our design and observed an enhanced shift current response once virtual transition via nearby bands are allowed. Furthermore, by tuning the system's filling, we predict a giant response at a small frequency proportional to the energy gap of the nearby band. Then, we point to twisted multilayer graphene (TMG) as the natural platform to experimentally realize such an effect. We identify the twist angles at which the shift current response is maximized via virtual transitions for each multilayer graphene and highlight the importance of TMG as a promising material to achieve a giant shift current response at terahertz frequency. Finally, we emphasize that our proposed mechanism is generally applicable to other systems and can serve as a guiding principle for designing materials exhibiting large shift current response.