Twisted photovoltaics at terahertz frequencies from momentum shift current

The bulk photovoltaic effect (BPVE) converts light into a coherent dc current at zero bias, through what is commonly known as the shift current. This current has previously been attributed to the displacement of the electronic wave function center in real space, when the sample is excited by light. We reveal that materials like twisted bilayer graphene (TBG) with a flatband dispersion are uniquely suited to maximize the BPVE because they lead to an enhanced shift in the momentum space, unlike any previously known shift current mechanism. We identify properties of quantum geometry, which go beyond the quantum geometric tensor, and are unrelated to Berry charges, as the physical origin of the large BPVE we observe in TBG. Our calculations show that TBG with a band gap of several meV exhibits a giant BPVE in a range of 0.2-1 THz, which represents the strongest BPVE reported so far at this frequency in a two-dimensional material and partially persists even a room temperature. Our paper provides a design principle for shift current generation, which applies to a broad range of twisted heterostructures with the potential to overcome the so-called terahertz gap in THz sensing.

Automated summary

This study reveals that twisted bilayer graphene (TBG) with a flatband dispersion is uniquely suited to maximize the bulk photovoltaic effect (BPVE) through an enhanced shift in momentum space. Quantum geometry properties, beyond the quantum geometric tensor and unrelated to Berry charges, are identified as the physical origin of the large BPVE observed in TBG. The study shows that TBG with a band gap of several meV exhibits a giant BPVE in the 0.2-1 THz range, which is the strongest reported in a two-dimensional material at this frequency and partially persists even at room temperature. The research provides a design principle for shift current generation in a wide range of twisted heterostructures, with the potential to bridge the so-called terahertz gap in THz sensing.