Large shift current generated via in-gap and charge-neutral exciton excitations in BN nanotubes and single BN layer

Yi-Shiuan Huang^1,3*, Yang-Hao Chan^2,3, Guang-Yu Guo^1,2

¹Department of Physics and Center for Theoretical Physics, National Taiwan University, Taipei, Taiwan
²Physics Division, National Center for Theoretical Sciences, Taipei, Taiwan
³Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan

* Presenter:Yi-Shiuan Huang, email:r10222054@ntu.edu.tw

We investigate the exciton shift current in single-walled zigzag [(5,0),(6,0),(7,0),(8,0)] BN nanotubes as well as BN sheet by performing ab initio many-body calculations utilizing the state-of-the-art GW plus Bethe-Salpeter equation approaches with the newly developed efficient algorithms. Our GW-BSE calculations reveal a giant in-gap peak in the shift current spectrum in all the studied BN systems due to the excitation of the A exciton. The peak value of the excitonic shift current is more than three times larger than that of the quasiparticle shift current, and is attributed to the gigantic enhancement of the optical dipole matrix element by the A exciton resonance. Contrary to the previous model studies [1,2], the direction of the shift current is constant, thus not following the rule of sgn(J) = mod(n, 3). A bundle of zigzag BN-NTs would have either zero or very small net shift current if the rule is valid. This finding of the constant direction of the shift current is thus significant from an application standpoint. On the other hand, our results indicate that the exciton shift current spectra in BN nanotubes have a red shift as the diameter decreases. The red shift is caused by the orbital rehybridization generated by curvature. Our work thus suggests that the curvature and many-electron effects play important roles in the shift current generation in BN nanotubes and BN sheet.

References
[1] P. Krl, E. J. Mele, and D. Tomnek, Photogalvanic Effects in Heteropolar Nanotubes, Phys. Rev. Lett. 85, 1512 (2000).
[2] S. Konabe, Exciton effect on shift current in single-walled boron-nitride nanotubes, Phys. Rev. B 103, 075402 (2021).
[3] Y.-S. Huang, Y.-H. Chan, and G.-Y. Guo, Large shift currents via in-gap and charge-neutral excitons in a monolayer and nanotubes of BN, Phys. Rev. B 108, 075413 (2023).

Keywords: Excitons, First-principles calculations, Photocurrent, Nanotubes, GW method