Carrier-Induced and Coherent-Phonon-Controlled Band Renormalization in Quasi-One-Dimensional α-Bi4Br4
Peng Tan, Yuantao Chen, Yuqi Zhang, Hanyan Cheng, Guoyu Xian, Ming Cheng, Minghong Sun, Jiaxin Yin, Feifan Wang, Yaxian Wang, Yanjun Liu, Mingyuan Huang, Zhiwei Wang, Yugui Yao, Sheng Meng, Li Huang, Yanan DaiAbstract
Directional control of band structures at ultrafast time scales is a key requirement for nanoscale optoelectronics. Here, we realize cooperative band engineering in the quasi-one-dimensional (quasi-1D) van der Waals topological insulator α-Bi4Br4 through joint carrier-phonon band renormalization. Femtosecond excitation drives quasi-1D carrier accumulation, generating a strongly nonlinear many-body band renormalization that redshifts an optical transition by up to ∼25 meV, with >90% polarization anisotropy. Superimposed Ag coherent phonons further modulate the same transition by ∼1 meV on intrinsic lattice time scales. By implementing phase-controlled dual-pump excitation, we achieve deterministic and reversible positioning of the renormalized band edge with ∼100 μeV precision within a ∼1.5 meV tuning window. First-principles calculations with charge-neutral photoexcitation constraints qualitatively reproduce the carrier-induced narrowing of the relevant interband transition, while phonon-mode analysis reveals that coherent Ag phonons modulate the same transition through interchain hopping and orbital polarization. These results establish cooperative carrier-phonon band renormalization as a scalable, all-optical mechanism for ultrafast, polarization-sensitive band engineering in quasi-1D van der Waals nanomaterials, enabling dynamically reconfigurable optoelectronic functionality.