Spin-polarization anisotropy controlled by bending in tungsten diselenide nanoribbons and tunable excitonic states

A WSe2 monolayer shows many interesting properties due to its spin-orbit coupling induced spin splitting in bands around the Fermi level and the spin-valley configuration. The orientation of the spin-polarization in the relevant bands is crucial for the nature of exciton states and the optical valley selectivity. In this work, we studied the electronic properties of the WSe2 nanoribbons under different mechanical bending curvatures and electron/hole doping using density functional theory and their optical absorption and excitonic states using many-body perturbation GW and BSE (Bethe-Salpeter equation) methods. We found that the WSe2 nanoribbons can exhibit an enhanced SOC effect and a spatially varying spin-polarization in bands around the Fermi level under bending conditions. The spin-polarization can show an anisotropy (or asymmetry) in these nearly degenerate bands, leading to a controllable magnetism via bending and electron/hole doping of the nanoribbons, suggesting a potential application in compact and controllable magnetic nanodevices and spintronics. The optical absorption spectrum of the nanoribbon presents a large tunability with bending within the near infrared region of about 0.4 to 1.5 eV, showing an enhanced absorption under a large bending condition. The exciton states generally show mixed or various spin configurations in the electron and hole pairs that are controlled by bending, potentially useful for applications in spin-based quantum information processes.