Spin-selective orbital reconfiguration and colossal nonlinear anisotropy in defect-engineered atomically thin quantum dots

Abstract

Spin-texture and wavefunction modification is still a central problem in the attempt to control spin orientation and distribution, realize effective spin transport, minimize energy dissipation, and improve functionality in spin-based information processing. Here, we studied a specific combination of atomically thin, nonmagnetic, group-13-based post-transition metal chalcogenide (PTMC) quantum dots (M₁₀X₁₂; M = Ga, In, Tl; X = S, Se, Te) with a single transition metal (Zr, Mo, Mn) introduced at the central site using density functional theory. It is observed that the spatial distributions of frontier orbitals are not only spin-selective but also site-selective in real-space due to the induced TM–X₃ hybridization, which could allow independent tuning of both spin and spatial characteristics simultaneously. The normalized density overlap can differ by up to 10⁻³, whereas the normalized signed-amplitude overlap of the corresponding wavefunctions can reach values less than or equal to 10⁻⁴. The local and effective magnetic moments of the M₉TMX₁₂ structure can range from 1 → 7µ_B and 1.62 → 20.93µ_B, respectively, while the spatial extent of identical-spin orbitals can differ by up to ∼197 a.u. Moreover, the energy gap of the pristine combination ranges between 0.89 and 5.02 eV, and it widens upon TM substitution for E^↑_g (2.6–5.41 eV) while narrowing for E^↓_g (1.62–5.36 eV), which demonstrates that one spin channel can consistently remain energetically more accessible. A clear directional imbalance in the nonlinear optoelectronic response is observed, with the induced polarization switching between in-phase and out-of-phase (phase inversion) based on the field direction. The hyperpolarizability components reach maximum values of up to ∼10⁶ (β_‖) and ∼10⁹ (γ_‖, γ_⊥), while the relative distribution between parallel and perpendicular directions remains consistent. Such multifunctional responses resulting from a single-site TM perturbation facilitate fundamental insight into how localized electronic changes simultaneously modulate spin selectivity, orbital anisotropy, and nonlinear polarization as interdependent quantum variables in low-dimensional nanoflakes, which holds promise for the design of multivariable quantum information processing architectures and next-generation nano-spintronic materials.